Quasi-brittle fracture during structural impact of AA7075-T651 aluminium plates

The stress–strain behaviour of the aluminium alloy 7075 in T651 temper is characterized by tension and compression tests. The material was delivered as rolled plates of thickness 20 mm. Quasi-static tension tests are carried out in three in-plane directions to characterize the plastic anisotropy of the material, while the quasi-static compression tests are done in the through-thickness direction. Dynamic tensile tests are performed in a split Hopkinson tension bar to evaluate the strain-rate sensitivity of the material. Notched tensile tests are conducted to study the influence of stress triaxiality on the ductility of the material. Based on the material tests, a thermoelastic–thermoviscoplastic constitutive model and a ductile fracture criterion are determined for AA7075-T651. Plate impact tests using 20 mm diameter, 197 g mass hardened steel projectiles with blunt and ogival nose shapes are carried out in a compressed gas-gun to reveal the alloy's resistance to ballistic impact, and both the ballistic limit velocities and the initial versus residual velocity curves are obtained. It is found that the alloy is rather brittle during impact, and severe fragmentation and delamination of the target in the impact zone are detected. All impact tests are analysed using the explicit solver of the non-linear finite element code LS-DYNA. Simulations are run with both axisymmetric and solid elements. The failure modes are seen to be reasonably well captured in the simulations, while some deviations occur between the numerical and experimental ballistic limit velocities. The latter is ascribed to the observed fragmentation and delamination of the target which are difficult to model accurately in the finite element simulations.     

Normal impact of truncated oval-nosed projectiles on stiffened plates

The objective of this paper is to investigate the perforation capability of projectiles against stiffened plates and to determine how many stiffened plates can be perforated by projectiles. Some important experimental results on the perforation of stiffened plates, of a variety of configurations, by truncated oval-nosed projectiles at normal impact are introduced. A four-stage analytical model is formulated for the dynamic perforation of stiffened plates by rigid projectiles. By adopting an energy method, the model can be used to predict accurately the residual velocity of the projectiles. Numerical simulations have been performed for projectiles against single and layered plates adopted in the experiments. The perforation process is explored and deformation and failure modes are obtained. Good agreement is obtained between the numerical simulations, theoretical predictions and experimental results.     

Mechanical and microstructural investigation of friction spot welded AA6181-T4 aluminium alloy

Friction spot welding (FSpW) is a solid state welding process suitable for spot joining lightweight low melting point materials like aluminium and magnesium alloys. The process is performed by plunging a rotating three-piece tool (clamping ring, sleeve and pin) that creates a connection between sheets in overlap configuration by means of frictional heat and mechanical work. The result is a spot welded lap connection with minimal material loss and a flat surface with no keyhole. FSpW has been performed in a 1.7 mm-thick AA6181-T4 aluminium alloy using different welding parameters (rotation speed and joining time) aiming to produce high quality connections in terms of microstructure and mechanical performance. Microstructural features of the FSpW connections were analysed by optical microscopy; while mechanical performance was investigated in terms of hardness and tensile testing. Connections with shear strength close to 7 kN were obtained with high reproducibility. The results also showed that geometric features of the connection play an important role on the fracture mechanism and hence on the mechanical performance of the connections.