Stress distribution: mode of failure

The mode of failure in a test may be adhesive, cohesive or mixed (see Stress distribution mode of failure). Sometimes authors talk as if a test only measured adhesion in the first of these cases. Such a distinction does not seem helpful as none directly measures fundamental adhesion and all give a number that reflects indirectly the interfacial forces. 

Depending on the locus of failure (see Stress distribution mode of failure), either the work of adhesion Wa or the work of cohesion Wc will be appropriate tA represents all the other energy-dissipation terms. 

Minford studied the effect of contaminating aluminium with a commercial lubricant. The results are summarized in Fig. 1. It was found that the lubricant had little effect on the adhesion level until its thickness exceeded about 10 p,m. The adhesion level then fell sharply and the mode of failure (see Stress distribution mode of failure) changed from cohesive within the adhesive to apparent interfacial failure. Thus the epoxide, which required a fairly long cure at a high temperature, was able to absorb quite large quantities of the lubricant. 

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 reflection technique is more limited in resolution and magnification than is transmission. Reflection optical microscopy of adhered surfaces is often revealing, but the very limited depth of field is a severe handicap. Nevertheless, examination of surfaces after testing or failure is a simple, but important, first step in determining locus of failure (see Stress distribution mode of failure). Surface reflectivity of specimens is often improved by deposition of a layer or metal. The reflection microscope is especially useful when operated in dark-field mode, where topographical differences are accentuated. 

Bikerman maintained that the mode of failure of an adhesive bond was always cohesive and that adhesive failure (interfacial failure, see Stress distribution mode of failure) could never occur. When adhesive failure appeared to occur, more careful examination would reveal a thin layer of adhesive remaining on the substrate. When failure occurred close to the interface, it was often in a weak boundary layer. 

Scanning electron microscopy is essentially a technique for the observation of surfaces and is thus especially applicable to the examination of adherends and failure surfaces, observations of the results of surface preparation of metals and polymers, investigations of failure modes (see Stress distribution mode of failure), and examinations of finishes on fibres are typical of work reported in the literature. 

Other relevant articles include Stress distribution bond thickness. Stress distribution Poisson s ratio. Stress distribution stress singularities. Stress distribution shear lag solution and Stress distribution mode of failure. 

The durability of the bonded joints was greatly influenced by the nature of the adhesive the best performers in all climates were epoxy-novolak and nitrile-phenolic formulations. A tropical, hot-wet climate was the most damaging to bonded stmctures and the combination of high humidity and applied stress was particularly deleterious. During exposure to natural environments, the failure mode of aluminium joints was found to change gradually from wholly cohesive, within the adhesive, to include increasing amounts of interfacial failure (see Stress distribution mode of failure). 

Stress distribution mode of failure D A DILLARD Locus of failure depends on energy and stress distribution... 

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