This article will investigate the various ways in which mechanical testing techniques can be applied to engineering materials. We will examine the equipment and procedure for the mechanical tests and analyse a typical set of results from the test to determine what this tells us about the engineering material properties.
A tensile test is carried out on a sample engineering material to determine the maximum tensile strength of the material and its elongation at fracture (or failure). This is a common test for materials and perhaps one of the most useful testing techniques. A specimen will be deformed under tension (a 'pull' force), usually until the fracture point.
The testing method requires the use of a tensile testing machine equipped with an extensometer and a material specimen. The specimen consists of a material with known dimensions such as the gauge length and diameter, which will give us the cross-sectional area, and is shown in figure 1. Once the material is in place a gradually increasing force is applied uniaxially in the longitudinal direction via the moving cross head. The specimen is elongated at a constant rate during the test where the applied instantaneous load is measured by the load cell and the elongation is measured by the extensometer. The output of the test is recorded on a chart with the load applied versus the elongation of the material.
The method is known as destructive testing as the test specimen is permanently deformed and often fractured after the test. The resulting graph that is plotted is usually converted to the engineering stress versus the engineering strain using the original dimensions of the material. The stress and the strain can be found with the following formulas:
σ = stress (Pa)
F = Force applied (N)
A = Specimen cross sectional area (
ε = strain (no units)
x = material extension (m)
L = Original specimen length (m)
The resulting graph will look similar to that in figure 2 where the elastic and plastic regions can be identified. The elastic region represents the maximum stress that can be applied before the material permanently deforms.
Another key feature from the graph is the Young’s modulus, E. This can be found from the gradient of the straight line, up until the proportional limit is reached. A material with high Young's modulus requires a higher force to produce a deformation and is more rigid.
The Young's (or Elastic) Modulus, E, is given by:
Another key feature of a stress strain curve relates to the area under the curve, which can be related to the toughness of the material. Figure 3 shows how example tests for ceramics, metals and unreinforced polymers may look like. As can be seen metals have the expected highest toughness values compared to the smallest in polymers meaning.