Double diaphragm forming (DDF) is an important preforming route for multi-ply dry textile reinforcements. Still, the coupled effects of interface friction and tool geometry on inter-ply sliding and wrinkling remain insufficiently quantified. This paper presents a numerical study of DDF for multi-ply non-crimp fabrics, with emphasis on the relationship between interface friction, tool geometry and defect formation. An Abaqus/Explicit finite element framework is used to simulate the DDF process, and dedicated post-processing procedures are applied to quantify inter-ply sliding and out-of-plane wrinkling. The coefficients of friction at the main DDF interfaces are varied systematically, with particular focus on the fabric–fabric and fabric–diaphragm interfaces. Representative double dome and spar-like tool geometries are then used to examine how geometry modifies the friction response and whether the dominant mechanism remains transferable across different forming conditions. The results show that fabric–fabric friction is the dominant parameter governing the global sliding–wrinkling trade-off. Increasing fabric–fabric friction suppresses inter-ply sliding but promotes wrinkling by restricting a key deformation-accommodation mechanism within the laminate. Fabric–diaphragm friction has a weaker and more geometry-dependent influence, while diaphragm–diaphragm and diaphragm-tool friction have limited effects on the global volume-based response metrics within the investigated range. Tool geometry strongly affects the magnitude and spatial localisation of both sliding and wrinkling, although the primary fabric–fabric friction mechanism remains consistent across spar-like variants. These findings indicate that inter-ply friction should be treated as a process-design variable in DDF, and that friction selection and tool-feature optimisation should be considered together when seeking to control wrinkling and relative ply motion in multi-ply NCF preforming.
This study presents a novel experimental methodology for measuring nominal mode III fracture resistance curves and cohesive laws for a unidirectional glass/epoxy composite. The proposed method adapts a double cantilever beam (DCB) configuration to apply out-of-plane moments, thereby generating nominal mode III loading. The experiments ran stably for all specimens, and mode III cohesive laws were obtained using a J-integral approach. Experimental characterisation of mode III delamination remains limited, particularly compared with nominal modes I and II. For this reason, it is common to assume that mode II and mode III cohesive laws are the same; however, this work shows that this assumption is erroneous for the delamination of unidirectional composites. Fractographic images and acoustic emission measurements support this finding. In the present study, the R-curves and cohesive laws of all three nominal fracture modes are compared for the same material interface for the first time.
