Optimisation of local in-plane constraining forces in double diaphragm forming

Abstract

Rigid blocks (risers) were introduced in the double diaphragm forming (DDF) process to change the local in-plane strain distribution in the diaphragms, aimed at reducing wrinkling defects in the production of fabric preforms. A two-step optimisation method was developed to determine the position and dimension of each riser. In Step I, optimisation of the riser position was conducted using a simplified finite element (FE) model coupled with a genetic algorithm (GA). The height of each riser was optimised in Step II using a detailed FE model with the optimised riser positions from Step I. For demonstration, a hemisphere preform was manufactured by DDF using the optimum riser arrangement established by the optimisation routine. Results indicate that the optimum riser pattern (shape and position relative to the component boundary) can dramatically improve the preform quality through reduction of out-of-plane wrinkles, validating the feasibility of the two-step routine.

Simulation of the forming process for curved composite sandwich panels

“…The predicted shear angles in the reinforcement of the sandwich skins (Fig. 15b) are in the range from −30°to 12°, which does not exceed the critical shear angles for onset of wrinkling (42°in positive shear and − 50°in negative shear [23,25]). Hence, simulations indicate that no out-of-plane wrinkling occurs in either of the skins after forming.…”

“…2) as a consequence of the pillar stitch pattern. This has previously been modelled by the authors using a homogenised non-orthogonal constitutive model [21][22][23][24][25], which exhibited sufficient fidelity for fabric forming process modelling and optimisation.…”

“…Friction coefficients were calculated from the ratio of the tangential (pulling) force and the applied normal force (10 N, corresponding to a normal pressure of 2 kPa) during relative movement at a constant velocity (100 mm/min). The average value was calculated from five repeat tests for each surface pairing [25]. The friction behaviour does not depend on which face of the NCF is in contact with the honeycomb core, as the pillar stitch pattern is identical on both faces of the NCF.…”

“…While this assumption may affect the precision of the simulation results, it greatly reduces complexity since it enables the in-built isotropic Coulomb friction model to be used in Abaqus/Explicit. Friction coefficients were previously obtained by the authors for other surface pairings, including 0.23 for tool-fabric contact and 0.36 for tool-core contact [23][24][25].…”

“…High-fidelity predictive modelling of reinforcement forming (draping) is well advanced. Typically, woven and non-crimp fabrics can be modelled using a non-orthogonal constitutive model, incorporating multiple plies and frictional effects [21][22][23][24][25][26][27][28][29][30][31][32]. The multi-axial deformation behaviour of the core is complex, requiring a material model that reflects the forming characteristics arising from the distortion of the cellular structure and through-thickness crushing of the core, resulting from inelastic buckling and folding of the cell walls.…”