The Fracture Behavior of Aluminum Foam Sandwich Structures Based on Fiber Reinforced Thermoplastics

The fracture properties of a range of novel sandwich structures based on an aluminum foam core and fiber reinforced thermoplastic skins have been investigated. Initially, the mixed-mode interlaminar fracture properties of the thermoplastic-matrix composites were investigated using the mixed-mode flexure test geometry. Here, values of G _I/IIc ranged from 2400 J/m ² for the glass fiber/nylon composite to 3300 J/m ² for a glass fiber/polypropylene composite. The fracture properties of the thermoplastic interlayer materials used to bond the skin and core materials were also investigated and found to vary between approximately 1200 J/m ² for a PP-based system to 6800 J/m ² for a neat PEEK interlayer. Tests using the single cantilever beam sandwich (SCB) specimen showed that the degree of adhesion between the composite skins and the foam core was good at both quasi-static and dynamic rates of loading. An optical examination of the PP and PEEK-based SCB samples indicated that the primary crack propagated through the core material whereas failure in the nylon-based system occurred through the interlayer material at the skin-core interface. The fracture energies associated with both types of failure were found to be significantly lower than those of the plain composite and modified samples containing the thermoplastic interlayer. The impact response of three types of sandwich structure was investigated by conducting low velocity impact tests on simply supported beams. Here, damage in the form of fiber fracture and splitting in the composite skins and localized crushing in the composite core were observed at intermediate and high incident energies. Flexure tests to characterize the residual load-bearing properties of the sandwich structures highlighted the superior mechanical properties of the carbon fiber/PEEK-based system.