When laminated composite materials in modern aircraft structures are subject to impact loads, they are typically not unloaded but under a certain state of prestress. Therefore, in this study the effect of a compressive preload on the low velocity impact behaviour of three different carbon fibre-reinforced plastic (CFRP) materials is investigated. An experimental test programme is documented first, including the design of a preload test device, the specimen manufacture and the results description. An increased deflection and energy absorption for composite plates with a preload of 80% of the buckling load could be observed. Non-destructive inspections showed a large extent of delaminations occurring between individual plies, being an important energy absorption mechanism. The development of numerical simulation methods for this impact scenario using the commercial explicit finite element code LS-DYNA is described in detail. The focus is on the composite material, delamination and preload modelling. The final simulation results showed a good correlation to the experimental data in terms of force and energy plots or the evaluated interlaminar and intralaminar damage, although these numerical results proved to be strongly influenced by simulation parameters like mesh size or the number of shell element layers.

Although good progress was made by two international benchmark exercises on in-plane permeability, existing methods have not yet been standardized. This paper presents the results of a third benchmark exercise using in-plane permeability measurement, based on systems applying the radial unsaturated injection method. 19 participants using 20 systems characterized a non-crimp and a woven fabric at three different fiber volume contents, using a commercially available silicone oil as impregnating fluid. They followed a detailed characterization procedure and also completed a questionnaire on their setup and analysis methods. Excluding outliers (2 of 20), the average coefficient of variation (c v) between the participant's results was 32% and 44% (non-crimp and woven fabric), while the average c v for individual participants was 8% and 12%, respectively. This indicates statistically significant variations between the measurement systems. Cavity deformation was identified as a major influence, besides fluid pressure/viscosity measurement, textile variations, and data analysis.

The influenc e of the loading rate on the material behaviour of glass fibre reinforced phenolic composites and phenolic resin-impregnated aramid paper (Nomex ® ) honeycomb structures was investigated experimentally. The composite specimens were made of woven fabric plies and loaded in tension and shear. Two types of Nomex ® honeycomb specimens (hexagonal and over-expanded) were loaded in uniaxial compression in all three material directions. Quasi-static test results were compared to dynamic test data obtained on a drop tower, where different strain rates from 10 s -1 to 300 s -1 were tested.The glass/phenolic composite material showed a remarkable strain rate effect at higher loading rates with over 80% increase in tensile strength. Also for the Nomex ® honeycomb an increase of the stress level of up to 30% was observed. These material characteristics should be taken into account in case of dynamic analysis (e.g. crash, impact) of honeycomb sandwich panels for public transport applications, which are usually made from these phenolic materials.

The characterisation of the mechanical behaviour of folded core structures for advanced sandwich composites under flatwise compression load using a virtual testing approach is presented. In this context dynamic compression test simulations with the explicit solvers PAM-CRASH and LS-DYNA are compared to experimental data of two different folded core structures made of aramid paper and carbon fibre-reinforced plastic (CFRP). The focus of the investigations is the constitutive modelling of the cell wall material, the consideration of imperfections and the representation of cell wall buckling, folding or crushing phenomena. The consistency of the numerical results shows that this can be a promising and efficient approach for the determination of the effective mechanical properties and a cell geometry optimisation of folded core structures.

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This paper reports the results of an international benchmark exercise on the measurement of fibre bed compaction behaviour. The aim was to identify aspects of the test method critical to obtain reliable results and to arrive at a recommended test procedure for fibre bed compaction measurements. A glass fibre 2/2 twill weave and a biaxial (±45°) glass fibre non-crimp fabric (NCF) were tested in dry and wet conditions.All participants used the same testing procedure but were allowed to use the testing frame, the fixture and sample geometry of their choice. The results showed a large scatter in the maximum compaction stress between participants at the given target thickness, with coefficients of variation ranging from 38 % to 58 %. Statistical analysis of data indicated that wetting of the specimen significantly affected the scatter in results for the woven fabric, but not for the NCF. This is related to the fibre mobility in the architectures in both fabrics. As isolating the effect of other test parameters on the results was not possible, no statistically significant effect of other test parameters could be proven. The high sensitivity of the recorded compaction pressure near the minimum specimen thickness to changes in specimen thickness suggests that small uncertainties in thickness can result in large variations in the maximum value of the compaction stress.Hence, it is suspected that the thickness measurement technique used may have an effect on the scatter.

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