Influence of Elastic Couplings on Delamination of Fiber Reinforced Laminates

This PhD thesis is devoted to investigation of any possible influence that elastic couplings have on delamination phenomenon in continuous-fiber reinforced polymer (FRP) laminates. Results presented in the dissertation have been published in journals from the Journal Citation Reports (JCR) list as a series of five articles that are thematically related. The papers give an overview of the effects elastic couplings have on both initiation and propagation phenomena of delamination processes in laminates subjected to different fracture schemes in macroscopic sense, as well as on the applicability of standardized test methods to determination of the critical strain energy release rates (c-SERRs). In particular, the first article presented experimental research conducted on glass/epoxy composite laminates subjected to the double cantilever beam (DCB) tests. In the second article the effect of stiffness matrices terms on behavior of laminates subjected to the mode I fracture scheme were described, as well as the real delamination front shapes were investigated through fractographic analysis. The third article presented experimental determination of fracture toughness for carbon/epoxy laminates with different delamination interfaces and specific stacking sequences - exhibiting the bending-twisting (BT) and the bending-extension (BE) elastic couplings, subjected to the DCB tests with respect to the ASTM D5528 Standard. The experiments on delamination resistance were additionally monitored with the acoustic emission (AE) equipment. After the stand tests, fractographic observations in microscale by using the scanning-electron microscope (SEM) were performed. In the fourth article, which is an individual work of the PhD Candidate, experimental research on influence of elastic coupling phenomena on delamination in carbon/epoxy laminates subjected to the mode II end-notched flexure (ENF) tests were presented. The respective experiments followed the ASTM D7905 Standard; they were supported by acoustic emission, as well. Similarly, delamination surfaces after the ENF tests were analyzed using the SEM . The fifth article describes numerical analysis of the effect elastic couplings have on the strain energy release rate (SERR) distributions along delamination front at the very moment of crack initiation for laminates subjected to the mode III fracture scheme – in a macroscopic sense, by using the split cantilever beam (SCB) configuration. Numerical simulations were conducted in the Abaqus/CAE finite element software environment by using the virtual crack closure technique (VCCT) at different boundary conditions (BCs) attributed to the laminate beam model. Additional analysis performed for hybrid – glass/epoxy-carbon/epoxy laminates turned out to be advantageous from the point of view of limiting undesirable effects of elastic couplings. Application of hybrid configurations to coupled laminates is being developed and examined as an object of prospective research.