Three-dimensional failure modes and criteria of textile composites and sandwich structures

Two major obstacles in the realization of the great potential possessed by composite materials are their susceptibly to damage and complex failure behavior. Fiber reinforced composites are increasingly used in structural applications and detailed design parameters are needed to minimize weight and cost while maintaining reliability. Out-of-plane loading investigations on composite sandwich structures and thick composite laminates represent an area in which experimental characterization can help engineers design better structures and optimize failure prediction models. The current study addresses impact loading on sandwich structures and through-thickness characterization of thick textile laminates.; The low velocity impact behavior of a composite sandwich panel is investigated. The facesheets are composed of woven carbon fabric/epoxy and the core is polymeric foam. Panels are impacted with a falling mass from increasing height until damage is induced. Dynamic displacement, strain and load histories are recorded during the impact event for subsequent analysis. A sandwich panel is also statically loaded to compare its response to impacted panels. Comparisons of the load-strain response and damage behavior indicate that low velocity impact on the panel is quasi-static in nature. Damage was localized near impact and consisted of facesheet delamination. Delamination is confirmed to be detrimental to the structure's residual compressive strength.; In an effort to better understand and predict out-of-plane failure in composites, the through-thickness properties of a thick textile composite are investigated in the study. Specimens are tested in through-thickness compression, tension and shear with various loading orientations. Delamination failure strengths and fracture behavior are recorded. Experimental results are compared to various existing failure criteria in relation to off-axis loading angles and failure envelopes in stress-space. The material exhibits increased interlaminar shear strength with the presence of compressive loading. Existing failure criteria do not adequately characterize this response and thus are lacking.; A new failure theory is put forth that better captures the delamination failure response of a textile composite. In addition, the theory is shown to work well for predicting the compressive delamination response of a quasi-isotropic carbon/epoxy laminate under through-thickness loading.