Author: Gang Liu
Publisher: National Library of Canada = Bibliothèque nationale du Canada
ISBN: 9780612720688
Category : Metals
Languages : en
Pages : 420

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Author: Nathan Spulak
Publisher:
ISBN:
Category : Deformations (Mechanics)
Languages : en
Pages : 0

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Materials experiencing impact loading deform under complex three dimensional states of stress and at high strain rates. Accurately simulating impact events using finite element modeling requires material models capable of depicting the material behavior under these same conditions. In order to create accurate material models, this material behavior must first be determined experimentally. It is of particular interest to determine the equivalent plastic fracture strain at stress states consisting of in-plane biaxial tension and out-of-plane compression, and the plastic stress-strain response at strain rates on the order of 104 s-1. Both of these conditions are found during impact loading, and are outside the scope of current testing techniques. A new test technique is used to investigate Aluminum 2024, Titanium 6Al-4V, and Inconel 718 under in-plane biaxial tension and out-of-plane compression. The test consists of a small spherical or elliptical punch that is advanced into a thin specimen plate to induce in-plane biaxial tension on the back surface of the specimen. A second plate of an appropriate material is placed against the back surface of the specimen plate during loading in order to create out-of-plane compression. The equivalent plastic fracture strain at these stress states is determined from the experimental data and simulations using the commercial finite element software LS-DYNA. The same materials mentioned above are also tested using a modified, direct impact split-Hopkinson bar testing technique to induce strain rates greater than 104 s-1. For these tests, a small cylindrical specimen is placed in contact with the end of a larger cylindrical bar. The specimen is then impacted with a free flying cylindrical projectile to compress the specimen at a high rate of deformation. The stress-strain response of the material at these high strain rates is then investigated from the experimental data and in conjunction with LS-DYNA finite element simulations.

Author: Stephane Jean Marie Marcadet
Publisher:
ISBN:
Category :
Languages : en
Pages : 173

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In engineering practice, sheet metal often fails after complex strain paths that deviate substantially from the widely studied proportional loading paths. Different from previous works on the ductile fracture of sheet metal, this thesis research addresses the experimental and modeling issues related to the crack initiation in advanced high strength steels after loading direction reversal. The main outcome of the present work is a fracture initiation model for proportional and non-proportional loading. The starting point of this thesis is a first chapter on the development of a micromechanically-motivated ductile fracture initiation model for metals for proportional loading. Its formulation is based on the assumption that the onset of fracture is imminent with the formation of a primary or secondary band of localization. Motivated by the results from a thorough unit cell analysis, it is assumed that fracture initiates after proportional loading if the linear combination of the Hosford equivalent stress and the normal stress acting on the plane of maximum shear reaches a critical value. A comprehensive fracture initiation model is then obtained after transforming the localization criterion from the stress space to the space of equivalent plastic strain, stress triaxiality and Lode angle parameter using the material's isotropic hardening law. Experimental results are presented for three different advanced high strength steels. For each material, the onset of fracture is characterized for five distinct stress states, including butterfly shear, notched tension, tension with a central hole, and punch experiments. The comparison of model predictions with the experimental results demonstrates that the proposed Hosford-Coulomb model can predict with satisfactory accuracy the instant of ductile fracture initiation in advanced high strength steels. In a subsequent chapter, experimental methods are developed to perform compression tension experiments. In addition, a finite strain constitutive model is proposed combining a Swift-Voce isotropic hardening law with two Frederick-Armstrong kinematic hardening rules and a Yoshida-Uemori type of hardening stagnation approach. The plasticity model parameters are identified from uniaxial tension-compression stress-strain curve measurements and finite element simulations of compression-tension experiments on notched specimens. The model predictions are validated through comparison with experimentally-measured load-displacement curves up to the onset of fracture, local surface strain measurements and longitudinal thickness profiles. The extracted loading paths to fracture show a significant increase in ductility as a function of the compressive pre-strain. The Hosford-Coulomb model is therefore integrated into a non-linear damage indicator modeling framework to provide a phenomenological description of the experimental results for monotonic and reverse loading. Another extension of the modeling framework is presented in a third chapter inspired by the results from loss of ellipticity analysis. It is demonstrated that the Hosford-Coulomb model can also be expressed in terms of a stress-state dependent critical hardening rate. Moreover, it is shown that the critical hardening rate approach provides accurate predictions of the instant of fracture initiation for both proportional and non-proportional loading conditions. Enhancements of the finite strain constitutive model are also proposed to enable a fast identification of all model parameters. The plasticity model parameters are identified from stress-strain curve measurements from shear loading reversal on specimens with a uniform thickness reduced gage section. The model is used to estimate the local strain and stress fields in fracture experiments after shear reversal. The extracted loading paths to fracture show a significant increase in ductility as a function of the strain at shear reversal, a feature that is readily predicted by the prosed critical hardening rate model.

Author: P. F. Thomason
Publisher: Pergamon
ISBN:
Category : Technology & Engineering
Languages : en
Pages : 240

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An account of the recent developments in research into ductile fracture in metals and alloys. Aspects covered include localized fracture at the root of notches and sharp cracks, and fracture in bulk plastic-deformation processes of the metal and metal forming type. Also discusses various theoretical

Author: Bradley Dodd
Publisher: Academic Press
ISBN:
Category : Technology & Engineering
Languages : en
Pages : 328

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Author: W. Böhme
Publisher:
ISBN:
Category : Charpy-V test
Languages : en
Pages : 15

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Dynamic tensile tests and instrumented impact tests with Charpy-V-notch specimens and precracked Charpy specimens (SENB) are performed and simulated. Two strain-rate dependent micromechanical models based on the modified Gurson flow function are compared by simulating different dynamically loaded specimens. One purpose of the study is to investigate the influence of strain rate on the parameters of the micromechanical models and thus to check the applicability of these models for dynamic loading. Another purpose is to find out the relation between the micromechanical approach and conventional fracture mechanics.

Author: Vasanth S. Kothnur
Publisher:
ISBN:
Category : Stainless steel
Languages : en
Pages : 290

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Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 75

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This report results from a contract tasking Institute for Fundamental Technological Research of the Polish Academy of Sciences as follows: The effect of strain rate on ductile fracture is one of the least understood phenomena in modern fracture mechanics. At the same time, information on ductile fracture under dynamic loading is very important for reliable prediction of fracture and fragmentation of high consequence structures. High-consequence structures are understood here as components of turbofan engines for fixed-wing aircraft and/or rotorcraft dynamic components. A comprehensive theoretical, experimental, and numerical research project will be undertaken to resolve some of the fundamental issues and construct a dynamic fracture locus suitable for engineering applications and implementation into the FE codes. It is envisioned that the project will be broken into three interrelated tasks: Hopkinson bar tensile fracture tests on small, flat specimens using a unique apparatus developed at IPPT; Drop tower fracture tests on specially designed specimens subjected to a combined shear/compression and shear/tension loading; Finite element simulation of dynamic experiments and construction of the dynamic fracture envelope for a few typical materials. The second and third task will be a joint endeavour between IPPT and MIT where the funding for the work at MIT will come from GE Global Research Center and the funding for the IPPT will come from this grant. No government furnished equipment or data will be used by the researchers at IPPT.

Author: David Kin-Ming Shum
Publisher:
ISBN:
Category :
Languages : en
Pages : 406

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Author: Jessica Papasidero
Publisher:
ISBN:
Category :
Languages : en
Pages : 147

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