Failure, adhesive stress cracking

The work discussed here has related not only to structure relationships but also to means of protection of the macromolec-ular material and the protective functions of these materials. There are many modes of failure, by chemical reaction, failure by fracture, environmental stress cracking and creep. Further there are complicating interactions arising from chemical reaction during relaxation of polymer networks, and in multiphase polymer systems and cos osites, failure at interfaces by adhesive failxire or stress-stress dilation. 

Besides delamination, actual cracking of silicon die is a failure mode that can occur from excessive adhesive stresses, voids, and moisture absorption. Residual stresses in adhesive-attached single-chip devices become critical as the size of the device increases and dissimilar die and leadframe materials are used. Several types of fractures that can occur within the die or within the adhesive are shown in Fig. 6.8. A hairline crack in an 1C chip that resulted from adhesive stress is shown in Fig. 6.9. 

For the monomer polymerization at room temperature, the adhesive was augmented with a redox system of 3% BP and 0.75% DMA. To study, explain, and predict the development of the elastic failure of the polymer in the adhesive interlayer, an improved method of investigating adhesive layer crack resistance with modeling of the formation and growth of a crack at the adhesive-honded joint loading was used [119]. Five adhesive-bonded joints with the adhesive mixture compositions shown in Table 3.1 were subjected to static tests for crack resistance at room temperature. The characteristics of the static crack resistance of the adhesive-bonded joint Kic is the coefficient of the stresses intensity Gic is the intensity of the elastic energy release ic is the opening in the crack tip) were determined at the moment of onset of the crack in double-cantilever specimens DCB (Fig. 3.5). The specimen cantilevers were made of PMMA of TOCH type. 

Scarf joint. The importance of testing adhesive fracture under mixed-mode stress conditions has been noted. In the scarf joint (Fig. 4.14(e)), the applied load is resolved in the bondline to Mode I and Mode II components and their ratio changes with the scarf angle, < >. Bascom et al. 26, 70) investigated the effects of bondline thickness and test temperature, and G,i. n)c for < ) = 45° was calculated from the failure load and crack length using a finite-element analysis. A complex behaviour pattern emerged, as discussed... 

Cracks or other such flaws present in the adherend surface may also serve as foci for crack propagation under stress leading to apparent adhesive failure. In joints or welds that undergo cyclic mechanical (bending) or thermal (hot-cold-hot cychng) stress may also develop stress cracks at or near a joint or... 

Strength, unlike elastic modulus, is not even theoretically a readily determinable quantity. Overall elastic-plastic deformation in a structural adhesive might be describable in terms of intermolecular forces and models of viscous flow, but not at the discontinuous moment of fracture. In fact overall behaviour loses sight of the fact that it is normally isolated phenomena that control the magnitude of joint strength and the locus of failure (see Stress distribution mode of failure). The term isolated phenomena refers to voids, cracks, second phase material, and so on, which can act as stress concentrators. Clearly, it would be unwise to suggest that an adhesive bond tester should merely locate and size voids and cracks, as whether or not such a defect is active depends upon where it lies in the working stress pattern of the structure. 

The adhesives will bond almost all materials (though a primer may be needed with some), except polyolefin plastics (eg Polythene) and other low surface-energy types such as fluoropolymers (eg Teflon) and silicone-based rubbers. Alkaline glass may cause premature bond failure and all glasses should be silane primed if at all possible, as this considerably improves the joint s humidity resistance. May stress crack stressed mouldings or susceptible plastics - polycarbonate, for example. 

The first stage was considered to be a slow environmental stress crack (ESC), produced by slight chemical attack in the weld. ESC may be defined as a reduction in the tolerance of mechanical stress caused by special chemical environments, or, alternatively, as a reduction in the time to fracture under a fixed strain. Other examples have been the failure of blow-moulded polyethylene bottles when filled with detergent, and the weakening of crash helmets made of polycarbonate when affected by petrol or adhesive badges. 

Tape wrap (two layer) Simple application Poor shear stress resistance Many documented failures related to corrosion and stress crack corrosion Shielding of cathodic protection Adhesives subject to biodegradation... 

In a typical GMOD experiment, as in real-world exposure, when coatings on black and clear substrates are exposed to the same conditions, they experience different stresses when exposed to the same radiation dose at the same ambient temperature, a black substrate will reach a higher temperature than a clear substrate. The entitlement experiment eliminated this tanperature difference between the clear and black samples since the ESPEC heated the samples by convection. In the entitlement experiment, all of the clear and black samples should have experienced the same temperature and humidity stresses. Accordingly, it was expected that there would be no difference in adhesion or cracking times to failure in the entitlement experiment between the black and the clear samples. 

Adhesives can also stress crack or craze certain plastics and this can also indnce premature failure of the substrate. This generally happens whilst the adhesive is uncured and the plastic part is pre-stressed due to abrupt changes of section. The liquid adhesive softens and weakens the plastic leading to the formation of cracks and the liqnid adhesive then penetrates these cracks causing fnrther damage. With fast-curing adhesives, this is less likely to occur as once the adhesive has cured it is essentially a thermoset plastic (or thermoplastic) itself and is therefore inert. Amorphons thermoplastics are more prone to stress cracking than others and so it is important therefore to ensure that the adhesive is compatible with the substrate. 

Fracture mechanics (qv) affect adhesion. Fractures can result from imperfections in a coating film which act to concentrate stresses. In some cases, stress concentration results in the propagation of a crack through the film, leading to cohesive failure with less total stress appHcation. Propagating cracks can proceed to the coating/substrate interface, then the coating may peel off the interface, which may require much less force than a normal force pull would require. 

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