Double diaphragm forming simulation for complex composite structures

Abstract

A finite element (FE) material model has been developed to simulate the double diaphragm forming (DDF) process, to identify potential defects when forming complex 3D preforms from 2D biaxial non-crimp fabric plies. Three different metrics have been introduced to predict and characterise defects, which include local shear angles to determine ply wrinkling induced by over-shear, compressive strains in the primary fibre directions to determine bundle wrinkling, and tensile stresses in the primary fibre directions to determine fabric bridging . The FE simulation is in good agreement with experiments performed on a demonstrator component. Results indicate that fabric bridging occurs in large-curvature regions, which is the dominant defect in DDF. The axial tensile stress in fibres has been used as a measure to identify suitable positions and orientations for darts, to alleviate fabric bridging and improve surface conformity, whilst minimising the effect on the mechanical performance of the component.

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.…”

“…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.…”

Characterization of intra‐ply shear behaviors of unidirectional prepregs during hot diaphragm forming process

“…The finite element method has been developed to optimize the process parameters without the requirement of expensive trial and error tests. [3][4][5][6] For the hot diaphragm forming simulation, an important challenge is to capture the deformation behavior of prepreg stacks.…”