Flow failure adhesion

Thicker bonds fail more easily in shear whether adhesive or cohesive. For cohesive failure this is undoubtedly due to the narrower gap through which the adhesive is flowing. For adhesive failure, it is presumably due to an increase in peel forces at the top of edge of the bond. 

A continuous intact film of water-resistant paint forms an effective electrical resistance to the flow of a corrosion current (a resistance of over lO flcm through the film is easily achieved). Underfilm corrosion can then only occur if a channel of electrolyte connecting anode and cathode can be established by local adhesion failure between the coating and the metal substrate. 

Materials science associated with fracture mechanics has mainly been confined to composite materials such as concrete, ceramics and metals. Much of the emphasis of the research has been on preventing fatigue and failure rather than designing for it to occur. The way a structure deforms and breaks under stress is crucial for properties such as flow and fracture behaviour, sensory perception of structure, water release and the mobility and release of active compounds. In the case of foods, the ability to break down and interact with the mouth surfaces provides texture and taste attributes. The crack propagation in a complex supramolecular structure is highly dependent on the continuous matrix, interfacial properties and defects and the heterogeneity of the structure. Previous structure-fracture work has dealt with cellular plant foods, and it has been demonstrated that the fracture path differs between fresh and boiled carrots due to cellular adhesion and cell wall strength as well as cell wall porosity and fluid transport (Thiel and Donald 1998 Stoke and Donald 2000 Lillford 2000). 

T.I.Fowle ° has pointed out the potential for molybdenum disulphide to reduce wire-wool failures of plain bearings. This type of failure can occur when a hardened adhesive wear particle becomes embedded in a relatively soft bush, causing severe abrasive wear of the counterface. The formation of the initial wear particle can arise from a transient failure of the hydrodynamic lubricant film, caused, for example by temporary interruption of a drip feed or coolant flow. Addition of a small amount of molybdenum disulphide, say 3% or more, to the lubricant can prevent the short-term occurrence of adhesive wear, and thus the formation of the initiating particle. 

Yet, for systems A and C, the measured fracture energies remain low compared with the critical fracture energy of the bulk aluminum 10 J Moreover, we do not observe islands of passivation material on the A1 fracture surface and, inversely, we do not observe A1 on debonded surfaces of the passivation films. This suggests that the loss of interfacial adhesion is close to a brittle fracture process despite the influence of plasticity of the A1 substrate and crack blunting at the interface. This sort of brittle mode of interfacial failure, including plastic flow in a ductile material (the substrate), has been observed or discussed for a sapphire/Au interface. ... 

Lastly, both static (incipient powder failure) and dynamic (continued-flow) yield loci may be measured, giving both static and dynamic values of wall and powder friction angles as well as wall adhesion. 

Fluoropolymers have been used as processing aids because small quantities can reduce signih-cantly the overall viscosity and thus facilitate extrusion. Feng et al. [1996] examined the mechanism of viscosity reduction in the capillary flow of HDPE/fluoroelastomer blends. X-ray photoelectron spectroscopy, used to characterize the composition of the extmdates surface, indicated only very small traces of the fluoroelastomer on the extrudate, pointing to the fact that the viscosity reduction is due to adhesive failure between the fluoropolymer layer and HOPE. 

Finally, once a failure site or failure mode had been found, a mechanism based on knowledge of electrical, chemical, and physical reactions can be postulated, simulated, and verified and corrective action taken. Figure 6.1 is a flow diagram of some tests used in a PoF study for electronic modules containing adhesive-attached components. 

The function of a structural adhesive joint is to transmit an external load to the structural member. If the joint fails to function as it is intended, it will undergo damage or failure. The damage could be actual fracture of the structure, excessive elastic deformation, or excessive inelastic flow. The criteria for what constitutes structural failure depend on the performance requirements of the joint. The fundamental problem in the mechanics of adhesives and joints is to obtain some relationship between the loads applied to the joint and a parameter that will adequately describe the criteria for strucmral failure. The most common criterion for such failure of lap-type joints is actual fracmre of the joint. For a given combination of adherend and adhesive, the stress analyst must decide what the mode or theory of failure would be if the applied loads become large enough to cause failure. The decision as to which theory would realistically determine the mode of failure is usually based on past experience, or upon some form of experimental evidence. ... 

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. 

There are two further constraints to be considered. These relate to the hardness or viscosity of the strip and to the stack height of the strip in its container Uncured rubber is thermoplastic and will flow under the pressure of its own weight. This flow behaviour can lead to strip adhesion problems on the lower levels of the stack. The effect is time and temperature related, and must be controlled through the size of container and accurate scheduling to avoid prolonged storage of the prepared rubber. Failure to control properly the stock presentation will provide the press operator with extra work separating strips, and cause component rejects. 

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