Sébastien Kohler - Academia.edu (original) (raw)

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Papers by Sébastien Kohler

Driven by the industry requirement for materials exhibiting high strength and stiffness to weight... more Driven by the industry requirement for materials exhibiting high strength and stiffness to weight ratios, carbon-fibre reinforced polymers (CFRP) have been increasingly used in recent years. Their usage is however subjected to some caveats, as their damage modes and failure mechanisms are hard to predict. Furthermore, the multi-axial laminates used in industrial applications often exhibit a first ply failure as low as 30% of the theoretical fibre strength, seriously hampering the theoretical weight savings. The recent introduction of tow-spreading techniques to the market allowed the emergence of the so-called thin-ply laminates, where the individual laminae thicknesses are typically comprised between 20 µm and 150 µm. The design space that this new class of materials offers is much larger than the one of traditional thicker plies and is of particular interest when applied to very thin structures such as sandwich skins. Additionally, these materials exhibit outstanding strength properties thanks to a ply confinement effect allowing certain laminates to reach the ultimate translaminar fracture of the 0°plies with no premature sign of damage. Unfortunately, the full potential of these new materials is currently not exploited due to the lack of accurate prediction of this strength increase with respect to ply thickness for very light plies, as well as its dependency on the constituents of the composite. This work therefore focuses on the understanding and the modelling of the damage mechanism encountered in thin-ply CFRP composite laminates to understand and characterise the cause responsible for the discrepancy observed between LEFM strength predictions and experimental results. To this effect, the damage dependency on ply thickness is investigated experimentally for three different thin-ply CFRP quasi-isotropic laminates, with constituents spanning a large range of properties. It is shown that free edge damage mechanisms, observed with in-situ microscopy, change with decreasing ply thickness. Whilst free-edge delamination dominates the damage behaviour when thicker plies are used, transverse cracking (TC) dominates the free edge behaviour of thinner plies with an increasing onset of damage with decreasing ply thickness. With some material constituents, this onset is increased so much that the translaminar fracture of the 0°plies precedes it and no transverse cracking is detected any more. Bulk damage, recorded by acoustic emission, is linked to major propagation of TC from the free edge through the width of the sample. It is shown to happen at higher strains than the free edge damage for all but the thickest plies tested, where free edge delamination prevails. Its onset is also shown to scale differently with respect to ply thickness than what is observed v Abstract at the free edge. Assuming LEFM hypotheses, a critical ERR associated with transverse cracking is identified by inverting the in-situ strength model. A ply dependency of this apparent toughness is shown for all material tested with a decrease of its value with decreasing ply thickness. A multiscale embedded-cell FE model is developed to gain more insight in the intricate damage mechanisms present at the micro-scale. All material properties used are obtained or derived from experimental tests, save for the fibre-matrix interface. A good agreement with the obtained experimental onset of TC in the bulk as well as at the free edge is achieved. A dominance of the interface properties on the TC development is demonstrated, and its appearance is shown to be triggered by debonding coalescence, leading to matrix micro-filament bridging. The crucial effect of thermal residual stresses is highlighted, as values leading to debonding or matrix failure after curing can be reached at the micro-scale for high temperature curing composites. An inversely proportional influence of the curing temperature on the onset of TC under the assumed modelling hypotheses is shown. Using this calibrated model to identify a TC-related critical ERR, the same trend of a decreasing apparent toughness with decreasing ply thickness than in the semi-experimental case is observed. In light of these results, a mesoscale model is proposed with all the damage lumped into a single TC modelled using two superposed cohesive element zones, the first one representing debonding and the second one representing matrix micro bridging. An optimisation scheme is used to identify the two linear traction-separation laws implemented to match the experimentally measured AE onsets. The maximum cohesive stresses thus identified as well as critical ERR predictions are in good agreement with the values obtained from the micromechanical model. It was noticed that a simple linear cohesive law could predict the observed scaling of TC onset with respect to ply thickness with enough accuracy, which opens the way for new, improved in-situ strength models that could properly account for the finite size of the process zone without requiring a much larger experimental dataset.