Cohesive technology applied to the modeling and simulation of fatigue failure

Estimation of fatigue and fracture properties of materials is essential for the safe life estimation of aging structural components. Standard ASTM testing procedures being time consuming and costly, computational methods that can reliably predict fatigue properties of the material will be very useful. Toward this end, we present a computational model that can capture the entire Paris curve for a material. The model is based on a damage-dependent irreversible cohesive failure formulation. The model relies on a combination of a bilinear cohesive failure law and an evolution law relating the cohesive stiffness, the rate of crack opening displacement, and number of cycles since the onset of failure. Threshold behavior of the fatigue crack propagation is determined by the initial value of the damage parameter of the cohesive failure law, while the accelerated region is the natural outcome of the cohesive formulation. The Paris region can be readily calibrated with the two parameters of the proposed cohesive model. We compare the simulation results with the NASGRO material database, and show that the threshold region is adequately captured by the proposed model. We summarize a semi-implicit implementation of the proposed model into a cohesive-volumetric finite element framework, allowing for the simulation of a wide range of fatigue problems

Grain-Level Simulation of Dynamic Failure in Ceramic Materials

137 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2002.To demonstrate the capabilities and versatility of the grain-based CVFE code, we investigate four dynamic fracture problems. The first one is concerned with the propagation of dynamic intergranular cracks under mode I loading, with special emphasis on the effect of the microstructure on the branching instability of the crack motion. The second problem is that of dynamic fracture under mode II loading and the capture of the associated elastic bridging of the crack by partially or completely dislodged grains. The third application is that of dynamic fragmentation of ceramic materials primarily under tensile conditions. The emphasis here is to understand the effect of the microstructure and cohesive strength of the material on the onset of the failure event and on the fragment size distribution. Finally, we present the results of a preliminary study of a dynamic single grit scratch test used to simulate the damage created by a single grinding particle on the machined ceramic component.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Recycling Krylov Subspaces for Sequences of Linear Systems

Many problems in engineering and physics require the solution of a large sequence of linear systems. We can reduce the cost of solving subsequent systems in the sequence by recycling information from previous systems. We overview two different approaches. For several model problems, we demonstrate that we can reduce the iteration count required to solve a linear system by a factor of two. We consider both Hermitian and non-Hermitian problems, and present numerical experiments to illustrate the effects of subspace recycling

Electromagnetic interference shielding of carbon nanotube/ethylene vinyl acetate composites

Single-walled carbon nanotube (SWCNT) and ethylene vinyl acetate (EVA) composites were synthesized in an internal mixer by melt mixing. The electrical conductivity as well as electromagnetic interference (EMI) shielding effectiveness (SE) over the X-band (8-12 GHz) and microwave (200-2,000 MHz) frequency ranges of these composites were investigated. It was observed that the electrical conductivity of composites increases with increasing SWCNT loading. A percolation threshold of about 3.5 wt.% was obtained and the electrical conductivity of EVA was increased by ten orders of magnitude, from 10 -14 to 10-4Ω-1 cm-1. The effect of sample thickness on SE was investigated. The correlation between SE and conductivity of the composites is discussed. The experimental data showed that the SE of the composites containing higher carbon nanotube loadings (above 10 wt.%) could be used as an EMI shielding material and lower SWCNT loadings could be used for the dissipation of electrostatic charge. © 2008 Springer Science+Business Media, LLC

Computationally efficient black-box modeling for feasibility analysis

Computational cost is a major issue in modern large-scale simulations used across different disciplines of science and engineering. Computationally efficient surrogate models that can represent the original model with desired accuracy have been explored in the recent past. However, with the exception of few efforts, most of these techniques rely on a reduced order representation of the original complex model, resulting in a loss of information. In this paper we demonstrate the applicability of high dimensional model representation (HDMR) technique in addressing this issue while preserving the original model dimension. We will discuss the applicability of this surrogate modeling technique in the field of feasibility analysis drawing examples from process systems and materials design. It will be shown that the original physical models can be essentially considered as a black box, and same methodology can be applied across all the examples studied. It is found that the accuracy of the surrogate models depends on the order of the approximation and number of sampling points employed. While first-order approximation is largely inadequate, second-order approximation is sufficient for the model systems studied. Sampling requirement is also dramatically low for the construction of these surrogate models. © 2010 Elsevier Ltd

Evolution of shear bands in bulk metallic glasses under dynamic loading

Shear band spacing in Zr-based bulk metallic glasses (BMGs) under dynamic loads is found to vary with position and local strain rate in the indented region. To investigate the dependence of shear band evolution characteristics on local strain rate and normal stress, a micromechanical model based on momentum diffusion is proposed. The thermo-mechanical model takes into account the normal stress dependence of yield stress, the free volume theory and the associated viscosity change within the shear band region. Temperature rise is obtained from the balance between the heat diffusion to the adjacent regions from a shear band and the heat generation due to the accumulated plastic work in a shear band. The parametric study has revealed that thermal effects play a minor role when the critical shear displacement is below 10 nm (as in nanoindentation) but become significant when the shear displacement accumulated in a shear band is of the order of hundreds of nanometers (as in uniaxial compression and in dynamic indentations). Finally, it is found that the normal stress plays a crucial role in the deformation behavior of BMGs by not only decreasing the time for shear band formation but also increasing the temperature rise significantly. © 2008 Elsevier Ltd. All rights reserved

Local heating and viscosity drop during shear band evolution in bulk metallic glasses under quasistatic loading

Based on the facts that the thickness of a shear band in bulk metallic glasses (BMGs) is a few tens of nanometers, the shear displacement across the band is few micrometers, and the time for their formation is in submicrosecond duration, the local strain rates within the shear band can be as high as 109 s. To capture such dynamic effects, a thermo-micromechanical model based on momentum diffusion mechanism, free-volume theory, and heat diffusion analysis is proposed. The model has been shown to capture the characteristic rate effects, i.e., significant local temperature rise and a dramatic drop in viscosity during shear band evolution in BMGs. The model also takes into account the effects of normal stress component on the deformation behavior of BMGs. While the predicted maximum temperature rise under quasistatic deformation in the absence of normal component of stress is low (300 K), significant temperature rise well above 1000 K accompanied by a sudden drop in viscosity has been predicted under dynamic loads at high normal pressures. It is also predicted that temperature rise and viscosity drop are negligible during the early phase of shear band formation but increase significantly towards the final phase of shear band evolution and cause subsequent fracture, as has been theorized by many researchers in the literature. © 2007 American Institute of Physics

Effect of microscopic deformation mechanisms on the dynamic response of soft cellular materials

Cellular materials show progressive or uniform collapse during impact loading depending on their microstructural and material properties. It is generally agreed that a complex interplay among microinertia, microbuckling and microbending of the cell walls of these materials plays an important role in determining their macroscopic stress-strain response. However, an evaluation of the dependency of the overall deformation behavior on these parameters requires sophisticated modeling approach due to extremely fast and complex wave propagation events occurring during dynamic deformation. We have developed a transient finite element based computational framework that can examine the contribution of each of these effects on the deformation history of this class of materials. An in-depth parametric study for different loading, microstuctural and material parameters has been undertaken in this study. Our significant finding is that at high strain rate, shorter pulse rise times lead to higher microinertial stress enhancement due to an increase in apparent microbuckling strength. A variation of cell size shows insignificant effect of microinertia and microbuckling at initial stage but localization can be found at later stage of deformation due to increasing microbuckling and microbending activities. Deformation localization occurs in lower Young\u27s modulus specimens due to lower buckling and bending strength of the cell walls. A significant inertial stress enhancement can be noticed in the specimens with higher bulk density of the constituent material leading to increased microbuckling activities resulting in localized collapse at the impact end

A generalized cohesive element technique for arbitrary crack motion

A computational method for arbitrary crack motion through a finite element mesh, termed as the generalized cohesive element technique, is presented. In this method, an element with an internal discontinuity is replaced by two superimposed elements with a combination of original and imaginary nodes. Conventional cohesive zone modeling, limited to crack propagation along the edges of the elements, is extended to incorporate the intra-element mixed-mode crack propagation. Proposed numerical technique has been shown to be quite accurate, robust and mesh insensitive provided the cohesive zone ahead of the crack tip is resolved adequately. A series of numerical examples is presented to demonstrate the validity and applicability of the proposed method. © 2009 Elsevier B.V. All rights reserved

Mechanical properties of PECVD thin ceramic films

Silicon dioxide (thickness 350 nm and 969 nm) and silicon nitride (thickness 218 nm) films deposited on silicon substrate using plasma enhanced chemical vapor deposition process were investigated using a Berkovich nanoindenter. The load-depth measurements revealed that the oxide films have lower modulus and hardness compared to the silicon substrate, where as the nitride film has a higher hardness and slightly lower modulus than the substrate. To delineate the substrate effect, a phenomenological model, that captures most of the \u27continuous stiffness measurement\u27 data, was proposed and then extended on both sides to determine the film and substrate properties. The modulus and hardness of the oxide film were around 53 GPa and 4-8 GPa where as those of the nitride film were around 150 GPa and 19 GPa, respectively. These values compare well with the measurements reported elsewhere in the literature. © 2009 Elsevier Ltd. All rights reserved