Stress wrinkle

Stress wrinkles n. Distortions in the face of a laminate caused by uneven web tensions, slowness of adhesive setting, selective absorption of the adhesive, or by reaction of the adherents with materials in the adhesive. 

An indirect estimate of surface tension may be obtained from the change in lattice parameters of small crystals such as magnesium oxide and sodium chloride owing to surface tensional compression [121] however, these may represent nonequilibrium surface stress rather than surface tension [68]. Surface stresses may produce wrinkling in harder materials [122]. 

In addition to high permselectivity, the membrane must have low-elec trical resistance. That means it is conductive to counterions and does not unduly restrict their passage. Physical and chemical stabihty are also required. Membranes must be mechanically strong and robust, they must not swell or shrink appreciably as ionic strength changes, and they must not wrinkle or delorm under thermal stress. In the course of normal use, membranes may be expec ted to encounter the gamut of pH, so they should be stable from 0 < pH < 14 and in the presence of oxidants. 

A Kelvin foam model with planar cell faces was used (a. 17) to predict the thermal expansion coefficient of LDPE foams as a function of density. The expansion of the heated gas is resisted by biaxial elastic stresses in the cell faces. However SEM shows that the cell faces are slightly wrinkled or buckled as a result of processing. This decreases the bulk modulus of the... 

A big rubber brain sits atop my computer monitor at work. I squeeze it from time to time to reduce stress. Sometimes I imagine myself as a brain-snatcher—the Chief Curator of a futuristic collection of famous brains. The brains are stored on shelves in a cavern, beneath the basement of my home. I walk down lamp-lit corridors filled with gray, wrinkled things stored in formalin-filled jars to prevent decay. Stalagmites support some of the shelves. The coolness of the cavern contributes to the brains remarkable state of preservation. 

Elastic recovery or resilience is the recoveiy of length upon release of stress after e xtension or compression. A fiber, fabric, or carpet must possess this property in order to spring back to its original shape after being crushed or wrinkled, Polyolefin fibers have poorer resilience than nylon this is thought to be partially related to the creep properties of the polyolefins. 

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. 

The wrinkled or corrugated texture of the skin is illustrated in Fig. 2.4. The skin elongates under lateral stress and relaxes when stress is removed. A liquid bandage applied to the skin must be able to adhere to the skin and possess a stress/strain property similar to skin in order to remain attached to the skin. 

Fig. 1 SEM image of sub-micron periodicity wrinkles created by plasma oxidation of strained polydimethylsiloxane and subsequent stress release...

Fig. 3 A typical situation in which wrinkles occur in the presence of a macroscopic stress is schematically depicted A thin sheet is exposed to a uniaxial macroscopic deformation (1). As a consequence, the sheet is compressed in the direction perpendicular to the elongation axis and the reacts by a buckling instability. Wrinkles are formed, which however relax as the macroscopic strain is released (2), unless plastic deformations occur in the macroscopically stressed state...

Fig. 4 In order to obtain wrinkles that are stable in the absence of macroscopic stresses, a macroscopic substrate of large thickness has to be modified in stressed state as explained below. While the substrate does not react to compressive strains by wrinkling (1), the thin membrane that was created in the stressed state wrinkles when the substrate is relaxed (2). While the system is still under tension microscopically, no macroscopic stress is necessary to maintain the wrinkle structure and long-term stable wrinkles can be created...

We have recently introduced embossing as a possibility to generate circular wrinkled patters [38], Here, the substrate is uniform, but in the embossing process, stresses vary locally. Thus, complex patterns like those depicted in Fig. 5 are feasible. 

It may be concluded that shape retention, wrinkle recovery and pleat and crease retention depend on well-known viscoelastic properties the existence of a transition range, stress relaxation, creep and permanent deformation, and a possible resilience by a change in external conditions. 

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