Stress, moisture gradients

Failure in an adhesive joint can occur in one of two ways (1) adhesive failures that occur at the interfaces between the adhesive and adherends, and (2) cohesive failures, which occur either in the adhesive or in the adherends. The determination of the strength, failure, and reliability of an adhesive joint requires both an understanding of the mechanisms of adhesion and a knowledge of deformation and stresses in the joint. The mechanisms of adhesion are closely related to chemical and physical properties of the adhesive polymers. The deformation and stress states can be determined once the geometry, loading, boundary conditions, and mechanical properties of the constituent materials of the joint are known. The mechanical properties of the adhesive and adherend materials enter the stress analysis via constitutive models, which relate strains, temperature and moisture gradients, and density to stresses and fluxes in the joint. The chemical, physical, and mechanics aspects of the constituent materials enable the formulation of appropriate constitutive models for adhesive joints. The determination of stresses allows the prediction of the strength, failure, and reliability, in a macromechanics sense, of adhesive joints. 

It may be expected that concrete adjacent to the outer surface of the vessel will lose moisture at a greater rate than in the general body of the vessel. This moisture gradient through the thickness of the vessel will cause differential drying shrinkage and the tensile stresses, hence, are introduced in the outer surface of the vessel. These stresses are effectively controlled by means of bonded reinforcement. 

In view of (6.27) and (6.30), relations (6.31) and (6.32) are extremely complicated even at T = Tq and Cy = cTy the complexity is due to the presence of m in D and Vf (through and C(r) in the compliances listed in (6.27). However, in spite of the cumbersome details and the paucity of information, several features of the form of (6.32) emerge. Accordingly, the diffusion equation contains (1) nonlinear terms in the moisture gradient dm/dxc, (2) follows a time-retardation process akin to mechanical viscoelastic response (3) varies nonlinearly with external stresses ai] (4) exhibits an aging behavior characteristic of glassy polymers. 

The water vapour absorbed in the polymeric components of a module can act as reaction partner in chemical (e.g. hydrolysis) or physical (delamination by thermomechanical stresses) degradation processes. A principal difference to energy transfer by heat conductivity (temperature) or radiation transfer (UV) is the need for mass transport of the water vapour molecules that is based on permeation, especially diffusion, processes. There are two possibilities for acceleration—increasing the moisture gradient and increasing the temperature. The second way uses the temperature dependence of the diffusion coefficient (mostly according to the Arrhenius law). 

In sulphur concretes, the mechanism of deterioration caused by frost action has been attributed to entirely different causes to those above. The material has low permeability to moisture and as water is not used in mixing, it was not considered that water played a major role in deterioration. Sulphur has a very high coefficient of thermal expansion (a - 55 x 10 6/°C) and low thermal conductivity (0.27 W/m K). Hence the poor durability performance in cyclical freezing and thawing has been attributed to the development of high stresses due to thermal gradients (5,... 

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