TITLE: TENSILE TEST

OBJECTIVE:

The objective of this experiment is to study the fracture characteristics and deformation of a sample under tension. We then will determine the modulus of elasticity (Young’s modulus), yields stress and ductility of the material from the curve obtained.

APPARATUS:

By using an Instron testing machine model 4206 you will obtain a Force-Displacement Curve, Vernier Caliper and specimen.

Instron testing machine model 4206

Vernier Caliper Specimen

THEORY:

a) Yield stress

b) Ultimate tensile stress

c) Elastic or Young’s modulus Є of a material can all be determined from the stress – strain curve for that material.

The curve below is a typical metallic behavior.


The engineering stress is plotted (force divided by the original cross-sectional area). At small strain values (the elastic region). The relationship between stress and strain is nearly linear. Within this region, the slope of the stress-strain curve is defined as the elastic modulus. Since many metals lack a sharp yields point, i.e. a sudden, observable transition between the elastic region and the plastic region, the yields points is often defined as the stress that gives rise to a 0.2% permanent plastic strain. By this convention, a line is drawn parallel to the elastic region of the materials, starting at a strain level of 0.2% strain. The point at which this line intersects the curve is called the yield point or yield stress. The ultimate tensile strength (stress), in contrast is found by determining the maximum stress, reached by the materials.

The ductility of the material can be measure most accurately from the tensile test using the definition.

\

Ductility = change in x – sectional area

Original x – sectional area

This measure of ductility is closely related to one based upon the change in gauge length divided by the original gauge length, but after fracture the gauge length is difficult to measure.

METHOD AND PROCEDURE:

For specimen:

1. Clean the surface of specimen.

2. Using the vernier caliper, measure the initial gauge length and cross-sectional area of the test sample for later calculations of ductility.



For machine:

1. By using vernier caliper, the initial gage length, width and the thickness of specimen were measured. The cross-sectional are to be calculate by multiplying the value of width and thickness of specimen.

2. Then the specimen is mounted to the machine’s gripper. Fit it properly.

3. Set load gage to zero.

4. The preloading load is given to specimen to fix it to the gripper by turning the hand wheel counter clockwise. Turn it slowly until the load gage showed a small reading.

5. Again set the load gage to zero. Also set the dial gage to zero.

6. Then turned the hand wheel counter clockwise, extension to 0.2mm per turning.

7. Data are taken from both load gage and dial gage measurement value.

8. Step 6 and 7 are repeated until the specimen broke (failure).

Note that:

- Counter clockwise turning for tension, clockwise turning for compressive.

Ratio for load gage: 1cm = 0.5N, dial gage: 1cm = 0.01

After each test is complete, remove the sample from the machine and observe the surface characteristic at the fracture point. Make note for your report. Note the nature of the plastic deformation in each material (i.e. necking, brittle fracture, extensive plastic flow). Finally, estimate the final cross-sectional area of each fracture surface and place the two halves of the sample together and estimate values for the final gauge length.

EXPERIMENT RESULT:

1. The material with complete dimension.


Data from the specimen:

L = 36.44 mm

b = 3.12 mm

h = 1.2 mm

A = bh

= 3.72

The results were tabulated in the table as shown below:-

EXTENSION (mm)

LOAD (N)

STRAIN

STRESS

0.1

7.75

0.00

2.08

0.2

8.1

0.00

2.17

0.3

8.35

0.00

2.24

0.4

8.4

0.00

2.25

0.5

8.55

0.01

2.29

0.7

8.6

0.01

2.31

0.8

8.9

0.02

2.39

0.9

8.9

0.02

2.39

1.0

8.9

0.02

2.39

1.5

9.9

0.04

2.66

2.0

9.95

0.04

2.67

2.5

9.95

0.06

2.67

3.0

10.0

0.08

2.68

3.5

10.0

0.08

2.68

4.0

10.5

0.11

2.69

4.5

10.5

0.12

2.69

5.0

10.5

0.13

2.69

5.5

10.5

0.15

2.69

6.0

10.5

0.16

2.69

6.5

10.5

0.17

2.69

7.0

10.5

0.19

2.69

7.2

10.0

0.19

2.68

7.4

10.0

0.20

2.68

7.6

10.0

0.20

2.68

7.8

10.0

0.20

2.68

8.0

10.0

0.21

2.68

8.51

9.55

0.23

2.56

9.0

8.4

0.24

2.55

9.1

7.6

0.24

2.04

9.2

6.6

0.25

1.77

9.3

5.9

0.25

1.58

9.4

4.65

0.25

1.25

9.5

3.7

0.25

0.99

9.52

3.45

0.26

0.92

9.54

3.05

0.26

0.81

9.56

2.9

0.26

0.77

9.58

2.7

0.26

0.72

9.6

2.55

0.26

0.68

9.62

2.45

0.26

0.65

9.64

2.0

0.26

0.53

9.69

1.5

0.26

0.40

9.7

1.45

0.26

0.39

9.72

1.25

0.26

0.33

9.74

1.15

0.26

0.30

9.76

1.1

0.26

0.29

9.78

1.05

0.26

0.28

9.8

0.95

0.26

0.25

9.82

9.0

0.26

0.24

9.84

0.22

0.26

0.22

9.86

0.75

0.27

0.20

Table 1

CALCULATION

Sample of Calculation

Stress,σ = F/A Stress,σ = F/A

= 8.55/ 3.72 = 9.78 / 3.72

= 2.29 N/mm2 = 2.62 N/mm2

Strain,ε = δL/Lo Strain,ε = δL/Lo

= 2.5 / 36.44 = 9.82 / 36.44

= 0.06 = 0.26

Slope of the stress-strain curve is defined as the elastic modulus.

Slope of the stress-strain graph

=7.2MPa

The actual energy that needed to break the specimen is:

Energy

After the graph of stress versus strain was analyzed, get that modulus elasticity of the material is 7.2MPa. The energy needed to break the specimen is 0.1075 Joule. The value defined by analyzes the graph of Load against extension.

From the graph had drawn, it is represent the tensile stress-strain behavior for ductile materials loaded to fracture. So, from this representation, it can be conclude that this material being test is ductile. Ductility of the material is a measure of degree of plastic deformation that has been sustained at fracture. If the material experience very little or no plastic deformation upon fracture is termed brittle.


The graph Load (N) versus Extension (mm) drawn using Microsoft Excel as shown below:-

The Graph Stress versus Strain drawn using Microsoft Excel as shown below:-

DISCUSSION:

  1. The specimen being tested follows any international standards because to get a true result.
  2. To minimize errors;

a) Parallax error – An error that occurs because of wrong way of looking at the measurement tool’s reading. This can avoided by making sure that to read a reading of a tool, the eye must be parallel to it.

b) Sensitive machine – The machine is very sensitive, the readings of loads can be easily changed just by touching it. Avoid this by keeping a distance with the machine.

c) Zero Parallax – An error of a measurement tool’s scale where when it should be zero, it shows a reading. Avoid this error by avoiding the usage of old measurement tools. In a case the error containing tools is used, calculate first the percentage of the error to make correction on your reading afterwards.

d) Every person must pay attention.

e) They also must have only one work to do.

f) For get a actual value like length and diameter, you must take two or more value and calculate the average value.

  1. The type and surface texture of the broken specimen;

a) Finally, when the material fracture the last surface have a cone and cup


CONLCUSION:

After completing the experiment, we can study the characteristics of material (mild steel) in elastic deformation and plastic deformation, and materials failure (fracture) (samples under tension). Besides that, we could also obtain the modulus of elasticity (E) by calculating the slope of the stress-strain graph. There were also a phenomenon occurred around the fractured area called ‘the necking’.

REFFERENCES

1. Material Science and Engineering An Introduction, sixth edition (William D. Callister, Jr)

2. Mechanics of Materials, Sixth Edition in SI units (R.C. Hibbeler)

3. http://www.gunt.de/static/s7_1.php?p1=&p2=&pN=