Creep Fracture mechanisms

In Chapter 17 we showed that, when a material is loaded at a high temperature, it creeps, that is, it deforms, continuously and permanently, at a stress that is less than the stress that would cause any permanent deformation at room temperature. In order to understand how we can make engineering materials more resistant to creep deformation and creep fracture, we must first look at how creep and creep-fracture take place on an atomic level, i.e. we must identify and understand the mechanisms by which they take place. 

There are two mechanisms of creep dislocation creep (which gives power-law behaviour) and diffusiona creep (which gives linear-viscous creep). The rate of both is usually limited by diffusion, so both follow Arrhenius s Law. Creep fracture, too, depends on diffusion. Diffusion becomes appreciable at about 0.37 - that is why materials start to creep above this temperature. 

In this book no prior knowledge of plastics is assumed. Chapter 1 provides a brief introduction to the structure of plastics and it provides an insight to the way in which their unique structure affects their performance. There is a resume of the main types of plastics which are available. Chapter 2 deals with the mechanical properties of unreinforced and reinforced plastics under the general heading of deformation. The time dependent behaviour of the materials is introduced and simple design procedures are illustrated. Chapter 3 continues the discussion on properties but concentrates on fracture as caused by creep, fatigue and impact. The concepts of fracture mechanics are also introduced for reinforced and unreinforced plastics. 

Fracture mechanics specimens (CT, DENT, FNCT, DCB) are a convenient means to study creep behaviour and slow crack growth. 

Structural failure may occur when the overall structural cross-section cannot support the applied load or, when the critical flaw size ac is exceeded by preexisting discontinuity or by reaching the critical crack size through fatigue, stress corrosion cracking or creep mechanisms. Using fracture mechanics the stress at a crack tip can be calculated by a stress-intensity parameter K as,... 

Structural failures in general occur by a combination of crack initiation followed by subcritical crack growth mechanism such as stress corrosion, fatigue, and creep until a critical crack size is reached. Thus, fracture mechanics is useful in evaluating ac that leads to failure by fracture. The application of fracture mechanics in failure analysis throws light on the progress from subcritical crack growth to fracture, the amount of load... 

H. E. Evans, Mechanisms of Creep Fracture, Elsevier Applied Science Publishers, London, U.K., 1984. 

Analogous to the arguments of small-scale yielding in linear elastic fracture mechanics, small-scale creep conditions (i.e., the situation where the size of the creep zone ahead of the fatigue crack tip is small compared to the characteristic dimensions of the test specimen, including the size of the crack and that of the uncracked liagment) can be assumed to exist when... 

H. Riedel and J. R. Rice, Tensile Cracks in Creeping Solids, in Fracture Mechanics Twelfth Conference, ASTM Technical Publication 700, ed. P. C. Paris, American Society for Testing and Materials, Philadelphia, PA, 1980, pp. 112-130. 

Crack growth models in monolithic solids have been well document-ed. 1-3,36-45 These have been derived from the crack tip fields by the application of suitable fracture criteria within a creep process zone in advance of the crack tip. Generally, it is assumed that secondary failure in the crack tip process zone is initiated by a creep plastic deformation mechanism and that advance of the primary crack is controlled by such secondary fracture initiation inside the creep plastic zone. An example of such a fracture mechanism is the well-known creep-induced grain boundary void initiation, growth and coalescence inside the creep zone observed both in metals1-3 and ceramics.4-10 Such creep plastic-zone-induced failure can be described by a criterion involving both a critical plastic strain as well as a critical microstructure-dependent distance. The criterion states that advance of the primary creep crack can occur when a critical strain, ec, is exceeded over a critical distance, lc in front of the crack tip. In other words... 

A. Saxena, Creep Crack Growth under Non-Steady-State Conditions, in Fracture Mechanics Seventeenth Volume, eds. J. H. Underwood, R. Chait, C. W. Smith, D. P. Wilhem, W. A. Andrews, and J. C. Newman, ASTM STP 905, American Society for Testing and Materials, Philadelphia, PA,... 

A. Saxena, Mechanics and Mechanisms of Creep Crack Growth, in Fracture Mechanics Microstructure and Micromechanisms, eds. S. V. Nair, J. K. Tien, R. C. Bates, and O. Buck, ASM Materials Science Seminar, ASM International, OH, 1987, pp. 283-334. 

J. L. Bassani, D. E. Hawk, and F.-H. Wu, Crack Growth in Small-Scale Creep, in Nonlinear Fracture Mechanics Volume I—Time Dependent Fracture, eds. A. Saxena, J. D. Landes, and J. L. Bassani, ASTM STP 995, American Society for Testing and Materials, Philadelphia, PA, 1988, pp. 68-95. 

Pinter G, Lang RW (2001) Fracture mechanics characterisation of effects of stabilisers on creep crack growth in polyethylene pipes. Plast Rubber Compos 30(2) 94-100... 

Several cautions are, however, in order. Polymers are notorious for their time dependent behavior. Slow but persistent relaxation processes can result in glass transition type behavior (under stress) at temperatures well below the commonly quoted dilatometric or DTA glass transition temperature. Under such a condition the polymer is ductile, not brittle. Thus, the question of a brittle-ductile transition arises, a subject which this writer has discussed on occasion. It is then necessary to compare the propensity of a sample to fail by brittle crack propagation versus its tendency to fail (in service) by excessive creep. The use of linear elastic fracture mechanics addresses the first failure mode and not the second. If the brittle-ductile transition is kinetic in origin then at some stress a time always exists at which large strains will develop, provided that brittle failure does not intervene. 

By studying the sample dilatation versus strain in uniaxial tension creep tests. Buck-nail is able to determine the operative niechanism in each system. Fracture mechanics is used to evaluate the toughness parameters of the various systems. 

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