This paper focuses on the effect of fiber orientation and stacking sequence on the progressive mixed mode delamination failure in composite laminates using fracture experiments and finite element (FE) simulations. Every laminate is modelled numerically combining damageable layers with defined fiber orientations and cohesive zone interface elements, subjected to mixed mode bending. The numerical simulations are then calibrated and validated through experiments, conducted following standardized mixed mode delamination tests. The numerical model is able to successfully capture the experimentally observed effects of fiber angle orientations and variable stacking sequences on the global load-displacement response and mixed mode inter-laminar fracture toughness of the various laminates. For better understanding of the failure mechanism, fracture surfaces of laminates with different stacking sequences are also studied using scanning electron microscopy (SEM).

This paper analyses the progressive mixed mode delamination failure in unidirectional and multidirectional composite laminates using fracture experiments, finite element (FE) simulations and an analytical solution. The numerical model of the laminate is described as an assembly of damageable layers and bilinear interface elements subjected to mixed mode bending. The analytical approach is used to estimate the total mixed mode and decomposed fracture energies for laminates with different stacking sequences, which is also validated through experiments. It is concluded that the interlaminar fracture toughness of multidirectional laminates is considerably higher than that of the unidirectional ones. The effect of initial interfacial stiffness and element size is studied and it is also shown that their value must not exceed a definite limit for the numerical simulations to converge. The model can also be further extended to simulate the mixed mode fracture in hybrid fiber metal laminates.

The increase usage of polymer composite materials in the aerospace and automotive industry has generated considerable interest in using composite materials for crashworthy structures that absorb impact energy through a controlled failure in progressive crushing. By tailoring the fiber type, matrix type, fiber-matrix interface, fiber stacking sequence and fiber orientation, composite crashworthy structures have been shown to have excellent energy absorption performance characteristics. To understand the energy absorption and failure mechanisms of crashworthy structures, DLR has developed a chamfered tube segment specimen, which is easy to fabricate and gives reproducible axial crush failures under quasistatic and dynamic loading conditions. This may be used for screening of different energy absorbing composite materials and provides design data for crashworthy design studies, see [1].The paper describes a successful methodology for crush testing of chamfered tube segment specimens in both quasi-static and dynamic loading conditions with an analysis method to compare crushing characteristics and energy absorption performance. The specimen is a flanged circular segment which is self-stabilising without edge supports and when triggered fails by steady crushing without buckling, as seen in Fig. 1. The specimens are easily fabricated in open moulds and are clamped in the test at their base, which avoids special support fixtures used in similar tests with flat plate specimens. A test programme is described [2] which compares quasi-static crush behaviour with tests carried out at impact velocities of 2 -10 m/s in a high rate servo-hydraulic test machine. Tests on carbon fibre fabric/epoxy test specimens are presented in detail, with particular emphasis on the influence of loading rate on energy absorbed and crush failure mechanisms. The testing methodology included the use of a high speed camera to capture the crushing behaviour of the specimen during the crush tests. In addition to capturing the crushing behaviour of the specimen during the crush tests, Highresolution Computed Tomography (HRCT) scanning of the specimen was performed to enable a detailed analysis of crushed specimen using the nanotom® CT system by phoneix‫|‬xray. This permits detailed observation of the composites failure surface by X-ray without the need for sectioning and microscopic analysis. The quasi-static crush specimen exhibited the most favourable energy absorption performances in terms of Crush force

a b s t r a c tCyclic mixed mode delamination in multidirectional composite laminates subjected to high cycle fatigue loading has been investigated by numerical simulations and cyclic mixed mode bending experiments. The numerical model includes lamina and interface elements. The description of the delamination crack growth rate is based on the cyclic degradation of bilinear interface elements linking the evolution of the damage variable with the delamination crack growth rate. The constitutive cyclic damage model is calibrated by means of mixed mode fatigue experiments and reproduces the experimental results successfully and with minor error. It is concluded that only with implementing a cyclic damage variable in the cohesive interface element the experimentally observed crack growth and stiffness degradation can be captured properly. Scanning electron microscopy of fracture surfaces after cyclic loading revealed that abrasion of crack bridging surface roughness is the main microscopical cause of weakening and degradation of the interface.

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