materials testing
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What is materials testing?
Materials testing measures the properties and behavior of substances like metals, ceramics, or plastics under different conditions to determine if they are suitable for specific applications, such as construction or packaging. It helps designers estimate anticipated stress and specify materials that can endure them.
Materials testing includes mechanical, thermal, and electrical testing, as well as testing for corrosion, radiation, and biological deterioration resistance, and nondestructive testing. Testing machines measure tensile strength, ductility, hardness, impact strength, and more. The International Organization for Standardization (ISO) and the American Society for Testing and Materials (ASTM) have established standard test methods.
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Why do scientists and engineers test materials?
Scientists and engineers test materials to measure their characteristics and behavior under different conditions. The data obtained helps determine if the materials are suitable for specific applications, such as construction or packaging. Material testing helps designers estimate the amount of stress a material can withstand to prevent failure from fracture or deformation. The tests also help to measure a material's propensity to propagate a crack. Mechanical property tests indicate which materials may be safely employed.
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What are some basic types of materials tests?
Materials testing assesses substance traits like metals, ceramics, or plastics under varying conditions to see if they are suitable for particular uses. Materials testing includes mechanical, thermal, and electrical testing, as well as testing for corrosion, radiation, and biological deterioration resistance, and nondestructive testing.
Some common tests include:
* Impact test Measures a material's capacity to withstand impact, often using a swinging pendulum to strike a notched bar.
* Hardness test Measures a material's resistance to localized penetration using a standardized indenter.
* Static tension and compression tests Measures how materials respond to pulling or crushing forces.
* Fatigue test Measures how materials behave under repeated stress.4 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How does a tensile test work?
A tensile test determines how a material reacts when pulled apart. The test involves stretching a test piece until it breaks, measuring its breaking point and elongation, known as strain. Strain is the change in length per unit length.
The process requires a test piece, a machine to apply and measure loads, and grips to hold the test piece. The machine stretches a small part of the test piece, and an extensometer measures the length at different loads to compute strain. The tensile strength is the maximum load the material can handle without breaking, divided by its original cross-sectional area.
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How do materials tests help in designing safe buildings and products?
Materials testing is crucial for designing safe buildings and products. These tests measure how materials behave under different conditions, such as stress, tension, and shear, to determine their suitability for specific applications like building and aircraft construction.
Stress analysis and mechanical property tests help designers estimate anticipated stress and select materials that can withstand it. Tests also measure resistance to corrosion, radiation, and biological deterioration. Nondestructive tests are employed on costly materials to detect defects without destroying the sample. The data obtained ensures that structures and machines can endure expected stresses, preventing failure due to fracture or deformation. Standard test methods established by organizations like ISO and ASTM further ensure reliability.
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What is the difference between destructive and non-destructive materials testing?
Destructive testing assesses material properties by testing to failure, which destroys the sample. This is suitable when there is a plentiful supply of the material.
Nondestructive testing is used on costly or difficult-to-fabricate materials, or on finished products, using methods such as infrared, magnetic, or liquid penetrant techniques to detect flaws without destroying the material.
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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 applicationsāe.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.
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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 (101ā100)/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.
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Static compression tests determine a materialās 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.