Literature study report of plasticity induced anisotropic damage modeling for forming processes

A literature study report covering the topics; micromechanics of damage, continuum damage mechanics (gurson model and effective variable concept) and the dependence of damage on strain rate and temperature

Parameter identification of a mechanical ductile damage using Artificial Neural Networks in sheet metal forming.

In this paper, we report on the developed and used of finite element methods, have been developed and used for sheet forming simulations since the 1970s, and have immensely contributed to ensure the success of concurrent design in the manufacturing process of sheets metal. During the forming operation, the Gurson–Tvergaard–Needleman (GTN) model was often employed to evaluate the ductile damage and fracture phenomena. GTN represents one of the most widely used ductile damage model. In this investigation, many experimental tests and finite element model computation are performed to predict the damage evolution in notched tensile specimen of sheet metal using the GTN model. The parameters in the GTN model are calibrated using an Artificial Neural Networks system and the results of the tensile test. In the experimental part, we used an optical measurement instruments in two phases: firstly during the tensile test, a digital image correlation method is applied to determinate the full-field displacements in the specimen surface. Secondly a profile projector is employed to evaluate the localization of deformation (formation of shear band) just before the specimen’s fracture. In the validation parts of this investigation, the experimental results of hydroforming part and Erichsen test are compared with their numerical finite element model taking into account the GTN model. A good correlation was observed between the two approaches

Strain localization analysis using a large deformation anisotropic elastic-plastic model coupled with damage

Sheet metal forming processes generally involve large deformations together with complex loading sequences. In order to improve numerical simulation predictions of sheet part forming, physically-based constitutive models are often required. The main objective of this paper is to analyze the strain localization phenomenon during the plastic deformation of sheet metals in the context of such advanced constitutive models. Most often, an accurate prediction of localization requires damage to be considered in the finite element simulation. For this purpose, an advanced, anisotropic elastic-plastic model, formulated within the large strain framework and taking strain-path changes into account, has been coupled with an isotropic damage model. This coupling is carried out within the framework of continuum damage mechanics. In order to detect the strain localization during sheet metal forming, Rice's localization criterion has been considered, thus predicting the limit strains at the occurrence of shear bands as well as their orientation. The coupled elastic-plastic-damage model has been implemented in Abaqus/implicit. The application of the model to the prediction of Forming Limit Diagrams (FLDs) provided results that are consistent with the literature and emphasized the impact of the hardening model on the strain-path dependency of the FLD. The fully three-dimensional formulation adopted in the numerical development allowed for some new results - e.g. the out-of-plane orientation of the normal to the localization band, as well as more realistic values for its in-plane orientation.ArcelorMittal & Projet Européen CEC

Prediction of material damage in orthotropic metals for virtual structural testing

Models based on the Continuum Damage Mechanics principle are increasingly used for predicting the initiation and growth of damage in materials. The growing reliance on 3-D finite element (FE) virtual structural testing demands implementation and validation of robust material models that can predict the material behaviour accurately. The use of these models within numerical analyses requires suitable material data. EU aerospace companies along with Cranfield University and other similar research institutions have created the MUSCA (non-linear MUltiSCale Analysis of large aero structures) project to develop virtual structural testing prediction. The MUSCA project focuses on static failure testing of large aircraft components. It aims to reduce laboratory tests using advanced numerical analysis to predict failure in order to save overall cost and development time. This thesis aims to improve the current capability of finite element codes in predicting orthotropic material behaviour, primarily damage. The Chow and Wang damage model has been implemented within ABAQUS as a VUMAT subroutine. This thesis presents the development of a numerical damage prediction model and an experimental study to develop a damage material characterisation process that can easily be performed using standard tensile test specimen and equipment already available in the aerospace industry. The proposed method makes use of Digital Image Correlation (DIC), a non-contact optical strain field measurement technique. Experiments were conducted at Cranfield University material testing facility on aerospace aluminium alloy material AA-2024-T3 and AA-7010-T7651. After thorough literature survey a complete new method was formulated to implement Chow and Wang damage model in Abaqus Explicit numerical code. The damage model was successfully implemented for isotropic and orthotropic behaviour using single element model, multi-element coupon test model and a simple airframe structure. The simulation results were then verified with the similar experimental results by repeating the experimental procedure using simulation for each material type and found matching results. The model is then compared with experimentally determined orthotropic material parameter for AA2024 and AA7010 for validation and found agreeable results for practical use. The material characterisation of damage parameters from standard tensile specimen using DIC technique was also demonstrated and the procedures were established. In this research the combination of experimental work and numerical analysis with clear and simpler calibration strategy for damage model is demonstrated. This is the important contribution of this research work and the streamlined procedures are vital for the industry to utilise the new damage prediction tools. The damage model implementation and test procedures developed through this research provide information and processes involved in fundamentally predicting the ductile damage in metals and metal alloys. The numerical damage model developed using the well-defined verification and validation procedures explained in this research work with new streamlined damage material characterisation using recent contact less DIC technique has wider implication in the material model development for ductile metals in general. The thesis ultimately delivered a fully verified, validated robust damage model numerical simulation code with a new DIC damage characterisation procedure for practical application. The model is now used by the aerospace industry for predicting damage of large aircraft structures

Strain localization analysis using a large deformation anisotropic elastic-plastic model coupled with damage

Sheet metal forming processes generally involve large deformations together with complex loading sequences. In order to improve numerical simulation predictions of sheet part forming, physically-based constitutive models are often required. The main objective of this paper is to analyze the strain localization phenomenon during the plastic deformation of sheet metals in the context of such advanced constitutive models. Most often, an accurate prediction of localization requires damage to be considered in the finite element simulation. For this purpose, an advanced, anisotropic elastic-plastic model, formulated within the large strain framework and taking strain-path changes into account, has been coupled with an isotropic damage model. This coupling is carried out within the framework of continuum damage mechanics. In order to detect the strain localization during sheet metal forming, Rice's localization criterion has been considered, thus predicting the limit strains at the occurrence of shear bands as well as their orientation. The coupled elastic-plastic-damage model has been implemented in Abaqus/implicit. The application of the model to the prediction of Forming Limit Diagrams (FLDs) provided results that are consistent with the literature and emphasized the impact of the hardening model on the strain-path dependency of the FLD. The fully three-dimensional formulation adopted in the numerical development allowed for some new results - e.g. the out-of-plane orientation of the normal to the localization band, as well as more realistic values for its in-plane orientation.ArcelorMittal & Projet Européen CEC

Ductile damage prediction in sheet metal forming and experimental validation

Tese de doutoramento. Engenharia Mecânica. Universidade do Porto. Faculdade de Engenharia. 201

Advanced damage modelling of free machining steels

The current available damage models do not accurately predict effective plastic strain to failure in low triaxiality stress states. A damage model was developed for low triaxiality that is appropriate to hot rolling of steel. This work focuses on nucleation and growth of damage as well as the effect of the strain and stress path. The latter is especially important for the rolling of bar and other complex cross-section products. A study of damage mechanisms and methods to model them has been undertaken. It is pointed out that the many models are only useful under certain conditions but can be used when the expected damage mechanisms are active. Several test types were evaluated to assess their ability to simulate stress state in rolling. A program has been written to evaluate the stress state for plane and axisymmetric tests, which allows one to choose the most appropriate test-piece geometry. A test has been designed and implemented. Thermal and mechanical data was gathered, which has been used to relate the stress triaxiality to damage growth and identify appropriate damage growth models. The size and spacing distributions of inclusions in free cutting steels have been measured. The different distributions have an effect on the ductility of the different steels. This effect has been found to change at different strain rates and temperatures. By better accounting for the effect of inclusions on damage growth under a range of test conditions, the damage model can be significantly improved. Free cutting steels that contained different additions of heavy metals were tested. The ductility and damage mechanisms were compared in each of the steels. The effect of the precipitation of the different heavy metals at the inclusion to matrix boundary was highlighted. The same damage mechanisms were observed in each steel but the ability to accommodate damage varied between the steels. Ex-situ synchrotron x-ray micro-tomography was used to better measure and quantify the distribution of inclusions and damage evolution in a free cutting steel. Localised damage coalescence away from the centre of the uniaxial tensile test-piece was attributed to the effect of inclusion clustering. This research was used to develop a realistic damage model, which can predict damage growth and coalescence for a range of forming parameters and different stress-state conditions related to hot rolling applications. The micro-mechanics based model includes the effects of inclusion distribution on damage. The model is calibrated using twenty six temperature based material constants

Multiscale Analysis of Contact in Smooth and Rough Surfaces: Contact Characteristics and Tribo-Damage

This dissertation is comprised of two major interrelated foci. The first focus is to investigate the effect of surface roughness on the behavior of dry contacting bodies through both deterministic and statistical approaches. In the current research, different statistical micro-contact models are employed together with the bulk deformation of the bounding solids to predict the characteristics of the dry rough line-contact and elliptical point-contact including the apparent pressure profile, contact dimensions and real area of contact. Further, based on the results of numerical simulations, useful relationships are provided for the contact characteristics. In addition, a robust approach for the deterministic prediction of pressure and tangential traction distributions in dry rough contact configuration subjected to stick-slip condition is presented with provision for elastic-fully plastic asperity effects. The second focus of this research involves the assessment of three of the most common types of degradation processes that are observed in contact mechanics. The first contact failure mechanism studied is the rolling/sliding contact fatigue wear. In this research, the principles of continuum damage mechanics (CDM) are applied to predict the rolling/sliding contact fatigue crack initiation, and the effect of variable loading on the fatigue behavior of rolling contact with provision for non-linear damage evolution is investigated. The estimated numbers of cycles to crack initiation are compared to the available experimental results revealing good agreement. The second contact degradation phenomenon involves the study of the adhesive wear for unlubricated and lubricated contacts. A method is presented that applies the principles of CDM to predict the Archard adhesive wear coefficient for unlubricated contacts. By carrying out pin-on-disk experiments, wear coefficients for a specific material are obtained and compared with the predicted values showing good agreement. Further, the load sharing concept is applied to develop an engineering model for lubricated wear with the consideration of the thermal effects. The third type of degradation studied is the so-called fretting fatigue which is a failure phenomenon observed in contacting bodies subjected to very small amplitude oscillatory motion. Using the deterministic model developed for stick-slip contact condition, the effect of surface roughness on the crack initiation risk in a fretting contact is investigated and compared with experimental observations. In order to investigate the last two degradation phenomena, the results obtained from the first objective are directly utilized