Scientific Events

Two-dimensional (2D) materials, such as Graphene, hBN, and MoS₂, are promising candidates in a number of advanced functional and structural applications owing to their exceptional electrical, thermal, and mechanical properties. Understanding the mechanical properties of 2D materials is critically important for their reliable integration into future electronic, composite, and energy storage applications. In this talk, we will report our efforts to study the fracture behaviors of 2D materials. Our combined experiment and modelling efforts verify the applicability of the classic Griffith theory of brittle fracture to graphene [1]. Strategies on how to improve the fracture resistance in graphene, including a nanocomposite approach, and the implications of the effects of defects on mechanical properties of other 2D atomic layers will be discussed [2, 3]. More interestingly, stable crack propagation in monolayer 2D h-BN is observed and the corresponding crack resistance curve is obtained for the first time in 2D crystals [4]. Inspired by the asymmetric lattice structure of h-BN, an intrinsic toughening mechanism without loss of high strength is validated based on theoretical efforts, enabling stable crack propagation not seen in graphene. Finally, we will also discuss some of our recent efforts in evaluating the mechanical properties of 2D covalent organic frameworks (COFs) [5, 6] and the fracture behaviors of ultrathin van der Waals solids [7] [more]

The defactant concept allows to predict why at higher temperature the formation energy of vacancies, dislocations and surfaces is no longer decreased by hydrogen, because it is not trapped to these defects any more. Thus failure due to hydrogen embrittlement is not present at high temperatures. At low temperatures the diffusion of hydrogen to defect generated by deformation will be reduced and, therefore, the decrease of defect formation energy by segregated hydrogen will not occur. Based on these scenarios equations for crack growth or strain to failure are derived and compared with experimental result for power law creep, stress-strain tests and fatigue. [more]

Abstract: Some of the most pressing challenges in engineering science arise when materials are adjacent to one another - from the bottom of an impacting droplet to the separating faces of a crack. Here I will discuss two vignettes on these important topics: first, a calibrated, nano-scale, direct measurement of the intervening air film at the critical juncture of contact formation during droplet impact, and second, the toughening that can result from geometric complexity at the tip of a propagating crack. These seemingly disparate systems are deeply connected on a variety of levels, from their sensitivity to defects to the propagating singularity that defines the mathematical problem of contact line and crack propagation. The talk will conclude with a discussion of perspectives and some puzzles that remain open despite recent progress. [more]