On slip and kink band localization in metallic single and polycrystals
Samuel Forest (Mines ParisTech, France)
: Strain localization is ubiquitous in the plastic deformation of metallic single and polycrystals. Discrete intense slip and kink bands are observed in grains of cubic and hexagonal metals for instance, and correspond to the activation of one single slip system in the localization zones. It is well-known that such localization modes can be predicted by continuum crystal plasticity theory . Slip and kink bands have been observed at cracks and notches in single crystal Nickel-based superalloys. However, it turns out that the relative proportion of slip and kink bands in FCC polycrystals as predicted by classical continuum crystal plasticity, is not in agreement with experimental observations . The consideration of Cosserat and strain gradient plasticity will be shown to improve these predictions, thus illustrating the role of geometrically necessary dislocations in driving strain localization phenomena.
Impact response of materials: From metals to soft gels
D. Rittel (Technion, Israel)
: Strain The impact response of materials is of prime importance in a variety of scenarios that can involve structural destruction on the one hand, but also structural or bodily protection on the other. This presentation will address two different, yet complementary, aspects of the research carried out in the Dynamic Fracture laboratory at Technion. Starting with metals, we will discuss the adiabatic shear failure of metals, as subject on which we have been working for some 15 years. While the classical paradigm explains adiabatic shear formation in terms of strain-rate sensitivity vs. thermal softening, we will present a different viewpoint in which local softening leading to the formation of a shear band (ASB) is not of thermal but rather of microstructural origin. Namely, we will show the early formation of dynamically recrystallized islands in the prospective locus of the future shear band. Those islands are stronger than the matrix surroundings on the one hand (Hall-Petch effect) but are also characterized by a lack of strain-hardening, causing local softening. In addition, local real-time temperature measurements using HgCdTe-IR detectors clearly shows that until the structural load collapse stage, the temperature rise is quite minor, and probably not of a sufficient extent to trigger significant thermal softening as commonly believed. The thermomechanical coupling-microstructure relationship will also be addressed. The outcome of those works points out to the microstructurally stored energy of cold work as a potential factor for a dynamic failure criterion related to adiabatic shear failure.
Turning next to impact energy mitigation, we will show results on thermo-reversible dilute aqueous Methyl Cellulose gels, when those liquid gels solidify upon heating as opposed to most known materials. The endothermic nature of the process, which has also been identified to occur under strong shocks, acts to absorb a significant part of the impact energy. Results will be shown about the extent of the impact mitigation that make those simple gels powerful candidates for future bodily (or other structural) protection in which one wants to mitigate the strong elastic accelerations that can be very damaging to internal organs.
Mechanics of robotic matter
Chiara Daraio (Caltech, USA)
: Morphing two-dimensional sheets into three-dimensional objects is a classical problem in mechanics, mathematics and art, pursued over centuries of human history. Today, the ability to manufacture materials with an almost arbitrary materials’ distribution, architecture and pre-stress opens the door to new approaches for bending sheets into complex forms or actuating complex three-dimensional structures. In this talk, I will discuss recent progress in the design of micro- and macro-scale, nonuniform materials that can bend into freeform objects, in response to stimuli or with simple application of point loads. Engineering the distribution of residual stresses, stiffness gradients and/or cut patterns, we control the sheets’ buckling at both local and global scales. The designed distribution of responsive materials in the sheets provides a time dependent control of the developing shapes. Programming 2D sheets into rigid, 3D geometries or reprogrammable curves expands the potential of existing manufacturing tools for efficient and versatile production of 3D objects and will expand the potential of autonomous soft robots.