Failure Modes of Materials

Liberty Ship - Hull Failure
Brittle fracture of the U.S. Liberty Ship Esso Manhattan

In materials science, material failure is the loss of 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.The failed component requires, in the best case, 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 mecahnical failure modes: fracture, fatigue, and creep. In addition, be familiar with appropriate design principles that may be employed to prevent inservice failures. This is the main task for very important scientific field known as the 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 the understanding of surface science and tribology. The complexity of the nature of the subject requires the embracing of various engineering disciplines, to succeed high process performance and effective root-cause analysis, which is the core and the central objective of the failure investigation process. The following section is addressed to simple fracture (both ductile and brittle modes), fundamentals of fracture mechanics, fracture toughness testing, the 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. There are fractures (e.g. brittle fracture), which occur under specific conditions without warning and can cause major damage to materials. Brittle fracture occurs suddenly and catastrophically without any warning. This is a consequence of the spontaneous and rapid crack propagation. However, for ductile fracture, the presence of plastic deformation gives 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, fatique is the weakening of a material caused by cyclic loading that results in progressive, brittle and localized structural damage. Once a crack has 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 contacting substance may consist of another surface, a fluid, or hard, abrasive particles contained in some form of fluid or suspension, such as a lubricant for example. In most of the 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 fracture. This is because wear is usually anticipated.
  • Corrosion. Corrosion is usually a negative phenomenon, since it is associated with 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 the noble metals, such as gold, are subject to corrosive attack in some environments.
  • Creep. Creep, known also 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 with in limit of yielding and is more severe in material that are subjected to heat for long time. Creep is a very important phenomenon if we are using materials at high temperature. Creep is very important in power industry and it is of the highest importance in designing of jet engines. For many relatively short-life creep situations (e.g. turbine blades in military aircraft), time to rupture is the dominant design consideration.
References:

Materials Science:

  1. U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
  2. U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 2 and 2. January 1993.
  3. William D. Callister, David G. Rethwisch. Materials Science and Engineering: An Introduction 9th Edition, Wiley; 9 edition (December 4, 2013), ISBN-13: 978-1118324578.
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  5. Gaskell, David R. (1995). Introduction to the Thermodynamics of Materials (4th ed.). Taylor and Francis Publishing. ISBN 978-1-56032-992-3.
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  7. Ashby, Michael; Hugh Shercliff; David Cebon (2007). Materials: engineering, science, processing and design (1st ed.). Butterworth-Heinemann. ISBN 978-0-7506-8391-3.
  8. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.

See above:
Metals