Showing papers on "von Mises yield criterion published in 2009"

A Consideration of Allowable Equivalent Stresses for Fatigue Design of Welded Joints According to the Notch Stress Concept with the Reference Radii r ref = 1.00 and 0.05 mm

Abstract: In the literature, allowable stresses (FAT-values) for the fatigue design of welded joints are established according to the notch stress concept with the reference radii rref = 1.00 mm for thick connections (t ≥ 5 mm) and 0.05 mm for thin connections (t < 5 mm). However, it is not clear for which strength hypothesis they are valid. As local equivalent stresses may be calculated by the principal stress or von Mises hypotheses, it is necessary to distinguish between the applied hypotheses. The FAT-values according to the principal stress and von Mises hypotheses are compiled for steel, aluminium and magnesium for the reference radii rref = 1.00 and 0.05 mm. The allowable stresses are derived from normal as well as from shear stresses. However, the values derived from pure normal loading (axial or bending) and from pure torsion are not compatible when the principle stress or the von Mises hypotheses are applied. Therefore, in case of biaxial loading, the stated incompatibility between the values obtained from different loading modes should be overcome by the Gough-Pollard relationship.

Anisotropic micro-sphere-based finite elasticity applied to blood vessel modelling

Abstract: A fully three-dimensional anisotropic elastic model for vascular tissue modelling is here presented. The underlying strain energy density function is assumed to additively decouple into volumetric and deviatoric contributions. A straightforward isotropic neo-Hooke-type law is used to model the deviatoric response of the ground substance, whereas a micro-structurally or rather micro-sphere-based approach will be employed to model the contribution and distribution of fibres within the biological tissue of interest. Anisotropy was introduced by means of the use of von Mises orientation distribution functions. Two different micro-mechanical approaches -- a, say phenomenological, exponential ansatz and a worm-like-chain-based formulation -- are applied to the micro-fibres and illustratively compared. The passage from micro-structural contributions to the macroscopic response is obtained by a computational homogenisation scheme, namely numerical integration over the surface of the individual micro-spheres. The algorithmic treatment of this integration is discussed in detail for the anisotropic problem at hand, so that several cubatures of the micro-sphere are tested in order to optimise the accuracy at reasonable computational cost. Moreover, the introduced material parameters are identified from simple tension tests on human coronary arterial tissue for the two micro-mechanical models investigated. Both approaches are able to recapture the experimental data. Based on the identified sets of parameters, we first discuss a homogeneous deformation in simple shear to evaluate the models' response at the micro-structural level. Later on, an artery-like two-layered tube subjected to internal pressure is simulated by making use of a non-linear finite element setting. This enables to obtain the micro- and macroscopic responses in an inhomogeneous deformation problem, namely a blood-vessel-representative boundary value problem. The effect of residual stresses is additionally included in the model by means of a multiplicative decomposition of the deformation gradient tensor which turns out to crucially affect the simulation results. (Less)

A Coupled Thermal-Mechanical Analysis of Ultrasonic Bonding Mechanism

Abstract: A three-dimensional (3-D) finite element model has been developed to simulate the coupled thermal-mechanical fields in ultrasonic welding of aluminum foils. Transient distributions and evolution of the in-process variables, including normal stress, shear stress, slide distance, heat generation, temperature, and plastic deformation on the contact interface, and their interactions have been studied in detail. The von Mises plastic strain from the simulation has been correlated with the measured bonded area of ultrasonic joints. A possible mechanism for ultrasonic bond formation is proposed. The severe, localized, plastic deformation at the bond region is believed to be the major phenomenon causing bond formation in ultrasonic welding.

Pitting Life Prediction Based on a 3D Line Contact Mixed EHL Analysis and Subsurface von Mises Stress Calculation

Abstract: Surface pitting due to contact fatigue is a major failure mode of many mechanical components, such as various gears and rolling element bearings. Pitting life prediction, therefore, is vital to design and performance/reliability improvements. Conventional prediction methods, commonly found in industrial standards, are based on the Hertzian contact theory under assumptions that surfaces are ideally smooth with no lubrication. The present study aims to develop a von Mises stress-based pitting life prediction approach based on the 3-D line contact mixed elastohydrodynamic lubrication (EHL) model recently developed for analyzing various gears, roller/needle bearings, cams and followers and other line-contact components. Pitting life evaluation employs the fatigue life model developed by Ioannides-Harris and Zaretsky et al, using the von Mises stress field obtained. Sample cases are analyzed for model validation, and the life prediction results are compared with available transmission gear test data.

An assessment of the Gurson yield criterion by a computational multi‐scale approach

Abstract: Purpose – The purpose of this paper is to assess the Gurson yield criterion for porous ductile metals.Design/methodology/approach – A finite element procedure is used within a purely kinematical multi‐scale constitutive modelling framework to determine estimates of extremal overall yield surfaces. The RVEs analysed consist of an elastic‐perfectly plastic von Mises type matrix under plane strain conditions containing a single centered circular hole. Macroscopic yield surface estimates are obtained under three different RVE kinematical assumptions: linear boundary displacements (an upper bound); periodic boundary displacement fluctuations (corresponding to periodically perforated media); and, minimum constraint or uniform boundary traction (a lower bound).Findings – The Gurson criterion predictions fall within the bounds obtained under relatively high void ratios – when the bounds lie farther apart. Under lower void ratios, when the bounds lie close together, the Gurson predictions of yield strength lie sli...

The dynamic natures of implant loading.

Abstract: Statement of problem A fundamental problem in fully understanding the dynamic nature of implant loading is the confusion that exists regarding the torque load delivered to the implant complex, the initial force transformation/stress/strain developed within the system during the implant complex assembly, and how the clamping forces at the interfaces and the preload stress impact the implant prior to any external loading Purpose The purpose of this study was to create an accurately dimensioned finite element model with spiral threads and threaded bores included in the implant complex, positioned in a bone model, and to determine the magnitude and distribution of the force transformation/stress/strain patterns developed in the modeled implant system and bone and, thus, provide the foundational data for the study of the dynamic loading of dental implants prior to any external loading Material and Methods An implant (Branemark Mark III), abutment (CeraOne), abutment screw (Unigrip), and the bone surrounding the implant were modeled using HyperMesh software The threaded interfaces between screw/implant and implant/bone were designed as a spiral thread helix assigned with specific coefficient of friction values Assembly simulation using ABAQUS and LS-DYNA was accomplished by applying a 32-Ncm horizontal torque load on the abutment screw (Step 1), then decreasing the torque load to 0 Ncm to simulate the wrench removal (Step 2) The postscript data were collected and reviewed by HyperMesh A regression analysis was used to depict the relationships between the torque load and the mechanical parameters Results During the 32-Ncm tightening sequence, the abutment screw elongated 133 μm The tightening torque generated a 554-N clamping force at the abutment/implant interface and a 522-N preload The von Mises stress values were 248 MPa in the abutment at the abutment-implant interface, 765 MPa at the top of the screw shaft, 694 MPa at the bottom of the screw shaft, 1365 MPa in the top screw thread, and 21 MPa in the bone at the top of the implant-bone interface This study also identified various characteristic isosurface stress patterns The maximum stress magnitude to complete the von Mises stress joint pattern in the present model was 107 MPa during screw tightening, and was reduced to 104 MPa with removal of the wrench Various specific stress patterns were identified within all elements of the implant complex during the assembly simulation Conclusions During the torque moment application, the abutment screw was elongated, and every 10-μm elongation of the screw was equivalent to a 479-N increase of the preload in the implant complex The ideal index to determine the preload amount was the contact force at the interface between the screw threads and the threaded screw bore The isosurface mode identified various characteristic stress patterns developed within the implant complex at the various interfaces during the assembly simulation These patterns are the (1) spiral and ying-yang pattern of the XY stress, (2) spring, cap, clamping, and preload pattern of the ZZ stress, and (3) bone holding and joint pattern of the von Mises stress (J Prosthet Dent 2009;101:359–371)

Considering damage history in crashworthiness simulations

Abstract: Summary: On behalf of improvements for crashworthiness simulations, great effort has been done throughout the past years regarding the treatment of crack formation and propagation. Current state of the art here is the use of failure models that accumulate damage on an incremental basis. Most models are based on the observations of Bridgman [1], who found that failure strain in metallic materials depends on the hydrostatic pressure. Examples of models in use are the Gurson model with extensions by Tvergaard and Needleman [2], and the failure model of Johnson and Cook [3]. As a shortcoming, the mechanical properties of sheet metal parts for crashworthiness calculations are usually assumed to be as in material delivery state. This disregards the changes in properties resulting from previous treatment in the process chain of sheet metal part manufacturing, including deep-drawing. Namely, an increase in flow stress due to work hardening can be expected, which can play an important role especially for low-speed impact cases. Since plastic pre-straining also results in a reduction of the remaining strain to failure, plastic pre-damage has to be taken into account. This leads to the need of considering damage in forming simulations. For crashworthiness calculations, the constitutive models used are usually isotropic and based on the von Mises flow rule. For forming simulations, a more sophisticated and anisotropic description of yield loci is considered important, which makes it necessary to use different constitutive models for both parts of the process chain. A damage model suitable to be used for both disciplines therefore has to be able to correctly predict damage regardless of the details of the constitutive model formulation. In the following, the damage model GISSMO (Generalized Incremental Stress-State dependent damage MOdel), which is currently under development at Daimler will be presented. The main issues of the model are a combination of the proven features of failure description provided by damage models for crashworthiness calculations, together with an incremental formulation for the description of material instability and localization. Yet, a user-friendly and simplified input of material parameters is intended, which will be achieved by a phenomenological formulation of ductile damage. Special attention is paid to considering the point of instability or localization, as is a central issue in forming simulations. For crashworthiness simulations of ductile materials, the correct description of instability and localization can also greatly influence calculation results. The constitutive model used in the actual working state is the anisotropic yield locus by Barlat, Lege and Brem 1991 [4], which is used to allow for a consideration of anisotropic yield loci in crashworthiness calculations also.

A Nonlinear Rubber Material Model Combining Fractional Order Viscoelasticity and Amplitude Dependent Effects

Abstract: A nonlinear rubber material model is presented, where influences of frequency and dynamic amplitude are taken into account through fractional order viscoelasticity and plasticity, respectively The problem of simultaneously modeling elastic, viscoelastic, and friction contributions is removed by additively splitting them Due to the fractional order representation mainly, the number of parameters of the model remains low, rendering an easy fitting of the values from tests on material samples The proposed model is implemented in a general-purpose finite element (FE) code Since commercial FE codes do not contain any suitable constitutive model that represents the full dynamic behavior of rubber compounds (including frequency and amplitude dependent effects), a simple approach is used based on the idea of adding stress contributions from simple constitutive models: a mesh overlay technique, whose basic idea is to create a different FE model for each material definition (fractional derivative viscoelastic and elastoplastic), all with identical meshes but with different material definition, and sharing the same nodes Fractional-derivative viscoelasticity is implemented through user routines and the algorithm for that purpose is described, while available von Mises' elastoplastic models are adopted to take rate-independent effects into account Satisfactory results are obtained when comparing the model results with tests carried out in two rubber bushings at a frequency range up to 500 Hz, showing the ability of the material model to accurately describe the complex dynamic behavior of carbon-black filled rubber compounds

Mixed method and convex optimization for limit analysis of homogeneous Gurson materials: a kinematical approach

Abstract: A fully kinematical, mixed finite element approach based on a recent interior point method for convex optimization is proposed to solve the limit analysis problem involving homogeneous Gurson materials. It uses continuous or discontinuous quadratic velocity fields as virtual variables, with no hypothesis on a stress field. Its modus operandi is deduced from the Karush-Kuhn-Tucker optimality conditions of the mathematical problem, providing an example of cross-fertilization between mechanics and mathematical programming. This method is used to solve two classical problems for the von Mises plasticity criterion as a test case, and for the Gurson criterion for which analytical solutions do not exist. Using only the original plasticity criterion as material data, the method proposed appears robust and efficient. providing very tight bounds on the limit loadings investigated. (C) 2008 Elsevier Masson SAS. All rights reserved.

Finite-element analysis on wafer-level CMP contact stress: reinvestigated issues and the effects of selected process parameters.

Abstract: Contact stress uniformity is a key issue for the performance of wafer-level chemical–mechanical planarization (CMP) and has been extensively studied during the past two decades. However, contact-stress-related issues are not consistently presented in the literature. In addition, a number of topics remain to be addressed in wafer-level contact analysis. The objective of this article is in twofold. First, it aims to provide a more detailed discussion and stress analysis of the inconsistent issues, including the definition of CMP uniformity, the stress indicator, and the effect of carrier films. Second, contact stress analyses of several important but rarely touched problems are also investigated. Topics to be investigated include: the effects of material hyperelasticity, the effects of a grooved pad, the effects of wafer warpage due to residual stress, and finally the possible advantages of a multizone loading manner. For the first category, this work proposes a new definition of CMP uniformity based on the width of the relatively flat zone. In addition, the contact stress distribution in terms of both von Mises and normal stress are also investigated and their relationship is qualitatively established. Furthermore, the importance of the carrier films is reinvestigated, and the conclusion indicates that their importance is not as significant as previously reported. The hyperelasticity of pad material primarily affects the pad deformation. The presence of pad grooves results in a net increase of contact stress, but the global tendency is unchanged. A warped wafer significantly reduces the contact stress uniformity. By contrast, the multizone loading manner can effectively improve the uniformity of stress distribution. Finally, the stress analyses presented are integrated with a graphic user interface to form a CMP computer-aided design system for further applications. The issues addressed and the conclusions obtained are important for improvement of CMP performance.

Stress Analysis on Layered Materials in Point Elastohydrodynamic-Lubricated Contacts

Abstract: For multilayered or coated substrates in elastohydrodynamic-lubricated (EHL) contacts, the subsurface stress distributions under a normal load combined with shear traction have been analyzed in this article through computer simulations. The Papkovich-Neuber potentials and Fourier transform are adopted to deduce the pressure–displacement, pressure–stress, and shear traction–stress response functions in frequency domain for the coated substrates, and to calculate distributions of pressure and subsurface stress. The results from the analysis of EHL contacts on coated substrates are compared with those from dry contact model in which shear traction is assumed to obey Coulomb’s law. Effects of the Young’s modulus of coatings, the properties of lubricants, and the magnitude of traction are discussed. Similar to the results in dry contacts, hard coatings in lubricated cases tend to increase the von Mises stress, whereas soft coatings decrease the stress. Shear traction makes the max von Mises stress increasing and moving closer to surface. However, the changes in subsurface stress due to shear traction are less obvious in lubricated contacts. Comparison between EHL and dry contact models reveals that lubrication can reduce the von Mises stress in the coating layer due to smaller shear traction. The analyses show that pressure, film thickness, and subsurface stress distributions are influenced by surface coatings, sliding velocity, rheological models, and pressure–viscosity behaviors.