Stress redistribution mechanism

Mai (1985) has also given a review of the fracture mechanisms in cementitious fiber composites. The total fracture toughness, / i, is given by the sum of the work dissipation due to fiber pull-out, fiber and matrix fraetures, fiber-matrix interfacial debonding and stress redistribution, i.e.. 

The initial intent of this review is to address the mechanisms of stress redistribution upon monotonic and cyclic loading, as well as the mechanics needed to characterize the notch sensitivity.5 13 This assessment is conducted primarily for composites with 2-D reinforcements. The basic phenomena that give rise to inelastic strains are matrix cracks and fiber failures subject to interfaces that debond and slide (Fig. 1.1).14-16 These phenomena identify the essential constituent properties, which have the typical values indicated in Table 1.1. 

Fig. 1.2 Three prevalent damage mechanisms occurring around notches in CMCs. Each mechanism allows stress redistribution by a combination of matrix cracking and fiber pull-out.

The analytical approach developed by Schadler and Noyan, allows calculation of the stress redistribution in cracked triple layer systems. This approach assumes mechanical equilibrium of the cracked coating and the interlayer through perfectly adhering interfaces which transfer the applied stress to the substrate. It is thus possible to deduce expressions for stress distribution normal to the cracked film and shear stress distribution at the interlayer ... 

It is not surprising that a model which imposes only tectonic loading and coseismic stress redistribution, produces no aftershocks, because it is likely that aftershocks are due to additional mechanisms triggered by the mainshock. A discussion on candidates for such mechanisms is given in [58]. A common feature is the presence of postseismic stress which generates aftershock activity. In [22], for instance, postseismic stress has been attributed to a viscoelastic relaxation process following the main-... 

How large a cavity can exist at depth in an intact sandstone or in a dilated and yielded sand This question is a coupled mechanics-flow issue involving seepage forces acting at the local scale (cm), pore pressure variations acting at a larger scale (m), and stress redistribution at all scales. 

Is sand bursting in conventional wells and shear events in conventional wells chaotic Is it linked to stress redistributions and how do we predict longterm behavior Microseismic monitoring could help quantify these mechanical coupling issues. 

The fundamental difference between mechanical stresses and tliermal stresses lies in the nature of the loading. Thermal stresses as previously stated are a result of restraint or temperature distribution. The fibers at high temperature are compressed and those at lower temperatures are stretched. The stress pattern must only satisfy the requirements for equilibrium of the internal forces. The result being that yielding will relax the thermal stress. If a part is loaded mechanically beyond its yield strength, the part will continue to yield until it breaks, unless the deflection is limited by strain hardening or stress redistribution. The external load remains constant, thus the internal stresses cannot relax. 

All polymer materials used in reinforced plastics display some viscoelastic or time-dependent properties. The origins of creep in composites stem from the behaviour of polymers under load together with local stress redistributions between fibre and matrix as a function of time. There is little creep at normal temperatures in the reinforcing fibres. The origin of the creep mechanisms is related to the nature and levels of internal bonding forces between the chains of the polymer, which are influenced by temperature and moisture. 

The mechanism proposed by F. Bueche comprises the following fundamental steps 1) the splitting of the chain fragments located between two particles of filler 2) recovering to a statistical distribution of tensions, after 3) tensions relaxation and 4) stress redistribution on the network chains. 

The observation that fibre reinforcement of slabs avoids their brittle collapse once the first crack appears, considerable stress redistribution begins due to the ability of the fibres to tie the cracks together. Thus the slabs can carry further loads until a collapse mechanism occurs when the fibres are no longer able to bridge the cracks effectively. 

In this chapter we shall first outline the basic concepts of the various mechanisms for energy redistribution, followed by a very brief overview of collisional intennoleciilar energy transfer in chemical reaction systems. The main part of this chapter deals with true intramolecular energy transfer in polyatomic molecules, which is a topic of particular current importance. Stress is placed on basic ideas and concepts. It is not the aim of this chapter to review in detail the vast literature on this topic we refer to some of the key reviews and books [U, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, and 32] and the literature cited therein. These cover a variety of aspects of tire topic and fiirther, more detailed references will be given tliroiighoiit this review. We should mention here the energy transfer processes, which are of fiindamental importance but are beyond the scope of this review, such as electronic energy transfer by mechanisms of the Forster type [33, 34] and related processes. 

In aerobic composting, an air blower distributes air under the pile and maintains most of the pile in aerobic conditions for faster degradation. The piles are turned daily to redistribute material and moisture and to maintain porosity of the pile. The mechanical stress imposed by turning the compost piles facilitates... 

This chapter is concerned with the influence of mechanical stress upon the chemical processes in solids. The most important properties to consider are elasticity and plasticity. We wish, for example, to understand how reaction kinetics and transport in crystalline systems respond to homogeneous or inhomogeneous elastic and plastic deformations [A.P. Chupakhin, et al. (1987)]. An example of such a process influenced by stress is the photoisomerization of a [Co(NH3)5N02]C12 crystal set under a (uniaxial) chemical load [E.V. Boldyreva, A. A. Sidelnikov (1987)]. The kinetics of the isomerization of the N02 group is noticeably different when the crystal is not stressed. An example of the influence of an inhomogeneous stress field on transport is the redistribution of solute atoms or point defects around dislocations created by plastic deformation. 

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