Adhesive joints nonuniform shearing

This type of analysis is based on the study of stresses and strains in adhesive joints. By considering the stress state in single lap joints, which are commonly used in industry and have a simple geometry, it can be seen that a complex state of stress is present in adhesive joints. When loaded in tension, the shear stresses in the adhesive layer in single lap joints have a nonuniform distribution owing to the differential straining of the adherends. This shear lag effect, as shown inO Fig. 25.9, causes maximum stresses at the ends of the overlap. 

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 nonuniform material characteristics, stress concentrations or localized partial failures, creep and plastic yielding, etc. It is extremely difficult to accurately measure the strains in adhesive joints with such small glue line thicknesses and such relatively inaccessible adhesive. Extensometers, strain gauges, and photoelasticity are being used with limited success [12]. 

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... 

Fig. 14 shows the instrument with which these curves are obtained. The aluminum-to-aluminum specimen is quite thick, 0.375 inch or sometimes 0.5 inch thick, to create a close-to-uniform state of shear stress in the adhesive. Even so, precise analyses show significant nonuniformities. Nevertheless, the shear stresses are far more uniform than they are in typical adhesively bonded joints between uniformly thin adherends. 

The key to the success of these designs was the acknowledgement that the adhesives shear stresses were, and should be, highly nonuniform. There are enormous differences between the way adhesives behave, under what appear to be the same external loads, in short-overlap test coupons and long-overlap structural joints, as explained in Fig. 44.16. 

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