Energy Absorption of Monolithic and Fibre Reinforced Aluminium Cylinders

Summary accompanying the thesis: Energy Absorption of Monolithic and Fibre Reinforced Aluminium Cylinders by Jens de Kanter This thesis presents the investigation of the crush behaviour of both monolithic aluminium cylinders and externally fibre reinforced aluminium cylinders. The research is based on analytical work, numerical work and experimental work, which together show a complete picture of the tube axial crush phenomenon. The research is relevant for transport safety. In specific for the automotive industry, however with a clear link to the aerospace industry, based on the materials used and current developments in aerospace crashworthiness. Materials used in the investigation are aluminium and fibre composite, both materials extensively used in the aerospace industry. Also the aerospace safety focus is widening nowadays. Where in the past only crash prevention was considered now also crash protection is of concern. Transport safety and also sustainability benefit from reduced mass of the vehicles. In case of crash structures, the mass can be reduced by having components with a higher specific energy absorption (energy absorbed per unit mass, SEA) . Further quantitative requirements for energy absorbing tubes are the load ratio, i.e. the peak load divided by the mean load and the stroke efficiency, i.e. the axial displacement divided by the original tube length. Qualitative requirements for the tubes are for example reliability, predictability and post-crash integrity. In search of improved crash tube performance this research followed a trajectory where aluminium tubes are reinforced by fibre composite. In order to gain full insight in the crush phenomena, first the monolithic aluminium tube crush behaviour is described. The aluminium crush behaviour is governed by a number of collapse modes, which depend on the diameter to thickness ratio of the tubes. Basically two collapse modes exist, the axisymmetric or concertina collapse mode and the diamond mode. The diamond collapse mode is characterised by the number of diamond lobes present. For tubes with diameter over thickness ratios in the range of 10 to 100 the number of diamond lobes may vary from two to four. Most of the tubes tested were made of aluminium alloy 6060-T66, with a diameter of 50 mm and thickness varying from 0.5 mm to 2.5 mm. Within this thickness range the folding takes place in the plastic regime of the material. The experimentally established folding behaviour is well reproduced by analyses with a number of different finite element codes. Abaqus implicit was used for the static simulation and both Abaqus explicit and PAM-CRASH were used for the dynamic situation. The dynamic tube crushing is characterised by a dynamic buckling phenomenon and a small increase of the loads compared to the static situation. The load increase is attributed to the strain rate sensitivity of the metal and due to inertia (of the laterally moving fold elements) . Also analytical models are presented for the metal tube crushing. These give good predictions of the mean loads and show the balance in the energy absorption between membrane stretching and fold bending. Shortcoming of the models is the validity over different collapse modes. For improving the SEA, the aluminium cylinders were reinforced by externally winding impregnated S2 glass fibres. A few tubes were reinforced by carbon fibres and some were reinforced with prepreg in the tube longitudinal direction as well. The fibre winding orientation and lay-up were varied in the experimental program. In the production process some minor variables, as used adhesive and fibre winding tension were examined. The adhesive was used between the aluminium tube and the fibre windings and had a beneficial effect on the energy absorption. The hoop winding reinforcements un-balance the stiffness distribution in the monolithic tube, giving high restoring membrane stresses in circumferential direction, which increase the buckling load and change the folding pattern from a concertina mode to diamond 3 lobe and 2 lobe. The relative increase in SEA is largest (about 20% ) for the single layer reinforcement, with a concertina collapse mode. The thick reinforcements ( 6 or more layers) caused 2 lobe folding, which has a large fold length and a relatively low SEA, making this configuration less efficient. By changing the orientation of the fibres different collapse modes may be initiated. The experimental program includes the hoop wound specimen (90) , hoop wound specimen with UD fibres in longitudinal tube direction (0 degrees/90 degrees) , helix wound specimen (plusminus 0 degrees) , and helix hoop wound specimen (plusminus 0 degrees /90 degrees) . The character of the collapse mode varied from global two lobe folding to small three and four lobe folding. The smaller the folds the higher the mean load and energy absorption. The balanced lay-up of the helix hoop wound specimen (plusminus 45 degrees/90 degrees ) proved best in energy absorbing performance. The SEA of the monolithic metal tube was increased by 65% , while the load ratio reduced from 2.0 to 1.5. The post crash integrity was diminished however as also cracks were present in the metal.