Determination of Interface Fracture Parameters in Thermoplastic Fiber Metal Laminates Under Mixed-Mode I+II

Thermoplastic fiber metal laminates (FMLs) are hybrid material systems that consist of a thin aluminum alloy sheet bonded to plies of fiber-reinforced adhesive. They provide excellent properties like fatigue strength, damage-tolerant properties, and inherent resistance to corrosion. However, they are still challenging materials in terms of the metal-composite interface, which is the weakest link in this material system. In this paper, an experimental-numerical method for the determination of the fracture stress and energy for metal-composite interlayer is presented and verified. The proposed method utilizes four different experimental tests: DCB test (interface opening-mode I), ENF test (interface shearing-mode II), MMB test (mixed-mode I+II-opening with the shearing of the interface) and three-point bending test (3PB). For each test, digital twin in the form of a numerical model is prepared. The established numerical models for DCB and ENF allowed us to determine fracture stress and energy for mode I and mode II, respectively. On the basis of the numerical and experimental (from the MMB test) data, the B-K exponent is determined. Finally, the developed material model is verified in a three-point bending test, which results in mixed-mode conditions. The research is conducted on the thermoplastic FML made of aluminum alloy sheet and glass fiber reinforced polyamide 6. The research presented is complemented by fundamental mechanical tests, image processing and Scanning Electron Microscopy (SEM) analysis. As an effect, for the tested material, fracture parameters are determined using the described method.