Research

The UTC conducts research in a number of areas relating to composites. These include projects include looking at through-thickness reinforcement, defects and features of composites, vibration and fatigue, novel structures and materials, woven textiles and composites manufacturing.

A full list of current research themes can be found here: Current UTC projects May18 (PDF, 62kB).

Case studies

Z-pin Reinforced Laminates

Z-pinning is an effective through-thickness reinforcement technology, whereby small diameter rods (Z pins) are inserted through the thickness of composite laminates to improve the interlaminar strength and toughness. A three dimensional micro-mechanical FE modelling tool has been established and validated by experimental tests for predicting the mixed-mode response of single Z-pins. The modelling approach is based upon a versatile ply-level mesh, which takes into account the significant micro-mechanical features of Z-pinned laminates. The effect of post-cure cool down is also considered in the approach. The Z-pin/laminate interface is modelled by cohesive elements and frictional contact. The progressive failure of the Z-pin is simulated considering shear-driven internal splitting, accounted for using cohesive elements, and tensile fibre failure, modelled using a Weibull statistical criterion. The modelling methodology is suitable for optimising the Z-pin configuration in order to maximise the apparent toughness of through-thickness reinforced laminates.

For more information see the full article Micro-mechanical finite element analysis of Z-pins under mixed-mode loading.

Failure investigation in laminates containing embedded wrinkle defects

This study has investigated the influence of embedded wrinkle defects to cause early failure in laminates loaded under quasi-static tension/compression and tension-tension fatigue. Laboratory specimens with artificially induced wrinkles of controlled wave severity were manufactured and tested to failure, with a focus on understanding the failure mechanisms and their interaction in the defect region, and also the influence of the defect severity. The test findings were then used to build detailed finite element models which accurately captured the defect geometry and failure modes. Intra-laminar failure was captured using continuum damage and discrete crack modelling techniques, while inter-ply failure was simulated using traditional cohesive zone models. A novel crack-tip tracking based formulation was used to simulate delamination fatigue. The models correlated very well with tests, and thus can be applied to give reliable failure predictions under a variety of manufacturing induced defect scenarios.

For more information see the full article Compressive failure of laminates containing an embedded wrinkle; experimental and numerical study.

Integrated modelling of woven materials

Woven composites is a promising technology that offers an economical and mechanically efficient alternative to unidirectional composites. These composites are characterized by complex internal yarn architectures, leading to complex deformation and failure development mechanisms. The internal yarn architecture, which dominates the mechanical behaviour, is dependent on the final structural geometry and the manufacturing process. An integrated multi-scale modelling framework has been developed to predict the mechanical performance of woven composites taking into account the internal architecture. This framework is comprised of a set of integrated modelling approaches which simulate the complete manufacturing process of 3D woven composites. Using multi-scale kinematic models, the weaving, draping and compaction manufacturing phases are simulated. Next, the mechanical performance of these structures is calculated taking into account the defects and deformations introduced to the internal yarn architecture during the manufacturing phase. In the mechanical modelling phase, a combination of homogenization and global/local analysis techniques are used to build accurate and computationally efficient models of woven composites.

For more information see the presentation Integrated modelling of woven materials (PDF, 1,922kB).

Severely tapered laminates

A highly complex specimen, representative of a typical composite dove-tail joint, has been designed, manufactured and tested. This has been both an exercise to understand the nature of the failure mechanisms and behaviour and to validate finite element models. The specimen, produced from unidirectional carbon fibre/epoxy pre-preg, reduces down from 147 plies at the thick end to 45 plies at the thin end. The manufacture of the specimens was in itself a complex task, with great care being taken to ensure the accuracy of the terminated ply placement and specimen symmetry. The finite element model used was a ply-by-ply representation of the layup with cohesive interface elements between each ply to be able to capture the delamination failure mode. Results show the close working of numerical modelling and experimental testing to better understand and predict the failure of a complex specimen which in turn will improve future component design.

Find out more (PDF, 3,056kB)

Surface Cut Ply specimen

The Surface Cut Ply specimen is a convenient test coupon for the assessment of mixed-mode fracture in composite materials. It has been designed, manufactured, tested and verified by FE Analysis for measuring the mixed-mode Gc, the critical mixed-mode Energy Release Rate (ERR), for laminated composites. The desired mode mixity can be achieved through selection of the continuous to cut ply ratio; current work has concentrated on the 5/5 configuration which is a 10 plies thick specimen consisting of 5 cut and 5 continuous plies. The specimen is loaded in the 4 Point Bending configuration and a new analytical solution has been developed to compute the mixed-mode ERR from the specimen geometry and the recorded load - deflection test data. The simplicity of the test and that the need to measure the crack length is avoided are key advantages over other mixed-mode tests.

Find out more (PDF, 748kB)

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