Papers by Keyword: Damage

Abstract: In order to reduce the rate that child passengers injury their chest and head in a collision, parameter analysis of child restraint system is performed. Based on multi-body dynamics software MADYMO, child occupant crash simulation model is built. In accordance with the laws and regulations, the validity of the model is verified. Damage criterions, such as head the centroid synthesis of acceleration, HIC36 and resultant chest acceleration, are used to analyze the influence of different coefficients on childs injury, including friction between booster seat and vehicle seat, safety belt stiffness and friction between the dummy and safety belt. Results show that increasing the coefficient of friction of boost seat bottom and seat belt by coating them with friction material can reduce childs head and chest injuries significantly during the collision.

Abstract: Large deformation problem under dynamic loading has been an issue for mechanical engineer and researcher who study continuum mechanics because of its complex behavior. This study focuses on a large deformation problem based on nonlinear viscoplasticity. Therefore, the Johnson-Cook constitutive model and the Zerilli-Armstrong constitutive model are used to analyze and to understand this stretch example under dynamic loading. Numerical results for stretch example using both constitutive models were compared with the results from ABAQUS/Explicit that considered mesh sensitivity analysis, and were then applied to model failure processes of OFHC copper, with consideration of geometric nonlinearity based on concepts of progressive damage mechanics. This approach helps us to understand and to predict the behavior of large deformation and to perform damage failure analysis.

Abstract: Through analyzing the real collision process of a steel plate rolling sliding off a lorry and box girder flange of an overpass, the damage degree of box girder may be known, and in this article the collision process is also simulated by means of finite element method so that it may be obtained how the damaged girder collapses and the cracks propagate. Furthermore, the dynamic characteristics are studied during the collision process and the reliability of overpass section collision is analyzed from the angle of energy transfer and dissipation.

Abstract: Acoustic Emission (AE) is a sensitive technique which can be used to characterise damage in high strength composite plate. This paper describes an extension to an earlier piece of research work carried out by the ERC which resulted in the successful development of a novel source location methodology for the said material. The previous work concentrated on the source location in plate-like composite structures using acoustic emission. The work presented in this paper focuses on establishing the correlation between the different damage types suffered in the material namely de-lamination, matrix cracking, fibre rupture and stringer to skin debonding with key signal features of the AE activities. Controlled bending tests were initially carried out on laterally grooved slender composite specimens to progressively propagate damage in the weakened region of these specimens. The composite laminate plate itself is made from 16 plies of carbon fibre twill weaved in an epoxy matrix with bidirectional fibre alignments in the 0° and 90° directions with 60/40 fibre-matrix volume composition. These prepared samples were fully instrumented with broad band (100 kHz to 1MHz) Physical Acoustic AE sensors linked to the necessary signal conditioning hardware. The AE events were recorded using a high speed DAQ card accessed by customised software written in LabVIEW^TM. Gathered raw data were analysed off-line for key signal features including energy and frequency contents and subsequently correlated to actual damage types. It can be concluded from the empirical evidence that feature vectors are distinct to the type of damage. Results gathered from additional test on the progressive skin-stringer debonding of the same material to failure confirmed the uniqueness of the AE feature trends. An integrated system which is capable of both in-situ location of compromised sites and the diagnostic of flaw types in composite plate can potentially find engineering applications including the structural health monitoring of composite aircraft parts.

Abstract: In petroleum wells, the adhesion between the steel and the cementitious coating material is responsible for ensuring the efficiency of the mechanical point of view and of the thermodynamic stability of steels, protecting them against corrosion, preventing the escape of fluids inside and hydraulically isolating the structure against infiltration. The push-out test is used to measure the level of adhesion between the steel and cement. In this paper, the numerical simulation of steelcement interface was performed to reproduce the mechanical behavior of this interface used a cohesive zone model combined with Coulomb's law for friction. The proposed model was implemented in CAST3M software. The numerical results obtained with the proposed model were compared with experimental results of push-out test. The comparison between the force versus displacement curves, obtained experimentally and numerically, validated the proposed model.

Abstract: Fracturing by ductile damage occurs quite naturally in metal forming process due to the development of microcracks associated with large straining or due to plastic instabilities associated with material behavior and boundary conditions. Metal forming processes generally introduce a certain amount of damage in the material being formed. Predictions of the damage formation and growth in a series of forming steps may assist in optimizing the individual operations and their order. This is particularly true for operations such as cutting and blanking, which rely on the nucleation of damage and cracks in order to separate material. In this work numerical simulation of the blanking process, using Deform 2D, taking in account the damage, has been performed. In order to evaluate the accuracy of the numerical solution, experimental test have been performed. Furthermore a numerical – experimental correlation has been carried out.

Abstract: Damage of metals subjected to large plastic deformations typical for forming processes is mainly governed by void nucleation, growth and coalescence. An opposite process may occur in deformation processes with negative stress triaxialities: the closure of strain-induced defects under large hydrostatic pressure. Understanding the mechanisms of damage growth and healing under plastic deformation of metals is still an urgent problem. In order to solve it a theoretical framework for anisotropic ductile damage based on a physically motivated concept for changes in the void volume and shape was recently developed [6]. Strain-induced damage was experimentally determined during uniaxial compression of cylindrical metallic specimens with artificial voids represented by fully-trough drilled holes. It was revealed that the governing physical mechanism of failure is a change in void shapes due to compressive stresses at low negative stress triaxialities in contrast to the growth of voids volume due to high positive stress triaxialities in the processes with dominating tensile stresses. The tensorial model presented in [6] proved to be able to describe kinetics of ductile damage, failure as the ultimate damage, and the closure of voids at negative stress triaxialities.

Abstract: In the field of sheet metal forming traditional forming processes are used. However, a quasi-static forming process combined with a high speed forming process can enhance the forming limits of a single one. In this paper, the investigation of the process chain quasi-static deep drawing – electromagnetic forming by means of a new coupled damage-viscoplasticity model for large deformations is performed. The finite strain constitutive model, used in the finite element simulation combines nonlinear kinematic and isotropic hardening and is derived in a thermodynamically consistent setting. The anisotropic viscoplastic model is based on the multiplicative decomposition of the deformation gradient in the context of hyperelasticity. The kinematic hardening component represents a continuum extension of the classical rheological model of Armstrong–Frederick kinematic hardening. Hill-type plastic anisotropy is modelled by expressing the yield surface as a function of second-order structure tensors as additional tensor-valued arguments. The coupling of damage and plasticity is carried out in a constitutive manner according to the effective stress concept. The constitutive equations of the material model are integrated in an explicit manner and implemented as a user material subroutine in the commercial finite element package of LS-Dyna with the electromagnetical modul. Aim of the work is to show the increasing formability of the sheet by combining quasi-static deep drawing processes with high speed electromagnetic forming process.

Abstract: 2D C/SiC ceramic matrix composite (CMC) displays significant damage characteristic coupled with inelastic strain under tension and shear loads, which should be considered in the constitutive model. In this study, a continuum damage mechanics (CDM) model was proposed for this material, in which the process degradation of the material property was described by introducing a set of scalar damage variables, and the damage-coupling effect was also considered. Meanwhile, isotropic hardening theory was applied to form the evolution rule of inelastic strains. The model was then implemented into the UMAT in ABAQUS software and validated by comparison between the simulation and experiment results.

Abstract: This paper considers damage development mechanisms in composite laminates subjected to tensile loading. The continuum damage mechanics is the most widely used approach to capture the non linear behaviour of laminates due to cracking. In this study, a continuum damage model based on ply failure criteria, which is initially proposed by Ladevèze has been extended to cover all plies failures mechanisms using an accurate numerical model to predict the equivalent damage accumulation. However, this model requires a reliable representation of the elementary damage mechanisms which can be produced in the composite laminate. To validate this model, a numerical application has been carried on the cross-ply laminates of type [0_n/90_m]_s._. A shear lag model was adapted to calculate the average stress of the 0° and 90° plies. The solution presented is obtained by using finite element analysis which implements progressive failure analysis. The effect of the stacking sequences has been done by varying the thickness of the 90° plies.