Bonded joints adhesive shear stress distribution

Figure 5.38 Adhesive shear stress distribution in intact bonded joints (based on reference 5.31). (Copyright ASTM reprinted with permission).

Despite the importance of designing out potential induced peel stresses in maximizing the strength of adhesively bonded joints, by far the most important factor is that the adhesive shear stress distribution is naturally highly non-uniform and that such joints would have extremely limited durability if this were not so. The variability comes about as the result of enforcing compatibility of deformations, as is explained in Fig. 6. 

It is not uncommon for composites to be bonded to metals and there are one or two important points to be brought out. Figure 58 shows the adhesive shear stress distributions for double-lap joints between aluminium and unidirectional type II CFRP adherends. If the outer adherends are aluminium and the centre adherend is CFRP, the highest shear stress occurs at the compression end of the joint where the aluminium adherends are loaded. This is because the aluminium adherends have a lower tensile stiffness than the composite adherends. However, the adhesive stress concentrations at each end of the joint are similar (Table 6). Therefore, as far as the adhesive is concerned, the joint is well conditioned. Alternatively, if the outer adherends are unidirectional CFRP and the centre adherend is aluminium, then the higher shear stress that occurs at the tension end of a joint with similar adherends is increased still further by the adherend dissimilarity. The stress concentration at the tension end of the joint is now 3 6 times the stress concentration at the compression end, with the result that the joint efficiency, in terms of the strength of the aluminium alloy adherends, is reduced from 79% to 48%. 

Adhesive shear stress distribution for bonded joint with no defects... 

The design analysis of a scarf may be considered similar to a single lap bonded joint, detailed analysis of which can be found in MIL-HDBK 17-3E 3. The analysis of a bonded joint is made complex, however, by the modulus difference of the adhesive compared to the adherends and the relative thicknesses of both which causes a non-linear distribution of the shear forces in a lap joint with peak stresses at the ends [1]. Scarf repairs provide a more uniform stress distribution however, to achieve this an adequate scarf angle is required [24] shown in Eigure 14.6. 

Typically, the yardstick for qualitatively measuring the internal resistance of an adhesive bond to an external load has been the determination of the strain distribution in the adhesive and adherends. This is a difficult task. Even in simple lap joints, the actual stress-strain distributions under load are extremely complex combinations of shear and tensile stresses, and are very prone to disturbance by non-uniform material characteristics, stress concentrations or locaUzed partial failures, creep and plastic yielding, etc. It is extremely difficult to accurately measure the strains in adhesive joints with such small glue Une thicknesses and such relatively inaccessible adhesive. Extensometers, strain gauges, and photoelasticity are being used with limited success." ... 

Shear tests are very common because samples are simple to construct and closely dupUcate the geometry and service conditions for many structural adhesives. As with tensile tests, the stress distribution is not uniform and, while it is often conventional to give the failure shear stress as the load divided by the bonding area (Table 11.1), the maximum stress at the bond line may be considerably higher than the average stress. The stress in the adhesive may also differ from pure shear. Depending on such factors as adhesive thickness and adherend stiffness, the failure of the adhesive shear joint can be dominated by either shear or tensioa ... 

A typical shear stress or strain distribution in an adhesively bonded lap joint is shown in Figure 5.38. If fasteners are placed in conventional positions (i.e. metal bolted connections), they are located in the area of minor shear stresses. Therefore, when the joint is undamaged the fasteners will be only moderately loaded. 

Second, around the periphery of the joint, both tensile and shear stresses act (see Stress distribution bond thickness). As Fig. 2 shows, their magnitude depends on the aspect ratio of the joint and also varies throughout the thickness of the adhesive layer. On the adhesive - substrate interface, there is always a stress concentration at the edge of the substrate. 

As mentioned, shear tests will usually produce nonuniform stress distributions in the bonded joint. This deviation from a pure shear condition results in misleading adhesive strengths. To determine the shear strength and modulus of a bonded system in which peel stresses, bending stresses, and other nonuniformities are eliminated, ASTM E229-70 has been... 

The effect of adhesive non-linearity on the behaviour of FRP adhesively bonded joints was adequately addressed in Section 10.3. However, it is useful to recall that considering adhesive non-linear representation in FEM has a negligible effect on the predicted shear and peel stresses at service load levels of the FRP composite joint, because their distribution along the whole bondlength is mainly linear-elastic. Interfacial adhesive non-linearity has a substantial effect on the accuracy of quantitative FEM predictions of both stresses, and thus joint capacity, only at loads nearing joint failure where all practical adhesives, even brittle ones, exhibit non-linear behaviour. 

A critical issue when designing adhesively bonded joints mating pipes is when the joint undergoes torsion, which could often be experienced by pipes and risers in their service lives. Again, without elaborate mathematical derivations, the distribution of the shear stress in the adhesive with reference to the geometry shown in Fig. 18.8 can be evaluated by the following equation ... 

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