Showing papers by "Michael R Wisnom published in 2013"

Demonstration of pseudo-ductility in high performance glass-epoxy composites by hybridisation with thin-ply carbon prepreg

Abstract: A new approach and material architecture is presented in order to overcome the inherent brittleness and unstable failure characteristic of conventional high performance composites. The concept is the use of thin-ply hybrid laminates. Fracture mechanics calculations were carried out to determine the critical carbon layer thickness for stable pull-out in a three layer unidirectional hybrid laminate, which can provide a pseudo-ductile failure. Unidirectional hybrid composites were fabricated by sandwiching various numbers of thin carbon prepreg plies between standard thickness glass prepreg plies and tested in tension. Specimens with one and two plies of thin carbon prepreg produced pseudo-ductile failure, whereas ones with three and four plies failed with unstable delamination. An explanation of the different failure modes is given in terms of the different energy release rates for delamination in various specimens. The observed damage characteristics agreed well with the expectations according to the estimated critical carbon layer thickness.

A new mechanism for the formation of ply wrinkles due to shear between plies

Abstract: The formation of out-of-plane ply deformation causes significant reductions in the mechanical properties of composites. In-plane fibre misalignments also cause reductions in the compressive strength, yet the origins of these defects are misunderstood. This paper presents a new mechanism for the formation of wrinkles, which is based upon the shear forces generated as a result of mismatches in the coefficient of thermal expansion of composite and tool, as well as the process of ply slippage that occurs during consolidation into radii. Using a U-shaped tool, defects in composite spars have been characterised using light microscopy, showing that the tool geometry and prepreg bridging leads to “instability sites,” which lead to wrinkles up to 750 μm in height as well as in-plane misalignment of 0° plies of up to 50°. Increasing the frictional shear stress through omission of release film prevents the formation of wrinkles, supporting the mechanism presented in this paper.

Modelling failure of composite specimens with defects under compression loading

Abstract: Composite structures exhibit many different failure mechanisms, but attempts to model composite failure frequently make a priori assumptions about the mechanism by which failure will occur. Wang et al. [1] conducted compressive tests on four configurations of composite specimen manufactured with out-of-plane waviness created by ply-drop defects. There were significantly different failures for each case. Detailed finite element models of these experiments were developed which include competing failure mechanisms. The model predictions correlate well with experimental results – both qualitatively (location of failure and shape of failed specimen) and quantitatively (failure load). The models are used to identify the progression of failure during the compressive tests, determine the critical failure mechanism for each configuration, and investigate the effect of cohesive parameters upon specimen strength. This modelling approach which includes multiple competing failure mechanisms can be applied to predict failure in situations where the failure mechanism is not known in advance.

Defects in woven preforms: Formation mechanisms and the effects of laminate design and layup protocol

Abstract: Defects, such as in-plane waviness and out-of-plane tow wrinkles, cause significant reductions in the mechanical performance of RTM-manufactured composite parts based on woven preforms. To avoid this problem and achieve a greater acceptance rate in industrial processes, the mechanisms behind these defects must be understood. This paper presents a mechanism for the formation of these defects, which is supported through layup trials of woven preforms. Laminate design and layup protocol were found to be significant drivers behind the mechanism. Defect severity can be controlled through intelligent stacking sequence design and reducing ply bridging by manual forming actions and ply–ply adhesion during layup.

Measurement and modelling of interlaminar shear strength enhancement under moderate through-thickness compression

Abstract: Interaction between compressive through-thickness stress and out-of-plane shear can enhance the shear stress at which delaminations initiate in composite materials and components. In complex loading situations this may become significant, either resulting in an under prediction of strength if it is not taken into account or even a failure to predict the correct failure mode. This effect is investigated experimentally, analytically and numerically in this work. A symmetric version of the double-notch shear test is proposed for direct measurement of the interlaminar shear strength under moderate through-thickness compression. The test has successfully measured shear strength enhancement under increasing through-thickness compression. A range of failure criteria presented in the literature has been compared to the results obtained. Finally the tests have been modelled using finite element analysis. User defined interface elements which take account of the effect of compression enhancement allow the experimental results to be accurately predicted.

A new semi-empirical law for variable stress-ratio and mixed-mode fatigue delamination growth

Abstract: A new semi-empirical equation that describes the fatigue delamination growth in fibre reinforced toughened epoxies is presented and validated against data available in the literature. The new law accounts for the simultaneous effects of the stress-ratio and mode-mixity on the interlaminar crack propagation. If delamination propagation thresholds are ignored, the proposed semi-empirical equation allows describing interlaminar crack propagation employing only three material dependent parameters, whereas alternative models presented in the literature require four. If reliable threshold data are available from experimental tests, the new semi-empirical law can be extended to a unified description of stress-ratio, mode-mixity and thresholds effects using six material dependent parameters.

Investigation into failure of laminated composite T-piece specimens under bending loading

Abstract: This paper presents an experimental and numerical investigation into failure of T-shaped laminated composite structures. Three out-of-plane bending cases with loads at angles of 0°, 45° and 90° are studied. It is found that very high free-edge maximum principal transverse tensile stresses perpendicular to the fibre direction occur at the failure locations. The use of the “High Stress Concentration method” demonstrates that these are responsible for the specimens’ failure. It is also demonstrated that this tool is very useful for stress engineers who mainly rely on linear elastic analyses to design complex laminated composite components.

Impact damage detection in composite plates using deflectometry and the Virtual Fields Method

Abstract: This paper presents a new method for detecting damage in layered composite materials using a simple measurement technique, deflectometry. The aim is to locate the damage in a specimen and provide a “signature” from the measured surface slopes. Firstly, this method is applied to plate specimens using numerical and experimental data. Secondly, damage indicator based on a new application of the virtual fields method to compute local apparent gaps in equilibrium is presented and used to process the measurements on plates. It is shown that the damage indicator is very sensitive, allowing detection of damage that is difficult to identify directly from the strain maps.

A finite element based statistical model for progressive tensile fibre failure in composite laminates

Abstract: This paper presents a two-parameter integrated Weibull model that is implemented in a finite element analysis to capture the well known “size effect” in composite materials. It can thus account for the effect of randomly distributed defects on fibre dominated tensile failure in a general loading situation. Here it is combined with cohesive interface elements to include the interaction with delamination. As a verification of the modelling method, scaled four-point bend tests from previous research were simulated. The models gave excellent results for both the strains to failure and the final failure modes and captured the observed size effect which cannot be represented by conventional progressive damage models.

Failure mechanisms under compression loading in composites with designed out-of-plane fibre waviness

Abstract: Compressive strength reduction due to fibre waviness is a key concern in composite structures. Specimens with fibre misalignments representative of those that can occur at ply drops and resin rich areas in components were manufactured by incorporating either two or 10 discontinuous plies oriented either in 0° (‘longitudinal’) or 90° (‘transverse’). Longitudinal specimens did not fail in compression but failed by delamination initiating at the ply drops. Transverse specimens did not fail directly due to the waviness: specimens with 8° waviness failed at similar stresses and strains as fully aligned specimens, while 30° waviness specimens failed in the aligned plies at a slightly reduced overall stress due to redistribution of load from the less stiff wavy plies. None of the specimens failed directly in compression due to misalignment in the wavy plies, indicating that this is not necessarily the critical failure mechanism, and highlighting the importance of delamination and load redistribution at p...