Thermal expansion and mechanical properties of urethane-modified epoxy bonded CFRP/steel joints at low and high temperatures for automotive

Long-term reliability is critical in automobiles owing to their frequent exposure to the external environment and lengthy operational durations. Using dissimilar materials as adhesives to reduce the weight of vehicles results in a mismatch of coefficients of thermal expansion (CTE). Difference in CTE causes constant internal stress at the adhesive interface when the temperature changes, leading to delamination of the adhesive at the interfaces, microcrack initiations, and crack propagations. Therefore, a blend of linear urethane prepolymer and epoxy was tested to alleviate stress concentration. Dynamic mechanical analysis (DMA) was performed to determine the modulus according to the urethane content, while tensile strength was measured to evaluate changes in the physical properties. It was observed that increasing the urethane content decreased the modulus but progressively enhanced the tensile strength elongation rate. The lap shear strength specimen was thermally shocked to validate the decrease relative to the initial value based on the cycles. Consequently, combining urethane and epoxy successfully enhanced the elongation and reduced the internal stress caused by temperature changes.