Rupture mechanics, adhesion

Long-term durability of adhesively bonded joints may require resistance to a number of individual or combined degradation modes, including environmental attack, fatigue and time-dependent failures. Time-dependent failure mechanisms are often characterized nsing either a strength approach, involving creep and creep-rupture tests, or a fracture approach, in which debond rate is determined. In creep-rupture tests, adhesive joints are subjected to... 

A more subtle effect of temperature is also displayed in Fig. 17 where experiments [34] show a rapid increase of the adhesion energy with temperature, related to a change in the rupture mechanism from interfacial fracture by crack propagation to failure by fibrillation and debonding of the fibrils. The interpretation of the authors is that the temperature change is responsible for a modification of the adhesive-probe contact area, which, in turn, results into different types of rupture mechanisms in the two temperature ranges. 

Figure 29 Different modes of mechanical stresses on adhesives (or sealants) joints. Modes of rupture of adhesives and sealants bonds.

Fig. 21 Proposed mechanisms of lipoplex formation (a) vesicle titration (DNA initially in excess) -DNA coats the vesicle surfaces as the latter are added to the DNA solution - with increase of the vesicle concentration, clusters of DNA-coated vesicles form and consequently rupture (b) DNA titration (lipid initially in excess) - DNA encounters with bare membranes result in vesicle associations - vesicle-DNA-vesicle adhesion generates stresses, which lead to vesicle rupture, followed by continued aggregation and growth of the complex upon further addition of DNA. (reproduced with permission from [67] copyright (2000) Biophysical Society)...

Mechanism. Basically, fretting is a form of adhesive or abrasive wear, where the normal load causes adhesion between asperities and oscillatory movement causes ruptures, resulting in wear debris. Most commonly, fretting is combined with corrosion, in which case the wear mode is known as fretting corrosion. For example, in the case of steel particles, the freshly worn nascent surfaces oxidize (corrode) to FejO, and the... 

In effect, the physical chemistry of drying from solution is a problem in mechanics. In practice, instead of warping the substrate, a typical lacquer develops stresses (in excess of o as calculated by Equation 12, because some stress relief is brought about by bending) that may exceed a threshold value, whereupon the system ruptures. Inasmuch as the system for all practical purposes is a two-dimensional laminated structure, rupture will occur at or near the interface. Laminar failure does not necessarily signify poor adhesion, for the failure may be cohesive as indicated by ESCA and SEM detection of residual polymer on separated surfaces (66). In some cases, failure is, indeed, adhesive as revealed by the absence of radioactive-tagged molecules (67). 

The concept of adhesive interaction of contacting surfaces is already familiar to us from previous discussion of the adhesive mechanism of friction (Chapters 8 and 12). If the two bodies participating in the adhesive junction are in motion relative to each other, in particular tangential motion, the junction is ruptured shortly after it is established. Rupture of the junction at a location other than the original interface results in transfer of material from one body to the other. According to the broad definition of wear of Section 13.1, each body has been worn—one by loss of material, the other by gain—but there has been no net loss or gain in the system as a whole. 

The theory that we have developed does not demand that all fibrils and their bases be the same size, or that their mechanical properties be identical. Chain entanglement is a statistical property, and (as already noted) the number of chains per fibril is small enough that serious fluctuations in local properties are to be expected. Even if, in a particular case, the vast majority of fibrils detach cleanly and without leaving any polymer on the solid, a small fraction of the fibrils may rupture, in an experiment of "adhesive" separation. Likewise in a frictional experiment even when, at the majority of points of adhesional attachment, detachment occurs without appreciable fibril drawing, there may be fibrildrawing and rupture at a few sites and consequently, wear will occur. 

When sand molds or cores made with low ratio (<3.5) silicate binders get dry either by exposure to a dry atmosphere or by heating, they become harder. On the other hand, when sand molds or cores made with very high ratio silicate as binders get dry either by exposure to a dry atmosphere or by heating they tend to become weak and friable. This is because the overall strength of the mold or core is primarily dependent on the mechanical properties of the solid film formed by the silicate adhesive when it sets. The separation of adhesive bonds is rarely the breaking away of the solid-hquid interface but more generally a rupture either within the adhesive film or within the body of the material to which the adhesive was apphed. Cracks or other faults within the adhesive film are more likely to account for low bond strength than mpture at the interface. 

Mechanical properties of fatty and wax encapsulates have been less explored compared to others mentioned earlier. The rupture forces as well as adhesion forces increased with particles diameter and the structure of the particles could have been weakened by the surfactant in case of particles made from hydrogenated palm oil matrix. ... 

Bilayers on surfaces, including SAM of thiols with hydroxyl end groups, can be formed by vesicle fusion [175,176[. The process, as depicted schematicaUy in Fig. 18, can be followed conveniently by in situ AFM measmements [177, 178]. As shown by various authors, the vesicle surface coverage, the mechanism of adsorption and bilayer formation, and the vesicle dimensions are directly accessible. Using appropriate models, the adhesion potential and the critical rupture radius of the vesicles can be calculated [ 179]. 

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