In the wind turbine and aerospace sectors recent innovations, including larger and more sophisticated structures, have driven the need for better understanding of failure of composite structures containing waviness defects. Real manufactured components contain a range of stress concentrators. The aim of the project is to understand and model how such defects affect the strength of the structure. The project has three main strands: (i) characterising realistic defects in industrial components and in controlled laboratory specimens, (ii) identifying mechanisms of compressive failure under fatigue loading and developing predictive models for failure at waviness defects, validated with experiments, (iii) modelling of defect formation during processing. The project is co-funded by EPSRC and DSTL and is in collaboration with the University of Bristol and industrial partners. [Photo acknowledgement: Mandell et al, 2003]

This project has explored the micromechanics of a microbuckling in composites in collaboration with Norman Fleck, other colleagues in Cambridge, and various industrial partners, with funding from DSTL and EPSRC. Experiments have explored the initiation and propagation of microbuckles in un-notched and notched components, with corresponding modelling work. An image analysis algorithm has been developed to quantify fibre waviness orientation and this has been applied to various specimens. The resulting measurements have been used to give statistical predictions of compressive strength. Current work looks at the generation of microbuckling during textile forming processes and the influence of friction on that.

The project considers scientific modelling of the spun-bonded process to produce non-woven material. The emphasis of the project is on exploring development of non-uniformity and mechanical properties of the web, relating them to process conditions and micromechanical models. This project, which is funded by Fitesa, involves a mixture of experimental and modelling work.

This project explored impact of two 3D woven CFRP composites, a layer-to-layer and an orthogonal weave, and 2D braided material. A range of experimental techniques were used to characterise failure, including c-scanning and vibration modal analysis. The effect of glass content in the 2D braided material was also examined. The project was undertaken by Guiseppe Zumpano and Carlos Monroy Aceves with EPSRC funding and in collaboration with the Universities of Nottingham and Cranfield and various other academic and industrial partners .

Although composite materials have become well-accepted in many applications, the widespread take-up and efficient use of these materials is limited by the problems associated with design and manufacture of composite components. One of the most important considerations facing a designer is understanding how the fabric behaves as it is draped over a mould. As the fabric is draped, it can undergo large in-plane deformations, changing the fibre orientation angles and the lamina thickness. Experimental measurements of the deformation of the tows in a five-harness satin weave fabric were made to understand this process. A novel 'intermediate' drape model was developed to model in a relatively robust and rapid way the drape deformation, validated by a range of benchmark characterisation tests. This model was used in a process optimisation procedure and applied to a case study of a composite helmet. The project was undertaken with EPSRC funding and collaboration with the University of Nottingham and industrial companies by Shrikant Sharma and Alex Skordos. Ongoing work with Simulayt explores development of benchmark tests.

Failure of sandwich honeycomb structures under indentation loading was considered. A failure criterion for Nomex honeycombs subjected to combined compressive and shear stresses was determined using biaxial tests. By combining this with a theoretical calculation of the stress distribution in the core due to indentation loading, found from a high-order sandwich beam theory (HOSBT), the indentation failure load of the sandwich beam due to core failure can be predicted. Short beam three-point bending tests are used to validate the theoretical predictions, using beams made with GFRP skins and Nomex cores with densities between 29 and 128 kg/m^3. Theoretical predictions of indentation failure load were in excellent agreement with measured values. Inclusion of shear stresses in the failure criterion significantly improved the predictions, correctly modelling the observed stronger behaviour of cores with a longitudinal ribbon direction. The project was undertaken as a PhD project by Achilles Petras.

The aim of the project was to develop advanced process models and integrate these within a design/optimisation framework. Materials analysed included a 4×4 twill weave carbon/epoxy prepreg, a 5-harness satin weave carbon/epoxy prepreg and a 2×2 twill weave glass/polypropylene commingled fabric in dry form. Advanced draping models were developed including wrinkling and variability and these were incorporated into a process optimisation procedure. Results were validated by the University of Nottingham with practical forming experiments. A design selection methodology was developed to assist designers with the selection of a shortlist of composite structures designs from a large number of alternatives, taking into account conflicting design objectives or constraints (e.g. weight, performance and cost). The procedure is based on the generation of a database containing results from an exhaustive search of a wide range of possible solutions in the design space The project was undertaken with EPSRC funding and collaboration with the University of Nottingham and industrial companies by Carlos Monroy Aceves and Alex Skordos.