Papers by Keyword: Damage Modelling

Abstract: In the present work, a cellular automata finite element model (CAFE) was developed to model the ductile-brittle transition of a Grade A ship plate steel. Therefore, ductile and brittle cellular automata (CA) arrays of cells were created in the model to integrate material data at microstructural level, along with the ductile and brittle fracture processes. Microstructural data was analysed with Weibull distributions and incorporated in CAFE model using random number generators, along with ductile and brittle fracture parameters. Ductile fracture was modelled with Rousselier damage model; hence damage model parameters were calibrated with experimental data. Brittle fracture was modelled with Beremin model, and four different cleavage particles, found in a Grade A ship plate steel, were incorporated in CAFE model in order to model a competition of particles nucleating microcracks of critical size in the damage regions of Impact Charpy tests and four-point double-notch bend tests performed at low temperature. The mechanical properties the plate steel was measured in the transition region and incorporated in CAFE model, along with ductile-brittle transition rules. The present CAFE model was able to simulate distributions of microcracks in the notch region of four-point double-notch bend models (in the transition region), which correlated with experimental data. CAFE model was also able to simulate microvoids in the notch region of Charpy specimens along with the load-displacement Charpy curve for room test temperature, with very good agreement with experimental data. Once CAFE model was validated at micro and structural level, it was applied to model the typical scatter of impact Charpy energy values in the transition region of Grade A ship plate steel with good agreement with the measured ductile-brittle transition curved of the plate steel. Keywords: cellular automata, finite element modelling, ductile-brittle transition, damage modelling.

Abstract: The latest demands in reduction of emissions compel the automobile industry to lighten the structure of vehicles using third generation advanced high strength steels. Due to the novelty of these steels, there is a need to characterize its fracture behavior during the forming process. This paper presents a study of strain field, crack locus and instant of failure for 980 grade third generation advanced high strength steel using defined tests with two specimens. Numerical simulations and experiments have been performed to evaluate and to compare the obtained results for this steel. Numerical simulations with implemented Hosford-Coulomb damage model use the extended finite element method to predict the fracture occurrence. According to results, numerical simulation predicts crack locus similar to experimental tests. Failure of the material shows a high sensitivity to damage evolution law.

Abstract: Advanced High Strength Steels (AHSS) are widely used in today's automotive structures for lightweight design purposes. FE simulation is commonly used for the design of forming processes in automotive industry. Therefore, besides the description of the plastic flow behaviour, also the definition of forming limits in order to efficiently exploit the forming potential of a material is required. AHSS are prone for crack appearances without prior indication by thinning, like exemplary shear fracture on tight radii and edge-fracture, which can not be predicted by conventional Forming Limit Curve (FLC). Stress based damage models are able to do this. However, the parameterisation of such models has not yet been standardised. In this study a butterfly specimen geometry, which was developed at the Institute for Forming Technology and Machines (IFUM), was used for a stress state dependent fracture characterisation. The fracture behaviour of two AHSS, CP800 and DP1000, at varied stress states between pure shear and uniaxial loading was characterised by an experimental-numerical approach. For variation of the stress state, the specimen orientation relative to the force direction of the uniaxial testing machine was orientated at different angles. In this way, the relevant displacement until fracture initiation was determined experimentally. Subsequently, the experimental tests have been numerically reproduced giving information about the strain and stress evolution in the crack impact area of the specimen for the experimentally identified fracture initiation. With the help of this testing procedure, two different stress-based damage models, Modified Mohr-Coulomb (MMC) and CrachFEM, were parameterised and compared.

Abstract: Damage due to atmospheric corrosion on metal structures is a significant aspect for both the design of new constructions and the maintenance of existing buildings. The paper discusses the corrosion depth trends for steel structures comparing an experimental campaign of measurements, given by Fratesi in 2002, with literature 2^nd level models calibrated from experiments on immersed elements, literature models based on testing in atmosphere and standard codes (i.e. EN ISO 9224 and EN 12500). Results show a significant variability of values using different models and codes. In addition, the paper underlines that literature studies and codes neglect specific models for nineteenth-century “wrought iron” constructions, that are very sensitive to corrosion phenomena. Based on this, the paper discusses results obtained by a new interpretative model developed by authors for the prediction of corrosion depth on wrought iron structures.

Abstract: Aluminum alloy wheels are the most commonly used wheel type for passenger cars for decades. Generally A356 alloy (including alloying elements of 7% Si and 0.3% Mg) is used and a T6 heat treatment (solutionizing and artificial aging) is applied for the wheels. The most commonly used casting method is the Low Pressure Die Casting method for the wheels. As a cast product, wheels are one of the most important safety parts of a car along with a huge visual impact on the car. Therefore a lot of proofing tests are applied on a wheel in order to ensure its reliability and to guarantee passenger safety. Inner rim compression test of aluminum alloy wheels is one of these important mechanical tests which is a quasi-static deformation test to determine the fracture and failure behavior of the wheel. In this test, wheel is fixed at its offset surface using lug nuts and a crosshead applies the load with an offset from the inner rim position applying the biggest stress to the valve hole section. This study comprises the efforts of simulation of this test. In the study, ABAQUS finite element software is used and results were compared with experimentally obtained results.

Abstract: Metal-ceramic composites offer many advantages over monolithic metals and their alloys such as high specific stiffness, strength and good thermal properties. In this paper, stress fields in a single-domain sample of metal-ceramic composite containing multiple cracks in the ceramic layer are investigated. In our previous studies the cracked microstructure for different stage of damage is modeled by analytical and computational approaches. In this paper the assumptions of the analytical model were verified using FE-models for different crack widening.

Abstract: Aluminum die casting components are widely used in vehicle constructions because they satisfy the conflicting requirements between weight reduction and mechanical property improvement. However, the analysis of deformation and damage behavior of aluminum cast components is very complex, since local mechanical properties in the components are inhomogeneous as a consequence of spatial distribution of microstructure. For crash simulation it is necessary to well predict the damage behavior which is strongly influenced by micro-defects especially by cast pores. The conventional continuum mechanics approaches often fail due to the statistical character of cast pores. In this work the Markov random field model (Ising) is used to describe the pore morphology. Markov random field classes are defined by porosity (macroscopic property) and equivalent pore size (microscopic property) and determined by micro computer tomography (CT) analysis.A multi scale approach was applied to map the results of the stochastic model to the FE models, which results in a distribution of porosity. A porosity dependent continuum model was developed based on results of representative volume elements with variation of porosity. It was shown that the continuum model with porosity distributions from the Ising model as initial conditions captures well the spatial material properties (i.e. fracture strain) and their variations in the bridging scale.

Abstract: The damage behavior of aluminum profiles depends strongly on the stress state. Many investigations have shown that both ductile and shear fracture have to be taken into account in damage analysis. Since fracture strains of aluminum profiles are relatively low, damage modelling has to be included in component simulations. However, it is an open question, which kind of damage model can be used for crash simulations and which tests should be performed in order to calibrate the model. An extruded aluminum profile with double chambers of AA6060-T79 was characterized under different stress triaxialities and shear ratios. The damage criteria IDS (Instability, Ductile and Shear fracture) in ABAQUS/Explicit were used for the simulations. An explicit relationship between triaxiality and shear ratio was derived for plane stress state. The influence of the model parameter on the overlapping of both criteria (ductile and shear fracture) was systematically studied for shell element applications. The applied damage model was validated by comparing experimental and calculated results of component tests.

Abstract: This paper provides an overview of recent developments in the modeling of progressive damage in fiber-reinforced composite laminates. Some insights into modeling the size effects of open-hole composite laminates under in-plane tension and compression, the significance of ply-blocking and delamination are discussed. Recent interest in the interaction and migration of matrix cracks and delamination, resulting in development of integrated XFEM-CE and floating node methods will also be presented.

Abstract: There have been many efforts to investigate and develop a numerical damage and failure models during metal forming process of lightweight alloys. Due to the difficulties experienced during experimental determination of the incurred damage during forming of lightweight alloys, many researchers have sought to predict the damage, failure and forming limit curves using numerical simulations. Conventional finite element analysis of metal forming processes for lightweight parts which have been subjected to a nonlinear strain history often breaks down due to numerical difficulties. Many scientific research works have attempted to use different mathematical methods to model the damage progression and failure of alloying material under large deformation. The damage initiation, progression and also failure of alloys are a result of accumulated damage under plastic deformation [1-3]. These models (single and multi-damage parameters) are generally based on energy and constitutive equations to simulate the fracture and failure processes in metal alloys. However, these methods have serious short comes in predicting the damage and failure in metal forming process with strain rate effects. In the present study, following the in-depth study of damage initiation and progression in lightweight alloys, a frame work has been setup to develop a numerical model for damage accumulation during forming process. Based on the existing damage theory, a mathematical extension for damage initiation and also damage accumulation under wide range of stress triaxiality (including near pure shear) has been developed. An experimental program has also been carried out using samples made from lightweight alloys. One of the main contributions of this paper is to show the advantages of using plasticity-based modified damage models to investigate the damage accumulation in cast aluminium alloys.