Hutchinson, John

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Hutchinson, John

john_hutchinson@harvard.edu

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Crossover Patterning by the Beam-Film Model: Analysis and Implications
(Public Library of Science, 2014)
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Crossing-over is a central feature of meiosis. Meiotic crossover (CO) sites are spatially patterned along chromosomes. CO-designation at one position disfavors subsequent CO-designation(s) nearby, as described by the classical phenomenon of CO interference. If multiple designations occur, COs tend to be evenly spaced. We have previously proposed a mechanical model by which CO patterning could occur. The central feature of a mechanical mechanism is that communication along the chromosomes, as required for CO interference, can occur by redistribution of mechanical stress. Here we further explore the nature of the beam-film model, its ability to quantitatively explain CO patterns in detail in several organisms, and its implications for three important patterning-related phenomena: CO homeostasis, the fact that the level of zero-CO bivalents can be low (the “obligatory CO”), and the occurrence of non-interfering COs. Relationships to other models are discussed.
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Cohesive Traction-Separation Laws for Tearing of Ductile Metal Plates
(Elsevier, 2012)
Nielsen, K. L.
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The failure process ahead of a mode I crack advancing in a ductile thin metal plate or sheet produces plastic dissipation through a sequence of deformation steps that include necking well ahead of the crack tip and shear localization followed by a slant fracture in the necked region somewhat closer to the tip. The objective of this paper is to analyze this sequential process to characterize the traction–separation behavior and the associated effective cohesive fracture energy of the entire failure process. The emphasis is on what is often described as plane stress behavior taking place after the crack tip has advanced a distance of one or two plate thicknesses. Traction–separation laws are an essential component of finite element methods currently under development for analyzing fracture of large scale plate or shell structures. The present study resolves the sequence of failure details using the Gurson constitutive law based on the micromechanics of the ductile fracture process, including a recent extension that accounts for damage growth in shear. The fracture process in front of an advancing crack, subject to overall mode I loading, is approximated by a 2D plane strain finite element model, which allows for an intensive study of the parameters influencing local necking, shear localization and the final slant failure. The deformation history relevant to a cohesive zone for a large scale model is identified and the traction–separation relation is determined, including the dissipated energy. For ductile structural materials, the dissipation generated during necking prior to the onset of shear localization is the dominant contribution; it scales with the plate thickness and is mesh-independent in the present numerical model. The energy associated with the shear localization and fracture is secondary; it scales with the width of the shear band, and inherits the finite element mesh dependency of the Gurson model. The cohesive traction–separation laws have been characterized for various material conditions.
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The Role of Nonlinear Substrate Elasticity in the Wrinkling of Thin Films
(The Royal Society, 2013)
The role of substrate nonlinearity on the stability of wrinkling of thin films bonded to compliant substrates is investigated within the initial post-bifurcation range when wrinkling first emerges. A fully nonlinear neo-Hookean bilayer composed of a thin film on a deep substrate is analyzed for a wide range of the film/substrate stiffness ratio, from films that are very stiff compared to the substrate to those only slightly stiffer. Substrate pre-stretch prior to film attachment is shown to have a significant effect on the nonlinearity relevant to wrinkling. Two dimensionless parameters are identified which control the stability and mode shape evolution of the bilayer: one specifying arbitrary uniform substrate pre-stretch and the other a stretch- modified modulus ratio. For systems with film stiffness greater than about five times that of the substrate the wrinkling bifurcation is stable, while for systems with smaller relative film stiffness bifurcation can be unstable, especially if substrate pre-stretch is not tensile.
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Three-dimensional Morphology Evolution of SiO2 Patterned Films Under MeV Ion Irradiation
(American Institute of Physics, 2006)
Otani, Kan
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Chen, Xi
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Chervinsky, John F.
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We have measured the evolving three-dimensional (3D) morphology of patterned SiO2 stripes on Si substrates induced by 3 MeV O++ ion irradiation. We develop a 3D constitutive relation to describe anisotropic deformation, densiﬁcation, and ﬂow. We use this constitutive relation in a ﬁnite element model that simulates the experimental morphology evolution, and we ﬁnd excellent agreement between simulated and measured proﬁles. The model should be useful in predicting morphology evolution in complex three-dimensional structures under MeV ion irradiation.
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Comparison of Mechanical Properties of Ni[sub]3Al Thin Films in Disordered FCC and Ordered L1[sub]2 Phases
(Elsevier Science B.V., Amsterdam, 2001)
Huang, Yucong
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Evans, Anthony G.
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Saha, R.
We report the results of several experiments isolating the effect of long-range order on mechanical properties of intermetallic compounds. Kinetically disordered FCC Ni3Al (Ni 76%) thin films were produced by rapid solidification following pulsed laser melting. For comparison, compositionally and microstructurally identical films with ordered L12 structure were produced by subsequent annealing at 550 °C for 2 hours. These FCC and L12 Ni3Al thin films were tested by nanoindentation for hardness and Young’s modulus, and the critical strain to fracture was measured by straining the substrate under four-point bending. Ni3Al thin films in the disordered phase were found to have nearly twice the critical strain to fracture, more than three times the fracture toughness, and about 20% lower hardness than in the ordered counterpart. Blunter crack tips and crack bridging observed in the disordered phase also illustrate increased ductility. The increased plasticity of Ni3Al due to chemical disorder is manifested both within the grains and at the grain boundaries. Young’s moduli of the ordered and disordered materials were found to be indistinguishable.
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Nanopore Fabrication in Amorphous Si: Viscous Flow Model and Comparison to Experiment
(American Institute of Physics, 2010)
George, H. Bola
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Tang, Yuye
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Chen, Xi
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Nanopores fabricated in free-standing amorphous silicon thin films were observed to close under 3 keV argon ion irradiation. The closing rate, measured in situ, exhibited a memory effect: at the same instantaneous radius, pores that started larger close more slowly. An ion-stimulated viscous flow model is developed and solved in both a simple analytical approximation for the small-deformation limit and in a finite element solution for large deformations. The finite-element solution exhibits surprising changes in cross-section morphology, which may be extremely valuable for single biomolecule detection, and are untested experimentally. The finite-element solution reproduces the shape of the measured nanopore radius versus fluence behavior and the sign and magnitude of the measured memory effect. We discuss aspects of the experimental data not reproduced by the model, and successes and failures of the competing adatom diffusion model.
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Buckling of a Pressurized Hemispherical Shell Subjected to a Probing Force
(ASME International, 2017-10-19)
Marthelot, Joel
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López Jiménez, Francisco
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Lee, Anna
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Reis, Pedro M.
We study the buckling of hemispherical elastic shells sub- jected to the combined effect of pressure loading and a prob- ing force. We perform an experimental investigation using thin shells of nearly uniform thickness that are fabricated with a well-controlled geometric imperfection. By systemat- ically varying the indentation displacement and the geome- try of the probe, we study the effect that the probe induced deflections have on the buckling strength of our spherical shells. The experimental results are then compared to finite element simulations, as well as to recent theoretical predic- tions from the literature. Inspired by a nondestructive tech- nique that was recently proposed to evaluate the stability of elastic shells, we characterize the nonlinear load-deflection mechanical response of the probe for different values of the pressure loading. We demonstrate that this nondestructive method is a successful local way to assess the stability of spherical shells.
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Numerical Simulation of Cropping
(ASME International, 2014)
Tvergaard, Viggo
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Cropping is a cutting process whereby opposing aligned blades create a shearing failure by exerting opposing forces normal to the surfaces of a metal sheet or plate. Building on recent efforts to quantify cropping, this paper formulates a plane strain elastic-plastic model of a plate subject to shearing action by opposing rigid platens. Shear failure at the local level is modeled by a cohesive zone characterized by the peak shear traction and the energy dissipated by shear failure process at the microscopic level. The model reveals the interplay between shear cracking and the extensive plastic shearing accompanying the cutting process. Specifically, it provides insight into the influence of the material’s microscopic shear strength and toughness on the total work of cropping. The computational model does not account for deformation of the cropping tool, friction between sliding surfaces and material temperature and rate-dependence.
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Strain gradient effects in periodic flat punch indenting at small scales
(Elsevier BV, 2014)
Nielsen, K.L.
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Niordson, C.F.
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Experiments on soft polycrystalline aluminum have yielded evidence that, besides the required punch load, both the size and shape of imprinted features are affected by the scale of the set-up, e.g. substantial details are lost when the characteristic length is on the order of 10µm. The objective of this work is to clarify the role played by strain gradients on this issue, and to shed light on the underlying mechanisms. For this, indentation by a periodic array of flat punch indenters is considered, and a gradient enhanced material model that allows for a numerical investigation of the fundamentals are employed. During a largely non-homogeneous deformation, the material is forced up in between the indenters so that an array of identical imprinted features is formed once the tool is retreated. It is confirmed that the additional hardening owing to plastic strain gradients severely affects both the size and shape of these imprinted features. In particular, this is tied to a large increase in the mean contact pressure underneath the punch, which gives rise to significant elastic spring-back effects during unloading.
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Response of Metallic Pyramidal Lattice Core Sandwich Panels to High Intensity Impulsive Loading in Air
(Elsevier, 2011)
Dharmasena, Kumar P.
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Wadley, Haydn N. G.
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Williams, Keith
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Xue, Zhenyu
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Small scale explosive loading of sandwich panels with low relative density pyramidal lattice cores has been used to study the large scale bending and fracture response of a model sandwich panel system in which the core has little stretch resistance. The panels were made from a ductile stainless steel and the practical consequence of reducing the sandwich panel face sheet thickness to induce a recently predicted beneficial fluid–structure interaction (FSI) effect was investigated. The panel responses are compared to those of monolithic solid plates of equivalent areal density. The impulse imparted to the panels was varied from 1.5 to 7.6 kPa s by changing the standoff distance between the center of a spherical explosive charge and the front face of the panels. A decoupled finite element model has been used to computationally investigate the dynamic response of the panels. It predicts panel deformations well and is used to identify the deformation time sequence and the face sheet and core failure mechanisms. The study shows that efforts to use thin face sheets to exploit FSI benefits are constrained by dynamic fracture of the front face and that this failure mode is in part a consequence of the high strength of the inertially stabilized trusses. Even though the pyramidal lattice core offers little in-plane stretch resistance, and the FSI effect is negligible during loading by air, the sandwich panels are found to suffer slightly smaller back face deflections and transmit smaller vertical component forces to the supports compared to equivalent monolithic plates.

Hutchinson, John

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