Experimental and Numerical Methods for Characterizing the Mixed-Mode Fracture Envelope for a Tough Epoxy

PR-2930 was developed by PPG Industries, Inc. to meet the challenging performance requirements of MIL-PRF-32662 Group-I-classified adhesives. PR-2930 is a high-strength, high-toughness, epoxy-based adhesive intended for automotive and aerospace applications. As PR-2930 functions as a structural adhesive, quantification of its mechanical properties and limit-states is a necessary task for designing joints bonded with the adhesive. The combination of both strength and ductility results in material non-linearities, making experimental characterization and numerical analyses more challenging. This work explores the quantification of fracture energy for PR-2930 bonded joints. Fracture can occur in one of three different modes, or in some combination. Many practical adhesive joints fail in the mixed-mode region involving both opening (mode I) and shearing (mode II) displacements. Mode I fracture was evaluated with double cantilever beam (DCB) tests, mode II fracture was characterized by end-notched flexure (ENF) tests, and varying degrees of mixed mode I/II fracture were assessed through single leg bend (SLB), single-lap joint (SLJ), and asymmetric DCB and SLB tests. Test specimens were fabricated by bonding Al 2024-T3 adherends, ranging from 1.6 mm to 25.4 mm thick, with a 0.25 mm thick PR-2930 adhesive layer. Digital image correlation (DIC) was used to experimentally measure local displacements and surface strains on the adherends. Standard data-reduction methods often used to determine fracture energies of bonded joint specimens were used to numerically analyze test results. These methods included the Corrected Beam Theory (CBT), the Compliance-Based Beam Method (CBBM), and the Paris and Paris J-Integral approach. Linear elastic fracture mechanics (LEFM) conditions must be valid to correctly apply these methods, however plastic deformations were observed in some adherends. Drawbacks of these approaches and their validity for analyzing PR-2930 joints were discussed. To account for non-linearities, more advanced numerical analysis was performed using finite element analysis (FEA) with cohesive zone models (CZMs) to model the adhesive layer. CZM parameters such as fracture energies and traction separation law (TSL) shapes were determined from experimental data and published literature. Results from CZMs were compared to experimental load, displacement, and strain data. Recommended TSLs for mode I and mode II fracture were formed in this work as well as a mixed-mode relationship using a Benzeggagh-Kenane damage evolution law. More ideal analytical methods were suggested to simplify analysis of joints using the same or similar material compositions.