Mesoscopical finite element simulation of fatigue crack propagation in WC/Co-hardmetal

WC/Co is an important technical material used in a wide range of industrial applications such as cutting tools, drilling bits and drawing dies due to its excellent combination of wear resistance, strength and toughness.The focus of this study is the numerical study of the microscale fatigue crack propagation in WC/Co. In this respect, a damage model based on a continuum damage mechanics (CDM) approach was implemented in the commercial FEM solver Abaqus/Explicit for simulating the crack propagation in the material. Separate damage laws based on brittle failure and accumulated plasticity driven fatigue are implemented for the WC and the Co phases, respectively.The material parameters for the carbides are taken from literature. On the other hand, to obtain the material parameters for the binder, a particular model alloy has been developed representing the composition of the binder. Experimental tests carried out with this binder alloy have been used to identify parameters for the appropriate plasticity and damage models.In order to evaluate the performance of the approach, a numerical model based on an experimental case was generated. The numerical model reflected strong agreement in comparison with the real crack pattern generated during the experiment. Moreover, results of this study indicate a strong dependence of the fatigue crack propagation on accumulated plasticity within the binder phase; this effect suggests a novel understanding of the fatigue mechanism of this material and provides a basis for microstructural simulation.     

An approach for development of damage-free drilling of carbon fiber reinforced thermosets

A new comprehensive approach to select cutting parameters for damage-free drilling in carbon fiber reinforced epoxy composite material is presented. The approach is based on a combination of Taguchi's experimental analysis technique and a multi-objective optimization criterion. The optimization objective includes the contributing effects of the drilling performance measures: delamination, damage width, surface roughness, and drilling thrust force. A hybrid process model based on a database of experimental results together with numerical methods for data interpolation are used to relate drilling parameters to the drilling performance measures. Case studies are presented to demonstrate the application of this method in the determination of optimum drilling conditions for damage-free drilling in BMS 8-256 composite laminate. A process map based on the results is presented as a tool for drilling process design and optimization for the investigated tool/material combination.     

Comparison of numerically and analytically predicted contact temperatures in shallow and deep dry grinding with infrared measurements

Contact zone thermal models of the grinding process are an important tool for the proper selection of process parameters to minimize workpiece damage while improving process efficiency. Validating contact zone thermal models with experimental measurements is difficult due to the high-speed and stochastic nature of the grinding process. In this work an infrared imaging system is used to validate two numerical thermal models, which are then compared to an established analytical contact zone thermal model. The two numerical thermal models consist of a shallow grinding model and a deep grinding model, where the deep grinding model takes the contact angle into account while the shallow grinding model does not. The results show that at small depths of cut both the numerical models and the analytical model perform well; however, as the depth of cut is increased the numerical models’ accuracy increases as compared to the analytical model. The increase in accuracy may be a result of the 2D solution of the numerical models as compared to the 1D solution of the analytical model. Additionally, it was found that the contact angle has very little effect on the contact temperatures. This work also reinforces Rowe's analytical work, using experimental and numerical results, which indicated that the workpiece temperatures are reduced by grinding at higher Peclet numbers for a given material removal rate.     

Theoretical and Experimental Investigation on SMA Superelastic Springs

The mechanical behavior of superelastic springs is investigated in this study. The goal is to evaluate the device response and to exploit the material superelastic behavior, main concerns being material and geometrical response nonlinearity. The investigation is made of two parts, i.e., an experimental campaign and a numerical model proposal. Experimental tests have been performed on superelastic SMA coil springs considering load history in tension and compression for three different spring geometrical configurations. Tested specimens experience a maximum elongation larger than the original spring axis length. The response is not symmetric and under compression it is affected by buckling instability. Nevertheless, experimental results show a very good superelastic behavior with no damage and with negligible residual displacements. Numerical analyses have been performed to reproduce the experimental campaign results. A simple finite element model is proposed. Experimental and numerical result agreement is very good. The numerical model turns out to be a powerful design tool even for the very complex geometrical and material nonlinear conditions under investigation. Hence, it is proposed as a useful tool for spring design validation and response prediction.     

Numerical analysis of shear deformation localization using a double-variable damage model

A double-variable damage model was introduced into the constitutive equations to demonstrate the effect of the material damage for the isotropic elastic, hardening, and damage states, and for the isothermal process. The shear damage variable D _s and the bulk damage variable D _b may be, respectively, used to describe the effect of shear damage and bulk damage for material properties without the superfluous constraint, D _b=D _s, that is found in the single-variable damage model. The double-variable damage model was implemented to form the finite element code for analyzing the effect of shear damage and bulk damage. In this article, two numerical simulation examples were completed to model the whole process of initiation and propagation of shear bands in an aluminum alloy. The numerical computational results are coincident with the experimental results.     

Model Investigation on Composite Failure Prediction of π-Joint Structures

This paper was concerned with the tensile mechanics behavior of the composite π-joint under static tensile loading. The numerical strength analysis methodology was presented containing the basis assumption for the analysis, the material modeling, and the selected element type. It was assumed that the composite ply had transverse isotropic material properties and the adhesive had linear elastic properties. With the goal of the strength analysis to determine the onset of the damage initiation and the ultimate...     

Grain level analysis of crack initiation and propagation in brittle materials

A study on the accuracy of cohesive models for capturing dynamic fragmentation of ceramic microstructures is presented. The investigation consists of a combined experimental/numerical approach in which microcracking and damage kinetics are examined by means of plate impact recovery experiments. The numerical analysis is based on a 2-D micromechanical stochastic finite element analysis. The model incorporates a cohesive law to capture microcrack initiation, propagation and coalescence, as well as crack interaction and branching, as a natural outcome of the calculated material response. The stochasticity of the microfracture process is modeled by introducing a Weibull distribution of interfacial strength at grain boundaries. This model accounts for randomness in grain orientation, and the existence of chemical impurities and glassy phase at grain boundaries. Representative volume elements (RVE) of ceramic microstructure with different grain size and shape distributions are considered to account for features observed in real microstructures. Normal plate impact velocity histories are used not only to identify model parameters, but also to determine under what conditions the model captures failure mechanisms experimentally observed. The analyses show that in order to capture damage kinetics a particular distribution of grain boundary strength and detailed modeling of grain morphology are required. Simulated microcrack patterns and velocity histories have been found to be in a good agreement with the experimental observations only when the right grain morphology and model parameters are chosen. It has been found that the addition of rate effects to the cohesive model results in microcrack diffusion not observed experimentally.     

Geometric,kinematical and thermal analyses of non-round cylindrical grinding

This paper is concerned with the analyses of grinding geometry and kinematics in the grinding zone and develops a thermal model, along with a chip-thickness-dependent value of specific grinding energy into the workpiece. The grinding geometry and kinematics are modeled for arbitrary non-round workpiece forms. Unlike other models, which are based on a fixed, constant geometry, the model presented here is based on first principles using a fundamental, transient, non-constant geometry and thus constitutes a much-needed advancement in grinding technology. A novel experimental approach is also taken, which uses the specific grinding energy into the workpiece, rather than the total specific grinding energy coupled with an estimate of the energy partition, an estimate which previously has proven difficult to achieve accurately. The model is verified with experimental work and predicted temperatures are in reasonable agreement with temperatures associated with the onset of thermal damage, determined via metallographic examination and Barkhausen noise. Finally, some of the challenges of using Barkhausen noise to evaluate thermal damage are investigated, namely the differing response characteristics of stressed and overtempered material vs. rehardened material, and how these can be overcome in practice.     

Prediction of damage in small curvature bending processes of high strength steels using continuum damage mechanics model in 3D simulation

Sheet metal bending of modern lightweight materials like high-strength low-alloyed steels (HSLA) is one major challenge in metal forming, because conventional methods of predicting failure in numerical simulation, like the forming limit diagram (FLD), can generally not be applied to bending processes. Furthermore, the damage and failure behaviour of HSLA steels are changing as the fracture mechanisms are mainly depending on the microstructure, which is very fine-grained in HSLA steels composed with different alloying elements compared to established mild steels. Especially for high gradients of strain and stress over the sheet thickness, as they occur in small curvature bending processes, other damage models than the FLD have to be utilised. Within this paper a finite element (FE) 3D model of small curvature bending processes is created. The model includes continuum damage mechanics model in order to predict and study occurring failure by means of ductile coherence loss of the material and crack formation with respect to influencing process parameters. Damage parameters are determined by inverse numerical identification method. The FE-model is strain based validated considering the deformation field at the outer bending edge of the specimen by using an optical strain measurement system. The Lemaitre based damage model is calibrated against the experimental results within metallographic analysis adapting the identified damage parameters to the bending process und thus adjusting the crack occurrence in experiment and simulation. Using this model the bendability of common HSLA steel, used for structural components, is evaluated with respect to occurring damage and failure by numerical analysis.     

Numerical and experimental study of dry cutting for an aeronautic aluminium alloy (A2024-T351)

In the present contribution, numerical and experimental methodologies concerning orthogonal cutting are proposed in order to study the dry cutting of an aeronautic aluminium alloy (A2024-T351). The global aim concerns the comprehension of physical phenomena accompanying chip formation according to cutting velocity variation.For the numerical model, material behaviour and its failure criterion are based on the Johnson–Cook law. The model proposes a coupling between material damage evolution and its fracture energy. A high-speed camera was used to capture the chip formation sequences. The numerical results show that the chip–workpiece contact and the tool advancement induce bending loads on the chip. Consequently, a fragmentation phenomenon takes place above the rake face when the chip begins to curl up. The computed results corroborate with experimental ones. The numerical results predict the residual stress distribution and show high values, along the cutting direction, on the machined workpiece surface.     

Damage growth triggered by interface irregularities in thermal barrier coatings

The efficiency and reliability of modern jet engines strongly depend on the performance of thermal barrier coatings (TBCs), which prevent melting and oxidation of the turbine blades’ structural core. The system’s lifetime is limited by cracks appearing in and in the vicinity of an oxide layer that grows in the TBC under thermal cycling. High replacement costs have led to an increased demand to identify, quantify and remedy damage in TBCs. An integrated experimental–numerical approach is presented for studying the main factors that contribute to damage, particularly interfacial irregularities. Damage at several stages of oxidation in TBCs is analyzed in samples with predefined interfacial irregularities. The model predicts the experimentally observed crack patterns, clearly quantifying the influence of imperfections and indicating that damage can be delayed by surface treatment.     

Modeling and FE Simulation of Quenchable High Strength Steels Sheet Metal Hot Forming Process

High strength steel (HSS) sheet metal hot forming process is investigated by means of numerical simulations. With regard to a reliable numerical process design, the knowledge of the thermal and thermo-mechanical properties is essential. In this article, tensile tests are performed to examine the flow stress of the material HSS 22MnB5 at different strains, strain rates, and temperatures. Constitutive model based on phenomenological approach is developed to describe the thermo-mechanical properties of the material 22MnB5 by fitting the experimental data. A 2D coupled thermo-mechanical finite element (FE) model is developed to simulate the HSS sheet metal hot forming process for U-channel part. The ABAQUS/explicit model is used conduct the hot forming stage simulations, and ABAQUS/implicit model is used for accurately predicting the springback which happens at the end of hot forming stage. Material modeling and FE numerical simulations are carried out to investigate the effect of the processing parameters on the hot forming process. The processing parameters have significant influence on the microstructure of U-channel part. The springback after hot forming stage is the main factor impairing the shape precision of hot-formed part. The mechanism of springback is advanced and verified through numerical simulations and tensile loading-unloading tests. Creep strain is found in the tensile loading-unloading test under isothermal condition and has a distinct effect on springback. According to the numerical and experimental results, it can be concluded that springback is mainly caused by different cooling rats and the nonhomogengeous shrink of material during hot forming process, the creep strain is the main factor influencing the amount of the springback.     

Numerical modeling of advanced materials

The finite element (FE) method is widely used to numerically simulate forming processes. The accuracy of an FE analysis strongly depends on the extent to which a material model can represent the real material behavior. The use of new materials requires complex material models which are able to describe complex material behavior like strain path sensitivity and phase transformations. Different yield loci and hardening laws are presented in this article, together with experimental results showing this complex behavior. Recommendations on how to further improve the constitutive models are given. In the area of damage and fracture behavior, a non-local damage model is presented, which provides a better prediction of sheet failure than the conventional Forming Limit Diagram.     

Fatigue analysis of closed-cell aluminium foam using different material models

The fatigue analyses of AlSi7 closed-cell aluminium foam were performed using a real porous model and three different homogenised material models: crushable foam model, isotropic hardening model and kinematic hardening model. The numerical analysis using all three homogenised material models is based on the available experimental results previously determined from fatigue tests under oscillating tensile loading with the stress ratio R=0.1. The obtained computational results have shown that both isotropic and kinematic hardening models are suitable to analyse the fatigue behaviour of closed-cell aluminium foam. Besides, the kinematic hardening material model has demonstrated significantly shorter simulation time if compared to the isotropic hardening material model. On the other hand, the crushable foam model is recognized as an inappropriate approach for the fatigue analyses under tension loading conditions.     

Deformation model of a thermomechanical gear finishing process

A low temperature thermomechanical precision finishing process is being developed in which ausforming and gear rolling are being combined to result in lower processing costs and improvements in final properties compared to conventional gear finishing processes. Since the ability to predict the deformations and loads is an important key to the success of the process, a numerical model is being developed to study the effect of forming operations on strains and displacements. This paper describes the current status of the program. The finite element program has been verified by solving a cylinder upsetting problem for which both experimental and numerical solutions are well documented. Nonlinear formulations due to material, geometry, and contact conditions have been modeled in the program and their application to the gear finishing processes has been demonstrated. The final tooth contour on both the approach and trailing sides of an 8.5 diametral pitch spur gear have been predicted.     

Semi-analytical modelling with numerical and experimental validation of electromagnetic forming using a uniform pressure actuator

In this study, an electromagnetic forming process using a uniform pressure actuator is investigated through electro-magnetic-mechanically coupled numerical simulations; a simplified analytical model to predict the forming pressure and shell theory for mechanical deformation; and experimental results, which include Photon Doppler Velocimetry to measure the deformation. Velocity and the final deformed part shape are compared between the numerical, analytical, and experimental methods and reasonably good agreement is demonstrated. However, accurate comparison is affected by the energy level used with the numerical simulations matching better for the lower energy case due to less material draw-in and the analytical model providing more precise results for the higher energy case.     

基于损伤力学的粉末高温合金FGH96疲劳寿命预测

以粉末高温合金FGH96为研究对象,提出采用损伤力学理论来建立寿命预测模型.对于不同夹杂物特征,粉末高温合金裂纹萌生有不同的表征参量,其数值变化为裂纹萌生的寿命预测提供思路.对粉末高温合金寿命预测的研究现状进行分析,然后利用损伤演变方程建立寿命预测模型;使用有限元软件(ANSYS)分别模拟夹杂物的不同位置、不同尺寸以及...     

Derivation,Parameterization and Validation of a Sandy-Clay Material Model for Use in Landmine Detonation Computational Analyses

A set of large-strain/high-deformation-rate/high-pressure material models for sand-based soils with different saturation levels and clay and gravel contents was recently proposed and validated in our study, and the same has been extended in this study to include clay-based soils of different saturation levels and sand contents. The model includes an equation of state which reveals the material response under hydrostatic pressure, a strength model which captures material elastic-plastic response under shear, and a failure model which defines the laws and conditions for the initiation and evolution of damage and ultimate failure of the material under negative pressure and/or shear. The model was first parameterized using various open-literature experimental results and property correlation analyses and, then, validated by comparing the computational results obtained in an ANSYS/Autodyn-based transient non-linear dynamics analysis of detonation of a landmine buried in sandy-clay with their experimental counterparts.