Mechanical loading response

In the last section we considered tire mechanical behaviour of polymers in tire linear regime where tire response is proportional to tire applied stress or strain. This section deals witli tire nonlinear behaviour of polymers under large defonnation. Microscopically, tire transition into tire nonlinear regime is associated with a change of tire polymer stmcture under mechanical loading. 

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... 

The above problems of fabrication and performance present a challenging task of identification of the governing material mechanisms. Use of nonlinear finite element analysis enables close simulation of actual thermal and mechanical loading conditions when combined with measurable geometrical and material parameters. As we continue to investigate real phenomena, we need to incorporate non-linearities in behavior into carefully refined models in order to achieve useful descriptions of structural responses. 

An issue sometimes raised in relation to bone maintenance in the elderly concerns the endogenous acid generated by the metabolism of dietary protein, which when regularly excreted in the urine can cause calciuresis via various mechanisms (Dawson-Hughes, 2003a,b). Opinions have varied in terms of the extent to which protein-induced increases in Ca excretion may impact bone mass via bone dissolution and/or resorption (Barzel and Massey, 1998 Heaney, 1998, 2002). Protein intake in conjunction with Ca supplementation has now been positively associated with bone accrual, and only more recently has the notion of Ca economy been considered as a compensatory mechanism in response to an acute increase in the dietary protein load (Bonjour, 2005 Kerstetter et ah, 2005). 

Mi. L.Y., Basu, M., Fritton, S.P., Cowin, S.C. (2004a) Analysis of avian bone response to mechanical loading Part One Distribution of bone fluid shear stress induced by bending and axial loading, in preparation. 

Throughout this text we refer to the relationship between external mechanical loading and cellular response that affects the physiology of ECM. Inasmuch as mechanical terms are used repeatedly, some of these terms are listed in Table 1.4. These terms refer to the ability of tissue to change its shape and to deform. Strain refers to the changes in tissue dimensions with respect to the initial dimensions, and extensibility refers to the strain a tissue can undergo before failure. The force per unit cross-sectional area of a tissue is referred to as stress the stress at which a tissue tears is... 

It is clear from these studies that external mechanical loading not only affects the cellular response through changes in genetic expression by cells, but it also sets into motion a cascade of events that balances cellular biochemical and biomechanical responses with changes in the environmental... 

The most well characterized of the p38 kinases, the final MAPK phosphorelay pathway, is p38a, which is expressed by most cells and regulates the expression of many cytokines. Interleukin 1, IL-1, a product of inflammatory cells, has been implicated in modulating the response to mechanical loading in a number of tissues. IL-1 is a product of inflammatory cells thought to be involved in cartilage destruction in osteoarthritis and in bone resorption associated with total joint implant failure. 

Brown RA, Prajapati R, McGrowther DA, Yannas IV, Eastwood M. Tensional homeostasis in dermal fibroblasts Mechanical responses to mechanical loading in three-dimensional substrates. J Cell Physiol. 1998 175 323-332. 

When mass loading is not the dominant sensing mechanism, sensor response may not be linear with concentration in the film. This departure from linearity has been observed with polymer films [61,64,86]. An investigation into the possible effect of this nonlinearity on the evaluation of D values fiom SAW frequency transients indicated that errors in D values (factor of two error) could be obtained if the nonlinearity of the response is large [86). However, using small steps in partial pressure, this nonlinearity in the response can be minimized, allowing the effective evaluation of difiiision coefficients based on AW frequency transients. 

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