Stress-induced adhesion

The typical interpretation of stress-induced adhesion differs significantly from the one given above. It is generally believed that the main effect of an applied tension is to reduce undulations. In the absence of undulations, the corresponding repulsion vanishes and the membranes can attract each other by, for example, van der Waals forces. 

Exposure of the SECs to pathogens or cytokines produced by other cells during stress induces activation of the SECs and subsequent production of cytokines, eicosanoids, and/or adhesion molecules. For instance, after activation with EPS, a main component of the walls of gramnegative bacteria and a major inducer of inflammation and non-specific immune functions [20], SECs produce a number of pro- and anti-inflammatory cytokines. Pro-inflammatory cytokines shown to be produced were tumour necrosis factor alpha (TNFa) [26] interleukin-1 alpha/beta(IL-lo/p) [27] the major inducer of acute phase proteins interleukin-6 (IL-6) [28] and the neutrophil chemo-attractant interleukin-8 (IL-8) [29]. Anti-inflammatory cytokines shown to be produced were interleukin-10 (IL-10) [27] and hepatocyte growth factor (HGF) [30]. 

Kamioka H, Sugawara Y, Murshid SA, Ishihara Y, Honjo T, Takano-Yamamoto T. 2006. Fluid shear stress induces less calcium response in a single primary osteocyte than in a single osteoblast implication of different focal adhesion formation. J Bone Miner Res 21 1012—21. 

Nomura S, Tandon NN, Nakamura T, Cone J, Fukuhara S, Kambayashi J. High-shear-stress-induced activation of platelets and microparticles enhances expression of cell adhesion molecules in THP-1 and endothelial cells. Atherosclerosis 2001 158 277-287. 

Three theories were proposed to explain wall-slip (a) adhesive failure at the wall, (b) cohesive failure within the material as a result of disentanglement of chains in the bulk and chains absorbed on the wall, and (c) the creation of a lubricating surface layer at the wall either by a stress-induced transition, or by a lubricating additive. If the polymer contains low molecular weight components or slip-additives, their diffusion to the wall will create a thin lubricating layer at the wall, generating apparent slip. 

Majors, A.K. et al., Endoplasmic reticulum stress induces hyaluronan deposition and leukocyte adhesion, J. Biol. Chem., 278, 47223, 2003. 

The manner by which shear stress-induced cellular changes occur in endothelial cells involves cell membrane and cytoskeletal molecules that lead to a shape change. The cytoskeleton contains actin filaments, intermediate filaments, and microtubules, all of which are restructured upon exposure to external force. Under stress conditions, actin filaments coalesce to form stress fibers that anchor at the focal contacts, which are adhesion sites at the cell substrate interface. 

It has also been shown in several studies that the combination of stress, temperature, and moisture can accelerate the hydrolytic instability of certain epoxy adhesives. In an FTIR study of the effect of moisture on DGEBA epoxy cured with nadic methyl anhydride, spectra changes were observed in stressed specimens aged for 155 days at 80°C and 100 percent RH.47 This was attributed to the slow, stress-induced hydrolysis of ester groups. In another study,... 

Impact Properties. Chemical Nature of the Rubber. If the rubber is too compatible with the matrix, it will dissolve in the rigid material and disperse on a molecular scale. Little or no reinforcement will occur since the rubber particles become smaller than the radius of the tip of a stress-induced propagating crack. However if it is highly incompatible, good adhesion between rubber and matrix cannot be obtained. For example polybutadiene rubber adheres poorly to a styrene/acrylonitrile copolymer, but a nitrile rubber adheres well to the SAN copolymer. If grafting techniques are used however, compatibility is less of a problem since the rubber is chemically bonded to the matrix. 

Figure 7.37 Schematic time-position dia- (Adapted from Ecault et al. (2013)). (a) Intact grams showing ID-shock wave propagation adhesion, (b) Partial interface debonding in a Ti6AI4V substrate coated with a Ti02 triggered by dynamic tensile stress induced bond coat and a hydroxyapatite top coat. by a laser-generated shock wave.

Epoxy-amine liquid prepolymers are extensively appHed to metallic substrates and cured to obtain painted materials or adhesively bonded stractures. Overall performances of such systems depend on the interphase created between the organic layer and the substrates. When epoxy-amine Hquid mixtures are appHed to a more or hydrated metaUic oxide layer (such as Al, Ti, Sn, Zn, Fe, Cr, Cu, Ag, Ni, Mg, or E-glass), amine chemical sorption concomitant with metaUic surface dissolution appear, leading to the organometaUic complex or chelate formation [1, 2]. Furthermore, when the solubility product is exceeded, organometaUic complexes may crystaUize. These crystals induce changes of mechanical properties (effective Young s modulus, residual stresses, practical adhesion, durability, etc.). 

A capacitor potting compound needs aggressive adhesion to the metal can edges, providing a positive seal despite stresses induced during thermal changes. 

Elevated temperatures are also commonly used to increase cleaning performance. An increase in temperature will lead to a corresponding increase in the solubilizing properties of the cleaning medium. There is of course a point at which the thermal stress induced by high temperature cleaning can damage the stencil adhesion. 

Closed form solutions. Structural adhesive joints are generally designed to be loaded in shear so that treatments of joint analyses are confined essentially to the transfer of load by shear, with some consideration of the transverse normal stresses induced by eccentricities in the load path. In the simplest case the adhesive and the adherends are assumed to behave elastically. The most refined analyses attempt to model the situation when the adhesive yields so that the adhesive and, eventually, the adherends behave plastically as the imposed load is raised. Closed-form analyses are difficult to apply to other than simple geometrical configurations, while a major difficulty with the elasto-plastic model is how to characterise the adhesive. 

Water uptake by polymers is accommodated largely by swelling. For uptakes of only a few mass per cent, volumetric swelling would be of a similar or lower order(98, 99), and barely measurable. Moire fringe interferometry has been used to quantify the swelling stresses developed in a layer of adhesive upon exposure to water(lOO), and Comyn(90) describes some other work related to calculations of the stresses induced in bonded joints by water sorption. 

A typical flaw, for purposes of the present discussion, would be an entrapped bubble of air or other contaminant that is itself relatively weak cohesively (Fig. 19.8). In the presence of such flaws, little or no energy can be transmitted through the flaw so that the stress becomes concentrated at the junctions of the flaws with the interface or in one of the bulk materials. The local stress, therefore, is greater than the average value and is more likely to exceed the adhesive or cohesive strength of the system near the flaw. As a result of that situation, the applied stress induces the formation of a crack that continues to propagate along the line of least resistance (with continued application of stress) until joint failure results. 

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