Investigation of Workpiece Distortion During the Photopolymerization of a PAAW Joint

As a workpiece is bonded to a photo-activated adhesive workholding (PAAW) fixture, the adhesive shrinks during photopolymerization. This leads to the buildup of residual stresses that may distort the workpiece or reduce the external load capacity of the adhesive joints. This research quantifies the impact of adhesive shrinkage on fractional thickness reduction and residual tensile force for a commercially available adhesive and gripper. These variables are quantified for typical ranges of adhesive joint thickness and workpiece interface stiffness. Empirical models are presented for relating these variables. This research reveals that once photopolymerization ceases, workpiece distortion and residual stresses are permanent, and are not diminished by adhesive stress relaxation. It also reveals that the fractional thickness reduction of a PAAW joint can range from close to zero to a value equivalent to the fractional volumetric shrinkage of the adhesive. Furthermore, it decreases with increasing workpiece interface stiffness and increasing adhesive joint thickness following a power law relationship. It is believed that necking within the adhesive joint has a significant influence on this relationship. For stiff workpiece interfaces, residual tensile forces can grow larger than 25% of the tensile break strength of an adhesive joint formed without restraint.