Traction-separation relations in delamination of layered carbon-epoxy composites under monotonic loads: Experiments and Modeling

▣ Speaker: John Botsis (Director, ME, EPFL)
▣ Venue: Lecture room A (#2204)
▣ Date: September 21st 2016 (Wed.), 10:30~11:30p.m.
Traction-separation relations in delamination of layered carbon-epoxy composites under monotonic loads: Experiments and Modeling
Abstract
It is well established that large scale bridging accompanying delamination and fracture in layered composites is among the most important toughening mechanisms. As a consequence, characterization of the tractions on the wake of the crack, the so-called bridging zone, is very important in the efforts to predict the loading response of composite structures.
In this seminar, experiments and modeling of delamination and fracture in layered carbon-epoxy composite specimens are presented. The experimental part consists of displacement controlled monotonic tests of interlaminar and intralaminar fracture. Selected specimens are equipped with wavelength multiplexed fiber Bragg grating (FBG) sensors to monitor crack propagation and strains over several millimeters in the wake of the crack. The modeling part involves an iterative scheme to calculate the traction separation-relation, due to bridging, using strains from the FBG sensors, parametric finite elements and optimization. The results demonstrate an important effect of specimen thickness in interlaminar and intralaminar fracture: the bridging length at steady state linearly increases with specimen thickness while the maximum bridging stress is independent of thickness in each case. Results of a similar study in cross-ply specimens, limited to a selected specimen thickness, show important effects of specimen width on the extent of large scale bridging.
On the basis of the experimental results on the morphology of the bridging zone, a micromechanics model is developed, using an embedded cell approach. The results of these studies predict the experimental findings and demonstrate that the traction-separation law is not a material parameter. The obtained traction-separation relations are employed in cohesive zone simulations to predict the corresponding load-displacement as well as the so called resistance curves.

References:

1. E. Farmand-Ashtiani, D. Alanis, J. Cugnoni, J. Botsis, ‘Delamination in cross-ply laminates: Identification of tractioneseparation relations and cohesive zone modeling’, Composites Science and Technology 119 (2015) 85-92.
2. E. Farmand-Ashtiani, J. Cugnoni, J. Botsis, ‘Specimen thickness dependence of large scale fiber bridging in mode I interlaminar fracture of carbon epoxy composite’, International Journal of Solids and Structures 55 (2015) 58–65.
3. G. Pappas , J. Botsis, ‘Intralaminar fracture of unidirectional carbon/epoxy composite: experimental results and numerical analysis’, International Journal of Solids and Structures 85–86 (2016) 114–124.
L.P. Canal, G. Pappas, J. Botsis, ‘Large scale fiber bridging in mode I intralaminar fracture. An embedded cell approach’, Composites Science and Technology 126 (2016) 52-59.

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