Correlating Fracture Toughness and Surface Roughness for a Ductile Epoxy Adhered to Aluminum Substrates

Adhesively bonded joints are used across multiple disciplines as an efficient and cost effective method for reinforcing, repairing, or creating new structures. Sufficient understanding of the bond line characteristics of the adhesive is necessary to properly design a reliable bonded joint and ensure a long service life. It is well understood that surface preparation has a significant impact on these interface characteristics as a given level of surface roughness achieves mechanical interlocking between the resin and metal and is important to prevent premature interfacial failure [1]. The goal of this study is to characterize the fracture toughness values for an adhesive bonded to aluminum substrates of varying surface preparation quality. Foundational equations are developed for relating surface roughness measurements to experimentally determined fracture toughness. Experimental tests are completed to determine the critical strain energy release rate, G_c, for mode I tension, G_Ic, mode II shear, G_IIc, and mixed-mode, G_I+II, loadings. The double cantilever beam (DCB), end-notched flexure (ENF), and single leg bend (SLB) tests are used for modes I, II, and I+II (mixed-mode) respectively with four types of surface preparations. Common data reduction methods are used for calculating G_c. Characterization of the interface surface, including quantification of void, cohesion, and adhesion failure mechanisms at the bondline, is studied to quantify each sample’s failure modes and void properties as it relates to surface roughness and fracture energy. The characterization is used to develop an analytical model of the relationship between these three parameters and the resulting fracture energy. Fundamental equations are developed that relates the surface roughness parameters to the fracture energies. Numerical simulations in Abaqus finite element software use a potential-based cohesive zone model to predict adhesive failure and simulate crack propagation of a mixed-mode case. These simulations are validated against the SLB experimental results for accuracy. The surface roughness measurements, coupled with the mathematical equations relating fracture energy to surface roughness, provides an input to the numerical models. The simulations are used to predict bondline performance within specified confidence intervals of the roughness measurement distributions and provides a basis for determining load carrying characteristics of the metal to adhesive interface.