Measures of ductility
- Key People:
- Charles Benjamin Dudley
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.
Fatigue
Materials that survive a single application of stress frequently fail when stressed repeatedly. This phenomenon, known as fatigue, is measured by mechanical tests that involve repeated application of different stresses varying in a regular cycle from maximum to minimum value. Most fatigue-testing machines employ a rotating eccentric weight to produce this cyclically varying load. A material is generally considered to suffer from low-cycle fatigue if it fails in 10,000 cycles or less.
The stresses acting upon a material in the real world are usually random in nature rather than cyclic. Consequently, several cumulative fatigue-damage theories have been developed to enable investigators to extrapolate from cyclic test data a prediction of material behaviour under random stresses. Because these theories are not applicable to most materials, a relatively new technique, which involves mechanical application of random fatigue stresses, statistically matched to real-life conditions, is now employed in most materials test laboratories.
Material fatigue involves a number of phenomena, among which are atomic slip (in which the upper plane of a metal crystal moves or slips in relation to the lower plane, in response to a shearing stress), crack initiation, and crack propagation. Thus, a fatigue test may measure the number of cycles required to initiate a crack, as well as the number of cycles to failure.
A cautious designer always bears the statistical nature of fatigue in mind, for the lives of material specimens tested at a common stress level always range above and below some average value. Statistical theory tells the designer how many samples of a material must be tested in order to provide adequate data; it is not uncommon to test several hundred specimens before drawing firm conclusions.
Measurement of thermal properties
Thermal conductivity
Heat, which passes through a solid body by physical transfer of free electrons and by vibration of atoms and molecules, stops flowing when the temperature is equal at all points in the solid body and equals the temperature in the surrounding environment. In the process of attaining equilibrium, there is a gross heat flow through the body, which depends upon the temperature difference between different points in the body and upon the magnitudes of the temperatures involved. Thermal conductivity is experimentally measured by determining temperatures as a function of time along the length of a bar or across the surface of flat plates while simultaneously controlling the external input and output of heat from the surfaces of the bar or the edges of the plate.
Specific heat
Specific heat of solid materials (defined as heat absorbed per unit mass per degree change in temperature) is generally measured by the drop method, which involves adding a known mass of the material at a known elevated temperature to a known mass of water at a known low temperature and determining the equilibrium temperature of the mixture that results. Specific heat is then computed by measuring the heat absorbed by the water and container, which is equivalent to the heat given up by the hot material.
Thermal expansion
Expansion due to heat is usually measured in linear fashion as the change in a unit length of a material caused by a one-degree change in temperature. Because many materials expand less than a micrometre with a one-degree increase in temperature, measurements are made by means of microscopes.
Measurement of electrical properties
An understanding of electrical properties and testing methods requires a brief explanation of the free electron gas theory of electrical conduction. This simple theory is convenient for purposes of exposition, even though solid-state physics has advanced beyond it.
Electrical conductivity involves a flow or current of free electrons through a solid body. Some materials, such as metals, are good conductors of electricity; these possess free or valence electrons that do not remain permanently associated with the atoms of a solid but instead form an electron “cloud” or gas around the peripheries of the atoms and are free to move through the solid at a rapid rate. In other materials, such as plastics, the valence electrons are far more restricted in their movements and do not form a free-electron cloud. Such materials act as insulators against the flow of electricity.
The effect of heat upon the electrical conductivity of a material varies for good and poor conductors. In good conductors, thermal agitation interferes with the flow of electrons, decreasing conductivity, while, as insulators increase in temperature, the number of free electrons grows, and conductivity increases. Normally, good and poor conductors are enormously far apart in basic conductivity, and relatively small changes in temperature do not change these properties significantly.
In certain materials, however, such as silicon, germanium, and carbon, heat produces a large increase in the number of free electrons; such materials are called semiconductors. Acting as insulators at absolute zero, semiconductors possess significant conductivity at room and elevated temperatures. Impurities also can change the conductivity of a semiconductor dramatically by providing more free electrons. Heat-caused conductivity is called intrinsic, while that attributable to extra electrons from impurity atoms is called extrinsic.
Conductivity of a material is generally measured by passing a known current at constant voltage through a known volume of the material and determining resistance in ohms. The total conductivity is then calculated by simply taking the reciprocal of the total resistivity.
