materials testing

Static tension and compression tests

When subjected to tension (pulling apart), a material elongates and eventually breaks. A simple static tension test determines the breaking point of the material and its elongation, designated as strain (change in length per unit length). If a 100-millimetre steel bar elongates 1 millimetre under a given load, for example, strain is (101–100)/100 = 1/100 = 1 percent.

A static tension test requires (1) a test piece, usually cylindrical, or with a middle section of smaller diameter than the ends; (2) a test machine that applies, measures, and records various loads; and (3) an appropriate set of grips to grasp the test piece. In the static tension test, the test machine uniformly stretches a small part (the test section) of the test piece. The length of the test section (called the gauge length) is measured at different loads with a device called an extensometer; these measurements are used to compute strain.

Conventional testing machines are of the constant load, constant load-rate, and constant displacement-rate types. Constant load types employ weights directly both to apply load and to measure it. Constant load-rate test machines employ separate load and measurement units; loads are generally applied by means of a hydraulic ram into which oil is pumped at a constant rate. Constant displacement-rate testing machines are generally driven by gear-screws.

Test machine grips are designed to transfer load smoothly into the test piece without producing local stress concentrations. The ends of the test piece are often slightly enlarged so that if slight concentrations of stress are present these will be directed to the gauge section, and failures will occur only where measurements are being taken. Clamps, pins, threading, or bonding are employed to hold the test piece. Eccentric (nonuniform) loading causes bending of the sample in addition to tension, which means that stress in the sample will not be uniform. To avoid this, most gripping devices incorporate one or two swivel joints in the linkage that carries the load to the test piece. Air bearings help to correct horizontal misalignment, which can be troublesome with such brittle materials as ceramics.

Static compression tests determine a material’s response to crushing, or support-type loading (such as in the beams of a house). Testing machines and extensometers for compression tests resemble those used for tension tests. Specimens are generally simpler, however, because gripping is not usually a problem. Furthermore, specimens may have a constant cross-sectional area throughout their full length. The gauge length of a sample in a compression test is its full length. A serious problem in compression testing is the possibility that the sample or load chain may buckle (form bulges or bend) prior to material failure. To prevent this, specimens are kept short and stubby.

Static shear and bending tests

Inplane shear tests indicate the deformation response of a material to forces applied tangentially. These tests are applied primarily to thin sheet materials, either metals or composites, such as fibreglass reinforced plastic.

A homogeneous material such as untreated steel casting reacts in a different way under stress than does a grained material such as wood or an adhesively bonded joint. These anisotropic materials are said to have preferential planes of weakness; they resist stress better in some planes than in others, and consequently must undergo a different type of shear test.

Shear strength of rivets and other fasteners also can be measured. Though the state of stress of such items is generally quite complicated, a simple shear test, providing only limited information, is adequate for most purposes.

Tensile testing is difficult to perform directly upon certain brittle materials such as glass and ceramics. In such cases, a measure of the tensile strength of the material may be obtained by performing a bend test, in which tensile (stretching) stresses develop on one side of the bent member and corresponding compressive stresses develop on the opposite side. If the material is substantially stronger in compression than tension, failure initiates on the tensile side of the member and, hence, provides the required information on the material tensile strength. Because it is necessary to know the exact magnitude of the tensile stress at failure in order to establish the strength of the material, however, the bending test method is applicable to only a very restricted class of materials and conditions.