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EXPERIMENT 1

TENSILE TEST 1.0 Objective

i. To obtain a general understanding of how different materials and cross sections behave under uniaxial tensile loading.

ii. To determine the stress-strain relationship and compare mechanical/material properties of various materials and cross section.

iii. To obtain the mechanical properties: the modulus of elasticity, the yield stress, the ultimate stress, the fracture stress and the ductility ratio.

2.0 Introduction The tensile test is the most commonly performed and is the simplest among of all the mechanical tests. In this experiment, a specimen is subjected to a gradually increasing uniaxial load until failure occurs. The typical testing procedure is to deform or stretch the material at a constant speed. A circular and rectangular cross section will be use as tested specimen which is made of steel and copper or aluminum. The load-deformation data is recorded during the experiment so this data can be plotted once the procedure is complete. The student will learn how to properly conduct a tensile test and obtain the relevant material properties from the results. Further, the student will discover how different materials as well as different cross section behave under similar loading conditions. 3.0 Background Mechanical testing play an important role in evaluating fundamental properties of engineering materials (i.e: modulus of elasticity, Poisson`s ratio, ultimate strength, yield strength, fracture strength, resilience, toughness, % reduction in area, and % elongation) as well as in developing new materials and in controlling the quality of materials for use in design and construction. Most of these engineering values are found by graphing the stress and strain values from testing. A number of experimental techniques are developed for mechanical testing of engineering materials subjected to tension, compression, bending and torsion loading. Ductile materials will neck down through the plastic range before rupture (Figure 1a). Brittle materials do not neck down significantly (Figure 1b). Instead they fail sharply and abruptly at the maximum load because brittle materials do not exhibit much plasticity.

a) Failure of ductile material b) Failure of brittle material

Figure 1: Typical of failure of materials

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When a specimen is loaded so that the resultant force passes through the centroid of the specimen cross section, the loading is called as axial and can be either tensile or compressive. The test measures force and change of length of the specimen which are used to calculate nominal stress and nominal strain. The term nominal (or engineering) is used to indicate that the stress is based on the original test specimen cross section area and the strain is based on the original gage length as shown in Figure 4. Stress is a measure of the intensity of an internal force. Stress is defined as the force P per unit area A:

Stress, AP

=σ (SI unit; N/m2

)

Strain is a measure of the deformation that has occurred in a material. In the case where the magnitude of deformation is the same over the entire length of a body, strain is defined as:

Strain, o

of

LLL −

=ε (m/m-i.e. dimensionless)

where: Lo L

= the initial length f

= final length

A typical stress-strain diagram from a tensile test for structural steel is shown in Figure 2. The particular properties are designated on the Figure 2 and are described as below: 1. Young`s Modulus (Modulus of elasticity), E Young`s Modulus is the ratio of stress to strain for the initial straight line portion of the stress-strain curve (slope of the straight line). Determined by:

p

p

εσ

=Ε

where: σp ε

= proportional limit stress p

= proportional limit strain

2. Proportional limit Proportional limit is the value of engineering stress (the load is divided by the initial cross-sectional area) at the point where the straight-line portion of the stress-strain curves ends. 3. Yield point Yield point is a point on the stress-strain curve, after which there is a significant increase in strain with little or no increase in stress. The corresponding stress is called the Yield strength/Stress of the material. For materials that do not possess well-defined yield point, “offset method” is used to determine it. 4. Elastic limit Elastic limit is the value of stress on the stress-strain curve after which the material deforms plastically (maximum stress for which stress will be directly proportional to strain).

5. Ultimate strength Ultimate strength is the highest value of apparent stress on the stress-strain curve. It is also known as the tensile (or compressive) strength. 6. Fracture strength Fracture strength is the value of stress at the point of final fracture on the stress-strain curve.

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7. Percent elongation Percent elongation is the measure of the deformation at the point of final fracture. Determined by:

100% xL

LLelongation

o

of −=

8. Percent reduction of area Percent reduction of area is the measure of the fracture ductility. Determined by:

100% xA

AARA

o

fo −=

where; Af A

= the final cross-sectional area at the location of fracture 0

= the initial cross-sectional area

9. Ductility Ductility is the characteristic of a material where the material can undergo large plastic deformations before fracture, especially in tension. Ductility of materials is measured by ductility ratio;

y

uductilityεε

µ =,

where; εu ε

= the ultimate strain y

= the yield strain

Figure 2: A typical stress-strain diagram for a ductile material

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4.0 Apparatus Universal testing machine, vernier caliper, steel ruler, two or three test specimens (steel, aluminum and brass)

Figure 3: Universal testing machine

Figure 4: A typical tensile test specimen 5.0 Brief Procedure 1. Measure the dimensions of the each test specimen before and after test and fill in the

table 1. Mark the gauge length on the test specimen. 2. Switch on the machine. 3. Mount the test specimen in the grips of the machine. 4. Apply and record load and the corresponding deformation 5. Repeat steps (1) to (4) for various type of the test specimen. Note: Important!! Step by step procedure to run the machine and experiments should be followed the instructions as stated on the machine.

(a) Undeformed specimen

Lf

(b) Deformed specimen

LO

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6.0 Result 1. The experimental data should be filled or can be printed from the machine. 2. Complete the tables as provided in the worksheet by using the appropriate equations

and experimental data. Find the reference values for the tested material of the specimen. 3. Plot the graph of load versus deformation and stress versus strain with suitable scales

for each tested specimen. (Stress on Y axis and Strain on X axis). Mark and label the elastic limit, upper yield point, lower yield point, yield stress, ultimate stress and fracture stress on curve.

4. Plot 0.2% offset line on the graph so that 0.2% offset yield stress can be determined. 5. Calculate the slope of the graph on the elastic limit region which is Modulus of

Elasticity. 6. Sketch the final condition of the specimen and showing the location of failure. 7.0 Discussion 1. Compare and discuss the results in table 4 to reference values and comment on the

possible reasons for discrepancies obtained for a tested specimen. 2. Compare and discuss the similarities and differences in mechanical/material properties

for the materials tested. 3. Distinguish between yield point and yield strength on a stress-strain curve. Which gives

the more accurate indication of a material`s fitness for a particular tensile application? 4. Distinguish between the proportional limit and the elastic limit for each material. Which

is the more important indicator of a material`s mechanical behaviour? 5. What are the advantages of stress-strain diagram over a load-deformation diagram for

showing the results of a test?

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WORKSHEET FOR TENSILE TEST Table 1: Dimension of the appropriate tested specimen Material: Steel/copper/aluminum Type: rectangular/round

Material Initial (unit: mm) Final (unit: mm)

L AO O

(mm2 d) bO hO LO Af f

(mm2 d) bf hf f

Steel

Copper

Aluminum

d = diameter; b = width; h = height(thickness); L = length; A = area Table 2: Determine the following observation for the tested specimen

No

Force (N) Elongation (mm) Stress (Pa) Strain

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Table 3: Determine the following observation load for the tested specimen

Material Load at Elastic Limit (N)

Load at Upper Yield Point (N)

Load at Lower Yield Point (N)

Ultimate Load (N)

Breaking Load (N)

Steel

Copper

Aluminum

Table 4: Determine the following properties for the tested specimen

Material Proportional Limit Stress

(Pa)

Nominal Fracture Stress

(Pa)

Actual Fracture Stress

(Pa)

% Reduction

in Area

Strain

% Elongati

on

Ductility

Steel

Copper

Aluminum

Table 5: Determine the % error of the tested specimen

Material Properties Modulus of Elasticity (Pa)

0.2% offset Yield Stress (Pa)

Yield Stress (Pa)

Ultimate Stress (Pa)

Steel Experimental

Reference

% Difference

Copper Experimental

Reference

% Difference

Aluminum Experimental

Reference

% Difference

Note: Yield stress = Yield load@Upper yield load / initial cross-sectional area Ultimate stress = Ultimate load / initial cross-sectional area Nominal fracture stress = Breaking load / initial cross-sectional area Actual fracture stress = Breaking load / final cross-sectional area