Intensive variable stress

During cell/stack operation, water content in the membrane is affected by the local intensive variables, such as local temperature, water vapor concentration in the gas phase, gas temperature and velocity in the channel, and the properties of the electrode and gas diffusion media. The power fluctuation can result in temperature variation inside the cell/stack, which will subsequently change the local membrane water content. As the water content in the membrane tends to be non-uniform and unsteady, this results in operation stresses. When the membrane uptakes water from a dry state, it tends to expand as there is no space for it to extend in plane and it can wrinkle up as schematically shown in Fig. 4 when the membrane dries out, the wrinkled part may not flatten out, and this ratcheting effect can cause the pile up of wrinkles at regions where membrane can find space to fold. The operation stress is typically cyclic in nature due to startup-shutdown cycles, freeze-thaw cycles, and power output cycles. 

Intensive variables Variables non-proportional to the amount of mass of the system, such as stress and temperature. 

For example, a change of internal energy due to mechanical work is denoted by an inner-product of stress, an intensive variable, and strain increment imder small strain conditions, which is an extensive variable A change of internal energy due to heat supply is denoted by a product of temperature, an intensive variable, and an entropy increment, which is an extensive variable. 

The principle of Le ChStelier asserts that in general a system will react to lessen the effect of a stress on an intensive variable, if it can do so. This effect was illustrated by considering the shift in equilibrium by changing the temperature or the pressure on a system and by adding a reactant or product to the system. The application of the thermodynamics of chemical equilibrium to biological processes was illustrated through a discussion of the coupling of chemical reactions and active transport. 

Composite materials have many distinctive characteristics reiative to isotropic materials that render application of linear elastic fracture mechanics difficult. The anisotropy and heterogeneity, both from the standpoint of the fibers versus the matrix, and from the standpoint of multiple laminae of different orientations, are the principal problems. The extension to homogeneous anisotropic materials should be straightfor-wrard because none of the basic principles used in fracture mechanics is then changed. Thus, the approximation of composite materials by homogeneous anisotropic materials is often made. Then, stress-intensity factors for anisotropic materials are calculated by use of complex variable mapping techniques. 

Whole body Variety and number of animals Chronic studies possible Minimum restraint Large historical database Controllable environment Minimum stress Minimum labor Messy Multiple routes of exposure skin, eyes, oral Variability of dose Cannot pulse exposure easily Poor contact between animals and investigators Capital intensive Inefficient compound usage Difficult to monitor animals during exposure Cleaning effluent air Inert materials Losses of test material Even distribution in space Sampling Animal care Observation Noise, vibration, humidity Air temperature Safe exhaust Loading Reliability... 

The recovery principle also presupposes that the environment can absorb and tolerate a certain amount of stress. The stressor should be limited to an intensity or concentration less than that at which long-term adverse impacts on ecosystem structure and functioning occur. From a scientific point of view, periodically occurring declines in population densities can be considered a normal phenomenon in ecosystems. In the course of evolution, organisms have developed a large variety of strategies to survive and cope with temporally variable unfavorable conditions such as desiccation,... 

Stresses. The main mechanical properties to consider are maximum stress or stress intensity factor, <7m ix or Kmax, cyclic stress or stress-intensity range, Act or AK, stress ratio R, cyclic loading frequency, cyclic load waveform (constant-amplitude loading), load interactions in variable-amplitude loading, state of stress, residual stress, and crack size and shape, and their relation to component size geometry.31... 

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