In materials science, material failure is the loss of the load-carrying capacity of a material unit. The design of a component or structure often calls upon the engineer to minimize the possibility of failure. Whether we like it or not, various components in service suffer failure (either breakage or change of shape) and cannot perform their designated function. The usual causes of failure are improper materials selection and processing and inadequate design of the component or its misuse. In the best case, the failed component requires replacement, and if we do not want a repetition of the failure, we must understand what caused it. Thus, it is important to understand the mechanics of the various mechanical failure modes: fracture, fatigue, and creep. In addition, be familiar with appropriate design principles that may be employed to prevent service failures. This is the main task in a very important scientific field known as failure analysis.
Failure analysis (FA) is a multidisciplinary scientific field connecting areas of engineering from diverse backgrounds and bodies of knowledge. From applied mechanics to electrochemistry and corrosion and from numerical modeling to understanding surface science and tribology. The complexity of the nature of the subject requires embracing various engineering disciplines to succeed in high process performance and effective root-cause analysis, which is the core and central objective of the failure investigation process. The following section is addressed simple fracture (both ductile and brittle modes), fundamentals of fracture mechanics, fracture toughness testing, ductile-to-brittle transition, pressurized thermal shocks, fatigue, and creep.
- Fracture of Material. A fracture is the separation of an object or material into two or more pieces under the action of stress. Engineers need to understand fracture mechanisms. Some fractures (e.g., brittle fractures) occur under specific conditions without warning and can cause major damage to materials. A brittle fracture occurs suddenly and catastrophically without any warning. This is a consequence of spontaneous and rapid crack propagation. However, for ductile fractures, the presence of plastic deformation gives a warning that failure is imminent, allowing preventive measures to be taken. A detailed understanding of how fracture occurs in materials may be assisted by the study of fracture mechanics.
- Fatigue of Material. In materials science, fatigue is the weakening of a material caused by cyclic loading, resulting in progressive, brittle, and localized structural damage. Once a crack has been initiated, each loading cycle will grow the crack a small amount, even when repeated alternating or cyclic stresses are of an intensity considerably below the normal strength. The stresses could be due to vibration or thermal cycling. Fatigue damage is caused by:
- simultaneous action of cyclic stress,
- tensile stress (whether directly applied or residual),
- plastic strain.
If any one of these three is not present, a fatigue crack will not initiate and propagate. The majority of engineering failures are caused by fatigue.
- Wear. In general, wear is mechanically induced surface damage that results in the progressive removal of material due to relative motion between that surface and a contacting substance or substances. A contracting substance may consist of another character, a fluid, or hard, abrasive particles contained in some form of liquid or suspension, such as a lubricant. In most technological applications, the occurrence of wear is highly undesirable, and it is an enormously expensive problem since it leads to the deterioration or even failure of components. In terms of safety, it is often not as serious (or as sudden) as a fracture. This is because the wear is usually anticipated.
- Corrosion. Corrosion is usually a negative phenomenon since it is associated with the mechanical failure of an object. Metal atoms are removed from a structural element until it fails, or oxides build up inside a pipe until it is plugged. All metals and alloys are subject to corrosion. Even noble metals, such as gold, are subject to corrosive attack in some environments.
- Creep. Creep, also known as cold flow, is the permanent deformation that increases with time under constant load or stress. It results due to long time exposure to large external mechanical stress within the limit of yielding and is more severe in materials that are subjected to heat for a long time. Creep is very important if we use materials at high temperatures. Creep is very important in the power industry and is of the highest importance in designing jet engines. Time to rupture is the dominant design consideration for many relatively short-life creep situations (e.g., turbine blades in military aircraft).
