Polymer matrix composites are currently used in many structural applications that require a significant reduction in weight for energy and/or environmental reasons. However, despite all existing information and actual knowledge about these materials, their complex mechanical behaviour (highly non-linear, anisotropic and with different and novel failure mechanisms not found in traditional structural materials) requires greater research efforts to optimize their performance and take advantage of their full potential [1-2].

These efforts pass through the understanding of the effect of manufacturing defects in the final mechanical properties and through the study of their failure mechanisms. IMDEA Materials contributions in these directions are based in the application of state-of-the-art, non-destructive, three-dimensional characterization methodologies, such as X-ray computed nanotomography. This technique is based on the computer-assisted reconstruction of three-dimensional bodies based on X-ray radiographies taken from various viewing angles. The development of new X-ray generation and detection techniques allow nowadays to achieve sub-micrometer resolutions, making this technique a valuable tool for studying the complex microstructure of advanced materials as well as the dominant fracture mechanisms. Moreover, three-dimensional images can be used to perform detailed quantitative analysis of microstructural features or used as input to model which take into account the actual material microstructure to predict the macroscopic mechanical behaviour.

[1] E. Totry, C. González, J. LLorca. Composites Science and Technology 68, 3128-3136, 2008.
[2] E. Totry, C. González, J. LLorca, J. Molina-Aldareguía. International Journal of Fracture 158, 197-209, 2009.
[3] A. Enfedaque, J. M. Molina-Aldareguía, F. Gálvez, C. González, J. LLorca. Journal of Composite Materials, 44, 2010. In press.

 

Figure 1. X-ray tomography analysis of the microstructural defects in a cross-ply glass-fiber epoxy-matrix composite manufactured in autoclave. Internal porosity (yellow) is found within the epoxy matrix
(semitransparent). Individual fibres of ≈ 10 µm in diameter are clearly distinguished. (F. Sket).

 

Figure 2. Deformation and fracture micromechanisms in front of a notch in a cross-ply glass-fibres epoxy-matrix composite. Fibre pull-out and fibre bridging are clearly visible in the lamina with the fibres perpendicular to the crack plane, while matrix cracking and fibres kinking are dominant in the lamina with fibre parallel to the crack (F. Sket, R. Seltzer, J. M. Molina-Aldareguía).

 

Figure 3. X-ray tomography sections of a glass-carbon fibre hybrid composite laminate subjected to out-of-plane low velocity impact with 63 J. (a) Cross-section under the impact axis, showing tensile fracture of the bottom plies and crushing of the top plies under the tup. The intraply cracks in the bottom and top plies grew upwards and downwards, respectively, leading to the development of delamination cracks, and final fracture took place by the formation of a crack through the laminate thickness. (b) Cross-section parallel to the lamina, showing the extent of delamination at the interply between carbon and glass fibre lamina [3].

 

By Dr. Jon Mikel Molina-Aldareguía and Dr. Federico Sket
Micromechanics and Nanomechanics Group