Creep mechanism

Depending on the temperature and the stress, different microscopic processes are important in determining creep behaviour. These will be discussed in this section. We will see that different processes are important at different temperature and stress values a fact that can be visualised using so-called deformation mechanism maps. 

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In the steady-state creep regime of ceramics, almost aU creep mechanisms fit a strain rate dependence of the form (18) ... 

It was found, [7], that selecting an optimum cooling regime, one can significantly change the amount of final bow due to energy dissipation by the creep mechanism, as can be seen in Fig. 1. 

The notion that the creep mechanism is responsible for the time - load dependence was previously recognized, [11]. Comparing the experimental and numerical results, a mismatch between two predictions was found to be eradicable in spite of the sensitivity of the simulation to the initial imperfection. This fact led to a revision of the model of the material... 

No.2, 2001, p.79-100 MODELING THE GAS-LOSS CREEP MECHANISM IN EVA FOAM FROM RUNNING SHOES... 

PP bead foams of a range of densities were compressed using impact and creep loading in an Instron test machine. The stress-strain curves were analysed to determine the effective cell gas pressure as a function of time under load. Creep was controlled by the polymer linear viscoelastic response if the applied stress was low but, at stresses above the foam yield stress, the creep was more rapid until compressed cell gas took the majority of the load. Air was lost from the cells by diffusion through the cell faces, this creep mechanism being more rapid than in extruded foams, because of the small bead size and the open channels at the bead bonndaries. The foam permeability to air conld be related to the PP permeability and the foam density. 15 refs. 

The predominant creep mechanism thus depends on the ratio... 

At high temperatures the glassy phase may become less viscous and even liquid and as a consequence may account for the plastic deformation. However, viscous flow creep is not regarded as a viable creep mechanism for superplasticity due to its limited deformation, which corresponds to the redistribution of the glassy phase and therefore to the squeeze of these secondary phases from grain boundaries subjected to compression.8... 

In the initial stage, known as primary creep, the strain rate is relatively high, but slows with increasing strain. The strain rate eventually reaches a minimum and becomes near-constant. This is known as secondary or steady-state creep. This stage is the most understood. The characterized creep strain rate , typically refers to the rate in this secondary stage. The stress dependence of this rate depends on the creep mechanism. In tertiary creep, the strain-rate exponentially increases with strain [1-9]. 

Solid-solid Adhesion Solid suspension, adhesion, cohesion, corrosion, passivation, epitaxial growth, wear, friction, diffusion, thin films, delamination, creep, mechanical stability, durability, solid state devices, blend and alloy, charge transfer, nucleation and growth abrasion... 

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,... 

R. Morrell and K. H. G. Ashbee, High Temperature Creep of Lithium Zinc Silicate Glass-Ceramics, Part 1, General Behavior and Creep Mechanisms, /. Mater. Sci., 8, 1253-1270 (1973). 

Fine-grained materials, when subjected to high temperatures and low applied stresses, deform by mutual accommodation of grains assisted by grain boundary sliding and transport of matter (diffusion). Under conditions where lattice diffusion dominates, the diffusional creep rate is reasonably well characterized by the Nabarro-Herring creep process. (For a review of this and other classical creep mechanisms, see Refs. 5 and 6.) Here the strain rate is expressed as... 

Two types of basic creep mechanisms have been identified in models for dislocation creep. (1) In glide-controlled creep, the obstacles to dislocation motion are on the scale of the dislocation core the obstacles are overcome by... 

These observations appear to be in contradiction with a creep mechanism for craze fibrillation, and the currently accepted description refers to the drawing-in mechanism due to Kramer [31,32], Kramer argued that fibrillation takes place within a thin layer (about 50 nm) at the craze/bulk interface, in which the polymer deforms into highly stretched fibrils similar to the mechanism of drawing of polymer fibers, as illustrated in Fig. 2. Craze thick-... 

Values for B, n, and A// vary significantly in the literature, depending on the respective material and testing condition [1.62-1.64]. They are characteristic for the respective dominating creep mechanism. 

The class of creep mechanisms of interest here are those that are mediated by stress-biased diffusion. If we are to consider the vacancy flux in a given grain within a material that is subjected to an applied stress, it is argued that the vacancy formation energy differs in different parts of the grain, and hence that there should be a gradient in the vacancy concentration leading to an associated flux. This... 

Detailed microstructural investigation [12] revealed enhanced precipitation of Nd-rich phases at the SiC/matrix interfaces in the QE 22 -SiC composite after T6 heat treatment and during creep (Fig. 9). Such precipitation can detrimentally affect the creep behavior in a similar way as to fiber-reinforced composites. Further, Moll et al. [11] have proposed that poor creep resistance of the QE 22-SiC composite may be explained by taking into account interfacial sliding as an additional creep mechanism... 

There is hardly any published data on tensile creep on these systems. Compressive creep rates for the Mo-base materials exhibited minimum creep rates of 1 x 10 9/s at 1200°C / 100 MPa and 1 x 10"8/s at 1300°C / 100 MPa. For Nb-based alloys the minimum creep rates are 5 x 10"8/s at 1200°C /100 MPa and 10 7/s at 1300°C / 100 MPa. Much work is needed in determining tensile creep mechanisms and damage in both these systems. 

Compressive set, creep, mechanical damping, and hysteresis all occur because, under mechanical stress, gas permeates out of the closed-cell walls to reduce the stress. When the mechanical stress is released, the gas permeates back only slowly and often incompletely. 

G. Sauthoff. Creep Behaviour and Creep Mechanisms in Ordered Intermetallics. In C.T. Liu, R.W. Cahn, and G. Sautholf (eds.) Ordered Intermetallics - Physical Metallurgy and Mechanical Behaviour. Kluwer Acad. Publ. Dordrecht (1992) 525-539. 

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