1. Introduction
We select materials for many components and applications by matching the properties of the material to the service condition required of the component.
“measure how a material withstands an applied force”
Tensile Test, Impact Test,
Fatigue Test, Creep Test
Hardness Test
The result of this test are the mechanical properties of the material.

2. Tensile Test
The most common way to assess the mechanical behavior of a material (strength and ductility)
The tensile test measure the resistance of material to a static or slowly applied force.
In a tension test, we collect force
(or load) vs. displacement (or time).
We use the resulting information
to assess “strength” and
“deformability”

3. Impact Test
In order to select a material to withstand a sudden intense load, we must measure a material’s resistance to failure in an impact test.

4. Fatigue Test
Stress may occur as a result of rotation, bending, or even vibration.
Even through the stress is below the yield strength, the metal may fail after a large number of applications of the stress.
This mode of failure is known as fatigue.

5. Creep Test
If we apply a stress to a material at a high temperature, the material may stretch and eventually fail, even though the applied stress is less than the yield strength at that temperature.

6. Hardness Test
The hardness test measures the resistance to penetration of the surface of a material by a hard object.

7. Tensile Test Specimen

8. Example (Tensile Test)
The test shown has a diameter of 12.5mm and a gage length of 50mm. The specimen is placed in the testing machine and force F is applied.

9. Engineering Stress
If we convert the force to stress and the distance between gage marks to strain.
Engineering Stress:  = F / Ao
Units for stress and conversion factors
Load per unit area (“force distribution”)
– PSI: 1 lb/in2 = x 10-3 MPa = x 10-4 kg/mm2
= 6.8 x 104 dynes/cm2
– MPa: 1 MPa = 1 MN/m2 = 1 N/mm2 = 1 x 106 N/m2

10. Engineering Strain Engineering Strain:  = ( l – lo ) / lo
Ao is the original cross-sectional area of the specimen
before the test begins.
lo is the original distance between the gage marks.
l is the distance between the gage marks after F is applied.
STRAIN IS UNITLESS!
It is a measure of the amount of distortion caused by the application of a force.
We express strain either as a fraction or as a percentage. Be careful when doing homework or solving real engineering problems. (ex. e = 0.02 is the same as 2% strain)

11. Engineering Stress and Strain

12. Engineering Stress and Strain

13. Elastic versus Plastic Deformation

14. Engineering Stress-Strain Curve

15. Key features
Key features : you should already be able to identify them
on the diagram.
– Proportional limit
– Ultimate tensile strength
– Offset yield stress
– Total strain to failure
– Uniform plastic strain
– Modulus of elasticity

16. Yield Strength
The yield strength is the stress at which slip becomes noticeable and significant.
Yield point = point at which plastic deformation begins, i.e. end of linear region of plot.
(Engineering) Ultimate tensile strength =

17. Modulus of Elasticity
The modulus of elasticity, or Young’s modulus, is the slope of the stress-strain curve in the elastic region.
This relationship is Hooke’s law

18. Ductility
Ductility is the ability of a material to deform plastically without fracture.
Percent of Elongation:
%EL = ( lf – lo)/lo x 100
Percent Reduction in Area:
%RA = (Ao – Af)/Ao x 100

19. Modulus of Resilience
The ability of a material to absorb energy when deformed elastically and to return it when unloaded.
UR = (Y)2 / 2 E

20. Modulus of Toughness
The ability of a material to absorb energy in the plastic range.
UT = u f

21. Theory

22. True Stress and True Strain
When materials are subjected to loads, they will deform or break.
When a material deforms, its volume is generally conserved.
Thus “shape changes with deformation.”
Gives rise to new definitions of stress and strain that take into account these changes.

23. True Stress and True Strain
The decrease in engineering stress beyond tensile point occurs because of our definition of engineering stress.
We used the original area Ao in our calculation, which is not precise because the area continually changes.
We define true stress and true strain by the following equations:
True stress: t = F / A
True strain: t =  dl/lo

24. True Stress and True Strain
Material which initially is equal to lo x A will remain same at any particular point in the tensile test, V = l x A
Hence,

25. Relationship between

26. Relationship between

27. True Stress and True Strain

28. Calculation Worksheets Load (N) Length (mm) Elongation l - lo
Eng. Stress P/Ao
Eng. Strain dl / lo
T. Stress P / A
T. Strain ln(l/lo)

29. Calculation

30. Analysis of Results

31. True Stress-Strain Curve