Shear crack initiation and propagation in foam core sandwich structures

• Main Author: Noury, Philippe
• Format: Dissertation
• Language: English
• Published: University of Southampton 2000
• Subjects: Cellular PVC

The subject of this thesis was on the fatigue and fracture behaviour of PVC cellular foam core materials used in sandwich constructions. The investigation was restricted to a special type of PVC called cross-linked after their chemical structure. The mechanical and physical characteristics of the material tested throughout the investigation were typical of the one used in the marine industry. The first investigation reports on mixed-mode fracture in rigid cellular PVC foam based on experimental and numerical analyses. Experiments were performed on sharp-cracked specimens using the compact-tension-shear (CTS) test loading device. Foams of three different densities were tested. The CTS specimen was, in association with a special loading device, an appropriate apparatus for experimental mixed-mode fracture analysis. Experimentally obtained fracture toughness results show good consistency. KIC fracture toughness was marginally different in different directions. The ratio KIIC/KIC was found to be between 0.4 and 0.65 depending on the foam density. For mixed-mode loading, Richard's criterion - using experimentally obtained KIIC and KIC - was the best in predicting accurately fracture locus and fracture angle. When no experimental data were used, the maximum tangential stress criterion predicted best kinking angle. The principal strain criterion predicted the best fracture locus. Fracture boundary curve and kinking angle were best predicted for low mode-II contribution. The theory appeared to be deficient in predicting accurately both fracture locus and fracture angle. The fatigue crack growth in rigid PVC cellular foam under combined mode-I and mode-II loading was then investigated. Experiments were performed on sharp-cracked specimens using the same CTS test technique, as in the previous investigation. Loading conditions ranging from pure mode-I (opening mode), mixed-mode, to pure mode-II (shearing mode), were generated using a simple loading device. Linear elastic fracture mechanics (LEFM) and plane strain conditions were assumed. Crack path γ, da/dN versus ΔK and crack growth behaviour for each loading condition were ascertained. Fatigue data were compared with results obtained from static tests in similar configurations. A refined fatigue and fracture process model was proposed.