How do you do a tensile test?
The ductility of a sample is determined by conducting a tensile strength test on a Universal Testing Machine like the one seen here (INTERMET, Lynchburg, VA). Samples of the material are placed in a Universal Testing Machine, gripped by the ends, and a vertical force is applied until they break; they are pulled apart. During the stretching process, the machine measures the load (ρ), or the force applied to the sample, and the displacement of the sample (s); along with the original cross sectional area of the sample (Ao) and the original length (Lo), an engineering stress-strain curve can be created. Stress (δ), computed by dividing the load by the cross sectional area, is plotted against strain (ε), derived by dividing the displacement (s) by the length:
δ = ρ/Ao, ε = s/Lo
When the graph is analyzed, it is found that the strain hardening of the material increases up to a certain maximum point, after which the strain begins to deform the material, softening it until it breaks. Graphically, it is the highest point on the engineering stress-strain curve. The maximum point is known as the Ultimate Tensile Strength, or UTS, and is used in measuring the ductility of metals. The |
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Tensile Test Values:
Young's Modules:
This is the slope of the linear portion of the stress-strain curve, it is usually specific to each material; a constant, known value.
Yield Strength:
This is the value of stress at the yield point, which is calculated by plotting young's modules at a specified percent of offset (usually offset = 0.2%).
Ultimate Tensile Strength:
This is the highest value of stress on the stress-strain curve.
Percent Elongation:
This is the change in gauge length divided by the original gauge length.
Calculated Fracture Energy:
This is a value calculated by using the definition of Energy = Force x Distance. By integrating the stress-strain curve and finding the area under the curve the energy required to fracture the specimen is determined.
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