Can a composite forming limit diagram be constructed?

The feasibility study has demonstrated that an experimental set-up using digital image correlation is able to provide data correlating fabric deformation with wrinkling. The strain measurements can be manipulated to find strains in critical directions, for example along the tows or in the direction of maximum compressive strain. For the NCF fabric considered, there does not appear to be a simple correlation between the observed strains and the onset of wrinkling. While the experimental work provides the tools to explore wrinkle development, meso-scale architecture-based FE modelling will be needed to guide a wrinkling criterion which can be used in conjunction with these measurements. A process-specific forming limit diagram, showing the range of shear strains and blank holder forces leading to wrinkling, has been drawn up from the measurements. In conjunction with simplified FE modelling, this could be applied in principle to different forming, using FE modelling to predict the corresponding variations in shear strain in these processes.In summary the proposed hybrid approach, of using experimental characterisation in conjunction with a simple FE model, shows considerable promise as a way of defining the forming limits for composite fabrics. Further work is needed, particularly on extending the range of deformation processes and understanding better the link between changes in tow architecture and wrinkling.

Aim and Objectives

The aim of the project was to demonstrate the feasibility of developing a forming limit diagram (FLD) for textile composites, capturing the limits imposed by defects such as macro-wrinkling, tow-level buckling and yarn sliding. The key objectives of the project were to:

1. Use existing measurements of wrinkle formation in woven and NCF fabrics to develop a preliminary forming limit diagram;

The project has successfully developed measurement techniques in order to allow development of a preliminary forming limit diagram. Difficulties in identifying appropriate failure criteria highlight the need for a better micromechanical model of wrinkling to inform the forming limit diagram development. A process-specific forming limit diagram has been produced, which can form the basis for a proposed hybrid experimental and modelling approach to FLD development.

2. Extend the range of test configurations to explore the generality of the derived forming limit diagrams;

Only preliminary work has been done in this area, due to the challenges of developing the forming limit diagram. However the FE model has successfully been applied to other test configurations, albeit without validation.

3. Examine the feasibility of using a range of canonical finite element calculations to interpolate and extrapolate the forming limit diagram from a limited set of tests;

This objective has not been met, with the focus of research remaining on the first objective

4. Use the results to inform a full-scale proposal which will develop the concept of forming limit diagrams to include a wider range of materials and forming situations.

The feasibility study has successfully identified an experimental route to forming limit diagram measurements, highlighting deficiencies in our understanding of wrinkling which need to be tackled to develop the concept further. Hence this key objective, of informing a full-scale proposal, has been met.

Research Proposal Plan

The aim is to produce an EPSRC responsive-mode proposal, in conjunction with colleagues at the University of Nottingham and industrial partners, to take forward the forming limit diagram approach. Rather than focus on the FLD, it is planned instead to shape a project which also builds on other EPSRC-funded work done by Cambridge and Nottingham including friction measurement and modelling. Consideration of the FLD will only be one work package within the programme, so that it is not appropriate to bid for a core proposal directly following on from the feasibility study. It is envisaged that the proposal will request funding for two 3-year post-doctoral researchers. In the meantime the existing links between Cambridge and CIMComp will be manifested via collaboration of the current PhD student working on the FLD approach. This will include a proposed package of experimentation at Nottingham.