Nonuniform adhesive failure

Compressive shear tests are similar to lap shear tests except that compressive, rather than tensile, forces are applied to the sample until failure. Compressive loading may provide better alignment of adhesive bonds and also may reduce stress nonuniformities caused by the bending adherends. 

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]. 

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