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This study investigates the influence of off-axis loading on the fracture mechanism and specific energy absorption (SEA) of glass/epoxy twill/weave composite box structures. In this regard, the off-axis angles of 5°, 10°, 20°, and 30° of loading direction with respect to the composite box axis is studied experimentally under quasi-static crushing process. At the off-axis angle of 5°, the crushing process behavior is similar to that of the axial crushing and fracture mechanisms, such as bundle fracture and interlaminar crack propagation in Mode-II are observed. These are characteristic of brittle fracture crushing mode. For crushing at an off-axis angle of 10°, additional interwall crack propagation at one side of the box is also found. This increases the energy absorbing capability of glass/epoxy composite box at this angle. The sidewall crack is a mixed-mode crack. To characterize this crack, the mixed-mode interlaminar fracture toughness, G I/IIC, is measured using asymmetric double cantilever beam (ADCB) test method. For other angles, the non-symmetrical fracture mechanisms are found at four sides of the composite boxes. The arriving of sustained crushing stage is delayed by increasing the off-axis crushing angle. Owing to this fact, the energy absorbing capability is reduced by increasing the off-axis loading angle. An analytical solution is proposed to predict the mean force of axial crushing in brittle fracture crushing mode. The off-axis crushing process of composite boxes is also simulated by finite element software LS-DYNA and the results are verified with the relevant experimental results.

The present paper reviews crushing process of fibre-reinforced polymer (FRPs) composites tubular structures. Working with anisotropic material requires consideration of specific parameter definition in order to tailor a well-engineered composite structure. These parameters include geometry design, strain rate sensitivity, material properties, laminate design, interlaminar fracture toughness and off-axis loading conditions which are reviewed in this paper to create a comprehensive data base for researchers, engineers and scientists in the field. Each of these parameters influences the structural integrity and progressive crushing behaviour. In this extensive review each of these parameters is introduced, explained and evaluated. Construction of a well-engineered composite structure and triggering mechanism to strain rate sensitivity and testing conditions followed by failure mechanisms are extensively reviewed. Furthermore, this paper has mainly focused on experimental analysis that has been carried out on different types of FRP composites in the past two decades.

Abstract. This research studies the low velocity impact behaviour of variable stiffness curved composite plates. Since variable thickness within composite structures is recognised as an important factor on the performance of the structures, significant mathematical modelling to predict the impact response of these types of structure is essential. Varying thicknesses of sections is widely found in aerospace and automotive composite sub structures. It has been observed that changing of geometry of these sections can vary the dynamic response of anisotropic composite structures under a range of monolithic and dynamic loading conditions. Here we have used first order shear deformation theory to predict the contact force history of curved composite plates and the same approach was used for variable thickness composite plates, which provides the main novelty of this research. It was shown that the model developed here is capable of successfully predicting the response of variable stiffness composite plates with a range of layups and geometry designs under impact loading conditions.

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