Name: Zdeněk P. Bažant, Northwestern University
Presentation Title: Phase-field, XFEM, cohesive, HRR, nonlocal and crack-band models: Restricted or enhanced by gap test of fracture and scaling?
Abstract: The recently conceived gap test1,2, along with its simulations by microplane models for concrete, shale, composites and metals, reveals that crack-parallel xx, zz and xz stresses can more than double the fracture energy Gf (or Kc, Jcr) of the material or reduce it to zero, and the effective Gf is strongly path-dependent. Neither the line crack (LEFM, XFEM, cohesive crack) nor the phase-field models can reproduce these major effects. To do so, the fracture process zone (FPZ) that has a finite width must be characterized by a tensorial constitutive damage law that can capture oriented microcracking or dilatant microslips in inclined directions, causing the FPZ to either strengthen or widen and weaken. For quasibrittle materials, these effects can be reproduced by the nonlocal and crack band models. After an update for T-stress, these effects are also reproduced, though incompletely, by the classical HRR theory of plastic-hardening metals. The gap test relies on the fracture scaling law, which was developed for quasibrittle materials long ago3. For the HRR theory, the scaling law is formulated4 and experimentally verified for aluminum. It takes into account the crack-parallel stresses, not only in the hardening-yielding zone of millimeter scale but also in the FPZ of micrometer scale. Accounting for both and exploiting the size effect enhances HRR applicability. Furthermore, the phase-field model is shown suffer from further problematic, severely restrictive, features. Finally, various practical problems in which the crack-parallel stress with its scaling matters are succinctly reviewed. They include beam or slab shear in reinforced concrete, shale fracking, pressure vessel or composite fuselage under biaxial tension, mode I fractures in composite airframes under shear, composite crush cans for cars, sea ice fractures, frictional mode II cracks in geology, and sideways cracks in composites.
Bio: Born and educated in Prague (Ph.D. 1963), Bažant joined Northwestern in 1969, where he has been W.P. Murphy Professor since 1990 and simultaneously McCormick Institute Professor since 2002, and Director of Center for Concrete Geomaterials (1981-87). He was inducted to NAS, NAE, Am. Acad. of Arts & Sci., Royal Soc. London, the academies of Austria, Japan, Italy, Spain, Czech Rep., Greece, India and Lombardy, and Academia Europaea. Honorary Member of: ASCE, ASME, ACI, RILEM. Received Austrian Cross of Honor for Science and Art; 7 honorary doctorates (Prague, Karlsruhe, Colorado, Milan, Lyon, Vienna, Ohio State); ASME Medal, ASME Timoshenko, Nadai and Warner Medals; ASCE von K´arm´an, Freudenthal, Newmark, Biot, Mindlin and Croes Medals, and Lifetime Achievement Award; SES Prager Medal; Outstanding Res. Award, Am. Soc. for Composites; RILEM Gold Medal; Exner Medal (Austria); Torroja Medal (Madrid); etc. He authored nine books: Scaling of Struct. Strength, Creep in Concrete Str., Inelastic Analysis, Fracture and Size Effect, Stability of Structures, Concrete at High Temp., Creep & Hygrothermal Effects, Probab. Mech. of Quasibrittle Str., QuasbrittleFrac. Mech.; He is one of the original top 100 ISI Highly Cited Scientists in Engrg. H-index: 137, citations: 81,000, i10 index: 660 (Google, incl. self-cit.). In 2019 Stanford U. weighted citation survey (see PLoS), he was ranked no.1 in CE and no.2 in Engrg. worldwide. In 2015, ASCE established ZP Bažant Medal for Failure and Damage Prevention. His 1959 mass-produced patent of safety ski binding is exhibited in New England Ski Museum, Franconia, NH. Read More About Zdeněk P. Bažant
Name: Glaucio H. Paulino, Georgia Institute of Technology
Presentation Title: Origami Engineering: Structures, Metamaterials, and Robots
Abstract: We study the geometric mechanics of origami assemblages and investigate how geometry affects behavior and properties. Understanding origami from a structural standpoint can allow for conceptualizing and designing feasible applications across scales and disciplines of engineering. We review the basic mathematical rules of origami and use 3D-printed origami legos to illustrate those concepts. We then present a reduced-order-model, which consists of an improved bar-and-hinge model, to simulate origami assemblages. We explore the stiffness of tubular origami and kirigami structures based on the Miura-ori folding pattern. A unique orientation for zipper coupling of rigidly foldable origami tubes substantially increases stiffness in higher order modes and permits only one flexible motion through which the structure can deploy. We couple compatible origami tubes into a variety of cellular assemblages that enhances mechanical characteristics and geometric versatility, leading to the design of structures and configurational metamaterials that can be deployed, stiffened, and tuned. We have designed, fabricated (using direct laser writing), and tested (SEM) this metamaterial at the micron-scale. This resulted not only in the smallest scale origami assembly, but also in a metamaterial with intriguing mechanical properties, such as anisotropy, reversible auxeticity, and large degree of shape recoverability. The presentation concludes with a vision toward the field of origami engineering, including origami robots with distributed actuation, allowing for on-the-fly programmability, and other interdisciplinary applications.
Bio: Professor Paulino is the Raymond Allen Jones Chair at the Georgia Institute of Technology. His seminal contributions in the area of computational mechanics include the development of methodologies to characterize the deformation and fracture behavior of existing and emerging materials and structural systems; topology optimization for large-scale multiscale/multiphysics problems; variational methods; deployable structures and origami engineering. Last year (2020), he received the Daniel C. Drucker Medal of ASME, the Raymond D. Mindlin Medal of ASCE, and the Reddy Medal from Mechanics of Advanced Materials and Structures (MAMS 2020). He also received the 2015 Cozzarelli Prize from the National Academy of Sciences, “which recognizes recently published PNAS papers of outstanding scientific excellence and originality.” He is a former President of the Society of Eng. Science (SES). Recently, he was elected to the US National Academy of Engineering (NAE). More information about his research and professional activities can be found here.