Very High Resolution Computed Tomography of Damage in Laminated Composites

Synchrotron radiation computed tomography (SRCT) has been used to study notch-tip damage in (90/0)S and (90/+45/-45/0)S carbon fibre-epoxy laminates. By exploiting near-field Fresnel diffraction (edge detection regime), high quality 3D images of cracks and delaminations have been obtained to a resolution of less than 1μm. 3D strain field mapping is facilitated by feature tracking/ image correlation of such data. These measurements are the first of their kind and are not achievable by other techniques. INTRODUCTION Many models have been developed that aim to predict damage growth, strength and life of fibre reinforced polymer composites [1, 2]. A review of the predictive capabilities of a selection of models carried out by Soden et al. [3] shows that the extent of variability between model predictions can be vast. Two likely explanations for the large discrepancy between model outputs are the lack of consistency with regard to damage interaction and growth, and the difficulty in defining an agreed, precise criterion under which failure is considered to have occurred. The work described in this report aims to clarify the failure mechanisms in (90/0)S carbon fibre-epoxy samples through in-situ 3D imaging of the samples in uni-axial tension. Observed damage can be separated into 3 groups: intralaminar matrix cracking, interlaminar delamination and fibre breaks [4, 5]. As the damage accumulates, their interaction and the order in which they initiate can be identified to characterise the response of the composite to applied load. In addition, through the use of image correlation, features embedded in the microstructure can be identified and tracked to determine strain fields through the laminate. Development of predictive models will be supported by the sub-micron resolution of the images, detailing crack propagation, crack displacements of the order of 100nm, matrix failure, regions of delamination and individual fibre breaks leading to ultimate failure. MATERIALS AND METHOD (90/0)S Double edge notched coupons were produced from Hexcel HexPly M21/T700GC carbon fibre-epoxy composite system. The specimens were imaged under two in-situ loading regimes. The first tests observed the propagation and interaction of damage. The second tests involved pre-loaded (pre-damaged) specimens and compared the unloaded and re-loaded damage states to determine the residual stress in damaged specimens. (90/+45/-45/0)S Laminate plates were prepared using Ciba Geigy’s Fibredux 914C/T300 carbon fibre-epoxy composite system. Centre notched coupons were stressed to 250MPa (80% of the average observed failure stress in identical coupons), and sectioned for high-resolution imaging. For each lay up configuration, SRCT imaging took place at the ID19 beamline at the European Synchrotron Radiation Facility (ESRF). Successive 2D X-ray projections of an object are reconstructed to produce a 3D image of the specimen. The attenuation of the synchrotron X-rays through the object during image acquisition is represented as a grey-scale histogram in the reconstructed volume allowing cracks and delaminations to be segmented and separately rendered. RESULTS AND CONCLUSIONS Figure 1 shows a 3D rendering of the segmented regions of damage in each ply and identifies the cracks, delaminations and fibre breaks at the notch tip in the (90/+45/-45/0)S configuration. There are strong correlations between the separate damage mechanisms and their interaction is evident from the images that have been produced. Information obtained using reconstructed SRCT volumes allows the microstructure of polymer composite materials to be characterised on a scale not previously obtainable in nondestructive testing, providing the basis for detailed micro-mechanical material analysis. ACKNOWLEDGEMENTSThe authors would like to acknowledge financial support from EPSRC grant EP/E003427/1and the assistance of Greg Johnson in the use of beamline ID19 at ESRF. REFERENCES1. Barbero, E.J., Abdelal, G.F., and Caceres, A., A micromechanics approach for damagemodeling of polymer matrix composites. Composite Structures, Vol. 67, No. 4, pp 427-436, 2005.2. Maimi, P., Camanho, P.P., Mayugo, J.A., Davila, C.G., A continuum damage model forcomposite laminates: Part I constitutive model. Mechanics of Materials, Vol. 39, No. 10,pp 897-908, 2007. 3. Soden, P.D., Hinton M.J., and Kaddour A.S., A comparison of the predictive capabilitiesof current failure theories for composite laminates. Composites Science and Technology,Vol. 58, No. 7, pp 1225-1254, 1998.4. Spearing, S.M. and Beaumont P.W.R., Fatigue damage mechanics of composite materials.I: Experimental measurement of damage and post-fatigue properties. Composites Scienceand Technology, Vol. 44, No. 2, pp 159-168, 1992.5. Kortschot, M.T. and Beaumont P.W.R., Damage mechanics of composite materials. I:Measurement of damage and strength. Composites Science and Technology, Vol. 39, No.4, pp 289-301, 1990.Fig. 1: SRCT volume showing segmented cracks, delaminations andfibre breaks ahead of the notch.