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Prof. Jean-François Molinari
Dynamic crack propagation in heterogeneous materials
The modeling of catastrophic failure of materials and structures is a long standing scientific challenge, with profound societal impact. Arguably, one of the most difficult to analyze and important damage mechanisms is the dynamic fragmentation of a contiguous body. The observation, or prediction, of fragment sizes has important implications on ballistic impact, crash performance, explosive drilling, and clustering of galaxies resulting from the big bang theory. Upon severe loading, multiple micro-cracks initiate at seemingly random locations and propagate at high velocities. Their paths may be tortuous, single cracks may form complex branches, but eventually the cracks coalesce, resulting in the formation of fragments.
In this presentation, we begin by investigating explosive fragmentation using the classic cohesive-element approach. This framework allows scalable parallel calculations and the convergence of fragmentation results for 3D simulations of brittle plate or hollow-sphere explosion [1-3]. A comparison between numerical results and analytical energy models reveal an identical scaling law exponent for the average fragment size as function of strain rate. However, our simulations include stress wave interactions, which yield a higher number of fragments. The calculations give also access to statistics on fragment shapes and orientations. We show that thin membranes generate quite structured fragments, whereas for larger membranes’ thickness, crack branching mechanisms bring random fragment orientations.
In the second part, using an elastodynamic boundary integral formulation coupled with a cohesive model, we restrict our attention to the problem of a single dynamic rupture front propagating along an heterogeneous plane . We show that small-scale heterogeneities facilitate the supershear transition of a mode-II crack. The elastic pulses radiated during front accelerations explain how microscopic variations of fracture toughness change the macroscopic rupture dynamics. Perturbations of dynamic fronts are then systematically studied with different microstructures and loading conditions. The process zone size is the intrinsic length scale controlling heterogeneous dynamic rupture. The ratio of this length scale to asperity size is proposed as an indicator to transition from quasi-homogeneous to heterogeneous fracture. Moreover, we discuss how the shortening of the process zone size with increasing crack speed brings the front to interact with smaller details of the microstructure. This study shines new light on recent experiments reporting perturbations of dynamic rupture fronts, which intensify with crack propagation speed.
Professor J.F. Molinari is the director of the Computational Solid Mechanics Laboratory (http://lsms.epfl.ch) at EPFL, Switzerland. He holds an appointment in the Civil Engineering institute, which he directed from 2013 to 2017, and a joint appointment in the Materials Science institute. He started his tenure at EPFL in 2007, and was promoted to Full Professor in 2012.
J.F. Molinari graduated from Caltech, USA, in 2001, with a M.S. and Ph.D. in Aeronautics. He held professorships in several countries besides Switzerland, including the United States with a position in Mechanical Engineering at the Johns Hopkins University (2000-2006), and France at Ecole Normale Supérieure Cachan in Mechanics (2005-2007), as well as a Teaching Associate position at the Ecole Polytechnique de Paris (2006-2009).
The work conducted by Prof. Molinari and his collaborators takes place at the frontier between traditional disciplines and covers several length scales from atomistic to macroscopic scales. Over the years, Professor Molinari and his group have been developing novel multiscale approaches for a seamless coupling across scales. The activities of the laboratory span the domains of damage mechanics of materials and structures, nano- and microstructural mechanical properties, and tribology. Prof. Molinari was a recipient of an ERC Starting Grant award in 2009.