materials testing
- Key People:
- Charles Benjamin Dudley
materials testing, measurement of the characteristics and behaviour of such substances as metals, ceramics, or plastics under various conditions. The data thus obtained can be used in specifying the suitability of materials for various applications—e.g., building or aircraft construction, machinery, or packaging. A full- or small-scale model of a proposed machine or structure may be tested. Alternatively, investigators may construct mathematical models that utilize known material characteristics and behaviour to predict capabilities of the structure.
Materials testing breaks down into five major categories: mechanical testing; testing for thermal properties; testing for electrical properties; testing for resistance to corrosion, radiation, and biological deterioration; and nondestructive testing. Standard test methods have been established by such national and international bodies as the International Organization for Standardization (ISO), with headquarters in Geneva, and the American Society for Testing and Materials (ASTM), Philadelphia.
Mechanical testing
Structures and machines, or their components, fail because of fracture or excessive deformation. In attempting to prevent such failure, the designer estimates how much stress (load per unit area) can be anticipated, and specifies materials that can withstand expected stresses. A stress analysis, accomplished either experimentally or by means of a mathematical model, indicates expected areas of high stress in a machine or structure. Mechanical property tests, carried out experimentally, indicate which materials may safely be employed.
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.
Measures of ductility
Ductility is the capacity of a material to deform permanently in response to stress. Most common steels, for example, are quite ductile and hence can accommodate local stress concentrations. Brittle materials, such as glass, cannot accommodate concentrations of stress because they lack ductility; they, therefore, fracture rather easily.
When a material specimen is stressed, it deforms elastically (i.e., recoverably) at first; thereafter, deformation becomes permanent. A cylinder of steel, for example, may “neck” (assume an hourglass shape) in response to stress. If the material is ductile, this local deformation is permanent, and the test piece does not assume its former shape if the stress is removed. With sufficiently high stress, fracture occurs.
Ductility can be expressed as strain, reduction in area, or toughness. Strain, or change in length per unit length, was explained earlier. Reduction in area (change in area per unit area) may be measured, for example, in the test section of a steel bar that necks when stressed. Toughness measures the amount of energy required to deform a piece of material permanently. Toughness is a desirable material property in that it permits a component to deform plastically, rather than crack and perhaps fracture.
Hardness testing
Based on the idea that a material’s response to a load placed at one small point is related to its ability to deform permanently (yield), the hardness test is performed by pressing a hardened steel ball (Brinell test) or a steel or diamond cone (Rockwell test) into the surface of the test piece. Most hardness tests are performed on commercial machines that register arbitrary values in inverse relation to the depth of penetration of the ball or cone. Similar indentation tests are performed on wood. Hardness tests of materials such as rubber or plastic do not have the same connotation as those performed on metals. Penetration is measured, of course, but deformation caused by testing such materials may be entirely temporary.
Some hardness tests, particularly those designed to provide a measure of wear or abrasion, are performed dynamically with a weight of given magnitude that falls from a prescribed height. Sometimes a hammer is used, falling vertically on the test piece or in a pendulum motion.
Impact test
Many materials, sensitive to the presence of flaws, cracks, and notches, fail suddenly under impact. The most common impact tests (Charpy and Izod) employ a swinging pendulum to strike a notched bar; heights before and after impact are used to compute the energy required to fracture the bar and, consequently, the bar’s impact strength. In the Charpy test, the test piece is held horizontally between two vertical bars, much like the lintel over a door. In the Izod test, the specimen stands erect, like a fence post. Shape and size of the specimen, mode of support, notch shape and geometry, and velocities at impact are all varied to produce specific test conditions. Nonmetals such as wood may be tested as supported beams, similar to the Charpy test. In nonmetal tests, however, the striking hammer falls vertically in a guide column, and the test is repeated from increasing heights until failure occurs.
Some materials vary in impact strength at different temperatures, becoming very brittle when cold. Tests have shown that the decrease in material strength and elasticity is often quite abrupt at a certain temperature, which is called the transition temperature for that material. Designers always specify a material that possesses a transition temperature well below the range of heat and cold to which the structure or machine is exposed. Thus, even a building in the tropics, which will doubtless never be exposed to freezing weather, employs materials with transition temperatures slightly below freezing.
Fracture toughness tests
The stringent materials-reliability requirements of the space programs undertaken since the early 1960s brought about substantial changes in design philosophy. Designers asked materials engineers to devise quantitative tests capable of measuring the propensity of a material to propagate a crack. Conventional methods of stress analysis and materials-property tests were retained, but interpretation of results changed. The criterion for failure became sudden propagation of a crack rather than fracture. Tests have shown that cracks occur by opening, when two pieces of material part in vertical plane, one piece going up, the other down; by edge sliding, where the material splits in horizontal plane, one piece moving left, the other right; and by tearing, where the material splits with one piece moving diagonally upward to the left, the other moving diagonally downward to the right.
Creep test
Creep is the slow change in the dimensions of a material due to prolonged stress; most common metals exhibit creep behaviour. In the creep test, loads below those necessary to cause instantaneous fracture are applied to the material, and the deformation over a period of time (creep strain) under constant load is measured, usually with an extensometer or strain gauge. In the same test, time to failure is also measured against level of stress; the resulting curve is called stress rupture or creep rupture. Once creep strain versus time is plotted, a variety of mathematical techniques is available for extrapolating creep behaviour of materials beyond the test times so that designers can utilize thousand-hour test data, for example, to predict ten-thousand-hour behaviour.
A material that yields continually under stress and then returns to its original shape when the stress is released is said to be viscoelastic; this type of response is measured by the stress-relaxation test. A prescribed displacement or strain is induced in the specimen and the load drop-off as a function of time is measured. Various viscoelastic theories are available that permit the translation of stress-relaxation test data into predictions about the creep behaviour of the material.
