mech of materials - lab 1

Table of Contents

Abstract.....................................................................................................2

Introduction...............................................................................................3

Discussion.................................................................................................5

Errors Faced And their effects..................................................................5

Conclusion................................................................................................6

Recommendation......................................................................................6

References.................................................................................................7

Appendix...................................................................................................8

1Mechanics Of Materials Lab

Abstract

We conducted the tensile test experiment to get the force elongation data and diagrams for the tested materials (like TH 4015 of 0.1% carbon steel). The diagram would include the stress-strain graph. The main aim of performing this experiment is to get the elasticity E, yield strength, ductility, modulus of toughness and ultimate strength. Finally, describe the specimen profile at each step in the test for the material.


Introduction

As we mentioned before, the aim of this experiment is to find the material properties of the selected specimen. How we are going to do that is by finding the stress- strain curves of the specimen. To find out stress and strain we are going to apply a certain force on the specimen and find out the corresponding deflection in the specimen.

So let us start by defining some important terms and concepts that we will face in this lab report.

Stress is the force applied on a specified area. Its unit is same as Pressure – Pascal. There are two types of stress – compressive and tensile stress.

Strain is the ratio of the change in length over the original length. It is a unit less property.

Young’s Modulus of Elasticity is the ratio of stress over strain before the limit of proportionality. It is discovered by a scientist named Thomas Young. It is called as the stiffness of a material – the bigger the Modulus value, the more the stiffness.

Limit of Proportionality is the limit till which point the Hooke’s Law is valid. Before that point stress is directly proportional to stress.

Elastic Limit is the limit after which a material experiences plastic deformation. The material will not return to the original length.

Ultimate Tensile Strength is the point where the maximum stress is applied. At this point the slope of the stress strain curve is zero and necking starts to occur.


Breaking Point/ Fracture - The point where the specimen breaks/ fractures.

Necking – The point at which even reduced loading can lead to increased extension.

Ductility – It is the ability of a material to withstand plastic deformation without breaking. It can be found by using any one of the two equations below.

Resilience – Area under the curve till the elastic limit.

Toughness – Total area under the curve.


Apparatus

1) Tensile Testing Machine – The most important apparatus in this lab is the tensile testing machine. Using this machine we can apply tensile force on the specimen and find out the corresponding extension. We can connect the machine to laptop to record the measurements of force applied and the corresponding extension.

2) Specimen – The specimen used in this experiment is 0.1 % Carbon Steel (TH 4015).

3) Vernier Caliper – To measure the thickness/ diameter and the gauge length of the specimen.


Figure 1 - Tensile Testing Machine

Figure 2 - 0.1 Percent Carbon Steel Specimen

Figure 3- Vernier Caliper

4) Laptop – A laptop is connected to the testing machine to collect all the measurements and give us the readings.


Figure 4- Laptop

Test Procedures1) Measure the original length of the gauge.

2) Calculate the cross sectional area of the specimen by finding its diameter using Vernier Caliper.

3) Put the specimen between the two chucks of the testing machine and fix it.

4) Start the software program to record the results.5) Increase the tension force slowly at a constant rate until the breaking point occurs. And

then report the changes in the specimen profile during testing.6) Use the software program to draw the diagram that will show the relationship between

the force and extension.7) Take out the tested specimen and examine its fracture surface pattern.8) Lastly, measure the final cross section area and the gauge length.


Results

Since the number of values measured is too much, we are not attaching the data file here. But we will attach the graph here.

0 0.05 0.1 0.15 0.2 0.25 0.30

50

100

150

200

250

300

350

400

450

500

Stress Vs. Strain Curve

Nominal Stress, σ (MNm-2)

Strain

Stre

ss (M

Pa)


Problems FacedThe original graph of the readings that we are given has some problems with it. That is why we had to edit the graph. Here are some problems we faced with the previous graph and the changes we made to it.

1) Extensions due to ZERO stress. The first few readings in the original graph shows an enormous extension with ZERO stress. Using logic, we know that this is IMPOSSIBLE. Thus we have removed the first few values and have taken the first extension point only when the stress is applied.

2) Wrong values of “total length” used to find the values of strain. We found that the formula used to find the values of strain had the wrong values of total length. It should be 83 mm instead of 173 mm. This gave us wrong strain values.


ZERO STRESS

Data Analysis

2) a)

Propotionality Limit=370MPa

b)

YoungModulus=∆ Stress∆Strain

youngmodulus= 300−1000.048−0.043

youngmodulus= 2000.005

youngmodulus=40000MPa

c)

UltimateStrength , σult=420MPa

d)

Resilience=Stress×Strain

Resilience=300×0.02

e)

Toughness=areaunder the curve

Toughness= (400×0.127 )

f)

% elongation=L2−LL

×100%


% elongation=ϵ ×100 %

% elongation=0.17×100 %

% elongation=17 %

% reduction∈area=A2−AA

×100 %


Discussion This experiment was intended to find the tensile strength of different material. To

conduct this experiment, we used Universal Testing Machine; the machine was set in a way that we can pull the specimen from each side and the force applied can be calculated. The elongation of the specimen was measured by extensometer attached to the upper side of the specimen. The process was continued until the specimen ruptured.

The values of force applied at different times and the respective extension was noted. With these tabulated values, we calculated the corresponding values of stress and strain by computing values in the above mentioned formulae. After getting values of stress and strain, we plot the graph of stress against strain. With the help of the graph, we will find and predict the properties of the material. From the graph, we predicted the proportionality limit, toughness, ultimate strength, and the ductility of the specimen material.

With respect to the results we got,


Conclusion

In this experiment, we find out the ability of the specimen under a force i.e. how much it can bear the force. This test reflects the kind of material whether it is good for the different applications. The very first step that was taken was to measure the length of the specimen and calculate its area. By using the tensile-testing machine tensile force was applied on the specimen until it breaks. Later, finding the stress, strain and the change in length that occurred after the test has been performed. Lastly, the software program produces the graph showing relationship between the force and the extension.

However, there were several factors that may have affected the results. Temperature can be one of the factors as due to change in temperature it may cause expansion and contraction. Thus, the dimensions may vary, and it can give us false results. The following picture below shows the break of the specimen. As it is clear from it that the breaking point didn’t occur from the middle. That is the machine used applied the tensile force from once side. Whereas, the correct elongation require the tensile strength on both sides of the specimen.

Recommendation

[how to remove the errors]


References


Appendix

Formalus and one example of finding the solution


mech of materials - lab 1

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