Effect of cooling rate on curing induced thermal strains in bonded joints

Measurement of strains in bondline of adhesive joints poses many challenges. The measurement technique should provide accurate and continuous measurements without affecting stress-transfer. Hence, fiber optic sensors with their inherently small size are best suited for bondline measurements. Furthermore, high-definition optically distributed sensors can provide strain measurements at every 1.2 mm. along the fiber length allowing to capture precise strain profiles in the bondline. One of the challenges in bonded joints is the lack of understanding of the effects of processing parameters on resulting mechanical behavior. In this work, the effect of cooling rate on the strains developed in the adhesive were experimentally measured. Lap-joints made of glass-fiber reinforced (GFRP) substrates were bonded with a thermoplastic adhesive (ABS). The fiber optic sensor was embedded along the length of the bondline in the center of the adhesive. It is experimentally cumbersome to try a large number of cooling rates with varying adhesives and substrates. Hence, three different cooling rates were selected in this work and strains developed were monitored over time. As expected, results indicate that the thermal residual strains in the adhesive increased with increase in cooling rates. At lower/gradual cooling rates, the viscoelastic nature of the adhesive allows for higher polymer chain relaxation, thus resulting in lower residual strains. In each case, the residual strains were observed to be higher at the joint free edges. The edges of the adhesive cool and solidify faster than the center leading to a ‘prestrain/pre-stress’ condition in the adhesive as result of processing conditions.