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Description
Carbon fiber reinforced polymer (CFRP) is a very popular material used in aerospace applications for its high strength, high stiffness and low weight properties. The brittle nature of CFRP’s lead to low impact resistance, damage tolerance and bearing strength. A form of hybrid composite laminate that substitutes CFRP plies with stainless-steel (SS) to reinforce the composite has been shown to overcome some of the problems in CFRP. The performance of the hybrid metal reinforced CFRP laminates is a strong function of the interlaminar strength and fracture properties between the SS and CFRP materials. This thesis presents the results of the double cantilever beam (DCB) and end notched flexure (ENF) tests conducted to determine fracture properties between SS and IM7/977-3 CFRP with different adhesive thicknesses. Characterizing the SS-CFRP interface in these hybrid composites required a modified specimen with a metal foil on the symmetry plane and a starter crack placed on one side of the SS-CFRP material interface. The resulting specimen leads to asymmetric laminate in one of the fractured legs in DCB and ENF specimens. The large mismatch in the coefficient of thermal expansion (CTE) between the SS and CFRP material leads to residual thermal strain induced curvature of the asymmetric specimen legs. Furthermore, the temperature changes experienced during the curing process combined with the CTE mismatches lead to additional strain energy created by residual thermal strains. These bending curvatures and residual thermal strain energy affect the DCB and ENF tests. Accurate estimation of fracture properties from test data (load versus crack opening) in hybrid composite laminates requires accounting for these thermal effects. Previously published work has provided some solutions for including thermal effects, however, they do not accurately account for the crack face contacts that developed in SS-CFRP laminate specimens during our testing. An improved formulation that accounts for the development of crack face contact is presented and used for characterization of fracture properties. This is then used to investigate the adhesive thickness effects on fracture properties.
