Cohesive laws of composite bonded joints under mode I loading

Mode I cohesive laws of carbon-epoxy composite bonded joints were obtained using the direct method applied to the double cantilever beam test. This method is based on the differentiation of the relation between the evolution of the strain energy release rate and the crack tip opening displacement (CTOD) during the test. An equivalent crack length method was used to obtain the strain energy release rate evolution. Following this procedure, several drawbacks inherent to crack length monitoring during the test are overcome. The CTOD was measured through digital image correlation technique and synchronized with the load-displacement data. Two different procedures were used to execute the pre-crack. It was verified that the use of a thin Teflon® film is more effective than a blade, although both procedures lead to consistent cohesive laws. It was concluded that the proposed methodology is adequate concerning the evaluation of the composite bonded joints cohesive laws.