Despite the best efforts of design engineers and materials specialists, and in spite of well-thought-out procedures for quality assurance and control, engineering components still fail in service from time to time. In most cases the failure does not lead to serious consequences and the component is merely replaced as having served its useful life and worn out, and in such cases optimization of the procedures of design and quality assurance have probably not been of unduly great concern, economic considerations dictating that avoidance of failure is not of overriding importance. However, in other cases, failure may lead to serious consequences which can cause serious financial loss, environmental contamination, or loss of life: in such situations and to minimize the risk of failure, the design and quality assurance procedures must have been very carefully studied and controlled, and in the event of such a failure occurring it is frequently necessary to establish the root causes in terms of design, choice, and quality of material, fabrication procedure, and so on.

Thus, the metallographer may be required to conduct an investigation to determine the mechanisms of failure, based on the appearance of the fracture surfaces and the microstructure of the components which have failed. It is the purpose of this paper to review the various mechanisms of fracture and to discuss the identifying morphological and microstructural features appropriate to each. There are two basic and distinct mechanisms of fracture, namely cleavage and ductile fracture. The former occurs under tensile stress and involves separation along crystallographic planes with little or no plastic flow taking place, and leads to a brittle appearance of the fracture surface, with a small amount of energy being dissipated: the latter involves plastic deformation by sl ip and the energy dissipation involved is much greater, but depends on the extent of the plastic flow. It will be large when extensive shear, necking, or void formation occurs, but relatively small when plastic flow is localized to the tip of a propagating crack, in which case the fracture may be termed brittle in an engineering sense but is ductile on a strictly mechanistic basis.

However, it is insufficiently informative to classify fractures strictly in terms of the mechanisms of cleavage or ductile fracture, and we must look at the various factors which allow and cause cracks to propagate gradually or in a stepwise manner until they reach a critical length at which rapid failure takes place under load, either by cleavage, ductile fracture or some combination of the two. Indeed, it is relatively uncommon to find fractures of a simple overload nature outside of the laboratory, and much more common to find that failure in service has taken place over a period under the normal working conditions of loading. Thus, we shall identify fatigue crack growth, intergranular cracking and environmentally assisted cracking as additional mechanisms leading to failure.

Fatigue crack growth occurs under repeated or cyclic tensile stress and is by far the commonest cause of failure in engineering components and structures. Intergranular cracking may arise in several ways: it may take place during creep deformation at high temperature either because of grain boundary sliding or from the growth of voids at grain boundaries because of vacancy deposition; or it may arise from segregation of specific elements at grain boundaries, so leading to an embrittled structure which can fracture under impact or monotonic loading. Environmentally assisted cracking relates to fracture under the combined action of stress and an environmental effect such as corrosion, the presence of an embrittling fluid such as hydrogen, or the adsorption of a chemical species on to the fracture surface. Thus, environmentally assisted cracking includes the phenomena of stress corrosion cracking, corrosion fatigue, hydrogen embrittlement and liquid metal embrittlement. In addition, we may extend it to include failure caused by radiation damage in conjunction with the service loading.

Two additional mechanisms of failure may be identified, corrosion and wear. These are both extremely important from a practical standpoint, although they are of a somewhat different nature to the mechanisms of fracture identified above.