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.

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”

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.

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.

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.

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

Tensile Test Specimen

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.

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 = 6.895 x 10-3 MPa = 7.032 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

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)

Engineering Stress and Strain

Engineering Stress and Strain

Elastic versus Plastic Deformation

Engineering Stress-Strain Curve

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

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 =

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

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

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

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

Theory

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.

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

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,

Relationship between

Relationship between

True Stress and True Strain

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)  

Calculation

Analysis of Results

True Stress-Strain Curve