A Finite Element Study of Bond-Line-Read-Out

Adhesive bonding of fiber-reinforced plastic (FRP) panels usually results in undesirable surface deformation due to adhesive cure and thermal shrinkage. Bond-line-read-out (BLRO) refers to the fact that the bond line is visible on the panel surface opposite to the side of adhesive application. This paper studies the surface deformation on a bonded FRP structure which results from the differential thermal shrinkage between the panel material and the adhesive using the finite element method. Sheet molding compound (SMC) is chosen as a typical FRP material in the analysis. Three geometrical configurations are considered, namely, a bare adhesive film, an SMC backing strip, and an SMC hat section, respectively, bonded to the underside of an SMC flat plate. In particular, the effects of (1) material properties, (2) part geometry, and (3) adhesive thickness and coverage, on BLRO are investigated. Based on this study, it is concluded that BLRO can be reduced by the following means: (a) reducing the difference between the thermal expansion coefficients of SMC and adhesive, (b) lowering the adhesive-cure temperature, (c) increasing the SMC/adhesive Young's modulus ratio, (d) increasing the SMC plate/adhesive layer thickness ratio, (e) eliminating or reducing the adhesive squeezeout, and (f) if possible, modifying part geometry or bond line layout.