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Abstract: The results of drop-weight impact tests and compression after impact (CAI) tests on carbon/epoxy laminates are presented. The experiments were carried out on specimens of two different geometries (rectangular and circular), according to two ASTM standards. Laminates of small thickness, thus prone to buckling under compression, were considered. Two different quasi-isotropic stacking sequences, obtained by cutting the specimens in two perpendicular directions, were also tested for each geometry, in order to study the influence of both these factors on the impact and post-impact response. In impact tests different dynamic behaviour, energy absorption and material damage were observed between coupons of different geometry; in the case of rectangular coupons, stacking sequence also affected the results. The behaviour of laminates under compression was always characterized by global buckling due to the small thickness. Different deformed shapes of the buckled specimens were observed, depending on geometry and lay-up, also by means of strain measurements. Finite element analysis was helpful in the interpretation of strain recordings in different positions of the laminates. Impact-induced damage did not affect the compressive behaviour and strength in every case, depending on the extent of delamination and the global buckling mode. In some cases the pre-existing damage was able to change the buckling mode and to lower the critical load as well as the ultimate compressive load. [Copyright &y& Elsevier]

Abstract: In this paper, the damage failure and behaviour of stitched composites under compression after impact (CAI) loading are experimentally investigated. This study focuses on the effect of stitch density and stitch thread thickness on the CAI strength and response of laminated composites reinforced by through-thickness stitching. Experimental findings show that stitched composites have higher CAI failure load and displacement, which corresponds to higher energy absorption during CAI damage, mainly attributed to greater energy consumption by stitch fibre rupture. The coupling relationships between CAI strength, impact energy, stitch density and stitch thread thickness are also revealed. It is understood that the effectiveness of stitching has high dependency on the applied impact energy. At low impact energy range, CAI strength is found to be solely dependent on stitch density, showing no influence of stitch thread thickness. It is however observed that stitch fibre bridging is rendered ineffective in moderately stitched laminates during compressive failure, as local buckling occurs between stitch threads, resulting in unstitched and moderately stitched laminates have similar CAI strength. The CAI strength of densely stitched laminates is much higher due to effective stitch fibre bridging and numerous stitch thread breakages. At high impact energy level, CAI strength is discovered to be intimately related to both stitch density and stitch thread thickness. Since CAI failure initiates from impact-induced delamination area, stitch fibre bridging is considerable for all specimens due to the relatively large delamination area present. Stitch threads effectively bridge the delaminated area, inhibit local buckling and suppress delamination propagation, thus leading to increased CAI strength for laminates stitched with higher stitch density and larger stitch thread thickness. Fracture mechanisms and crack bridging phenomenon, elucidated by X-ray radiography are also presented and discussed. This study reveals novel understanding on the effectiveness of stitch parameters for improving impact tolerance of stitched composites. [Copyright &y& Elsevier]

[Display omitted] This paper reports on a study of the damage tolerance of carbon fibre re- inforced polymer plates, used in aircraft fuselage, subjected to multi-site impacts. A new fixture was designed to perform sequential low-velocity impacts at different locations on plates with two different thicknesses, representing upper and lower fuselage panels. The residual strength in compression was measured using a new compression-after-impact rig designed to prevent global buckling of thin plates. Experimental results indicated that compared to single impacts, multi-site impacts can reduce the residual strength of the thin plates, but not the thick plates. To explain this difference, a detailed finite element model was developed, which enabled modelling of multi-site impacts and compression after impact in one simulation. Simulations sug- gested that the second impact damage changes the buckled shape prior to failure for the thin plates. A parametric analysis on key material parameters showed that they have a small effect on the residual compressive strength. This work can help to improve the design of aircraft panels with respect to threats posed by multi-site impacts. [ABSTRACT FROM AUTHOR]

Low velocity axial impact crush behaviours and energy absorption characteristics of thin-walled aluminium conical (AC) and E-glass/epoxy composite over-wrapped aluminium conical hybrid frustrated shells (CWAC) have been studied through experimental and numerical procedures. In this regard, 15°–24° semi-apical angled aluminium and hybrid conical test specimens of required thickness and fibre ply orientations were fabricated through hand layup method. The axial impact tests were performed at different strain rates on each AC and CWAC samples with the help of low velocity impact test setup in order to assess the energy absorption capacity of each specimen against impact loading. The influence of inertia, strain rate, addition of GFRP laminates over aluminium shell and GFRP fibre ply orientation towards the crashworthiness performances of various categories of conical frusta were analyzed. The low velocity impact tests were also simulated with the help of finite element analysis (FEA) techniques of ABAQUS® software in order to predict and compare the crashworthiness of each category of specimen model with experimental results. The obtained crashworthiness test results and collapse behaviours of FEA analysis are found in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: The influence of nanoclay on the impact damage resistance of carbon fibre–epoxy composites has been investigated using the low-velocity impact and compression after impact (CAI) tests. The load–energy vs. time relations were analyzed to gain insight into the damage behaviours of the materials. The CFRPs containing organoclay brought about significant improvement in impact damage resistance and damage tolerance in the form of smaller damage area, higher residual strength and higher threshold energy level. The presence of nanoclay in the epoxy matrix induced the transition of failure mechanisms of CFRP laminates during the CAI test, from the brittle buckling mode to more ductile, multi-layer delamination mode. Addition of 3wt% clay was shown to be an optimal content for the highest damage resistance. [Copyright &y& Elsevier]