A beam specimen to measure the face/core fracture toughness of sandwich materials under a tearing loading mode

• A new specimen is proposed as a mode III fracture test method for sandwich materials.
• The specimen design is based on finite element analysis and experimental testing.
• FEA shows that the crack loading is dominated by a rather uniform mode III.
• Crack propagation is visible in this mode III beam specimen.
• The measured face/core fracture toughness is in good agreement with FEA predictions.

A new test specimen named the sandwich tearing beam (STB) is proposed as a fracture test method to measure the face/core debond fracture toughness of sandwich materials loaded under mode III. The STB specimen consists of a sandwich beam made of a single laminated composite face sheet reinforced by a thick steel beam and bonded to the sandwich core, which is adhesively bonded to the edge of a metal base plate allowing for rotations of the face sheet. The sandwich beam has an initial crack introduced at the face/core interface and a tearing force (parallel to the crack front) is applied at the free end of the face sheet which extends beyond the length of the core. Finite element analysis (FEA) shows that the energy release rate distribution at the crack front of the STB specimen is highly dominated by loading mode III, with significant contributions of mode II only near the specimen edges. STB tests conducted to a steel beam-reinforced glass/vinyl ester face sheet bonded to an H100 PVC foam core show that crack propagation occurs as a sub-interface crack running parallel to the face/core interface. The measured compliance and face/core mode III fracture toughness are in good agreement with FEA predictions.