The goal of this paper is to evaluate the strain-based partitioning method (SBM), an analytical method based on beam analysis, for the mixed-mode fracture characterization of asymmetric cracks. The fracture energy is calculated at the crack tip of mixed-mode bending (MMB) test with asymmetric geometry and asymmetric material parameters. The fracture energy obtained using SBM is benchmarked against the widely applied Williams’ partitioning method (WM) and numerical results from finite element models using the virtual crack closure technique (VCCT). Results show that WM produces inaccurate fracture modes in asymmetric conditions. However, if using SBM, the fracture mode ratio of bi-material cracks was obtained with accuracy. SBM describes a coupling function between mode I and mode II fracture that allows the mode partitioning as long the specimen complies with a simple design criterion based on the longitudinal strain equivalence between arms. Experimental results of composite-to-metal bonded joints validate the results of the parametric study. SBM proves to be an easy and reliable solution for the mixed-mode fracture characterization of bi-material cracks.

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In this work, the influence of a layer of glass fiber mat (GFM) inserted in a bi-material bonded joint interface is investigated as a toughening mechanism to improve the fracture performance of bonded structures. Composite-to-metal bonded specimens were manufactured by hand lay-up using two different consolidation processes: at room pressure or using vacuum bagging. The fracture behavior was evaluated under quasi-static loadings using the mixed-mode bending (MMB) test. Test results revealed an increase of the fracture toughness with the insertion of an adjacent layer of GFM. The toughening mechanism is associated with a more irregular fracture surface profile. It was verified a relationship between the fracture surface roughness and the fracture toughness. The toughening effect showed to be more evident in higher mode II fracture conditions. The insertion of a layer of GFM and the use of vacuum pressure in the consolidation of the composite increase the fracture performance of composite repairs in metal structures.

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The dissemination of composite materials introduces applications of hybrid structures with composite and metal parts. The development of reliable methodologies to evaluate the performance of these structures is required. In this work, the mixed-mode fracture behaviour of a bi-material adhesively bonded joint is investigated. A new strain-based criterion for the design of the mixed-mode bending (MMB) bi-material specimen is suggested. A new analytical partitioning method based on the ‘global method’ is proposed and tested on a composite-to-metal bonded joint and compared with a finite element model using the virtual crack closure technique (VCCT). The results show that the proposed strain-based design methodology can be successfully used in MMB test for bi-material joints. The fracture mode partitioning is accurately predicted by the analytical method. However, the absolute values of the strain energy release rate (SERR) predicted by the analytical method are only accurate if the shear deformation in the test is not significant.

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The aim of this study is to assess the interlaminar adhesion of carbon-epoxy laminates under salt water condition. Carbon-epoxy laminate specimens were immersed in a salt water tank for 60 days. Some specimens were then dried at
room temperature for 280 days, until recovering their initial weight. Specimens were tested using the composite peel test, an adaptation of the floating roller peel tests for composite materials. The results showed a degradation of peel strength in some areas due to the ageing process. The drying process did not affect the test results. A scanning electron microscopic analysis carried out on the fracture surface of the specimens revealed a typical mode I failure microstructure. A mixture of matrix failure and fibre/matrix interfacial failure was observed in non-aged specimens. Finally, a chemical characterization of the fracture surfaces with energy-dispersive spectroscopy confirmed the penetration of salt water in regions near the edge of the specimens. A degradation of the fibre/matrix interface adhesion was observed in affected areas. Floating roller peel tests proved to be a fast and effective method to access the interlaminar adhesion performance of composite laminates.@en