research on tensile testing of materials

RESEARCH ON TENSILE TESTING OF MATERIALS

University of Lagos, Akoka

Tensile Testing of Materials (Mild Steel)

Dukor Kenechi Franklin

Mechanical Engineering

120404036

Author Note

The research on tensile testing of materials was performed on the June 13 2014 by

Dukor Kenechi Franklin and other members of Group 6. The research was carried

out at the Federal Institute of Industrial Research (FIIRO), Oshodi, Lagos. Nigeria.

This research project wouldn’t be a success without the help of the tensile testing

manager, Engineer Ojo.

Phone number(s): +2348031157806, +2348084717793.

E-mail: [email protected], [email protected]

Dukor Kenechi Franklin, Department of Mechanical Engineering, 120404036, [email protected], Group 6.

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mailto:[email protected]




TABLE OF CONTENT

Abstract 3

Nomenclature 4

Introduction 5

Literature Review 6

Research Design 12

Industrial Application of Tensile Testing 18

Conclusion 23

Acknowledgment 24

Reference 24


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I. Abstract

The research was performed to study the tensile strength of materials. The material used for the test was mild steel.

The material was subjected to tensile force. Due to the increasing tensile load, the specimen is continuously

stretched until rupture occurred on the material.

Graph and values were obtained from the test and compared with standard value of young modulus, Yield strength,

etc and it was discovered that the mild steel material obeys Hooke’s law of elasticity


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II. Nomenclature

(Nomenclature entries should have the units identified)

σ = engineering stress

ε = engineering strain

P = external axial tensile load

Aₒ = original cross-sectional area of the specimen

Lₒ = original length of the specimen

Lƒ = final length of the specimen


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III. Introduction

Tensile testing, also known as tension testing, is a fundamental material science test in which a sample is

subjected to a controlled tension until failure.

These results from the test are commonly used to select a material for an application, for quality control, and to

predict how a material will react under other type of forces.

Properties are that are directly measured via a tensile test are ultimate tensile strength, maximum elongation and

reduction in area.

From these measurements the following properties can also be determined. Young’s modulus, Poisson’s ratio,

yield strength and strain hardening characteristics

Uniaxial tensile testing is the most commonly used for obtaining the mechanical characteristics of isotropic

materials. For anisotropic materials such as composite materials and textiles, biaxial tensile testing is required

Typical applications of tensile testing are highlighted in the following sections on:

a) Aerospace Industry

b) Automotive Industry

c) Beverage Industry

d) Construction Industry

e) Electrical and Electronics Industry

f) Medical Device Industry

g) Packaging Industry

h) Paper and Board Industry


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IV. Literature Review

A. TENSILE TESTING

Uniaxial tensile test is known as a basic and universal engineering test to achieve material parameters such as

ultimate strength, yield strength, % elongation, % area of reduction and Young's modulus. These important

parameters obtained from the standard tensile testing are useful for the selection of engineering materials for any

applications required.

The tensile testing is carried out by applying longitudinal or axial load at a specific extension rate to a standard

tensile specimen with known dimensions (gauge length and cross sectional area perpendicular to the load direction)

till failure. The applied tensile load and extension are recorded during the test for the calculation of stress and strain.

A range of universal standards provided by Professional societies such as American Society of Testing and

Materials (ASTM), British standard, JIS standard and DIN standard provides testing are selected based on

preferential uses.

Each standard may contain a variety of test standards suitable for different materials, dimensions and fabrication

history. For instance, ASTM E8: is a standard test method for tension testing of metallic materials and ASTM B557

is standard test methods of tension testing wrought and cast aluminum and magnesium alloy products.


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B. STANDARDS IN TENSILE TESTING

Figure 1: Standard tensile specimens

A standard specimen is prepared in a round or a square section along the gauge length as shown in figures 1 a)

and b) respectively, depending on the standard used. Both ends of the specimens should have sufficient length and a

surface condition such that they are firmly gripped during testing.

Type specimen United State (ASTM) Great Britain Germany

Sheet (Lₒ / Aₒ) 4.5 5.65 11.3

Rod (Lₒ / Dₒ) 4.0 5.0 10.0

Table 1: Dimensional relationships of tensile specimens used in different countries.

The initial gauge length Lₒ is standardized (in several countries) and varies with the diameter (Dₒ) or the cross-

sectional area (Aₒ) of the specimen as listed in table 1. This is because if the gauge length is too long, the %

elongation might be underestimated in this case.

Any heat treatments should be applied on to the specimen prior to machining to produce the final specimen

readily for testing. This has been done to prevent surface oxide scales that might act as stress concentration which

might subsequently affect the final tensile properties due to premature failure.

There might be some exceptions, for examples, surface hardening or surface coating on the materials.

These processes should be employed after specimen machining in order to obtain the tensile properties results which

include the actual specimen surface conditions.


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C. STRESS AND STRAIN RELATIONSHIP

When a specimen is subjected to an external tensile loading, the metal will undergo elastic and plastic

deformation. Initially, the metal will elastically deform giving a linear relationship of load and extension. These two

parameters are then used for the calculation of the engineering stress and engineering strain to give a relationship as

illustrated in figure 3 using equations 1 and 2 as follows

σ= PAₒ

… (1)

ε=Lƒ−LₒLₒ

= Δ LLₒ

…(2)

Where:

σ … is the engineering stress

ε … is the engineering strain

P … is the external axial tensile load

Aₒ … is the original cross-sectional area of the specimen

Lₒ … is the original length of the specimen

Lf … is the final length of the specimen

The unit of the engineering stress is Pascal (Pa) or N/m2 according to the SI Metric Unit whereas the unit of psi

(pound per square inch) can also be used.


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Figure 3: Stress-strain relationship under uniaxial tensile loading


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D. EXPLANTION OF TECHNICAL TERMS IN TENSILE TESTING

i. Elastic modulus (Young's modulus) (E) : This is ratio of stress to strain below the elastic limit.

The engineering stress-strain relationship follows the Hook's Law and the slope of the curve indicates the

Young's modulus (E)

E=σε

… (3)

Young's modulus is of importance where deflection of materials is critical for the required engineering

applications. This is for examples: deflection in structural beams is considered to be crucial for the design in

engineering components or structures such as bridges, building, ships, etc.

The applications of tennis racket and golf club also require specific values of spring constants or Young's

modulus values.

ii. Yield strength, (σ y): This occurs when the tensile loading continues, yielding occurs at the beginning of plastic

deformation. The yield stress, σy, can be obtained by dividing the load at yielding (Py) by the original cross-

sectional area of the specimen (Ao) as shown in equation 4.

σ y=PyAₒ

…(4)

iii. Ultimate Tensile Strength, σ TS: Beyond yielding, continuous loading leads to an increase in the stress required to

permanently deform the specimen as shown in the engineering stress-strain curve. At this stage, the specimen

is strain hardened or work hardened. The degree of strain hardening depends on the nature of the deformed

materials, crystal structure and chemical composition, which affects the dislocation motion.


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iv. Fracture Strength, σ f: After necking, plastic deformation is not uniform and the stress decreases accordingly until

fracture. The fracture strength (σ fracture) can be calculated from the load at fracture divided by the original

cross-sectional area, Ao, as expressed in equation 5.

σfracture=PfractureAₒ

…(5)

v. Elongation: The strain at fracture expressed as a percentage; this is a measure of the ductility of the material.

vi. Modulus of resilience: The amount of energy (or work) stored per unit volume at the elastic limit.

vii. Modulus of toughness: The amount of energy stored per unit volume at fracture of the material; this is a measure

of the ductility of the material.

viii. Percent Area Reduction: Reduction in area at fracture in necking region with respect to original cross-section

area; this is a measure of the ductility of the material.

ix. Strain (engineering): the unit deformation of the material under load.


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V. Research Design

The Research Design is to Study the tensile strength of materials and to understand its importance

and application in industry today.

The industry where this research was carried out is Federal Institute of Industrial Research located

at, Oshodi, and Lagos, Nigeria (FIIRO). FIIRO is a well-known research institute in Nigeria which

aids to build and equip center for commercial production demonstration of developed technologies

e.g bio-technology unit, material research, technology transfer, development of Agro-based etc.

The research design was carried out on the following machines;

1. Mild Steel


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2. Universal testing Machine


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A. The procedure carried out during this research was:


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1. The engineer in charge of the machines Engineer Ojo introduced us to the Tensile testing

machine

2. After the introduction and discussion on the importance and application of Tensile testing,

we proceeded to perform a mini experiment on mild steel.

3. We started by fixing the mild steel material into the universal tensile testing machine.

Picture of apparatus with fixed mild before loading

4. We then subjected the mild steel material to tensile test until the material experienced

fracture.


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5. We then printed the readings and graph generated by the Universal tensile testing machine.

6. We studied the result obtained and drew some conclusions.

B. Picture of material after fracture

Picture of material after loading

C. Graph of stress against strain


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Graph plotted for stress against strain

D. Results Obtained From the Test


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VI. Industrial Applications of Tensile Testing


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Applications of Tensile Testing:

Tensile testing is used to guarantee the quality of components, materials and finished products within wide range

industries. Typical applications of tensile testing are highlighted in the following sections on:

i) Aerospace Industry

j) Automotive Industry

k) Beverage Industry

l) Construction Industry

m) Electrical and Electronics Industry

n) Medical Device Industry

o) Packaging Industry

p) Paper and Board Industry

q) Pharmaceuticals Industry

r) Plastics, Rubber and Elastomers Industry

s) Safety, Health, Fitness and Leisure Industry

t) Textiles Industry

A. Aerospace Industry

Applications of tensile testing in the aerospace industry include:

Peel tests on airframe composites

Shear and tensile strength testing of fasteners e.g. bolts, nuts and screws

Tensile & material strength testing of adhesive bonds, aircraft textiles and carpets, cables, hoses and tubing,

gaskets and o-rings, seat belts, welded and crimped joints, wiring looms and harnesses


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B. Automotive Industry

Applications of tensile testing in the automotive industry include:

Quality assessment through tensile testing of interior fittings including: airbags, carpets, dashboards,

electrical harness (incl. crimped terminals pull-off force), handles, laminated trim, mirrors, seals and

seatbelts and handbrake levers.

Quality assessment through tensile testing of exteriors fittings including: bumper mouldings and trims, door

and window seals, emblems and number plates, mirrors and mud flaps

C. Beverage Industry

Applications of tensile testing in the beverage industry include:

Peel strength of induction-sealed foils and labels

Tensile force required to open 'ring-pulls' on bevcans

Testing cork extraction force

D. Construction Industry

Applications of tensile testing in the construction industry include:

Bond strength testing of adhesives, mastics, sealants and bonds between brick and foam layers

Tensile and material strength testing of geotextiles and safety support netting

E. Electrical and Electronics Industry

Applications of tensile testing in the electrical and electronics industry include:

Connector withdrawal force

Pull-off forces of crimped, welded or soldered electrical contacts

Component-to-PCB pull-off force


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PCB material tensile strength.

F. Medical Device Industry

Applications of tensile testing in the medical device industry include:

Hypodermic needle-to-hub retention force

Tensile strength and elongation at break of medical tubing, bandages, dressings and tapes

Joint strength of IV connector fittings

Suture-to-needle crimps pull out test

Tensile strength of suture material and knotting

Joint strength and material elongation of respiratory masks

Elongation and tensile strength of examination gloves

Mechanical strength of orthopedic implant components

G. Packaging Industry

Applications of tensile testing in the packaging industry include:

Adhesive/peel testing of adhesive bonds, container seals and labels

Force associated with opening snap-caps, pop-caps and other push pull closures

Elongation of plastic packaging materials

H. Paper and Board Industry

Applications of tensile testing in the paper and board industry include:

Openability of card and paper based packaging

Folding characteristics of boxes and cartons

Force to separate multi-part documents


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Durability of documents

I. Pharmaceuticals Industry

Applications of tensile testing in the pharmaceuticals industry include:

Pull off force of phial caps

J. Plastics, Rubber and Elastomers Industry

Applications of tensile testing in the plastics, rubber and elastomers industry include:

Joint strength of interlocking plastic components

Assessment of material tensile properties

Adhesion / peel testing of plastic labels, ID and credit cards

K. Safety, Health, Fitness and Leisure Industry

Applications of tensile testing in the safety, health, fitness and leisure industry include:

Tensile testing of safety support netting

Ergonomic risk evaluations

Elastic properties of racquet strings

L. Textiles Industry

Applications of tensile testing in the textiles industry include:

'Pull-off' characteristics of buttons, stitched-on decorations, press studs, poppers, zip fasteners, hook-and-

loop fasteners

Strength testing of vulnerable seams


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VII. Conclusion

From experiment we performed, we noticed a gradually and slowly increasing tensile load applied on the mild

steel material through the tensile machine. Due to the increasing tensile load, the specimen is continuously stretched

until fracture occurred.

There was an increase in length of the mild steel material compared to it's original length. this increase in length

was used to calculate the stain in the material.


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It was confirmed that the material obeys Hooke’s law. Slope of this line provides information on the Young’s

modulus of the material.

Also, when the values we obtained was compared with standard values similarities were noticed.


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Acknowledgments

The research was carried out at the Federal Institute of Industrial Research (FIIRO), Oshodi,

Lagos. Nigeria.

This research project wouldn’t be a success without the help of the tensile testing manager,

Engineer Ojo.

References

[1] Hashemi, S. “Foundations of materials science and engineering,” 2006, 4th edition, McGraw-Hill, ISBN 007-1256903.

[2] Norman E. Dowling, “Mechanical Behavior of Materials”, Prentice-Hall International, 1993.

[3] W.D. Callister, Fundamental of materials science and engineering/an interactive e. text, 2001, John Willey & Sons, Inc.,

New York, ISBN 0-471-39551-x

[4] Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-100406-8.

[5] Czichos, Horst (2006). Springer Handbook of Material Measurement Method”, Berlin:Springer. Pp. 303-304. ISBN 978-3-

540-20785-6

[6] WIKIPEDIA: http://en.m.wikipedia.org/wiki/Tensile_testing


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