Department of Mechanical and Mechatronics Engineering

Laboratory Manual

Winter, 2013 Instructor: Mustafa Yavuz (E3-3011), ext. 32093, myavuz@uwaterloo.ca Lab Supervisor: Mark Griffett (E3-2119D)

Teaching Assistants: Dulal Chandra Saha, dcsaha@uwaterloo.ca, Tel: 226-606-0119, Room: E3-3110A Andrew Michael, a2michae@uwaterloo.ca, Tel: 519-497-8842, Room: E2-4403

Melody Liao, l2liao@uwaterloo.ca, Ext. 38971, Room: E3-1703B

Place: Materials Laboratory (E3-2119) Times: Tuesdays 2:30 or 5:20 pm Wednesdays 2:30 or 5:20 pm

Laboratory Exercises

1. Elastic Properties of Cantilevered Beams 2. Mechanical Properties of Metals 3. Fracture and Failure 4. Polymers and Non-metals

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INTRODUCTION

In ME 115, you are introduced to the atomic or molecular structure of metallic, non-metallic and composite materials and learn how these different structures influence their properties and performance. For example, the properties of a metal like iron are very different from a non-metallic material such as natural rubber. Many of these differences can be explained by examining the atomic structure of these materials. In addition to this, the important roles of atomic arrangements and crystallographic defects on mechanical properties of substances will be examined. Finally, common service failures due to creep, fatigue, or fast fracture will be examined in light of the atomic structure of the different materials. This term, four laboratory sessions will be performed, with formal reports will be written for each one. While an attempt will be made to cover all lab-related material in lectures before the labs take place, this may not always be possible due to scheduling restrictions. Therefore, you might perform some labs before the relevant material has been covered in the lectures. However, the key knowledge will be introduced in the beginning of the lab session, and reinforced through the practical work. For each of these experiments, each student will hand his/her own report. Since you are all responsible for the report's content, it is important that everybody be involved in the lab report write-up (think of the divide and conquer approach). To ensure you are satisfied with the quality of the overall report, it is recommended that you complete your section well in advance of the due date for your other members to review and provide feedback. The sign-up sheet for lab groups will be done online (see the course website). Any other important information will be posted on the course website throughout the term. Lengthy reports are not required, but a good discussion of the results is important. Normally, reports will be no more than 10 typed pages (12pt, double spaced) including graphs and diagrams. Handwritten reports are also acceptable providing they are in ink and neat. If multiple topics are covered in the lab (e.g., Lab #4), there should still be only one of each of the sections listed above; although subdividing these sections into subsections for the different topics is recommended. Projects and Laboratory reports will be marked for content and understanding.

Safe operation of all laboratory equipment is paramount and must be carried out under the supervision of a TA or technician. If you are unsure of how to use a piece of equipment or are having trouble with some equipment, please notify a TA or technician immediately! Finally, please be considerate and leave laboratory equipment in as good or better condition than you found it.

Project and Laboratory Report Format The laboratory reports must be written in the style of a formal engineering report as follows below. The report should be type-written and double-spaced. The report should also be written in the third person, avoiding terms like ‘I saw’ and ‘we measured …’, instead one would write “The deflection was measured to be .…”. More information on technical report writing can be found in the Introduction to Professional Engineering in Canada, 2nd ed., by G.C. Andrews, J.D. Aplevich, R.A. Fraser, C. MacGregor and H.C. Ratz, Pearson Prentice Hall, Toronto, ON, 2006, your ME 100 textbook. If in doubt – ask!

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Title Page: The title page should contain the following information: • University of Waterloo • Department of Mechanical & Mechatronics Engineering • Course • Project or Lab Title • Professor’s Name • Group Number and Author Name (As per Policy 19 Access to and Release of

Student Information, do not include both student names & ID numbers on the title page, just the student names)

• Date (Title page is considered page i, but should not be labeled)

Abstract: The summary provides the reader with a short overview of the report. The

information should include the purpose of the report, work done, important results or discussion points and major conclusions. Consider that the reader wants to know: “Why you did it, what you did, what you observed or measured what you found that was interesting, and what you concluded.” (Page ii)

Introduction: The introduction outlines the purpose the work described in the report. The

definition of “the problem”, the purpose and the objectives are presented in a clear, concise manner. Some background information is often provided to help the reader understand the purpose and content of the report. The introduction should be short. (Page 1)

Experimental Apparatus and Procedures: This section should contain a description of all

experimental equipment used and procedures that were undertaken during the project or lab that were additional to that described in the lab manual. If no new information is needed, simply state that the experimental apparatus and procedures outlined in the Lab Manual were used. This section should be explicit enough that another person could carry out the same project or lab.

Results: In preparation for the project and each lab, and to make sure that you collect and

record all of the data that you need, you should prepare a data sheet before you perform each lab. The results section will include tables, graphs, sketches and sample calculations. Scientific or engineering notation should be used with no more than three significant digits (or else you will lose marks). All results and observations should be described in paragraph form; however, they should refer to figures or exact data held in tables and graphs. The tables and graphs summarize data clearly and show trends in the results. If there are excessive amounts of raw data, they could be placed in a table in an appendix.

Examples of table and graph formatting are shown below in Table 1 and Figure 1. Note that table numbers and captions should be above the table and figure numbers and captions should be below the figure. The caption should start with a figure or table number that is referred to in the text and the table or figure should be placed as soon after it is cited in the text as possible. Tables and figures should be numbered in the order they are referred to in the text. Graphs should be scatter (x-y) plots, where the independent data is generally listed on the x-axis. Scatter plots should show data points and when appropriate, their trend should be

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described by a “best-fit” curve or line as shown in Figure 1. This line can be computer generated (or hand-drawn along with supporting regression data in an appendix for example), however it is highly recommended that you use Excel or MatLab to present data. If computer generated figures are not presented, they must be very neat hand-drawn sketches. Note that the content and salient details in a table or figure must always be described in the text after they are cited. Do not leave it to the reader to interpret the data and important information in a table or figure, or assume that only the instructor or lab TA will read it and simply know what was done. Always tell the reader what important information is in a table or figure and any conclusions that can be drawn from this information.

Table 1: Mass and volume measurements of all specimens, converted to

densities.

Sample Mass (g)

Volume (mm3)

Density (g/mm3)

A 10.2 3.8 2.7 B 8.4 1.1 7.6 C 21.7 2.4 9.0

Some raw data may be manipulated, to give relevant information. Calculations using the experimental data may be contained in the results section. For example, volume, V, and mass, m, measurements in Table 1 may be converted to density, ∆, through Equation 1:

Vm

=ρ (1)

All equations should be formatted and numbered as shown above and all variables used in an equation must be defined.

Discussion: The discussion is an interpretation of the results and observations. It may also

include comparisons between experimental and published data. This section should include a discussion of the trends in the data, their relevance, and explanations for “why” the experiment produced these results. This may include what is, and is not understood. It may also include calculations that compare the data to published data, or manipulate the data for further examination.

The discussion should also include sources of error as well as their effects on the

experimental results. Each source of error should be specific and have a specific influence on the outcome of the experiment. Simply stating, “human factors” is not acceptable, properly designed experiments should remove or minimize human factors, this is the whole point of good experimental design. Of course, error caused by one’s ability to read measurements on a ruler, etc. are accounted for by the number of significant digits given to the measured value. For example, the measurement on a ruler would be 12.4 cm instead of 12.425 cm.

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Figure 2: Comparison of experimental and predicted fatigue lives using the damage model. Conclusions: This section clearly states the important findings from your experiment. These

conclusions must be based on results obtained from the experimental results and discussion. The conclusions may also relate to any purposes or hypotheses stated in the introduction. The conclusions section should not introduce new information. It should be short and concise. It may even contain “bulleted” statements for each conclusion derived from the experiment. For example:

Through the preceding experiment, it was found that the hypothesis, “people smile when they are happy” was only partially correct. It was found that people do smile when they are happy, but that they also smile under some other specific conditions such as: responding to another person’s smile, when being tickled, or pretending to be happy. References: When writing a report, different sources may be used to research the topic; for

example, to obtain material properties, experimental procedures and other information. Using published material is acceptable, but only if you do not attempt to claim this work as your own. If you fail to give credit to your source, it is called plagiarism which is unacceptable and an academic offence! If you use something from another source, it must either be given a reference number, e.g., [1,2]. Each reference must be listed in the reference section at the end of the report in the order encountered in the text and should include author, book or paper title, (journal title), publisher, city and date. This ensures that the information you are using is properly credited, reliable and can be verified.

References 1. ME 115 – Structure and Properties of Materials Laboratory Manual, Department of

Mechanical and Mechatronics Engineering, University of Waterloo, Spring 2012. 2. D.R. Askeland and P.P. Phulé, the Science and Engineering of Materials, 5th ed.,

Thomson Canada Ltd., Toronto, ON, 2006. Appendices: are optional and may be added to include additional information such as additional

calculations, raw data and plots that were not directly referenced in the report.

0

20

40

60

80

100

120

0 1 10 100 1000 10000 100000 1000000 10000000Nf (Cycles to Failure)

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Experimental

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ME 115 Lab 1

ELASTIC PROPERTIES OF CANTILEVERED BEAMS INTRODUCTION In many cases, engineers find themselves needing to build things or conduct tests with minimal resources, requiring some improvisation (as shown in the following video: http://www.asminternational.org/portal/site/www/NewsItemVideo/?vgnextoid=a5cf58caf8a56310VgnVCM100000621e010aRCRD). The purpose of this project is to investigate the elastic behavior of three different materials by measuring the deflection of cantilever beams, using simple hardware and equipment. The term “elastic behavior” means that the material deforms when placed under a load, but returns to its original shape and dimensions when the load is removed. In this project, you will compare the elastic behavior of steel, aluminum and wood. Although a steel beam will normally support more load than an aluminum or a wooden beam of identical dimensions, the mass of the beam is often an important factor. In reality, when we are considering how to design a structure, it is really the strength, as well as the stiffness per unit of mass that is controlling the behavior of the design. When comparing the performance of these different materials, it is informative to examine this value, which is referred to as the specific modulus. The specific modulus is used to aid in material selection when both mass and stiffness are important design criteria, such as in the design of aircraft and other transportation vehicles. The specific modulus is defined as

)/()(

3mkgDensityPaModulusElasticEModulusSpecific ==

ρ (1)

In some applications, a material with the highest yield strength and lowest weight is required instead. The yield strength, σy (MPa), is the highest stress a material can support without permanent plastic deformation or permanent change of shape, i.e., up to this maximum stress, it behaves as an elastic solid, returning to its original shape when the load or stress is removed. In this case, the material with the highest specific strength is the most suitable material. The specific strength is defined as

)/()(

3mkgDensityPaStressYieldStrengthSpecific y ==

ρσ

(2)

It should be noted that while steel and aluminum normally have isotropic material properties, wood generally has very anisotropic material properties. In other words, the properties of wood vary depending on the direction of the applied load. Wood has the highest strength when the load is applied parallel to the grain. It has the lowest strength when the load is applied perpendicular to the grain. The goals of this project are:

1) To gain some hands-on experience with different materials from an engineering point of view.

2) To improve report writing skills. 3) To gain experience in experimental methods. 4) To begin to learn how to use the material selection software CES EduPack [1].

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You will be supplied with one piece of cold rolled, low carbon (AISI 1018, “mild” steel) plain carbon steel, one piece of extruded 6061-T6 age-hardenable, wrought aluminum alloy and one piece of pine wood. The approximate dimensions of these specimens are 38 mm wide × 3 mm thick × 305 mm long. The wood specimen will be 6 mm thick. You will also be supplied with a piece of 2" × 4" pine wood. If your samples do not have a hole for hanging the weights, you will have to do this in the Engineering Student Machine Shop (E3-2101). Mark-out and drill a 5 mm diameter hole in one end of the steel, wood and aluminum specimens located about 7 mm from one end and centred across the width of the specimen. This hole will be used to hang the weights. In the Materials Lab. (E3-2119), there are two measurement stations. At one of these stations: 1) Measure the dimensions of all three specimens, and then fix one end of the steel bar using the

clamp. Apply different loads to the end of the cantilever beam and measure the transverse deflection of the end of the steel beam as a function of the applied load. Note that you should measure the deflection at the same point that the load is applied. Plot your data on a load-deflection graph.

2) Repeat this procedure using the aluminum and the wood specimens. 3) In the Engineering Student Machine Shop, construct a wooden beam from the 2” × 4” pine

wood supplied that has the same load-deflection characteristics as the steel beam. Measure and plot the load-deflection behavior. Note: use the same length and width as the steel beam, but vary the thickness.

][ Thickness Beam][ Width Beam

:

)4(12

][Area of Moment Second][y Elasticitof Modulus

][ Beam of Length][ Force Applied

:

)3(3

3

4

3

mtmw

where

twI

andmI

PaEml

NFwhere

IElFy

==

=

=

===

=

Figure 1: Elastic deflection, y, at the end of a cantilever beam with a point load, F, applied at the end of the beam [2]. 4) Plot the theoretical load-deflection curves of all three materials using published values for the modulus of elasticity, E, of each material. The deflection of the end of the beam at the point of load application, y, can be calculated using the following diagram and equations [2]:

RESULTS 1) Report your findings using plots of load versus deflection for all four beams. Use scatter

plots for your experimental data, i.e., discrete data points, with a dashed line or curve of best-fit to represent your results. Also, using Equations (3) and (4), plot the predicted behaviour using published values for the modulus of elasticity (use continuous solid lines) found in the

l

F

y

lw

tF

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materials selection software, Cambridge Engineering Selector (CES) [1], available on NEXUS. (Hint: Use the Materials Universe, Level 2 data base of materials properties.)

2) Weigh each specimen, then calculate and tabulate the mass, density, measured Modulus of Elasticity, E, published values of the yield strength, σy [1], specific stiffness and specific strength of each material.

DISCUSSION 1) Compare the load-deflection curves for the wood, aluminum and steel beams. Describe how

you determined the dimensions for the wooden beam. Does Equation (3) accurately predict the behavior of each beam? If not, explain why. Calculate the modulus of elasticity for the wood, aluminum and steel using your experimental data and Equation (3). How do your calculated values compare to the published values available in CES?

2) Compare the mass, density, elastic modulus, yield strength, specific stiffness and specific

strength of the three beams? The most appropriate material for an application will depend on the design constraints and criteria for the part. For example,

1. An unsupported balcony is essentially a cantilever beam that must not deflect excessively when you walk on it or you will feel insecure. Assuming the specific stiffness is the most important material property for such applications, rank the three materials tested in order of preference for fabrication of balconies.

2. An aircraft wing is essentially a cantilever beam that must never fail under load! Assuming the specific strength is the most important material property for such applications, rank the three materials tested in order of preference for fabrication of aircraft wings.

3. If a cantilever beam must be durable, i.e., it must be maintenance free and resistant to corrosion, oxidation and other degradation due to exposure to the environment throughout its service life, rank the three materials tested in order of durability.

3) Predict the thickness of a wooden beam that would have the same mass as the steel beam.

Also, predict and compare the deflection of this wood beam and the steel beam under a force of 20 N.

NOTES 1) Metric units should be used throughout. 2) Forces should be in units of Newtons: (1N = 1 kg × 1 m/s2) or (9.81 N = 1 kg × 9.81 m/s2). References

1. CES EduPack 2008, Granta Design Ltd., Cambridge, UK. This materials data-base and materials selection software is available to you on NEXUS.

2. R.C. Hibbeler, Mechanics of Materials, 6th ed., Pearson Prentice Hall, Upper Saddle River, New Jersey, USA, 2005 (ME 219 Textbook)

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LABORATORY EXERCISE # 2

Mechanical Properties of Metals INTRODUCTION Metals are probably the most important engineering materials, especially for mechanical engineers. In order to determine the properties of a particular alloy or alloy shipment, a simple tensile test is commonly carried out. This test records the load required to achieve a given amount of deformation. Depending on the equipment used, the deformation may be that of the specimen alone or the deformation of the specimen and the machine. By measuring the dimensions of the specimen, these measurements can be converted to stress versus strain. In addition to furthering your understanding of tensile test methods, you will see how (i) the shapes of the stress-strain curves are different for different alloys and (ii) the stress-strain curve for a given alloy depends on the previous history of the part. Different metals may have different features in their stress-strain curves. All of them show an initial linear elastic deformation range. In the elastic region, the stress required to cause a given strain can be estimated knowing the elastic or Young's Modulus. The stress level separating the recoverable elastic deformation from the permanent plastic deformation region is given by the yield stress. The form of this elastic-plastic transition depends on the alloy. Alloying elements which dissolve interstitially (eg., carbon or nitrogen in iron) often result in a metal that exhibits a sharp upper yield point followed by a drop to a lower yield point and then a gradual increase in stress as plastic deformation begins. This is caused by strain aging. Pure metals and substitutional alloys (eg., copper and 70-30 brass) will most often exhibit a gradual transition from elastic to plastic behaviour. In this case, the yield stress is usually defined by the 0.2% offset yield stress. In manufacturing metal parts, ductility is an important property. There are several ways to measure ductility, including the strain to fracture, % elongation, and the reduction of area. It may also be important to know the ultimate tensile strength (UTS), especially if the metal is to be worked to a new shape without breaking. Plastic deformation of metals is accomplished by the motion of dislocations. Therefore, any process that either hampers or even completely prevents the motion of dislocations will inhibit plastic deformation. When dislocation mobility is restricted, the metal will possess a higher yield stress, a higher ultimate tensile strength and a higher hardness. Cold working or "strain hardening" is one of the methods used to restrict the motion of dislocations. Cold working increases the dislocation density to such a level that the interactions between the dislocations become strong enough to block further motion. The dislocations may only be made to move again if the applied stress is increased. When a metal is subjected to progressively increasing amounts of cold working, the hardness increases but the ductility decreases. Depending on the application, the remaining ductility or elongation of the cold worked metal may be insufficient to permit further required deformation; for example, to shape the sheet into a given part. Fortunately, the original soft properties can be restored by annealing. For a given alloy, the properties are also dependent on the thermal and mechanical history of the specimen, because these histories affect the material's microstructure. In mild steel (iron plus about 0.2 wt% carbon), for example, the upper and lower yield point may disappear on retesting if plastic deformation has just taken place. However, upon retesting some time later, the sharp

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upper and lower yield point may reappear. This phenomenon is called strain aging. In all alloys, the amount of plastic deformation possible depends on whether the specimen has been deformed previously at the same temperature. ANNEALING When a metal is cold worked, most (≈ 90%) of the energy expended on the metal is converted to heat, but a small amount is retained in the deformed metal structure in the form of strain energy. On heating, this stored strain energy acts as a driving force that tries to return the metal to the undeformed state. For this to occur, the metal must be heated to a temperature where the reactions can occur at an observable rate. During this reaction period, the cold worked metal may pass through three stages: 1. Recovery 2. Recrystallization and 3. Grain growth Recovery is the first stage where dislocations of opposite sign begin to annihilate each other. During the recovery stage, however, the grain structure remains unaffected and the hardness and other mechanical property values will show little change. Recrystallization occurs at higher temperatures or longer annealing times. During this stage, the distorted, cold-worked grains are transformed to smaller, strain-free equiaxed grains. Consequently, the hardness and strength will decrease, approaching their predeformed values. Full recrystallization occurs when the metal is completely composed of equiaxed, strain-free grains. The effects of time and temperature on recrystallization are similar, in that an increase in the annealing temperature has an analogous effect to an increase in the annealing time. However, quantitatively, they are different: doubling the temperature does not have the same effect as doubling the time. In this lab exercise, the effect of temperature will be demonstrated. Grain Growth of these new strain-free grains will occur if the temperature is kept high after recrystallization. Grain growth is usually not considered beneficial, because it will cause the hardness and strength of the metal to decrease slightly. This occurs because of the decrease in the number of grain boundaries that impede dislocation movement. APPARATUS - Rockwell superficial hardness testing machines (set for 15-T scale) - Electric furnaces - Instron testing machine with a 15,000 kg load cell - Strain gauge extensometer and X-Y plotter - Micrometer and Vernier calipers SAMPLES - 2 Mild Steel (AISI 1018 plain carbon steel alloy) Tensile Specimens - hot rolled - 1 Copper (C10200 alloy) Tensile Specimen - 1/2 hard - 1 Copper (C10200 alloy) Tensile Specimen - fully annealed (700°C for 1 hr. then air

cooled) - 1 70-30 Brass (C26000 alloy) Tensile Specimen - 1/2 hard - 1 70-30 Brass (C26000 alloy) Tensile Specimen – fully annealed (700°C for 1 hr. then air

cooled) - 1 machined 6061-T6 aluminum alloy tensile specimen from your project.

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EXPERIMENTAL PROCEDURE 1) At least one week prior to attending this lab exercise, each lab group should machine the

bar of aluminum used for the project into a tensile specimen (see the appendix at the end of the project manual for the specimen dimensions). This specimen will be tested to failure during this laboratory exercise. Also, each group should review the lab procedure carefully and prepare a data sheet for recording all measurements and other data obtained during the lab exercise.

2) Remove the Cu and brass specimen from the annealing furnace, allow to air cool, then

measure the width and thicknesses of all tensile specimens. Record these measurements in your data sheets.

3) Measure the 15-T Rockwell superficial hardness on the end tabs of the two 70/30 brass

specimens and the two copper specimens (See operating/testing instructions at the end of this lab). Note: You must sand the oxide off the top and bottom surfaces of the annealed Cu and brass specimens prior to hardness testing or you will get incorrect hardness readings.

4) Install the first steel specimen in the Table-top Instron Tensile Tester with the extensometer

and set the test conditions for an “elastic-region-only” test (cross head speed = 0.5 mm/min, extensometer readout to XY plotter). Set your plotter and test the specimen in the elastic region (i.e, do not exceed the yield stress). At the end of the test you should record the final load, extensometer reading and cross-head travel from the LCDs on the Instron controller. Unload the specimen and remove the extensometer. Use this data to calculate the Young’s Modulus of the steel.

5) Repeat step 4 using the machined aluminum specimen from your project. Use the data from

this test to determine Young’s modulus of the aluminum specimen. 6) Install the first steel specimen on the larger Floor-standing Instron tensile tester with no

extensometer and proceed to set the test conditions as for a “complete tensile test” (cross-head speed = 15 mm/min, machine readout to computer). Test the specimen to a load above the yield point, but before the UTS and observe the yielding behavior. Unload the specimen and heat-treat it at 200°C for at least an hour.

7) Install the second steel specimen in the large Instron and strain it beyond the yield point, but

before the UTS. Unload it to zero stress, measure the cross sectional area, then reload it to fracture. Note especially the yielding behavior of the new yield point.

7) Test to failure all remaining specimens using test conditions for a “complete tensile test”

(cross head speed=15 mm/min, machine readout to the computer). 8) After 1 hour at 200°C, test the heat-treated steel specimen to failure. Compare the yielding

behavior of this specimen with the reloading yielding behavior of the second steel specimen that was tested in step 7.

9) Measure the final cross-sectional areas of all specimens.

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RESULTS 1) From the extensometer tests, calculate Young's Modulus for your aluminium and steel

specimens. Calculate the percent difference between E measured from your aluminum and steel specimens and E reported in the literature for these metals, e.g., from CES EduPack [3] and ASM Metals Handbook, Vol. 1 [4] & Vol. 2 [5].

2) Calculate the yield stress, ultimate tensile stress, and fracture stress and % elongation for the

half-hard and annealed copper and brass specimens. 3) Sketch the engineering stress-strain curves for the partially annealed steel specimen

(200°C), the fully annealed copper specimen and the fully annealed brass specimen. 4) Plot an engineering stress-strain curve and a true stress-strain curve on the same graph for

the annealed copper specimen. DISCUSSION In your discussion, be sure to include the following: 1) Discuss how alloying affects the properties of metals with reference to your results. 2) Explain why mild steel can have a sharp upper and lower yield point, while copper, brass

and aluminum do not. 3) Discuss the differences in strain (or work) hardening behavior for the different alloys or

metals. Use steel (200oC), annealed copper and annealed brass to illustrate this. How does this relate to the strain hardening exponents in Askeland & Phulé [2] for these alloys or metals?

4) Discuss how the stages of annealing affect the properties of metals with reference to your

results. How is the microstructure (i.e., grain size) changing throughout the process of annealing? What happened to the steel specimen due to the partial anneal?

5) Explain the difference between engineering and true values for stress and strain and why the

UTS does not correspond to the maximum true stress. 6) Using CES EduPack [3], plot a graph of hardness versus ultimate tensile strength of just

metals and comment on the resulting trend(s). REFERENCES 1. Callister, Materials Science and Engineering: An Introduction 2. Askeland & Phulé, The Science and Engineering of Materials, 5th ed., 2006, Thomson

Canada Ltd., Toronto, ON, Canada 3. CES EduPack 2008, Granta Design Ltd., Cambridge, UK. This materials data-base and

materials selection software is available to you on NEXUS. 4. Metals Handbook, Vol. 1, 10th ed., Properties & Selection: Irons, Steels, and High

Performance Alloys, ASM International, Materials Park, OH 5. Metals Handbook, Vol. 2, 10th ed., Properties & Selection: Nonferrous Alloys and Special-

Purpose Materials, ASM International, Materials Park, OH 6. Metals Handbook, Vol. 8, 10th ed., Mechanical Testing, ASM Int’l, Materials Park, OH

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Rockwell Hardness Tests Hardness is a measure of the resistance of a material to plastic deformation due to penetration of the material by a sharp object or indenter. In general, the hardness of a metal is roughly proportional to the ultimate tensile strength; however, a hardness test can be performed in a fraction of the time and cost required to perform a tensile test. Therefore, this non-destructive test is frequently used as a quality control test of the strength of a metal part. The Rockwell hardness test, as described in the ASTM standard E 18, is a very common macroscopic hardness test [6]. The Rockwell hardness test has a number of different scales for testing a range of hard to soft materials. Each scale uses a specific shape of indenter such as a diamond cone for hard materials and a 1/16 inch diameter hardened steel ball for softer materials and different indenter weights. In all cases, the Rockwell hardness number is based on the difference of indenter depth from two load applications (see Figure 1). Initially, a minor load is applied, and a zero datum is established. A major load is then applied for a specified period of time, causing an additional penetration depth beyond the zero datum point. After the specified dwell time for the major load, it is removed while still keeping the minor load applied. The resulting Rockwell number represents the difference in depth from the zero datum position as a result of the application of the major load. Use of a minor load greatly increases the accuracy of this test, because it eliminates the effect of backlash in the measuring system and causes the indenter to break through slight surface roughness. The entire procedure requires only 5 to 10 s. Note that the plastic deformation that takes place in the material during indentation cold works (and hardens) a large volume of the material around the indentation. Therefore, to avoid erroneous hardness measurements, the indentation should be a minimum of three indenter diameters from the centre of any previous indentations or specimen edges, e.g., when using a 1/16 inch diameter steel ball indenter, the indentations should be at least 3/16 inch apart. Also, the top and bottom surfaces of the specimen/part must be clean and free of any oxides, mill scales, paints or other soft coatings.

Figure 1: The Rockwell hardness test is based on measuring the distance, d, that the indenter moves into the material when the major load is applied after the minor load. Although a diamond indenter is illustrated, the same principle applies for steel ball indenters and other loads. (taken from ASM Metals Handbook, Vol. 8 [6].)

d

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Rockwell Superficial Hardness Test Procedure using the 15T Scale

The Rockwell superficial hardness test uses a 1/16 inch diameter hardened steel ball indenter and a number of different major loads. Figure 2 shows the Rockwell hardness tester that will be used to measure the Rockwell superficial hardness of the copper and 70/30 brass tensile specimens used in this lab. To perform a superficial hardness test using the 15T scale, first check that the 1/16 inch steel ball indenter as shown in Figure 3 has been installed and then perform the following steps:

1) Clean all oxides, paints, etc., off the top and bottom surfaces of the specimen to be tested.

2) Insert the copper or brass tensile specimen with the burr side up between the anvil and indenter guard and raise the anvil by turning the spindle until the specimen is snug lightly in place as shown in Figure 3.

3) On the control panel above the anvil, push the MENU button on the bottom left and then START TEST button on the bottom right to get to the ROCKWELL SETUP screen shown.

4) Push the upper left button to SET SCALE. 5) Push the middle left button to select SUPERFICIAL hardness

test. 6) Push the top right button to select the 15T superficial

Rockwell hardness test. 7) Push the top left button to select the STEEL BALL 8) Push the bottom right button to start the TEST. The controller

will automatically lower the indenter, apply the minor load and then the major load and finally release of the major load. The measured superficial hardness will be displayed on the screen, e.g., MEAS 60 HR 15TS.

9) To perform another test, lower the anvil using the spindle, move the specimen so that the next indentation will be at least 3/16 inch away from any other indentation, raise the anvil and snug lightly in place and then press the bottom right button for TEST #2.

Figure 2: Microprocessor controlled Rockwell hardness tester.

Figure 3: Anvil, indenter and tensile specimen snugged lightly between the anvil and the indenter guard.

Figure 4: Rockwell hardness tester display screen and controller buttons.

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Notes on Machining Specifications for the Tensile Specimens

(As per ASTM E8M-04 Standard Test Methods for Tensile Testing of Metallic Materials)

Notes: 1) Do not alter the original specimen width. 2) Do not alter the original specimen thickness. 3) The radius should not be less than 12.5 mm, i.e., use a 25 mm (1 inch)

diameter or larger milling cutting tool. Also, there should not be any undercut at the radius as shown below. This will create a stress concentration and decreased cross-sectional area at this location that will cause the specimen to fail here rather than in the gage length thereby invalidating the tensile test.

GOOD BAD

12.5 mm

30 mm 100 mm 30 mm

160 mm

12.5 mm R min.See Note 3

See

Not

es 1

& 2

7.0 mm

5 mm Dia. Hole

16

LABORATORY EXERCISE # 3

Fracture and Failure (Creep, Fatigue, Ductile/Brittle Transition Temperature, and Fracture Toughness)

INTRODUCTION In this laboratory exercise, tests used to simulate certain types of common service conditions are examined and a case study carried out on a failed component. This laboratory exercise will include: A. Creep Testing - using a simple constant-load creep rig. Such tests are used to examine the

flow of materials with time at high temperatures, as well as creep embrittlement. B. Fracture Impact Test - using the Charpy impact test. These tests are used to study the

ductile/brittle transition in steel and other alloys. The impact behavior of aluminum will also be studied.

C. Fracture Toughness Test - using the Instron tensile testing machine. This test will be used

to study the fast fracture phenomenon and establish the critical fracture toughness of Plexiglas (with 5, 10, 15 mm deep edge notches).

A. CREEP

Service failures can arise at constant stress as well as after repeated cyclic loading. When a specimen is subjected to a load, an instantaneous elastic deformation results and, if the load is high enough, there will be some plastic flow. Due to strain hardening, this deformation will soon stop under normal conditions. However, if the test is carried out at a high temperature where recovery processes occur at a significant rate, there will be continued plastic flow with time. Creep may be defined as time dependent plastic flow under constant stress. For a constant stress, the characteristic creep curve for "flow" or strain against time is shown below in Figure 1. There are four main regions of this curve:

a) Initial instantaneous or time independent elastic deformation; b) Primary Creep - where the rate of strain decreases with time; c) Secondary Creep - a period of steady strain - the most important regime for engineering

design (since it can be characterized mathematically as discussed below); d) Tertiary Creep - where necking occurs and the rate of strain increases until final fracture

occurs.

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Figure 1. A characteristic creep deformation plot of strain versus time.

The following equation relates the secondary creep-rate, ἐII , to the creep stress, σ, absolute temperature, T (K), activation energy for creep, Q, and the Universal Gas Constant, R,

RT

Q exp nII

−

∝ σε (1)

The exponent n is a material dependent constant which is often close to 5 for metals. Another important mechanism for creep deformation is grain boundary sliding. With this time-dependent deformation, the deformation is concentrated along the grain boundary surfaces. As a consequence, it is common for materials to develop cracks and cavities at the grain boundaries during creep. Sometimes a very brittle fracture results, so that the creep toughness and ductility of materials can be very low and give rise to marked high temperature embrittlement. This type of failure arises due to local decohesion at points of stress concentration (e.g., inclusions) on the sliding grain boundary followed by linkage of such cavities and subsequent crack propagation. Thus, Inconel X-750, a high temperature gas-turbine alloy, exhibits a creep ductility of less than 1% and fails at 700°C in a brittle, catastrophic manner along the grain boundaries. In this lab exercise, the main experiment is concerned with the creep of lead in which the fracture is transgranular by a ductile void sheet process, with more plastic deformation prior to fracture. The purpose of this experiment is to study the stress dependence of creep of lead. Lead is chosen because the creep process can be usefully studied in this material at room temperature. Room temperature is a "high" temperature for lead, it being approximately half its absolute melting temperature. Creep of lead at ambient temperature is equivalent to the creep of steel at about 600°C.

STR

AIN

TIME

I II III Fracture

PrimaryCreep

SecondaryCreep

TertiaryCreep

Slope = ε =ΙΙ

recovery ratehardening rate = constant( )

TimeIndependantDeformation {

18

APPARATUS - dead-load creep apparatus. - 3, one meter long, 2.54 mm diameter lead wire specimens - Vernier calipers - timer EXPERIMENTAL PROCEDURE 1) Place the wire specimen in the apparatus and measure its diameter. 2) Apply a load (try about 14-15 lb.) to the wire and record the elongation at 10 second

intervals during the first minute, then at 1-1/2 minutes and at minute intervals from 2 minutes onwards until failure. Note the elongation at failure if possible.

3) Other groups will repeat (1) and (2) using other weights suggested by the instructor. RESULTS 1) Plot an engineering strain versus time curve for all specimens on a single linear graph and

determine as accurately as possible the strain rate (ἐII or dεII/dt) in the steady-state region of each curve. Include the additional data provided by the instructor in your plots.

2) Plot your data on a graph of log (strain-rate) versus log (stress) using the steady-state strain

rates obtained above. Determine the stress exponent, n, and the constant, A, from the plot for the empirical creep design relation:

σε nII A = (2)

3) Plot the total strain at failure versus the engineering stress for each specimen. DISCUSSION In your discussion, be sure to include the following: 1) Describe the three stages of creep. How can the effects of creep be minimized in a high

temperature application? 2) Discuss the results of your calculation of the empirical creep parameters including possible

sources of error in the experiment. 3) Consider the effect of load on the creep by discussing your findings from the strain at failure

versus engineering stress curve.

B. FRACTURE IMPACT TEST In an ordinary tension test, steels are normally ductile. But at high strain rates or impact conditions, steel, which has a body-centered cubic crystal structure, can behave in a brittle fashion. On the other hand, some other materials with face-centered cubic crystal structures such as aluminum and copper do not become brittle even at very low temperatures. The temperature at which a steel (or weld) becomes brittle is of considerable importance. An example of the current importance of brittle fracture in steel is in the Arctic pipelines - but there are many, many others - in ships (e.g., WWII Liberty Ships and the Titanic), aircraft, off-shore oil rigs, etc. Plastics can undergo similar transitions in fracture behavior. The notched-bar impact test has been widely used to measure the effect of a number of variables

19

on the ductile-to-brittle transition. The value of the impact test lies in the fact that it reproduces the ductile-to-brittle transition of steel in the same temperature range as it is actually observed in engineering structures. As may be seen in Figure 2 below, there is no single temperature at which an average BCC metal suddenly becomes brittle; the transition occurs over a range of temperatures. Still, as a matter of convenience, it is common practice to speak of the "transition temperature" of a metal. This term, however, needs to be carefully defined, as there are a number of different ways of expressing it. One is to take the temperature at which an impact specimen fractures with a half-brittle and half-ductile fracture surface. If the fractured surfaces of the impact specimen are examined after fracture, it is generally found that there is a reasonable correlation between the amount of the fracture surface that has broken in a ductile fashion and the energy expended in breaking the specimen. Therefore, the shift from ductile-to-brittle behavior can be followed by examining fractured surfaces of impact specimens. Completely ductile specimens exhibit surfaces that are rough or fibrous, while those of brittle specimens contain an irregular array of small bright facets, each corresponding to the surface of a cleaved crystal. In those specimens where the fracture is part ductile and part brittle, the brittle, or bright, area is found at the center of the cross-section. The completely ductile specimen shows a very large distortion of the specimen that occurred during fracture. In the completely brittle specimen,

the fracture cross-section is almost a perfect square, showing that fracture occurred with negligible plastic deformation.

Figure 2. Characteristic absorbed energy versus test temperature curve for a BCC metal. A second way of defining the transition temperature uses the average energy criterion: the temperature at which the energy absorbed falls to one-half the difference between that needed to fracture a completely ductile specimen, and that needed to fracture a completely brittle specimen. The temperature at which the specimen breaks with a fixed amount of energy, usually 15 or 20 ft-lb (20 or 27 J, respectively), is also a widely employed basis for the transition temperature. The composition of a ferrous (BCC) alloy has a very pronounced effect on the transition temperature. In general, other variables being constant, the higher the carbon content of a commercial steel, the more liable it is to brittle fracture near room temperature. Phosphorus has an even stronger deleterious effect on the transition temperature of steel. This is one reason why phosphorus is not desirable in ordinary carbon steels. Some elements, for example, manganese and nickel, seem to have the reverse effect and lower instead of raise the transition temperature. Recent trends in the design of steel for use in the hulls of ships have, therefore, tended toward a

Impact Test Temperature

AbsorbedEnergy

Brittle Failure Ductile Failure

TransitionTemperature

20

more liberal use of manganese in order to take advantage of the lower transition temperature. APPARATUS AND SPECIMENS - 5 AISI-1045 steel Charpy specimens. - 2 aluminum Charpy specimens [6061-T6 annealed at 775°F (440°C) for 3 hrs & air

cooled]. - impact testing machine. - furnace and hot plate. - low temperature baths. EXPERIMENTAL PROCEDURE 1. Perform the following impact tests: AISI-1045 steel at: 120°C (FURNACE) 60°C (OVEN) 0°C (ice bath) - 60oC (“dry ice”) -190°C (liquid nitrogen) Aluminum at: 60°C (OVEN) 0°C (ice bath) -190°C (liquid nitrogen) NOTES: a) The impact tester looks and is dangerous! These tests must always be carried out

under the supervision of a laboratory technician and/or TA demonstrator. b) The temperature at the root of the notch begins to change as soon as the sample is removed

from the heating or cooling medium. The ASTM standard for the Charpy impact test requires that the delay time in transferring the specimen and performing the test must be less than 5 seconds.

c) Record the energy required to fracture each specimen at the different temperatures. d) Examine all fractured surfaces. Assess the amount of ductile and brittle fracture for each

specimen. RESULTS 1) Plot the energy absorbed as a function of test temperature for both the steel and aluminum

specimens. Indicate the “upper shelf” and “lower shelf” energies for steel on the graph. 2) Plot the amount of ductile fracture as a function of test temperature. 3) Determine the ductile-to-brittle transition temperature for the steel. DISCUSSION In your discussion, be sure to include the following: 1) Discuss the transition temperature results for both methods. What are the “upper shelf” and

“lower shelf” energies? 2) Why do some metals not show low temperature embrittlement? 3) How can steels be made less brittle at low temperatures? 4) Discuss the accuracy of the results and possible sources of error.

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C. FRACTURE TOUGHNESS Charpy tests are useful to qualitatively compare different materials' ductile/brittle behavior, but they do not provide suitable numbers for design, partly because the specimen geometry is not suitable. Small cracks are present in most engineering structures, particularly those of welded construction. These cracks may or may not be detectable using standard non-destructive testing methods. When the structure is loaded during normal service, these cracks will either be stable or will propagate through the member at the speed of sound causing complete collapse of the structure. This fast fracture may occur even if the nominal applied stress is much less than the ultimate tensile stress of the base material. Many spectacular failures (bridges collapsing, ships sinking, etc.) have been attributed to fast fracture, so it is important that we gain an appreciation of this phenomenon. As shown in Figure 3 below, there are three modes of rapid fracture. These are distinguished by the direction of the applied load relative to the crack. In this experiment, we will be concerned with Mode I failure.

(a) (b) (c) Figure 3: Modes of rapid fracture; (a) Mode I, (b) Mode II, (c) Mode III. In a typical fracture mechanics test, a specimen with the geometry shown in Figure 3a is loaded under tension. If the crack depth, a, is large compared to the specimen thickness, t, a plane stress condition exists around the crack tip. Plane stress conditions are typically encountered in thin specimens where the stresses in the plane of the sheet may be significant, but the stress through the thickness of the part is zero, i.e., σz = 0. If the crack depth is much smaller than the specimen thickness, a condition of plane strain exists around the crack tip. Here, there is sufficient constraint from the surrounding material that εz = 0. The crack tip acts as a stress riser in that extremely high stresses are generated locally around the crack tip. From linear elastic theory, stresses near the crack tip when no plastic deformation occurs are given by

)(2

),( θπ

θσ fr

Kr = (3)

IIIIII

w

a

tF

F

22

where r and θ are polar coordinates radiating from the line of the crack tip, and K is a constant called a stress intensity factor. Note that the stress intensity factor is not a stress concentration factor. The stress pattern around a crack tip may be seen using photoelastic techniques. Here, a translucent polymer fracture specimen is loaded and viewed through a set of polarizing filters. A fringe pattern is produced which corresponds to the stress distribution within the material. For a Mode I geometry, the stress intensity factor, KI, is given by

afK I πσ= (4) where KI is the stress intensity under Mode I opening σ is the nominal stress = Force/area = F/tΑw, a is the crack length, t is the specimen thickness w is the specimen width and f is a geometric correction factor that takes into account the effect of

specimen geometry and crack size. Note that f is sometimes placed inside the square root term with a.

For an edge crack loaded in Mode I (see Fig. 3a), f may be calculated using the empirical equation [4]:

cos

sintan

•

w2a

w2a - 1

3 0.37 + (a/w) 2.02 + 0.752

w2a

a w2 = f

π

ππ

π (5)

Equation (5) will predict values of f within ±0.5% for any value of (a/t). The conditions under which one may expect rapid fracture to occur are given by the relationship:

CCICI afKK πσ== (6)

where KIC is the critical fracture toughness of the material in Mode I fracture and σC and aC are the stress and crack length when fast fracture occurs. KIC is a material property. Note that while KIC is a constant for any given material, there are an infinite number of combinations of σC and aC which will cause fast fracture. Thus, Equation (6) is a very simple but powerful relationship, because it allows us to predict the conditions under which fast fracture will occur without requiring detailed knowledge about the stress field about the crack tip. The critical fracture toughness is a characteristic of the material. A high KIC is indicative of a tough, ductile material which opposes crack propagation, i.e., high values of σ or a are required before brittle/fast fracture will occur. On the other hand, a low KIC is characteristic of a brittle material through which a crack will propagate very easily resulting in catastrophic failure of the member with very low values of σ or a. For example, KIC for mild steel is 140 MPa × m1/2 and for soda glass is 0.8 MPa × m1/2 at room temperature. KIC is dependent on both the yield strength of the material and the temperature. In engineering designs using fracture mechanics, three variables must be considered; the

23

minimum flaw size "a" that can be detected, the nominal stress "σ" and the critical fracture toughness of the material KIC. The value of "a" is given by the resolution of the detection equipment. The nominal stress is given by the mechanical design. Using Eqn. (4), a material must be selected with sufficient fracture toughness that the designed structure will not fail by fast fracture due to a crack which cannot be detected during a routine inspection procedure. THE EXPERIMENT The purpose of the following experiment is to study the fast fracture behavior of Plexiglas and to determine its critical fracture toughness. Plexiglas is used in these experiments because of its photoelastic characteristics. APPARATUS AND SPECIMENS - 3 Plexiglas (polymethyl methacrylate: PMMA) fracture specimens. - Instron tensile testing machine with the photoelastic apparatus. - micrometer and ruler EXPERIMENTAL PROCEDURE 1) Measure the dimensions of all three specimens. 2) Load each specimen in the Instron and observe the stress pattern that develops around the

crack tip by using the photoelastic apparatus. Make sure you rotate the polarization filters to obtain the best contrast possible.

3) Load each specimen to failure and record its load-displacement curve. RESULTS 1) Sketch and label the stress pattern that can be seen around the crack tip using the

photoelastic apparatus. 2) For each specimen, calculate the fracture toughness. Note: when calculating f, make sure

that your calculator is in radians. Compare the calculated values with each other and with a published value [5].

DISCUSSION In your discussion, be sure to include the following: 1) Discuss the results of the fracture toughness calculations for the 3 specimens. Are the

results the same? Why or why not? 2) Using CES EduPack [5] create a plot of yield strength versus fracture toughness and identify

and label the different family groups, e.g., metals, polymers, ceramics, etc. Which group of materials has the highest strength and fracture toughness? Which group has the lowest? Compare the calculated fracture toughness for Plexiglas (PMMA) with the published value and also with an age-hardened wrought aluminum alloy, with low carbon steels and with alumina (Al203), a ceramic [5].

3) Describe the fracture surface of the specimens. Is it a brittle or ductile fracture? 4) What are the three variables in fracture mechanics that affect the design of a component? REFERENCES 1. Askeland & Phulé, The Science and Engineering of Materials, 5th ed., 2006, Thomson

Canada Ltd., Toronto, ON, Canada 2. Meyers and Chawla - Mechanical Metallurgy 3. Callister - Materials Science and Engineering 4. From Hiroshi Tada, "The Stress Analysis of Cracks Handbook," 2nd ed., Paris Productions

Inc., St. Louis, Missouri, 1985, pp. 2.10-2.11

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5. CES EduPack 2008, Granta Design Ltd., Cambridge, UK. This materials data-base and materials selection software is available to you on NEXUS.

25

LABORATORY EXERCISE # 4

Polymers and Ceramics INTRODUCTION The use of polymers (plastics), ceramics and other non-metallic materials in engineering applications continues to increase tremendously. Materials such as polyethylene, nylon, Teflon, alumina, silica, and composites have replaced metals in many applications. The wide use of these non-metallic materials means that the modern engineer must be acquainted with their structure and properties. In addition, we must be fully aware of their strengths and limitations so that we can make wise decisions during the materials selection process. There are a number of different ways of classifying polymers. Polymers can generally be divided into thermoplastics, which can be melted and reshaped, or thermosets which cannot be reshaped by heating (but often can be machined). Thermosetting polymers are composed of a 3-dimensional network of covalently-bonded molecular chains which are joined at numerous locations along the length of these chains by "cross-links" of primary covalent bonds. Bakelite and epoxy resins are examples of thermosetting polymers. Thermoplastic polymers have few or no cross-links between their covalently-bonded molecular chains. At low temperatures, intermolecular Van der Waal’s bonds are strong and the thermoplastic behaves like a brittle glass. However, as the temperature is increased, these intermolecular bonds weaken and it becomes easier for the molecules to slide past each other. Thus, thermoplastics such as polyethylene or polyvinylchloride (PVC) can be heated and reshaped. Depending on the polymer's structure, the polymerization process, and the presence of other additives, a given polymer may have significantly different structures and hence properties. Different polymers also have different properties because of the influences of the architecture of their molecular chains. Chain architecture features which are important include; the side group(s) size, polar side groups, the regularity of side group(s) arrangement, "branches" of a chain in place of a side group, and cross-link density. Chains which have a regular arrangement of moderately-sized side groups, with few branches or cross-links, are able to form crystals by folding the chains into a regular 3-D arrangement when they are cooled. The regular packing of chains in such crystals makes them denser, stronger and stiffer than amorphous structures of the same polymer. The molecular chain architectures of polymers influence both their thermal and mechanical properties. Thermal Effects As the temperature is raised, the distance between the polymer chains increases more rapidly if the only bonds between the chains are weak secondary (Van der Waal's) bonds. Eventually these bonds are weak enough that the chains easily slide past each other; i.e., melting has occurred. The temperatures of melting, or (at lower temperatures) "softening" are increased by factors which make it more difficult for chains to move past each other; i.e., cross-links, branches, large side groups, stronger polar secondary bonding, and crystals. Below a certain temperature, the glass transition temperature, chain movement is so difficult that the material becomes "glassy", i.e., brittle. At higher temperatures, but below the melting temperature, comparison of the properties of different polymers shows the influence of chain architecture, crystallinity, and polymer additives. All of these factors will be demonstrated in this laboratory.

26

Mechanical Properties As indicated above, at a given temperature, different polymers will have different properties because of the effects of molecular chain architecture and arrangements. When the initial stress is applied in a tensile test, the secondary bonds between chains are generally strong enough to resist chain sliding. The amount of strain for a given stress will depend on the number and the strength of such secondary bonds. Polar side groups increase the strength of secondary bonds. In amorphous polymers, increased side group size also makes strain more difficult. Close packing of chains, in crystalline arrangements, also reduces the strain for a given stress. In a thermosetting polymer, the initial strain is resisted by the primary bonds of cross-links, increasing the elastic modulus compared to thermoplastics. As the applied stress is increased, chains may begin to uncoil (in elastomers) or move past one another (in thermoplastics). In thermoplastics much of this movement is not reversible, resulting in permanent deformation. The stress required to begin plastic deformation is increased by increased crystallinity, large side groups, etc. The onset of plastic deformation in a thermoplastic often results in a "neck" of considerably reduced cross-section. Unlike most metal alloys, this neck may increase in length once the alignment of chains within the neck strengthens it compared to the "undrawn" material. This can result in very large elongations (eg., > 1000% for polethylene versus 90% for pure, fully annealed Cu). In some cases, the yielding is accompanied by “crazing” - the formation of internal crack-like defects. These can change the appearance of the polymer, but greatly increase elongation. Eventually, the applied stress begins to pull against cross-links and primary bonds along chains, and the stress required for further deformation increases until final fracture occurs. The uncoiling, chain sliding and fracture processes described above are dependent on both temperature and rate of deformation. Higher strain rates require a higher stress to achieve a given strain, since less time is available for the deformation processes. If the strain rate is too fast, the material may become brittle instead of being very ductile. This strain-rate sensitivity is easily demonstrated with silly putty (polydimethyl siloxane) as well as other polymers. Ceramics and Glasses Ceramics contain both metals and nonmetals (often, but not always oxygen) and are mainly ionically bonded. Since the ionic bonds are stronger than the bonds in polymers, ceramic materials are generally stronger (at a given temperature) and have higher softening and melting temperatures than polymers. Nevertheless, there are many parallels between polymers and ceramics. Many ceramics may be noncrystalline (amorphous) if quickly cooled or crystalline if slowly cooled. An example is SiO2 will form silica glass if cooled quickly or crystalline quartz if allowed to cool very slowly. Additives or modifiers often have a lower average bond strength and usually interfere with the ability of the ceramic to form crystals. Hence, they are usually added to ceramic glasses to lower their softening temperature, thereby making it easier to shape the material (e.g., in glass blowing). At room temperature, most ceramics are below their glass transition temperature and are, therefore, brittle. At higher temperatures, however, their properties depend on both the temperature and the strain rate, much like thermoplastics.

27

EQUIPMENT - Table-Top Instron Tensile Test Machine - Hot Air Blower - Micrometer and Vernier Calipers - Oxygen/Gas Torch - Safety Glasses TENSILE SPECIMENS - Low density polyethylene (0.92 g/cm3) - Medium density polyethylene (0.94 g/cm3) - High density polyethylene (0.96 g/cm3) - Polycarbonate (Lexan) - Polyvinyl chloride (PVC) - Rubber - Polymethyl methacrylate (Lucite, PMMA) - Acrylic copolymer - Quartz and Pyrex glass - Silly Putty EXPERIMENTAL PROCEDURES Part 1 i) Measure the cross-sections of the polymer tensile specimens and then pull the tensile

specimens of the supplied polymers in the testing machine using crosshead speeds suggested by the TA. Test a second polyethylene specimen at a very different crosshead speed. Note the shapes of the load-deformation curves, and correlate changes in the shapes with the observed deformation stages (necking, etc.) of the materials. Note: It may be informative to devise new experiments, such as cutting off the ends of specimens which have been tested and retesting the remainder, or changing the testing conditions for a second specimen.

Part 2 i) Examine the mechanical properties of silly putty at room temperature when

deformed by slowly pulling on it versus quickly snapping it.

ii) Hold the thin strip of lucite in the hot air blower and observe the transition from "leathery" to "rubbery" to viscous behavior as it is heated. Allow it to cool and then break it.

iii) Investigate the behavior of polyethylene and rubber at room temperature and shortly

after being immersed in liquid nitrogen.

iv) NOTE: All group members must wear safety glasses for this part of the Lab. Observe the difference in softening temperature and behavior between different ceramic glasses when heated in a flame. Try to draw the Pyrex to a fine filament. Bend this filament. Heat the tip of the Pyrex glass tube and quench it in water. Do the same experiments with quartz. Try glass blowing with the different glasses.

28

RESULTS Polymers 1) Sketch the molecular structures of the different polymers investigated in the laboratory. 2) Calculate approximate elastic modulus, yield stress (if observed), and percent elongation for

each of the different tensile specimens, and tabulate the results. Compare the results with published property data for these materials, e.g., Ref. [3].

3) Using your tensile data, generate the engineering stress-strain diagrams for these materials. Glasses 4) Sketch the structures of quartz and Pyrex. DISCUSSION In your discussion be sure to include the following: 1) Discuss the effects of crystallization, type of monomer, and other chain architecture features

on the stress-strain behavior of the specimens tested in the experiment. Include the differences between high and low density polyethylene, and the effects of adding a plasticizer to PVC in your discussion.

2) Using CES EduPack [3], generate a plot of density versus Young’s Modulus for all

materials and label the family groups. Compare and discuss any trends and differences between the material groups. Elastomeric materials stand out on this graph. Can you explain the reason(s) for this?

3) Using CES EduPack [3], generate a plot of thermal conductivity versus electrical conductor or insulator. Label some of the materials to identify the main family groups of metals polymers and ceramics. Which material has the best thermal and electrical conductivity? Which type of material has the lowest thermal and electrical conductivity? Can you explain the reason(s) for this?

4) Describe the mechanisms of elastic and plastic deformation in thermoplastic materials, and

explain how the time dependence of some of the mechanisms gives viscoelastic behavior. 5) Discuss the strain rate dependence of polymers, including a schematic diagram of how the

strain rate affects the stress-strain curve(s). 6) Discuss the effects of temperature on the deformation behavior of elastomers and polymers,

referring to the laboratory results. Include a diagram of how temperature affects the modulus of elasticity of thermoplastics.

7) Discuss the recyclability of thermoplastic versus thermosetting polymers. 8) Discuss the differences in softening and formability of glasses due to the effects of glass

modifiers. 9) Discuss the effect of quenching on the different glasses with respect to thermal expansion

coefficients.

29

10) Describe glass tempering. Explain why tempered safety glass must always be cut to size in the fully annealed state, then tempered. Suggest another way to achieve toughening of glass.

REFERENCES 1. Askeland & Phulé, The Science and Engineering of Materials, 5th ed., 2006, Thomson

Canada Ltd., Toronto, ON, Canada 2. Callister - Materials Science and Engineering. 3. CES EduPack 2008, Granta Design Ltd., Cambridge, UK. This materials data-base and

materials selection software is available to you on NEXUS.