Mode-III fracture characterization of composites

Abstract

Composite monolithic laminates and sandwich materials are employed to build structures that demands a high stiffness to weight ratio, a high strength to weight ratio, good ther- mal and acoustic insulation and high capacity of energy absorption due to impact loads. Nowadays, composite materials are increasingly substituting metallic materials in different structural applications as in the aerospace, marine and wind energy sectors. A composite sandwich structure can develop a wide variety of damages under static and fatigue loads. The principal kind of damages observed during the in-service life of a sandwich structure are: face-core interface debonding, core indentation, core-shear failure, face wrinkling and dimpling, shear crimping and general buckling. The robustness of a structural component can be affected by the aforementioned damages. Therefore, it is of vital importance that each different damage mode is meticulously analysed and studied in order to be able to predict the remaining life of a sandwich structure. This PhD thesis deals with the analysis and fracture characterization of face-core inter- face debonding in foam-cored sandwich material systems and delaminations in monolithic laminates, when the defect is subjected to out-of-plane shear loads. The bonding strength of a disbond or a delamination can be assessed by determining the energy required to separate a unit area of the bonded material at the interface. This amount of energy is called fracture toughness and it is dependent on the mode-mixity present along the debond/delamination front. Several test fixtures and analytical models are present in the literature regarding interface fracture characterization of debonds/delaminations under mode-I, mode-II and mixed I-II modes. Instead, mode-III fracture characterization of debonds and delaminations is a less explored topic (especially for sandwich materials) and fewer test rigs and analytical models (respect to mode I-II characterization) are available in the literature. First of all, an objective of this PhD thesis was to make a literature review of the developed test rigs and data reduction methods for mode-III fracture characterization of composite monolithic laminates. The different characteristics of the known test fixtures are illustrated and commented. The shear torsion bending (STB) test rig was identified as the one that provides the most uniform distribution of energy release rate (ERR) mode- III component along the delamination front. Therefore, the design of the novel test rig for mode-III fracture characterization of foam-cored sandwiches was inspired by the STB fixture. A data reduction method is essential to compute the ERR in function of the external loads, material properties and geometry of the debonded/delaminated specimen. An im- proved data reduction method is first presented for the STB test designed for monolithic laminates. Subsequently, the applicability of the STB rig is extended to foam-cored sandwich composites subjected to out-of-plane shear loadings. The data reduction method consists in an analytical model based on first order shear deformation theory, Vlasov theory for non-uniform torsion of beams and near tips effects are also taken into account. A 3D fnite element model was build in order to verify the analytical model and generate both global and local predictions of the ERR distributions along the crack front. Experimental results are shown and discussed in the last part of the thesis with the direct application of the analytical model used to extract the experimental fracture toughness values.