Quality 101: Tensile Testing Basics
A tensile test is a static measurement of the effects of tensile force on a material or component, or to determine the bond strength of two materials that have been assembled together. By applying a tensile force on a material, one can find the tensile strength, yield point, yield strength, percent elongation, reduction in area and the modulus of elasticity. Tensile test results are used to indicate strength, ductility, stiffness and the correct parameters for heat treatment or processing.
In general, tensile testers or universal testing machines apply the load mechanically by a screw and gears, or hydraulically with a pump and motor. A load cell device or pressure transducer is used to indicate the mechanical load applied to the test specimen. An extensometer is a device for measuring the extension or elongation of the test specimen. A computer that automatically runs the test machine-either by a selected strain rate, load rate or position rate-controls most new equipment. In addition, the computer can automatically calculate all of the properties needed by the operator.
Tensile test specimens are normally shaped like dog bones-the center portion of the specimen is smaller in cross section than the two ends. Tensile specimens may be round or rectangular depending on the stock on which they are obtained. In most cases, the final results are reported in terms of pounds per square inch. Pounds per square inch is equal to the force divided by the cross sectional area.
Most mechanical tests have been derived from testing metals. However, in materials testing, as the load is applied and the specimen is stretched, a stress vs. strain curve is plotted. Many material properties can be found in this test such as yield strength, ultimate tensile strength, percent elongation and modulus of elasticity.
For most materials, the initial portion of the test, the relationship between the applied force, or load, and the strain or elongation of the specimen shows a linear relationship. Referred to as the proportional limit, Hooke’s Law or modulus line, it is where the material, if unloaded, would not show any permanent strain remaining when the stress is completely removed. Beyond this point is the yield point when strain occurs without an increase in stress. For metals and plastics the departure from the linear elastic region cannot be easily identified. Therefore, an offset method is used to determine the yield strength.
These methods are discussed in ASTM E8 (metals) and D638 (plastics). An offset is specified as a percent of stress that causes a specific amount of permanent strain-for metals, usually 0.2%. Starting at the origin of the curve, the offset is the horizontal distance between the modulus line and any line running parallel to it. Where this line intersects the stress strain curve is the offset yield point.
Another important property is the ultimate tensile strength. This is the peak or maximum load the specimen sustains during the test. Percent elongation measures the ductility of the material. Most samples prior to testing are marked with a gage length, for example, 1 or 2 inches. The broken sample is put back together and the new gage length is then measured. The original gage is subtracted from the final, divided by the original and then multiplied by 100 to calculate percent elongation. Modulus of elasticity is the ratio of stress to strain in the initial portion of the curve below the proportional limit. The greater the stress required to product a given strain, the stiffer the material.
- Tensile strength measures the force required to pull something such as rope, wire or a structural beam to the point where it breaks.
- Yield strength, or the yield point, is the stress at which a material begins to plastically deform.
- Percent elongation measures the ductility of the material.
- Modulus of elasticity is the mathematical description of an object or substance’s tendency to be deformed when a force is applied to it.