Inter-ply stitching optimisation of highly drapeable multi-ply preforms

Abstract

An efficient finite element model has been developed in Abaqus/Explicit to solve highly nonlinear fabric forming problems, using a non-orthogonal constitutive relation and membrane elements to model bi-axial fabrics. 1D cable-spring elements have been defined to model localised inter-ply stitch-bonds, introduced to facilitate automated handling of multi-ply preforms. Forming simulation results indicate that stitch placement cannot be optimised intuitively to avoid forming defects. A genetic algorithm has been developed to optimise the stitch pattern, minimising shear deformation in multi-ply stitched preforms. The quality of the shear angle distribution has been assessed using a maximum value criterion (MAXVC) and a Weibull distribution quantile criterion (WBLQC). Both criteria are suitable for local stitch optimisation, producing acceptable solutions towards the global optimum. The convergence rate is higher for MAXVC, while WBLQC is more effective for finding a solution closer to the global optimum. The derived solutions show that optimised patterns of through-thickness stitches can improve the formability of multi-ply preforms compared with an unstitched reference case, as strain re-distribution homogenises the shear angles in each ply.

Defect formation during preforming of a bi-axial non-crimp fabric with a pillar stitch pattern

“…There is a positive correlation between the shear deformation and the wrinkling strain, indicating the likelihood of wrinkles. This has been previously observed by the authors during fabric forming optimisation studies [3,15,17].…”

“…Predicted local shear angles for a woven material deviated by less than 3 % from experimental data [3], indicating high levels of accuracy within the repeatability of the experimental test.…”

“…A non-orthogonal constitutive model was extended here to define the asymmetric material behaviour of a bi-axial NCF, which was previously developed for woven fabrics [3,[15][16][17]. A user subroutine was previously developed by the authors in Abaqus/Explicit, defining the mechanical constitutive relations for woven fabrics, using a non-orthogonal relationship [3].…”

“…A user subroutine was previously developed by the authors in Abaqus/Explicit, defining the mechanical constitutive relations for woven fabrics, using a non-orthogonal relationship [3].…”

“…The stitch pattern can be adapted to either prevent unwanted shear deformation during handling through improved stability, or improve drapeability through relaxation of inplane constraints. In addition, multiple plies can be bonded together using local supplementary stitches to aid automated forming of complex lay-ups [3,4].…”