Tensile Test (Part 1)

The Tensile Test is used to identify mechanical properties or strength of materials by milling specimen as any testing standards. In this test, an increasing uni-axial load is continuously applied to the specimen by testing machine until it fractures. During testing, elongation of specimen will be measured continually and plotted as a load versus elongation diagram (engineering stress-strain curve) which shows relationship between the applied load and corresponding elongation. After that it will calculate the engineering values that are yield strength, ultimate tensile strength and percent elongation.

The engineering stress is calculated from applied load divided by the original cross sectional area of specimen that show in the unit of N/mm2, MPa, kgf/mm2, psi and ksi. The change in length of gage length divided by the original gage length, expressed as a percent, is the engineering strain or percent elongation.

True stress use actual cross section area which is reduced at any time for calculation instead original cross section of specimen in stress-stain curve. Although during testing, it has to change the dimension or cross section area of specimen. Especially in ductile material, the cross section area of specimen is decreasing rapidly in the test. This effect to the load required continuing deformation falls off. The average stress based on original cross section area likewise decreases, and this produces the fall off in the stress-strain curve beyond the point of maximum load. In fact, the metal will generate strain-hardening continually all the way up to fracture. So the stress requires to deform should be increased.