Mixed-mode Fracture Energy Research Articles

Mixed-Mode Fracture of Adhesively Bonded CF/Epoxy Composite Joints

Mixed-mode fracture behaviour of adhesively bonded CF/Epoxy composite joints was studied using a rubber-modified epoxy (Araldite® GY260) as the adhesive. Fixed-ratio mixed-mode specimens ( G I/ G II = 4/3) were applied in the experiments. Composite adherends were manufactured from carbon fibre/epoxy (5128 T300 BASF) prepreg. The global mixed-mode fracture energy, G c, of the joints with different bond thickness, t, was evaluated using compliance calibration obtained from the experiments, beam theory and finite element analysis. The crack path along the bond line in the adhesive joints was investigated using optical microscopy. It was found that G c, was dependent on both the bond thickness and the actual crack path in the joints. G c, increased mildly from thin bonds (t < 100 μm) to a plateau value for larger bond thickness (t > 100 Am). For poor bonding between the adhesive and the composite adherends, interfacial crack propagation was observed for very thin bonds with alternating crack jumping from one interface to another across the bond line. The crack propagated almost at one interface for large bonds with occasional kinking. However, for strong interfacial bonding, the crack was deviated from the mode-I precrack towards the composite adherend and propagated cohesively in the composite layer. Scanning electron microscopy on the fracture surfaces showed different morphology depending on the interfacial bonding between the adhesive and the composite adherend.

Mixed mode fracture of concrete

Strain-softening and strain-localization in cementitious and ceramic materials can be described by using a cohesive crack model, with closing forces at the crack tip, which represent plasticity, inclusion interlocking, fibre bridging and any kind of non-linear behaviour. In the present paper, the cohesive crack model is extended to mixed mode propagation and an experimental confirmation is provided by testing four-point shear specimens of concrete. A constant crack mouth sliding displacement rate is imposed, so that is is possible to control and detect the snap-back load vs deflection branches. The behaviour of the larger specimens is found to be the brittler. On the other hand, the brittleness in the numerical simulations is controlled through the crack length, which is certainly a monotonic increasing function during the irreversible fracture process. The experimental load vs deflection diagrams as well as the experimental fracture trajectories are captured satisfactorily by the numerical model. The mixed mode fracture energy results tend to be of the same order of magnitude as the Mode I fracture energy ℷF, each elementary crack growth step being produced by an opening mechanism along the curvilinear trajectory. This is particularly true for the larger specimens, where energy dissipation due to friction and interlocking is negligible if compared with the energy dissipated by separation.

Mixed mode fracture energy of concrete

An interpretation of the very complex failure mechanism in four point shear specimens of concrete is proposed. A size-scale transition between tensile strength failure and mixed mode crack propagation is demonstrated to occur. When the distance of the central supports is large, the tensile strength is overcome at the supports by flexure. When the same distance is small, the tensile strength is overcome in the middle of the specimen by splitting. Pure shear failure does not seem to activate locally. As a matter of fact, the mixed mode fracture energy is substantially equal to the mode I fracture energy G F , each crack growth step being produced by a mode I (or opening) mechanism.

Debonding Properties of Residually Stressed Brittle‐Matrix Composites

Trends in interface debonding have been calculated during fiber pullout for composites with interfaces subject to residual tension. The debond behavior is shown to depend sensitively on the thermal expansion mismatch. The results are used as the basis for designing a pullout test specimen suitable for measuring the mixed‐mode fracture energy of bimaterial interfaces. The solutions also provide the background needed to assess the role of debonding in the toughening of ceramics by fibers.

Apparent interfacial failure in mixed-mode adhesive fracture

Aluminium-epoxy adhesive specimens constructed with the bond at 45‡ to the direction of loading appear to fail very close to the interface. The actual locus of failure was investigated by14C labelling of the epoxy polymer and also by Auger spectroscopy profile analysis. Both techniques indicated a residual film of polymer a few hundred angstroms thick on the aluminium surface. The fracture energy of these specimens was determined and found to be affected by the surface roughness of the aluminium. The mixed-mode fracture energy (GI,II) C45° of these specimens in the absence of any surface roughness effect (polished surfaces) was 140 J m−2 compared to 136 J m−2 for the same polymer in simple opening-modeGI C adhesive fracture. The “interfacial” failure and the effect of surface finish on fracture are discussed in terms of the applied stress directing the failure toward the interface but the approach of the crack to the boundary being limited by the size of the crack tip deformation zone.