John Botsis obtained his diplôme in civil engineering at the University of Patras, Greece in 1979. He continued his education at Case Institute of Technology in Cleveland Ohio/USA, where he received his MS and Ph.D. 1984. After two years at the research centre for national defence in Athens he was nominated assistant professor at the University of Illinois in Chicago, associate in 1991 and full professor in 1995. In 1996, he was nominated professor of solids and structural mechanics at the EPFL. His activities cover experimental mechanics, fracture and fatigue of advanced materials including composites and biomaterials using novel experimental techniques, numerical methods and micromechanics. He has co-authored more than 150 journal papers, several book chapter and two textbooks. His research has been funded from the Swiss National Science Foundation, State Secretariat for Education and Research, Swiss commission for technology and innovation, EU and Swiss industry.
It is well known that large scale bridging in fracture of layered composites is one of the most important toughening mechanisms. The resulting resistance to fracture, however, is dependent on specimen geometry and microstructure rendering its modeling difficult. In this presentation, experimental results and modeling of fracture in layered composite specimens are discussed. The experimental part consists of monotonic tests of inter-, intralaminar fracture of unidirectional specimens as well as load-controlled fatigue of interlaminar specimens. 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 traction separation-relations, due to bridging, using the strains from the FBG sensors, parametric finite elements and optimization. The results demonstrate an important effect of specimen thickness in the fracture response under monotonic and fatigue loads and allow deducing scaling relationships due to large scale bridging. The obtained traction-separation relations are employed in cohesive zone simulations to predict very well the corresponding load-displacement and fracture resistance curves for each thickness. Large scale bridging in fatigue is also characterized using a similar methodology and crack propagation is found independent of specimen thickness if bridging zones are properly accounted for in terms of specimen thickness.
To elucidate further the phenomenological response, computational micromechanics models are developed to predict the specimen thickness effects on bridging and the large differences in inter- and intralaminar fracture. Analysis shows that if the traction separation relation is enriched with the local crack opening angle, the observed experimental response can be easily reproduced thus, suggesting traction separation relations with two-kinematic parameters as a physically sound model. Using real microstructures, the micromechanics models demonstrate that the matrix material between and within the composite plies plays important roles in fracture response and suggests a vast field of future research opportunities to predict toughness in composites.