Andrew Rhead

Compressive strength of delaminated aerospace composites.

An efficient analytical model is described which predicts the value of compressive strain below which buckle-driven propagation of delaminations in aerospace composites will not occur. An extension of this efficient strip model which accounts for propagation transverse to the direction of applied compression is derived. In order to provide validation for the strip model a number of laminates were artificially delaminated producing a range of thin anisotropic sub-laminates made up of 0°, ±45° and 90° plies that displayed varied buckling and delamination propagation phenomena. These laminates were subsequently subject to experimental compression testing and nonlinear finite element analysis (FEA) using cohesive elements. Comparison of strip model results with those from experiments indicates that the model can conservatively predict the strain at which propagation occurs to within 10 per cent of experimental values provided (i) the thin-film assumption made in the modelling methodology holds and (ii) full elastic coupling effects do not play a significant role in the post-buckling of the sub-laminate. With such provision, the model was more accurate and produced fewer non-conservative results than FEA. The accuracy and efficiency of the model make it well suited to application in optimum ply-stacking algorithms to maximize laminate strength.

Graphene oxide as a template for a complex functional oxide

We report the first use of graphene oxide (GO) as a sacrificial template for the structural direction of complex oxides. The superconductor yttrium barium copper oxide (YBCO) was used as a quarternary oxide test system, with the GO templates being used to create foams and layered paper-like structures which retained the superconducting properties of YBCO.

The influence of surface ply fibre angle on the compressive strength of composite laminates containing delamination

A combination of uniaxial compression tests and Strip Model and Finite Element analyses of laminates artificially delaminated to create circular [±θ] sublaminates is used to assess the influence of fibre angle on the compressive strength of composite laminates. Sublaminates with 0° < θ < 40° are found to fail by sublaminate-buckling-driven delamination propagation and provide poor tolerance of delamination. This is a consequence of their relatively high axial stiffnesses, low sublaminate buckling strains, Poisson’s ratio induced compressive transverse strains and extension-twist coupling which produces unexpected sublaminate buckling mode shapes. Sublaminates with 40° < θ < 60° are most tolerant to delamination; axial and transverse stiffnesses are minimal, formation of sublaminate buckles is resisted, high laminate buckling strains reduce interaction between laminate and sublaminate buckling mode shapes and extension-twist coupling is minimal. Sublaminates with 60° < θ < 90° are shown to produce varied tolerance of delamination. Sublaminate buckling is generally prevented owing to transverse tensile strains induced by mismatches between laminate and sublaminate Poisson’s ratios but may occur in laminates with low Poisson’s ratios.

Compressive Strength Following Delamination Induced Interaction of Panel and Sublaminate Buckling

A number of panels have been artificially delaminated to produce a range of thin anisotropic sub-laminates. Panels were subject to experimental compression testing in which sublaminate and global panel buckling modes were allowed to interact. A comparison of experimental results with previous results where panel buckling was restrained, demonstrates that interaction of buckling modes causes significant reductions in both panel buckling and delamination propagation strains. This indicates that the current industry practice of using separate tests to define allowable strains for panel buckling and damage tolerance may be unconservative. A combination of a Shanley model with a computationally efficient analytical Strip model, conservatively predicts values of compressive strain below which propagation of delaminations will not occur.

8.7 Optimum Design and Damage Tolerance of Compressively Loaded Laminates

In this chapter we will describe the different kinds of elastic coupling effects that can arise in anisotropic composite laminates before developing expressions that are used to determine laminate buckling loads. We then illustrate the optimization (or elastic tailoring) of laminates using straight fibers, followed by a presentation of the potential advantage of in-plane steering of fibers to achieve elastically tailored tows. Finally, we use an analytical model to examine the damage tolerance of a variety of straight fiber stacking sequences utilizing hybrid materials, highlighting improvements in compressive strength that can be made when plies are stacked in sequences that maximize damage tolerance.

Energy harvesting from coupled bending-twisting oscillations in carbon-fibre reinforced polymer laminates

Abstract The energy harvesting capability of resonant harvesting structures, such as piezoelectric cantilever beams, can be improved by utilizing coupled oscillations that generate favourable strain mode distributions. In this work, we present the first demonstration of the use of a laminated carbon fibre reinforced polymer to create cantilever beams that undergo coupled bending-twisting oscillations for energy harvesting applications. Piezoelectric layers that operate in bending and shear mode are attached to the bend-twist coupled beam surface at locations of maximum bending and torsional strains in the first mode of vibration to fully exploit the strain distribution along the beam. Modelling of this new bend-twist harvesting system is presented, which compares favourably with experimental results. It is demonstrated that the variety of bend and torsional modes of the harvesters can be utilized to create a harvester that operates over a wider range of frequencies and such multi-modal device architectures provides a unique approach to tune the frequency response of resonant harvesting systems.

IMaging and Probabilistic Assessment of Composite damage Threats (IMPACT)

Data store for all ultrasonic C-scan, X-ray Computed Tomography and impact test data created during the impact project. Data is organised into sets of data for each coupon tested. Coupon descriptions given in the table below will include stacking sequence, material, ply-thickness, coupon thickness and impact energy. Please cite this data set as: Rhead, A. and Pernice, F., 2016. IMaging and Probabilistic Assessment of Composite damage Threats (IMPACT). University of Bath. https://doi.org/10.15125/BATH-00195