Impulsive sheet metal forming based on standoff charge for conical geometry

Abstract

Recently, explosive forming has gained much attention from researchers to overcome problems of conventional methods in manufacturing complex geometries such as cone. Despite these developments, analytical studies especially on cone with sharp apex angle are rarely reported. Past analytical studies in explosive forming on cone ignored the effects of friction between the blank and the die, redundant work in the work sheet blank and strain rate on blank material behaviour. Likewise, in finite element (FE) method, Arbitrary Lagrangian Eulerian (ALE) approach, most frequently method in the past is very time consuming and costly especially for large number of simulation tests. An alternative to ALE, Coupled Acoustic-Structural Analysis (CASA) approach has been seen gradually applied to model damage on the marine structure subjected to under water explosion but reports on its applications in modelling of explosive forming is somehow very limited. Moreover, in the past reported works, estimation of explosive mass, deformation history and damage accumulation models were analysed independently which creates difficulties to predict all aspects of the blank behaviour simultaneously. An integrated model that addresses these three issues concurrently is however, not available. The main aim of this research is to establish a satisfactory explosive mass estimation equation for modelling cone forming behaviours under integrated conditions with reasonable number of trials, i.e. simulation and experimental. Analytical model based on the impulse method was adopted to estimate the explosive mass by considering the effects of deformation efficiency and strain rate during cone forming process. This was done prior to establishment of FE model. ABAQUS software was used to develop a FE model based on CASA approach. Both models were validated via a series of experimental tests. Three different circular blank materials were tested, i.e. AISI 1006, Cu-ETP and Al 6061-O subjected to C-4 explosive forming under water. Four geometrical parameters were varied in the experiments. They were blank diameter (100 and 110mm), blank thickness (0.8, 1 and 1.2 mm), standoff distance (130, 150 and 170 mm) and half apex angle of cone (45 and 60 degree). Height of deformed cone was measured after each test and these results was used an indicator for the right explosive mass determination. An analytical equation was established by taking into consideration the effects of strain rate, friction and redundant work during forming process. Verification via experimental tests showed that the error of explosive mass required for forming all blank materials into a complete cone is about 20% ± 2.91. The developed FE model was also able to predict concurrently the deformation history, thickness distribution and damage accumulation in a good agreement with experiments. In conclusion, this study provides very encouraging evidences that both impulse method and CASA approach can be used together for predicting material behaviours during explosive forming process.