Anisotropic Material Behavior Research Articles

Grasp synthesis for purposeful fracturing of object

This paper deals with the problem of purposefully failing (breaking) or yielding objects by a robotic gripper. Robotic harvesting is considered as an application domain that motivates this study. A definition of a failure task is first formulated using failure theories. Next, a grasp quality measure is presented to characterize a suitable grasp configuration and systematically control the failure behavior of the object. This approach combines the failure task and the capability of the gripper for wrench insertion. The friction between the object and the gripper is used to formulate the capability of the gripper for wrench insertion. A new method inspired by the human pre-manipulation process is introduced to utilize the gripper itself as the measurement tool and obtain a friction model. The developed friction model is capable of capturing the anisotropic behavior of materials which is the case for most fruits and vegetables. The evaluation method proposed in this study is formulated as a quasistatic grasp problem and can include both fully-actuated and under-actuated grippers. The proposed approach for purposefully breaking objects is validated using experimental results. Objects with different material properties are used to prove the generality of the method. KUKA LightWeight Robot IV is used as the manipulator.

Neural network based model for estimation of the level of anisotropy of unbound aggregate systems

Abstract Directional dependency of the materials properties in granular soils is an integral component of the analysis and design of pavement foundations. Laboratory determination of such anisotropic properties is often overlooked by design engineers due to the lack of available protocols, equipment, time constraints, and budgetary issues. This research strives to bridge this gap by investigating an alternative approach to provide a practical and reliable framework to estimate the level of anisotropy of unbound granular materials. To achieve this objective, an experiment design was developed to establish a comprehensive aggregate feature database. Nonlinear and cross anisotropic material properties were determined using Variable Dynamic Confining Pressure (VDCP) stress path tests in the laboratory. Particle geometry was characterized using the Aggregate Imaging System (AIMS). Scale parameters and shape parameters of the form, angularity and textural properties of the particles were incorporated in the aggregate database to account for the shape-induced anisotropy of particulate soils. Moisture state and density parameters were also incorporated in the database for further post processing. Several neural network models with different architectures were developed, and the performances of the models were assessed based on an unseen set of data. The impact of the neural network topologies on the performance of the models were also investigated in this research effort. The results indicate that increasing the number of hidden layers has negligible impact on the performance of the models. Additionally, a series of sensitivity analyses were performed to investigate the level of contribution of aggregate features on the anisotropic behavior of unbound granular materials. The results of the sensitivity analysis underscores the influence of aggregate angularity, fines content, surface macro-texture, as well as the parameters of the stress path tests, on the orthogonal load distribution capacity of aggregate layers.

Phase-Field Modelling of Crack Propagation in Anisotropic Polycrystalline Materials

Nowadays, products consisting of polycrystalline materials have been widely used in engineering applications, e.g. automobile and renewable energy. The macroscopic defects are generally strongly influenced by the fracture behavior of the polycrystalline materials at the meso- and microscopic level. In this paper, the proposed phase-field model for anisotropic fracture, which accounts for the preferential cleavage directions within each randomly oriented crystal, as well as an anisotropic material behavior with cubic symmetries, has been used to simulate the complex crack pattern in solar-grade polycrystalline silicon in a robust and straightforward manner. Furthermore, multi-field coupled finite element problems are performed with monolithic solution schemes. A representative numerical example for crack propagation in polycrystals is carried out. Finally, a summary of the numerical results in polycrystalline materials is presented and an outlook for next work steps is given.

Phase-Field Modelling of Crack Propagation in Anisotropic Polycrystalline Materials

Abstract Nowadays, products consisting of polycrystalline materials have been widely used in engineering applications, e.g. automobile and renewable energy. The macroscopic defects are generally strongly influenced by the fracture behavior of the polycrystalline materials at the meso- and microscopic level. In this paper, the proposed phase-field model for anisotropic fracture, which accounts for the preferential cleavage directions within each randomly oriented crystal, as well as an anisotropic material behavior with cubic symmetries, has been used to simulate the complex crack pattern in solar-grade polycrystalline silicon in a robust and straightforward manner. Furthermore, multi-field coupled finite element problems are performed with monolithic solution schemes. A representative numerical example for crack propagation in polycrystals is carried out. Finally, a summary of the numerical results in polycrystalline materials is presented and an outlook for next work steps is given.

Isothermal Fatigue and Creep-Fatigue Interaction Behavior of Nickel-Base Directionally Solidified Superalloy

The creep-fatigue interaction in directionally solidified nickel-base superalloy was analyzed with the modified Chaboche-based unified viscoplasticity constitutive model. The model features the anisotropic material behavior, hardening/softening, and stress relaxation. Simple low-cyclic fatigue and specified hold time experiments were conducted on a directionally solidified superalloy (DZ125) at temperatures over 760°C. The material parameters were optimized considering its tensile, cyclic and creep behavior with the Levenberg–Marquardt optimization procedure. The model was constructed in FORTRAN and integrated in FEA software UMAT/ABAQUS. The results show that experimental and simulated hysteresis loop size/shape, peak stresses, stress relaxation, and related areas are closely matched. The modified constitutive model was found to be instrumental for revealing the fatigue and creep-fatigue interaction behavior of such materials and can be used for practical applications.

Geometrically exact nonlinear analysis of pre-twisted composite rotor blades

Modeling of pre-twisted composite rotor blades is very complicated not only because of the geometric non-linearity, but also because of the cross-sectional warping and the transverse shear deformation caused by the anisotropic material properties. In this paper, the geometrically exact nonlinear modeling of a generalized Timoshenko beam with arbitrary cross-sectional shape, generally anisotropic material behavior and large deflections has been presented based on Hodges’ method. The concept of decomposition of rotation tensor was used to express the strain in the beam. The variational asymptotic method was used to determine the arbitrary warping of the beam cross section. The generalized Timoshenko strain energy was derived from the equilibrium equations and the second-order asymptotically correct strain energy. The geometrically exact nonlinear equations of motion were established by Hamilton’s principle. The established modeling was used for the static and dynamic analysis of pre-twisted composite rotor blades, and the analytical results were validated based on experimental data. The influences of the transverse shear deformation on the pre-twisted composite rotor blade were investigated. The results indicate that the influences of the transverse shear deformation on the static deformation and the natural frequencies of the pre-twisted composite rotor blade are related to the length to chord ratio of the blade.

On the Modeling of Textile Membranes with Nonlinear Anisotropic Material Behavior

AbstractCoated fabric membranes have been widely used in construction of light‐weight structures. The material behavior of textile membranes is highly nonlinear and anisotropic which may not be sufficiently represented by commonly used linear elastic orthotropic models. This contribution presents a formulation of a suitable nonlinear anisotropic polyconvex energy function for textile membranes in the geometrically nonlinear framework. Representation of total in‐plane material behavior is the main goal of this paper, however, the induced thickness change is also considered in the continuum model. The material parameters of the energy function are adjusted by introducing and minimizing an objective function enforcing the model response to be as similar as possible to the experimental data. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Material lattice orientation effect of local Si1-xGex stressors on the width dependence of high-k metal gate PMOSFETs

The novel material selection and fabrication became a major concern due to scaling limitations when the technology node was below 20 nm. Complex gate-last process step for high-k metal gate (HKMG) device was proposed to enhance device mobility gain through the proper assistance of boosting channel stress. However, the impact of anisotropic material behavior regarding SiGe and silicon device is rarely estimated. Thus, the foregoing mechanical responses combined with concerned device layout during gate etch, replacement metal gate must be investigated. In this study, a process-oriented simulation methodology was presented to investigate this critical issue by a vehicle of 20 nm short-channel HKMG p-type MOSFET with embedded SiGe alloy stressor. Results indicated that the poly gate etch process significantly improved the longitudinal channel stress (Sxx) of this device channel. Process-induced Sxx values of −306.94 and −1325.48 MPa were obtained when the gate-first and gate-last processes were estimated, and 100 nm gate width was achieved, respectively. Given the increase in gate widths from 100 nm to 4000 nm, the maximum carrier mobility of 116.57% and 111.01% were obtained when isotropic and anisotropic material properties were adopted.

Modeling of Anisotropic Behavior of Aluminum Alloys to Investigate Ductile Fracture for the Improved Formability in the Upsetting Process

Abstract In the bulk-forming operations, such as upsetting, the billets upon severe plastic deformation will fracture on reaching the threshold fracture stress. The fracture initiation and progression depends on the friction, height-to-diameter ratios (h/d), and the strength of the material. The anisotropic behavior of material is another issue that should be taken into consideration while designing the process. Macro fracture criteria, such as normalized Cockcroft, Brozzo, McClintock, and Oyane, can be better utilized to predict the fracture initiation. The Hill–von Mises criterion that incorporates the anisotropy factors (R0 and R90) was correlated with the macro failure criterion to evaluate the critical damage factor values at failure strain under plane stress condition. The process parameters, namely, friction, height-to-diameter ratios (h/d), and the strength of the material were considered, and the upsetting process was optimized using Taguchi’s design of experiment techniques with less anisotropy as desired output. After identifying the optimum result, an analysis of variance (ANOVA) analysis was performed and the contribution of each factor responsible for failure and anisotropy was also noted.

A Practical Anisotropic Vector Hysteresis Model Based on Play Hysterons

An anisotropic vector hysteresis model based on the improved isotropic vector play operator has been presented to predict the magnetization behavior of anisotropic hysteresis materials. The required parameters can be identified from the major hysteresis loops in all principal axes, which can be easily measured, or are directly available from magnetic material manufacturers. The presented model, which has been successfully implemented in 2-D and 3-D transient finite-element analysis, is validated by numerical experiments and measured data cited from TEAM problem 32, and is applied to simulate a synchronous reluctance motor with an axially laminated anisotropic rotor.

Microstructure effects on process outputs in micro scale milling of heat treated Ti6Al4V titanium alloys

This study investigates the influence of materials’ microstructural characteristics, including grain size and phase fractions, in micro end milling of heat treated Ti6Al4V titanium alloys. Micro milling process conditions such as feed, depth of cut, and the cutting edge radius of the micro end mill are in the same order of magnitude as the grain size of the material, which gives rise to the anisotropic behavior of the multiphase materials and their deformation characteristics considering their grain size, grain boundaries, and phase fractions. A good understanding of such relationships is believed to be instrumental in developing predictive models of machining based on computational techniques. The influence of micro milling process on the crystallographic texture and microstructure of Ti6Al4V alloys is the subject of this study. For this purpose, heat treatment was performed on the Ti6Al4V samples to obtain two different microstructures: fine equiaxed and enlarged equiaxed microstructures. Micro milling experiments were performed on each sample and process outputs such as cutting forces, areal surface texture, built-up edge (BUE) formation, and alterations in the microstructure were investigated. Electron backscatter diffraction (EBSD) analysis was used to investigate the microstructure of the machined surfaces. It was observed that smaller grain size (both α and β) and lower fraction of β phase in the material yielded higher cutting forces. BUE formation and its size were affected by the microstructure of the samples. The results of this study may be useful in developing microstructure-based, predictive modeling of micro milling process.

An improved viscoplastic constitutive model and its application to creep behavior of turbine blade

This paper improved Chaboche viscoplastic constitutive model in order to simulate the mechanical behavior of engineering components during service. The anisotropic tensor and Kachanov damage evolution equation were added to the constitutive equation to simulate the creep behavior of anisotropic material. Based on that, the Levenberg-Marquardt nonlinear optimization algorithm was used to obtain material parameters and the constitutive model was compiled as the subprogram of finite element software ABAQUS for its simulation application. Then the improved constitutive model was used to simulate and analyse the uniaxial tensile and creep behavior of specimens made of anisotropic material DZ125 at different loads, and the accuracy and feasibility of material parameters and constitutive model were verified by corresponding test results. Finally, the improved constitutive model was used to simulate the creep behavior of a hollow turbine blade, achieving its engineering application. And the calculated results have great significance for structure design and life prediction of turbine blade.

Swelling interactions of earlywood and latewood across a growth ring: global and local deformations

The sorption and swelling behavior of heterogeneous spruce wood samples is documented with high-resolution phase-contrast X-ray tomography combined with a dynamic vapor sorption analysis. Three samples extracted at the growth ring boundary contain almost equivalent volumes of earlywood and latewood tissues. The anisotropic swelling behavior of cellular wood material with different geometries and porosities is reported. In particular, the swelling anisotropy ratio in heterogeneous tissues is lower than the one seen in homogeneous wood specimens. This means that, due to the restraining effect of latewood on earlywood and vice versa, swelling of combined wood reduces and becomes less anisotropic. Hysteresis is no longer present when the affine swelling/shrinkage strain is considered as a function of moisture content, indicating that the same amount of moisture entering or exiting the cell wall material leads to the same swelling/shrinkage deformation of the cell material. Nevertheless, looking at non-affine deformations, using a modified B-spline algorithm, a local bending with respect to the latewood–earlywood interface is documented. In addition, the role of rays in the cellular structure in restraining the tangential swelling of thin-cell-walled earlywood is revealed. Finally, the swelling behavior of a greenwood sample is studied at cellular scale. The first desorption curve shows shrinkage strains that are not recovered under hygroscopic loading, and a similar, but of lesser magnitude, restraining effect is seen.

An adapted yield criterion for the evolution of subsequent yield surfaces

In numerical analysis of sheet metal forming processes, the anisotropic material behaviour is often modelled with isotropic work hardening and an average Lankford coefficient. In contrast, experimental observations show an evolution of the Lankford coefficients, which can be associated with a yield surface change due to kinematic and distortional hardening. Commonly, extensive efforts are carried out to describe these phenomena. In this paper an isotropic material model based on the Yld2000-2d criterion is adapted with an evolving yield exponent in order to change the yield surface shape. The yield exponent is linked to the accumulative plastic strain. This change has the effect of a rotating yield surface normal. As the normal is directly related to the Lankford coefficient, the change can be used to model the evolution of the Lankford coefficient during yielding. The paper will focus on the numerical implementation of the adapted material model for the FE-code LS-Dyna, mpi-version R7.1.2-d. A recently introduced identification scheme [1] is used to obtain the parameters for the evolving yield surface and will be briefly described for the proposed model. The suitability for numerical analysis will be discussed for deep drawing processes in general. Efforts for material characterization and modelling will be compared to other common yield surface descriptions. Besides experimental efforts and achieved accuracy, the potential of flexibility in material models and the risk of ambiguity during identification are of major interest in this paper.

Influence of process parameters on the formability of bead stamping part using advanced models

Beads are applied to deep drawn sheet metal parts to increase its stiffness. In order to form the bead, a two-stage manufacturing process including at least deep drawing and beading is performed. Thus, reductions of sheet metal thickness and consequently weight reduction can be effectively reached by numeric optimization. In order to apply the optimization algorithm to the production of non-virtual actual parts, the forming limits of the parts must be considered in both manufacturing steps. Bead formability is affected by process parameters such as blank holding force and lubrication. One way to investigate these influences is to use an accurate simulation model. In this study, the yield function named Yld2000-2D under non-associated flow rule is used to express anisotropic material behavior of blank and the Generalized Forming Limit Concept (GFLC) considering nonlinear strain paths is used to determine the formability. These models are validated by comparing the strain and bead forming limits measured in the Nakajima experiment. Then, the influences of mentioned process parameters are investigated by the validated model.

Capturing anisotropic constitutive models with WYPiWYG hyperelasticity; and on consistency with the infinitesimal theory at all deformation levels

The elastic nonlinear behavior of fiber-reinforced materials and soft biological tissues is analyzed using anisotropic hyperelastic models. Frequently, these models are not compatible with the corresponding infinitesimal theory, but some of them may be modified to accommodate that theory in the limit. WYPiWYG hyperelasticity is compatible with the infinitesimal theory at all deformation levels and capable of capturing exactly a complete set of experimental data, which reproduces all deformation modes at every strain level, under homogeneous deformations. In this work we study the relevance of recovering the infinitesimal theory at every deformed configuration and also the performance of the WYPiWYG method in predicting the behavior of anisotropic materials at large strains under nonhomogeneous deformations.

Consistent trilayer biomechanical modeling of aortic valve leaflet tissue

Aortic valve tissue exhibits highly nonlinear, anisotropic, and heterogeneous material behavior due to its complex microstructure. A thorough understanding of these characteristics permits us to develop numerical models that can shed insight on the function of the aortic valve in health and disease. Herein, we take a closer look at consistently capturing the observed physical response of aortic valve tissue in a continuum mechanics framework. Such a treatment is the first step in developing comprehensive multiscale and multiphysics models.We highlight two important aspects of aortic valve tissue behavior: the role of the collagen fiber microstructure and the native prestressing. We propose a model that captures these two features as well as the heterogeneous layer-scale topology of the tissue. We find the model can reproduce the experimentally observed multiscale mechanical behavior in a manner that provides intuition on the underlying mechanics.

Phase field modeling of fracture in anisotropic brittle solids

A phase field model of fracture that accounts for anisotropic material behavior at small and large deformations is outlined within this work. Most existing fracture phase field models assume crack evolution within isotropic solids, which is not a meaningful assumption for many natural as well as engineered materials that exhibit orientation-dependent behavior. The incorporation of anisotropy into fracture phase field models is for example necessary to properly describe the typical sawtooth crack patterns in strongly anisotropic materials. In the present contribution, anisotropy is incorporated in fracture phase field models in several ways: (i) Within a pure geometrical approach, the crack surface density function is adopted by a rigorous application of the theory of tensor invariants leading to the definition of structural tensors of second and fourth order. In this work we employ structural tensors to describe transverse isotropy, orthotropy and cubic anisotropy. Latter makes the incorporation of second gradients of the crack phase field necessary, which is treated within the finite element context by a nonconforming Morley triangle. Practically, such a geometric approach manifests itself in the definition of anisotropic effective fracture length scales. (ii) By use of structural tensors, energetic and stress-like failure criteria are modified to account for inherent anisotropies. These failure criteria influence the crack driving force, which enters the crack phase field evolution equation and allows to set up a modular structure. We demonstrate the performance of the proposed anisotropic fracture phase field model by means of representative numerical examples at small and large deformations.

Finite element model approach of a cylindrical lithium ion battery cell with a focus on minimization of the computational effort and short circuit prediction

In this research, a parameterized beam-element-based mechanical modeling approach for cylindrical lithium ion batteries is developed. With the goal to use the cell model in entire vehicle crash simulations, focus of development is on minimizing the computational effort whilst simultaneously obtaining accurate mechanical behavior. The cylindrical cell shape is approximated by radial beams connected to each other in circumferential and longitudinal directions. The discrete beam formulation is used to define an anisotropic material behavior. An 18650 lithium ion cell model constructed in LS-Dyna is used to show the high degree of parameterization of the approach. A criterion which considers the positive pole deformation and the radial deformation of the cell is developed for short circuit prediction during simulation. An abuse testing program, consisting of radial crush, axial crush, and penetration is performed to evaluate the mechanical properties and internal short circuit behavior of a commercially available 18650 lithium cell. Additional 3-point-bending tests are performed to verify the approach objectively. By reducing the number of strength-related elements to 1600, a fast and accurate cell model can be created. Compared to typical cell models in technical literature, simulation time of a single cell load case can be reduced by approx. 90%.

A time dependent equivalent stress function for proportional and non-proportional transient loaded and notched metallic components

The novel “Modified Mohr Mises” (MMM) Hypothesis allows the assessment of non-proportional stresses within a fully automated process, due to its invariant equivalent stress notation. Those non-proportional stresses are most common in complex structures of aircrafts, spacecrafts and vehicles for instance. The new MMM Hypothesis provides a physically reasonable sign determination derived directly from the effective stress components. Supporting effects through material and adjacent material areas and local geometries are considered in the theory as well. The use of a suitable tension shear strength ratio allows a consideration of anisotropic material behavior in the surface as well. The MMM Hypothesis and its predecessors were formulated to support the development of the fatigue strength assessment of the German FKM Guideline, in order to be able to utilize, among others, the Guideline material-related knowledge for non-proportional components with load-free and fluid-loaded, smooth and notched component surfaces.