Distributed Thermal Cracking of AC Pavement with Frictional Constraint

This paper develops a model to simulate the distributed thermal cracking of concrete structures with frictional constraint. This model is developed primarily for the thermal cracking asphalt-concrete (AC) pavement structures; however, with some modifications, it is also applicable to similar problems such as shrinkage cracking of concrete and cracking of reinforced concrete in uniaxial tension. This model reflects the multiscale nature of these problems: microcracking or damage on the mesoscale and localization or redistribution on the macroscale. Randomly distributed fictitious cracks are introduced to represent the inhomogeneities and damage in the material at the mesoscale. Friction is recognized as the mechanism leading to stress redistribution and, therefore, damage localization on the macroscale. When the problem is assumed to be 1D and Coulomb friction is used, a semianalytical numerical scheme is developed. The formation of stress-free open cracks is due to the combination of continuous crack growth and unstable jumps, which involve a nonlinear stability analysis. Equilibrium solutions and stability conditions are given in the paper. Displacement controlled analysis is used to follow the unstable equilibrium path after the structure has lost stability. Numerical simulations clearly show that, with slight mesoscale inhomogeneities and in the presence of a constraining frictional force, microcracking or damage on the mesoscale localizes and finally leads to open cracks distributed at a spacing on the order of the macroscale.     

Decomposition of Damage Tensor in Continuum Damage Mechanics

The principles of continuum damage mechanics are reviewed first for the case of uniaxial tension. The damage variable is then decomposed into two variables called crack damage variables and void damage variables. A consistent mathematical formulation is presented to decompose the damage tensor into two parts: one caused by voids and the other caused by cracks. In the first part of this work, isotropic damage in the uniaxial tension case is assumed. However, the generalization to three-dimensional states of damage is presented in the second part of this work, using tensorial damage variables. It is shown that the components of tensorial crack damage variables and void damage variables are not independent of each other, implying a coupling between the two damage mechanisms. This coupling may be obvious based on the physics of the problem, but a rigorous mathematical proof is given for it. Also, explicit relations governing the components of the crack and void damage variables are derived.     

Simulated Micromechanical Models Using Artificial Neural Networks

A new method, termed simulated micromechanical models using artificial neural networks (MMANN), is proposed to generate micromechanical material models for nonlinear and damage behavior of heterogeneous materials. Artificial neural networks (ANN) are trained with results from detailed nonlinear finite-element (FE) analyses of a repeating unit cell (UC), with and without induced damage, e.g., voids or cracks between the fiber and matrix phases. The FE simulations are used to form the effective stress-strain response for a unit cell with different geometry and damage parameters. The FE analyses are performed for a relatively small number of applied strain paths and damage parameters. It is shown that MMANN material models of this type exhibit many interesting features, including different tension and compression response, that are usually difficult to model by conventional micromechanical approaches. MMANN material models can be easily applied in a displacement-based FE for nonlinear analysis of composite structures. Application examples are shown where micromodels are generated to represent the homogenized nonlinear multiaxial response of a unidirectional composite with and without damage. In the case of analysis with damage growth, thermodynamics with irreversible processes (TIP) is used to derive the response of an equivalent homogenized damage medium with evolution equations for damage. The proposed damage formulation incorporates the generalizations generated by the MMANN method for stresses and other possible responses from analysis results of unit cells with fixed levels of damage.     

A 3D damage model for trabecular bone based on fabric tensors

Motivated by the role of damage in normal and pathological conditions of trabecular bone, a novel 3D constitutive law was developed that describes anisotropic elasticity and the rate-independent degradation in mechanical properties resulting from the growth of cracks or voids in the trabecular tissue. The theoretical model was formulated within the framework of continuum damage mechanics and based on two fabric tensors characterizing the local trabecular morphology. Experimental validation of the model was achieved by uniaxial and torsional testing of waisted bovine trabecular bone specimens. Strong correlations were found between cumulated permanent strain, reduction in elastic moduli and nonlinear postyield stress, which support the hypothesis that these variables reflect the same underlying damage process.     

Modeling and Simulation of Porous Elastoviscoplastic Material

Many materials exhibit elasto–visco–plastic behavior when subjected to loadings with certain strain rate. Examples include natural materials such as metals, clays, and soils and manmade materials such as some biomimic materials. Some voids may exist or be introduced in these materials. The effects of the voids on the material response are important in predicting the strength, reliability, and service life of structural systems containing these materials. This paper presents the results of applying a statistical micromechanical approach to describe the macroscopic behavior of elasto–visco–plastic materials containing many randomly dispersed spherical voids. Most existing micromechanics based models are only applicable to monotonic proportional loadings. The limitation is removed by integrating the material model into the framework of continuum plasticity. With the discrete integration algorithm and local return mapping algorithm, the proposed computation method is applicable to any loading and unloading histories and is ready for implementing into finite element analysis.     

The effect of SiC particle reinforcement on the creep behavior of 2080 aluminum

The influence of SiC particle reinforcement on the creep behavior of 2080 aluminum is investigated between 150 °C and 350 °C. The effect of particle size (F-280, F-600, and F-1000), volume fraction (10, 20, and 30 vol pct), and heat treatment (T6 and T8) on creep behavior is studied. In both the T6 and T8 conditions all composites are less creep resistant than similarly heat-treated monolithic materials when crept at 150 °C. These results contradict continuum mechanics predictions for steady-state creep rate, which predict composite strengthening. A high dislocation density is observed near SiC particles. It is proposed that strain localization near the reinforcements leads to microstructural breakdown and the subsequent reduction in creep resistance. When both materials are severely overaged or when they are tested at very high temperatures (350 °C), composite materials exhibit improved creep resistance relative to monolithic material. In these cases, the strengthening is consistent with continuum predictions for direct composite strengthening.     

Expressions for predicting the elasticity modulus of materials reinforced by second-phase grains

In the present study, expressions for predicting the elasticity modulus of the materials reinforced by the second-phase grains, which may be referred to as the granular composite for the sake of simplicity, are developed based on the solution of the overmatching problem in elastic mechanics. Taking the voids or defects in materials as the second phase with zero elasticity modulus, one can easily obtain the expressions for predicting the elasticity modulus and the threshold of the elastic percolation failure of materials containing voids. The values of the elasticity modulus of the granular composite and the materials containing random voids and the values of the threshold for the elastic percolation failure of the materials with voids predicted by using the above-mentioned expressions are in good agreement with results given in available literature.     

Three-Dimensional Micromechanical Finite-Element Network Model for Elastic Damage Behavior of Idealized Stone-Based Composite Materials

This paper presents a three-dimensional (3D) micromechanical finite-element (FE) network model for predicting elastic damage behavior of the idealized stone-based materials. Stone-based composite materials have multiphase structures: an aggregate (or stone) skeleton, a binding medium, fillers, and air voids. Numerical simulation of the micromechanical behavior of the idealized stone-based materials was accomplished by using a microframe element network model that incorporated the mechanical load transfer between adjacent particles. The elastic stiffness matrix of this special element was obtained from an approximate elastic stress-strain analysis of straight cement between particle pairs. A damage-coupled microframe element was then formulated with bilinear damage laws, including elastic and softening behavior based on the equivalent fracture release energy. Indirect tension and compression simulations were conducted with developed FE models on the idealized digital samples of the stone-based materials. These simulations predicted the internal microdamage distribution and global fracture behavior of these samples, which qualitatively agree with the laboratory observations. The results indicate that the developed FE models have the capability to predict the typical loading-related damage behavior observed from the stone-based materials.     

Microstructural Viscoplastic Continuum Model for Permanent Deformation in Asphalt Pavements

Permanent deformation is one of the major distresses in asphalt pavements. It is caused mainly by high traffic loads associated with high field temperatures. An anisotropic viscoplastic continuum damage model is developed in this study to describe permanent deformation of asphalt pavements. The model is based on Perzyna’s formulation with Drucker–Prager yield function modified to account for material anisotropy and microstructure damage. The material anisotropy is captured through microstructural analysis of aggregate distribution on two-dimensional sections of hot mix asphalt. A damage parameter is included in the model to quantify the nucleation of cracks and growth of air voids and cracks. A parametric study was conducted to demonstrate the sensitivity of the model to strain rate, aggregate distribution, and microstructure damage. Triaxial strength and static creep measurements obtained from the Federal Highway Administration Accelerated Loading Facility were used to determine the model parameters.     

低压冷喷涂铸造缺陷修复涂层性能研究

低压冷喷涂(LPCS)是一种拥有便携式冷喷涂系统的涂层技术。例如DYMET304K系统就应用于这项涂层技术中。通常情况下,压缩空气作为冷喷涂工艺中的运载气体。低压冷喷涂适用于喷涂金属基陶瓷复合粉末,如Cu基、Ni基、Zn基、Al基添加Al2O3粉的复合粉。硬质陶瓷相主要起到清洁喷嘴、增加表面活性和喷丸强化的作用,该方法在尺寸修复领域中具有优势。在这个领域里,修复铸造加工中的缺陷和气孔是一个很热门的应用。例如,Zn基复合材料就适用于防止电化学腐蚀和修复机械损伤造成的尺寸差异。本文对Zn+Al+Al2O3,Zn+Cu+Al2O3和Zn+Ni+Al2O3等复合材料做了实验研究。Zn和Al在腐蚀环境中起到阴极保护的作用,而Cu和Ni也有助于提高材料的机械性能。经过对微观孔蚀电位反应和力学性能(硬度和结合强度)的研究发现,涂层具有相对致密的结构和耐蚀性能。Zn在复合涂层中对其它金属起到阴极保护的作用。此外,在Fe52型铁基材料上的涂层有着足够的力学性能,硬度和结合强度较高。这一类涂层在修复宏观的铸造缺陷上具有很高的潜在应用价值。     

Effect of deformation heating and strain rate sensitivity on flow localization during the torsion testing of 6061 aluminum

The effect of deformation heating and strain rate sensitivity on flow localization during the torsion testing of 6061 aluminum was investigated both theoretically and experimentally. From the theoretical viewpoint, a simple analysis of the torsion test was carried out based on the torque equilibrium and one-dimensional heat transfer equations. The problem formulation was discretized to enable numerical solution of the governing equations and prediction of the effect of material properties on the development of deformation and temperature gradients. The analysis was validated by conducting high strain rate experiments on the 6061 alloy at a variety of temperatures. At low temperatures, at which the flow stress and temperature changes due to deformation heating are large and the strain rate sensitivity is low, marked flow localization occurred. The analysis modeled this behavior correctly, indicating that strain concentrations can occur solely as a result of the temperature gradients set up by heat transfer during testing, i.e. in the absence of geometric or deformation defects. At the higher temperatures, at which temperature changes due to deformation heating are small and the rate sensitivity is large, the flow remained nominally uniform until fracture intervened. The numerical simulations of these tests also showed good agreement with the observations.     

Deformation and fracture of miniature tensile bars with resistance-spot-weld microstructures

Plastic deformation of miniature tensile bars generated from dual-phase steel weld microstructures (i.e., fusion zone, heat-affected zone, and base material) was investigated up to final rupture failure. Uniaxial tensile true stress-strain curves beyond diffuse necking were obtained with a novel strain-mapping technique based on digital image correlation (DIC). Key microstructural features (including defects) in each of these three metallurgical zones were examined to explore the material influence on the plastic deformation and failure behavior. For weld fusion zones with minimal defects, diffuse necking was found to begin at 6 pct strain and continue up to 55 to 80 pct strain. The flow stresses of the weld fusion zones were at least twice those of the base material, and fracture strains exceeded 100 pct for both materials. The heat-affected zones exhibited a range of complex deformation behaviors, as expected from their microstructural variety. Only those fusion zones with substantial defects (e.g., shrinkage voids, cracks, and contaminants) failed prematurely by edge cracking, as signaled by their highly irregular strain maps.     

Characterization of Damage Evolution in SiC Particle Reinforced Al Alloy Matrix Composites by <Emphasis Type="Italic">In-Situ</Emphasis> X-Ray Synchrotron Tomography

We carried out a detailed investigation of the damage behavior of SiC particle reinforced 2080 Al alloy matrix composites by in-situ X-ray synchrotron tomography. We studied the tensile damage behavior of a peak-aged aluminum matrix composite. The main damage mode was SiC particle fracture with a very small contribution from void growth. The onset of damage takes place very close to the ultimate tensile strength of the composite. Particle fracture damage is stochastic in nature and is confined to a small distance from the fracture plane. Minimal void growth is observed, primarily at pre-existing microscopic voids from processing. Microstructure-based simulations, based on two-dimensional (2-D) images from the tomography data sets, show the importance of particle distribution and morphology on the evolution of plastic strain and damage in the composite.     

高氮不锈轴承钢高温旋弯疲劳性能及损伤机制

摘要：40Cr15Mo2VN高氮不锈钢在300、400℃条件下旋转弯曲疲劳试验，结果表明，300℃条件下安全疲劳极限强度为787MPa，400℃条件下疲劳极限强度为860MPa，300℃条件下安全疲劳极限较400℃下降85%。通过SEM观察断口发现，疲劳破坏类型均为表面缺陷起裂、夹杂物起裂及基体孔洞起裂。高温下，碳化物、晶界等在热力耦合作用下成为孔洞形核的位置，孔洞长大连接成微裂纹，成为裂纹萌生扩展的主要原因，300较400℃条件下安全疲劳极限下降的主要原因是蠕变孔洞聚集程度高，容易连接成微裂纹，导致疲劳失效。     

Effective 3D Constitutive Relations of Composites with Evolving Damage

In this study it is shown that a computational procedure, termed “discrete damage space homogenization method” (DDSHM), can predict the constitutive response of layered composite materials containing growing cracks. The fully 3D effective constitutive law for a specific layered composite architecture, as defined by the DDSHM, was integrated into the ABAQUS commercial finite-element program using the user-defined material feature. Calculations were carried out for flat laminates with penny-shaped microcracks to illustrate the computational efficiency of the method for general analysis of composite materials with growing cracks. Results show that, given basic information about the fracture toughness of the material, the DDSHM is able to predict important material parameters including the load at initiation of cracking, damage growth rate, and resulting effect on the macroscopic stiffness.     

Predicting Thermal Conductivity Evolution of Polycrystalline Materials Under Irradiation Using Multiscale Approach

A multiscale methodology was developed to predict the evolution of thermal conductivity of polycrystalline fuel under irradiation. At the mesoscale level, a phase field model was used to predict the evolution of gas bubble microstructure. Generation of gas atoms and vacancies was taken into consideration. Gas bubbles were predicted to form, grow, and coalesce around grain boundary (GB) areas. On the macroscopic scale, a statistical continuum mechanics model was applied to predict the anisotropic thermal conductivity evolution during irradiation. Microstructures predicted by the phase field model were fed into the statistical continuum mechanics model to predict properties and behavior. A decrease of thermal conductivity during irradiation was demonstrated. The influence of irradiation flux, the exposure time, and the grain microstructure were investigated. If the initial GB microstructure was isotropic, the thermal conductivity under irradiation would be similarly isotropic. If the initial GB configuration was anisotropic, anisotropy of thermal conductivity would intensify under irradiation as gas bubbles coalesce around GB areas. The prediction of microstructure and property evolution of polycrystalline materials under irradiation by bridging two models in different scales were demonstrated successfully. This approach provides a deep understanding from a basic scientific viewpoint.     

Three-Dimensional Microstructure Visualization of Porosity and Fe-Rich Inclusions in SiC Particle-Reinforced Al Alloy Matrix Composites by X-Ray Synchrotron Tomography

Microstructural aspects of composites such as reinforcement particle size, shape, and distribution play important roles in deformation behavior. In addition, Fe-rich inclusions and porosity also influence the behavior of these composites, particularly under fatigue loading. Three-dimensional (3-D) visualization of porosity and Fe-rich inclusions in three dimensions is critical to a thorough understanding of fatigue resistance of metal matrix composites (MMCs), because cracks often initiate at these defects. In this article, we have used X-ray synchrotron tomography to visualize and quantify the morphology and size distribution of pores and Fe-rich inclusions in a SiC particle-reinforced 2080 Al alloy composite. The 3-D data sets were also used to predict and understand the influence of defects on the deformation behavior by 3-D finite element modeling.     

Noninvasive photoelastic method to show distribution of strain in the hoof wall of a living horse

A photoelastic method used for materials testing in industry was adapted to show the distribution of strain through the hoof wall in the living horse. Strain was a change in length per unit length in the material of the loaded hoof wall compared with the unloaded condition. Coloured fringes appeared in the photoelastic plastic where there were differences in strain between adjacent sites (strain gradients) in the hoof. Strain distribution was observed in the shod and unshod hoof wall of the front hooves of 6 sound horses with hooves that appeared 'good' to visual inspection, and one unsound horse with hoof cracks. No significant differences in strains were apparent across the hoof walls of the sound horses when the horses were standing normally. Steep strain gradients were apparent in hooves, associated with defects such as cracks, unstable nail holes, and long toes.     

Kinetics of cyclic oxidation and cracking and finite element analysis of MA956 and sapphire/MA956 composite system

Sapphire fiber-reinforced MA956 composites hold promise for significant weight savings and increased high-temperature structural capability, as compared to unreinforced MA956. As part of an overall assessment of the high-temperature characteristics of this material system, cyclic oxidation behavior was studied at 1093 °C and 1204 °C. Initially, both sets of coupons exhibited parabolic oxidation kinetics. Later, monolithic MA956 exhibited spallation and a linear weight loss, whereas the composite showed a linear weight gain without spallation. Weight loss of the monolithic MA956 resulted from the linking of a multiplicity of randomly oriented and closely spaced surface cracks that facilitated ready spallation. By contrast, cracking of the composite’s oxide layer was nonintersecting and aligned nominally parallel with the orientation of the subsurface reinforcing fibers. Oxidative lifetime of monolithic MA956 was projected from the observed oxidation kinetics. Linear elastic, finite element continuum, and micromechanics analyses were performed on coupons of the monolithic and composite materials. Results of the analyses qualitatively agreed well with the observed oxide cracking and spallation behavior of both the MA956 and the Sapphire/MA956 composite coupons.