Adhesive stress distribution

Tsai M Y and Morton J (1995), The effect of a spew fillet on adhesive stress distributions in laminated composite single-lap joints . Compos Struct, 32(1 ), 123-131. 

Prickett and Hollaway(29) presented both classical and finite-element solutions for elastic-plastic adhesive stress distributions in bonded lap joints. Single, double, and tubular lap configurations having both similar and dissimilar adherends were considered. The results show how the development of adhesive yielding will occur as the joints are loaded to a failure... 

Fig. 6. Adhesive stress distributions for v = 0.5, for different values oiajh.

SAME ADHESIVE STRESS DISTRIBUTION IN EACH CASE... 

The bonded joints between typical thin structural elements have an extensive capability to tolerate the load redistribution that is caused by local flaws and porosity with no loss whatever in strength or durability. This derives from the same minimum overlaps needed to provide resistance to creep rupture that were discussed earlier. Figs. 26-29, taken from ref. [19], address this issue in the context of the longitudinal skin splices in the PABST forward fuselage, where the thickness was 0.050 inch of 2024-T3 aluminum alloy. Fig. 27 shows the adhesive stress distribution at room temperature for a load of 1000 Ibs./in., which corresponds with a 1.3 x P proof pressure load. Significantly, this load does not even exceed the elastic capability of the adhesive for this environment. 

Discussion of behaviour of adhesive stress distribution variation of adherend matenal and thickness, and of tap length. 

Standards ASTM D 4562 and ISO 10123 describe a shear test in which the specimen is a pin bonded inside a coUar. The test uses a press to force the pin through the collar, which rests on a support cylinder. The test results are the load required to initiate failure divided by the bonded area between the pin and the collar. This type of test is particularly suited to test anaerobic adhesives. The shear strength determined with this test is only an average value because the stress distribution is not uniform along the overlap (Neme et al. 2006 Martinez et al. 2008). ASTM E 229 also uses a pin-and-coUar type of specimen except that here torsional loadings cause failure. The adhesive stress distribution in this case is more uniform and may be used to determine the adhesive shear modulus and strength. However, the standard was withdrawn in 2003. 

Effect of the adhesive modulus on the adhesive stress distribution along the overlap... 

The apex (often referred to as bead filler) compound must be formulated for excellent dynamic stiffness to facilitate stress distribution and provide good car handling properties. The bead insulation compound must possess good adhesion to this most important component for enclosing the pHes of the tire and holding the tine to the rim. The chafer/rim strip compound protects the pHes from rim abrasion and seals the tire to the rim. 

Fig. 18. Adhesive contact of elastic spheres. pH(r) and pa(r) are the Hertz pressure and adhesive tension distributions, (a) JKR model uses a Griffith crack with a stress singularity at the edge of contact (r = a) (b) Maugis model uses a Dugdale crack with a constant tension aa in a < r < c [1111.

Recently, stress analysis has been carried out for the determination of stress distribution around inclusions in particulate filled composites. A model based on the energy analysis has led to the determination of debonding stress [8]. This stress, which is necessary for the separation of the matrix and filler, was shown to depend on the reversible work of adhesion (see Eq. 16) and it is closely related to parameter B. 

Occasionally debonding was observed at the interface (Figure 8) producing smooth, dewetted surfaces on the beads. This bead-matrix adhesion failure made a relatively minor contribution to the overall fracture mode and was limited to beads larger than about 150 /xm diameter. A critical diameter range for stress distribution at the interface may exist for certain matrix compositions and volume concentrations. 

The stress distribution analysis shows that maximum stress concentration develops in the radial direction at the pole of the particle, and shear yielding is initiated at around 45° on the surface of rigid particles. Debonding occurs at the pole of the particle, and extends to a critical angle [27]. In case of total adhesion, debonding does not occur and there is cavitation in the matrix, at some distance from the particle pole (not at the interface). 

Figure 10. Approximate stress distribution (oy) for L type specimen predicted for load P3 of lON/mm. Predictions based on 5 stepwise adhesive thicknesses from 0.8 mm to 0.08 mm at a = 4.8 mm, 5.6 mm, 6 mm and 6.4 mm. See text for details.

Figure 2.26 shows one of the reasons why spherical fillers give good performance in compounded materials. The birefringence patterns show stress distribution in the vicinity of various shapes of inclusions - only with a spherical shape and a good adhesion to the matrix, uniform stress distribution is observed. Stress distribution is an essential element of material design. 

Figure 8.42 shows one outcome of such studies. The addition of surface treated glass beads requires substantially higher stress to support the same growth rate of cracks compared with neat epoxy. Good adhesion and stress distribution are responsible for improvement. 

The centrical force transmission is obtained by gluing sections of the same adherend thickness onto the adherends in the force transmission area. The adherend extension will be eliminated by a larger adherend thickness (5 mm instead of 1.6 mm) and a reduced overlap length. Due to the homogeneous stress distribution, the respective test results are thus based on defined shear stress conditions. Adhesive layer values, to a large extent independent of the adherend, such as shear modulus, shear strength and deformation behavior, are available as a basis for precise calculations. 

In contrast to the relationships described in Section 10.2, the deformability of the adhesive layer and the related homogeneous stress distribution enable a simplified calculation of bonded constructions. Due the stress peaks at the overlap ends being omitted to a large extent, there is an approximate proportionality between overlap length and acting force. 

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