Types of Material Failure
A deformation failure is a change in the physical dimensions or shape of a component that is sufficient for its function to be lost or impaired. Cracking to the extent that a component is separated into two or more pieces is termed fracture.
Corrosion is the loss of material due to chemical action, and wear is surface removal due to abrasion or sticking between solid surfaces that touch. If wear is caused by a fluid (gas or liquid), it is called erosion, which is especially likely if the fluid contains hard particles. Although corrosion and wear are also of great importance, we shall primarily consider deformation and fracture.
Types of Material Failure
The basic types of material failure that are classified as either deformation or fracture are indicated in Fig. 1. Since several different causes exist, it is important to correctly identify the ones that may apply to a given design, so that the appropriate analysis methods can be chosen to predict the behavior. With such a need for classification in mind, the various types of deformation and fracture are defined and briefly described next.
Elastic and Plastic Deformation
Deformations are quantified in terms of normal and shear strain in elementary mechanics of materials. The cumulative effect of the strains in a component is a deformation, such as a bend, twist, or stretch. Deformations are sometimes essential for function, as in a spring. Excessive deformation, especially if permanent, is often harmful.
Deformation that appears quickly upon loading can be classed as either elastic deformation or plastic deformation, as illustrated in Fig. 2. Elastic deformation is recovered immediately upon unloading. Where this is the only deformation present, stress and strain are usually proportional. For axial loading, the constant of proportionality is the modulus of elasticity, E, as defined in Fig. 2(b). An example of failure by elastic deformation is a tall building that sways in the wind and causes discomfort to the occupants, although there may be only remote chance of collapse.
Elastic deformations are analyzed by the methods of elementary mechanics of materials and extensions of this general approach, as in books on theory of elasticity and structural analysis.
Plastic deformation is not recovered upon unloading and is therefore permanent. The difference between elastic and plastic deformation is illustrated in Fig. 2(c).
Once plastic deformation begins, only a small increase in stress usually causes a relatively large additional deformation. This process of relatively easy further deformation is called yielding, and the value of stress where this behavior begins to be important for a given material is called the yield strength, σo.
Materials capable of sustaining large amounts of plastic deformation are said to behave in a ductile manner, and those that fracture without very much plastic deformation behave in a brittle manner. Ductile behavior occurs for many metals, such as low-strength steels, copper, and lead, and for some plastics, such as polyethylene.
Brittle behavior occurs for glass, stone, acrylic plastic, and some metals, such as the high-strength steel used to make a file. (Note that the word plastic is used both as the common name for polymeric materials and in identifying plastic deformation, which can occur in any type of material.)
Tension tests are often employed to assess the strength and ductility of materials, as illustrated in Fig. 3.
Such a test is done by slowly stretching a bar of the material in tension until it breaks (fractures). The ultimate tensile strength, σu, which is the highest stress reached before fracture, is obtained along with the yield strength and the strain at fracture, εf .
The latter is a measure of ductility and is usually expressed as a percentage, then being called the percent elongation.
Materials having high values of both σu and εf are said to be tough, and tough materials are generally desirable for use in design.
Large plastic deformations virtually always constitute failure. For example, collapse of a steel bridge or building during an earthquake could occur due to plastic deformation. However, plastic deformation can be relatively small, but still cause malfunction of a component. For example, in a rotating shaft, a slight permanent bend results in unbalanced rotation, which in turn may cause vibration and perhaps early failure of the bearings supporting the shaft.
Buckling is deformation due to compressive stress that causes large changes in alignment of columns or plates, perhaps to the extent of folding or collapse. Either elastic or plastic deformation, or a combination of both, can dominate the behavior. Buckling is generally considered in books on elementary mechanics of materials and structural analysis.
Creep is deformation that accumulates with time. Depending on the magnitude of the applied stress and its duration, the deformation may become so large that a component can no longer perform its function.
Plastics and low-melting-temperature metals may creep at room temperature, and virtually any material will creep upon approaching its melting temperature. Creep is thus often an important problem where high temperature is encountered, as in gas-turbine aircraft engines.
Buckling can occur in a time-dependent manner due to creep deformation. An example of an application involving creep deformation is the design of tungsten light bulb filaments. The situation is illustrated in Fig. 4.
Sagging of the filament coil between its supports increases with time due to creep deformation caused by the weight of the filament itself. If too much deformation occurs, the adjacent turns of the coil touch one another, causing an electrical short and local overheating, which quickly leads to failure of the filament.
The coil geometry and supports are therefore designed to limit the stresses caused by the weight of the filament, and a special tungsten alloy that creeps less than pure tungsten is used.
Fracture under Static and Impact Loading
Rapid fracture can occur under loading that does not vary with time or that changes only slowly, called static loading. If such a fracture is accompanied by little plastic deformation, it is called a brittle fracture.
This is the normal mode of failure of glass and other materials that are resistant to plastic deformation. If the loading is applied very rapidly, called impact loading, brittle fracture is more likely to occur.
If a crack or other sharp flaw is present, brittle fracture can occur even in ductile steels or aluminum alloys, or in other materials that are normally capable of deforming plastically by large amounts. Such situations are analyzed by the special technology called fracture mechanics, which is the study of cracks in solids.
Resistance to brittle fracture in the presence of a crack is measured by a material property called the fracture toughness, KIc, as illustrated in Fig. 5. Materials with high strength generally have low fracture toughness, and vice versa. This trend is illustrated for several classes of high-strength steel in Fig. 6.
Ductile fracture can also occur. This type of fracture is accompanied by significant plastic deformation and is sometimes a gradual tearing process. Fracture mechanics and brittle or ductile fracture are especially important in the design of pressure vessels and large welded structures, such as bridges and ships.
Fracture may occur as a result of a combination of stress and chemical effects, and this is called environmental cracking. Problems of this type are a particular concern in the chemical industry, but also occur widely elsewhere.
For example, some low-strength steels are susceptible to cracking in caustic (basic or high pH) chemicals such as NaOH, and high-strength steels may crack in the presence of hydrogen or hydrogen sulfide gas. The term stress-corrosion cracking is also used to describe such behavior. This latter term is especially appropriate where material removal by corrosive action is also involved, which is not the case for all types of environmental cracking.
Creep deformation may proceed to the point that separation into two pieces occurs. This is called creep rupture and is similar to ductile fracture, except that the process is time dependent.
Fatigue under Cyclic Loading
A common cause of fracture is fatigue, which is failure due to repeated loading. In general, one or more tiny cracks start in the material, and these grow until complete failure occurs. A simple example is breaking a piece of wire by bending it back and forth a number of times. Crack growth during fatigue is illustrated in Fig. 8.
Prevention of fatigue fracture is a vital aspect of design for machines, vehicles, and structures that are subjected to repeated loading or vibration. For example, trucks passing over bridges cause fatigue in the bridge, and sailboat rudders and bicycle pedals can fail in fatigue.
Vehicles of all types, including automobiles, tractors, helicopters, and airplanes, are subject to this problem and must be extensively analyzed and tested to avoid it.
For example, some of the parts of a helicopter require careful design to avoid fatigue problems. If the number of repetitions (cycles) of the load is large, say, millions, then the situation is termed high-cycle fatigue.
Conversely, low-cycle fatigue is caused by a relatively small number of cycles, say, tens, hundreds, or thousands. Low-cycle fatigue is generally accompanied by significant amounts of plastic deformation, whereas high-cycle fatigue is associated with relatively small deformations that are primarily elastic. Repeated heating and cooling can cause a cyclic stress due to differential thermal expansion and contraction, resulting in thermal fatigue.
Cracks may be initially present in a component from manufacture, or they may start early in the service life. Emphasis must then be placed on the possible growth of these cracks by fatigue, as this can lead to a brittle or ductile fracture once the cracks are sufficiently large.
Such situations are identified by the term fatigue crack growth and may also be analyzed by the previously mentioned technology of fracture mechanics. For example, analysis of fatigue crack growth is used to schedule inspection and repair of large aircraft, in which cracks are commonly present. Such analysis is useful in preventing problems similar to the fuselage (main body) failure in 1988 of a passenger jet.
The problem in this case started with fatigue cracks at rivet holes in the aluminum structure. These cracks gradually grew during use of the airplane, joining together and forming a large crack that caused a major fracture, resulting in separation of a large section of the structure. The failure could have been avoided by more frequent inspection and repair of cracks before they grew to a dangerous extent.
Two or more of the previously described types of failure may act together to cause effects greater than would be expected by their separate action; that is, there is a synergistic effect.
Creep and fatigue may produce such an enhanced effect where there is cyclic loading at high temperature. This may occur in steam turbines in electric power plants and in gas-turbine aircraft engines. Wear due to small motions between fitted parts may combine with cyclic loading to produce surface damage followed by cracking, which is called fretting fatigue.
This may cause failure at surprisingly low stress levels for certain combinations of materials. For example, fretting fatigue could occur where a gear is fastened on a shaft by shrink fitting or press fitting.
Similarly, corrosion fatigue is the combination of cyclic loading and corrosion. It is often a problem in cyclically loaded components of steel that must operate in seawater, such as the structural members of offshore oil well platforms.
Material properties may degrade with time due to various environmental effects. For example, the ultraviolet content of sunlight causes some plastics to become brittle, and wood decreases in strength with time, especially if exposed to moisture. As a further example, steels become brittle if exposed to neutron radiation over long periods of time, and this affects the retirement life of nuclear reactors.
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