Adherend properties

Wernersson and Gustafsson [65] developed a nonlinear fraeture meehanies relationship based on pure shear for predicting the performance of lap joints of varying geometry and adherend properties based on the adhesive brittleness ratio ... 

JOINEXCELL (from AJI - The American Joining Institute). An adhesive selection expert system for use with composite materials. The system consists of a job-planner subsystem which creates a joint schedule, complete joint description, adhesive selection, surface preparation, etc. Also a recorder for Quality Assurance criteria. JOINADSELECT is part of the expert system and identifies appropriate adhesives from 15 major classes of engineering adhesives, given information about adherends and adhesives. Databases for composite adherend properties are included. 

The effects of adherend yielding were investigated by modelling the adherends with elasto-plastic properties. The adhesive maximum principal stress distributions are shown in Fig. 46 for a case where the adherend properties correspond to a relatively low strength alloy (a 0 -2% proof stress of 110 MPa) and the adhesive is linearly elastic. At a very low load of 0 01 kN the distribution is identical to that in Fig. 45, since the adherends are still elastic. Under the action of tension and bending, the adherends begin to yield at an applied load of approximately 1 5 kN. At 3 kN, the adherend plastic deformation has had two effects on the adhesive stresses. Firstly, it has led to a reduction in the peak stress concentration at the end of the overlap, at point A, over and above that for the elastic case, as a result of the enhancement of... 

Adherend properties also have a huge impact on the joint strength. The most important are the adherend modulus and its strength. The higher the adherend modulus, the lower will he its deformation at the ends of the overlap, where the load transfer takes place, and the lower will he the effect of the differential straining in the adhesive (Volkersen 1938). The adherend strength is also fundamental and can explain many joint failures. In the case of metallic adherends, the... 

Simple design rules for single-lap joints were proposed as a function of the main variables that influence joint strength adhesive and adherend properties, adhesive thickness, overlap, and residual stresses. The main rules are ... 

It is only in the context of the systematic variation of the properties of the adhesive and/or the adherend surface in a set of otherwise identical specimens subjected to a given mechanical testing procedure that it is reasonable to think of predicting relative interfacial strength. 

Fig. 2. Schematic of energy dissipation in a commonly used peel test. The energy dissipation can occur in the adhesive and/or the adherends. The extent of energy dissipation depends on the elasto-plastic properties of the adhesive and the adherends under the test conditions as well as the local stresses and strains near the crack tip.

Quantifying the effect of surface roughness or morphology is difficult, however. Surface preparations that provide different degrees of surface roughness also usually produce surfaces that have different oxide thicknesses and mechanical properties, different compositions, or different contaminant levels. The problem of separation of these variables was circumvented in a recent study [52] by using a modified microtome as a micro milling machine to produce repeatable, well-characterized micron-sized patterns on clad 2024-T3 aluminum adherends. Fig. 2 shows the sawtooth profile created by this process. 

There is a need for an accurate assessment of the mechanism of cathodic debonding of thick adherends. This would contribute to better control over the selection of formulations resistant to attack. One of the objectives of this work was to develop an accelerated screening test. The screening test was used to evaluate the relative performance profiles of a number of commercial adhesives which were recommended for marine applications. It is hoped that a screening test such as this one could be used to isolate measurable primer properties which can be related to the long term prospectus of the primer, and the variability of this prospectus under permutation of substrate type or additives. 

Metal ion modified polyimide films have been prepared to obtain materials having mechanical, electrical, optical, adhesive, and surface chemical properties different from nonmodified polyimide films. For example, the tensile modulus of metal ion modified polyimide films was increased (both at room temperature and 200 0 whereas elongation was reduced compared with the nonmodif ied polyimide (i). Although certain polyimides are )cnown to be excellent adhesives 2) lap shear strength (between titanium adherends) at elevated temperature (275 0 was increased by incorporation of tris(acetylacetonato)aluminum(III) (2). Highly conductive, reflective polyimide films containing a palladium metal surface were prepared and characterized ( ). The thermal stability of these films was reduced about 200 C, but they were useful as novel metal-filled electrodes ( ). 

Thanks to their multiphase constitution, block copolymers have the originality to add advantageously the properties of their constitutive sequences. These very attractive materials can display novel properties for new technological applications. In this respect, thermoplastic elastomers are demonstrated examples (l, 2, 3) they are currently used without any modification as elastic bands, stair treads, solings in the footwear industry, impact resistance or flexibility improvers for polystyrene, polypropylene and polyethylene whereas significant developments as adhesives and adherends are to be noted (5.). 

In addition to preferential adsorption of epoxy components at fiber or adherend surfaces, chemically different species can be added to the interphase to improve or alter an interfacial property. Among the species, that can be added to the composite, coupling agents have a great effect on the interphase structure and properties. 

The interphase in epoxy composites is an important material component and can have significant effects on over all composite performance. It is not a fiber (adherend) or matrix property but it is a product of the interaction of fiber and matrix. Its existence has been the subject of speculation primarily because commercial materials are optimized systems which have minimized the deleterious effects of an interphase and analytical models of composite behavior based on empiricle material properties artificially ignore it. 

The interphase provided by the adhesion promoter may be hard or soft and could affect the mechanical properties. A soft interphase, for example, can significantly improve fatigue and other properties. A soft interphase will reduce stress concentrations. A rigid interphase improves stress transfer of resin to the filler or adherend and improves interfacial shear strength. Adhesion promoters generally increase adhesion between the resin matrix and substrate, thus raising the fracture energy required to initiate a crack. 

Moisture can also degrade physical properties and cause dimension changes of certain adherends. 

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