Can a composite forming limit diagram be constructed?

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Can a composite forming limit diagram be constructed?

Host Institution: University of Cambridge

Lead Investigator: Michael Sutcliffe

Aims

Woven and non crimp fabrics are attractive in a range of composite applications because of their good mechanical performance and handling characteristics. Typically the dry fabric will be pre-formed at low pressure followed by a resin transfer moulding stage. Such a process route lends itself to automation with applications, for example, in the aerospace and automotive industries. However the formation of defects during draping of the fabric is a cause for concern.

Forming models for composite textiles have become reasonably mature. Wrinkling is commonly supposed to occur when the tows ‘lock-up’ as shear in the fabric reaches some critical value. However this over-simplifies the picture, with more sophisticated models and associated experiments highlighting the importance of the in-plane and bending components of the fabric forces in determining wrinkling. More sophisticated material models are needed to capture the drape and wrinkling behaviour of NCFs. While these simulations are encouraging, particularly in terms of the shear deformation modelling, the range of different modelling approaches make it difficult to define the extent of their applicability to different textile architectures and component shapes.

In sheet metal forming, wrinkling and necking tend to limit the range of shapes that can be formed in a single forming operation. Here the 'forming limit diagram' has been recognised as a useful way of characterising the material behaviour, plotting the limits of forming as a function of the local strains in the material; the combination of principal strains in a forming operation needs to lie below a critical upper envelope on the forming limit diagram to ensure a successful process. The concept is based on theoretical concepts of metal deformation, but its real value lies in the way that the forming limit diagram provides a methodology to relate a limited set of forming experiments performed with a given material to forming of complex parts using the same material.

The concept of a forming limit diagram for composites has been explored in the context of material shear deformation and applied to thermoplastic sheet forming. The hypothesis of this feasibility study is that the forming limit diagram concept can be extended to define the limits on forming to avoid defects typically found in composite textiles forming.

The aim of the project is to demonstrate the feasibility of developing a forming limit diagram 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 are to:
1. Use existing measurements of wrinkle formation in woven and NCF fabrics to develop a preliminary forming limit diagram;
2. Extend the range of test configurations to explore the generality of the derived forming limit diagrams;
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;
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.

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