Creep deformation mechanism

Mechanisms of Creep Deformation. Three creep deformation mechanisms occur ... 

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. 

This competition between mechanisms is conveniently summarised on Deformation Mechanism Diagrams (Figs. 19.5 and 19.6). They show the range of stress and temperature (Fig. 19.5) or of strain-rate and stress (Fig. 19.6) in which we expect to find each sort of creep (they also show where plastic yielding occurs, and where deformation is simply elastic). Diagrams like these are available for many metals and ceramics, and are a useful summary of creep behaviour, helpful in selecting a material for high-temperature applications. 

Frost, H.J. and Ashby, M.F. (1982) Deformation-Mechanism Maps The Plasticity and Creep of Metals and Ceramics (Pergamon Press, Oxford). 

The first section involves a general description of the mechanics and geometry of indentation with regard to prevailing mechanisms. The experimental details of the hardness measurement are outlined. The tendency of polymers to creep under the indenter during hardness measurement is commented. Hardness predicitions of model polymer lattices are discussed. The deformation mechanism of lamellar structures are discussed in the light of current models of plastic deformation. Calculations... 

The dynamic viscoelastic properties of acetylated wood have been determined and compared with other wood treatments in a number of studies. Both the specific dynamic Young s modulus (E /j) and tan S are lower in acetylated wood compared with unmodified wood (Akitsu etal., 1991, 1992, 1993a,b Korai and Suzuki, 1995 Chang etal., 2000). Acetylation also reduces mechanosorptive creep deformation of the modified wood (Norimoto etal., 1992 Yano etal, 1993). In a study of the dynamic mechanical properties of acetylated wood under conditions of varying humidity, it was concluded that the rate of diffusion of moisture into the wood samples was not affected by acetylation (Ebrahimzadeh, 1998). 

The applicability of the Gibson and Ashby approach, whereby deformation mechanisms are identified, to a range of thermoplastic polymer foams is explored. LDPE, EVA and PP foams were produced by the BXL Plastizote nitrogen expansion process. A full range of mechanical properties is discussed from the simpler aspects of modulus and strength to the complexities of creep and recovery performance. 8 refs. 

Figure 16.5 Deformation mechanism map for Ag polycrystal a = applied stress, p = shear modulus, grain size = 32 pm, and strain rate = IGF8 s 1. The diffusional creep field is divided into two subfields the Coble creep field and the Nabarro-Herring creep field. From Ashby [20].

Fig. 2.15 Illustration of three deformation mechanisms proposed for BCC spheres, depending on shear rate (Koppi et al. 1994) (a) slow shearing results in creep (b) at an intermediate shear rate, the generation of numerous defects leads to a loss of translational order (c) at high shear rates, the spheres, undergo an affine elastic deformation. The layers shown represent [110) planes of a BCC structure, y is the inverse relaxation time of the defects.

Keywords Crazing Creep Molecular deformation mechanisms Disentanglement Time-dependent strength Toughness... 

Numerous investigations that support the use of the C, parameter have been carried out by Saxena and co-workers,1,2,42 although there are many fundamental issues to be addressed. The latter essentially are related to the nonexistence of a strain-rate potential for simultaneous elastic and creep deformations. Nevertheless, based upon rigorous mechanics arguments, it is certain that neither C nor / 7 alone can adequately correlate crack growth in SSC. See the next section for more details on crack growth models. 

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... 

The first quantitative study of deformation mechanisms in ABS polymers was made by Bucknall and Drinkwater, who used accurate exten-someters to make simultaneous measurements of longitudinal and lateral strains during tensile creep tests (4). Volume strains calculated from these data were used to determine the extent of craze formation, and lateral strains were used to follow shear processes. Thus the tensile deformation was analyzed in terms of the two mechanisms, and the kinetics of each mechanism were studied separately. Bucknall and Drinkwater showed that both crazing and shear processes contribute significantly to the creep of Cycolac T—an ABS emulsion polymer—at room temperature and at relatively low stresses and strain rates. 

Yano et al. [53] studied acoustic properties of acetylated Sitka spruce by specific dynamic Young s modulus and by logarithmic decrement. For oven-dried specimens, both the modulus and the decrement have been found to increase. Meanwhile, mechanical properties are generally unchanged and adhesive strength is reduced by acetylation [2]. Furthermore, creep deformation of wood under humidity change is remarkably reduced by acetylation [54]. 

Much important information about deformation mechanisms has come from creep experiments, in which the stress is kept constant and the strain is measured as a function of time, producing a creep curve (Figure 9.5). In general, the creep curve exhibits three stages of deformation (i) transient creep, in which the strain-rate changes with time (this stage may be... 

In specimens deformed to several percent strain (or more) at low to intermediate temperatures and stresses, where neither work-hardening nor recovery processes predominate, dislocations tend to tangle into localized walls (Kirby and McCormick 1979 McCormick 1977 McLaren et al. 1970 Morrison-Smith et al. 1976). These walls behave as optical phase objects and give rise to the deformation lamellae that are commonly observed in deformed crystals by optical microscopy (see Section 1.3 and McLaren et al. 1970). Similar walls of tangled dislocations develop in metals in the power-law-breakdown creep regime where both recovery-controlled and glide-controlled deformation mechanisms are operative (see, e.g., Drury and Humphreys 1986). 

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