A 7-Year History of Necrobiotic Xanthogranuloma following Asymptomatic Multiple Myeloma: A Case Report

Necrobiotic xanthogranuloma (NXG) is a rare destructive xanthomatous granuloma with chronic, indolent, and progressive course. The morbidity and mortality are the results from wound complications and associated disorders. Because of its strong association with monoclonal gammopathy and multiple myeloma, early recognition of disease is mandatory to monitor and prevent systemic involvements of hematologic malignancies

Experimental Study of Particles Induced by Screw Tightening Process for Hard Disc Drive Assembly: Effects of ‘Bit’ Speed

The morphology of particles generated during screw tightening process in hard disc drive assembly was studied using a media installing tool kit under a class 100 clean room condition. The screws were made of martensitic 410 stainless steel and the ‘bit’ was made of S2 tool steel. The ‘bit’ speeds used during the screw tightening process can be divided into two steps: the beginning and the final speeds. The effect of both speeds on the morphology of particles generated was investigated. The studied parameters were the aspect ratio and the appearance cross-sectional area of particles. Particles with different sizes were found suggesting that there were different wear mechanisms. Small particles were caused by adhesive wear, while the larger particles were generated by fatigue wear. The appearance cross-sectional area of particles was found to decrease with increase in both speeds within the speed of 250 r/min, after which the appearance crosssectional area appeared to be constant. The effect of cold-weld at asperities was obvious resulting in an increase in aspect ratio at a higher speed. The understanding of the effect of bit speed on the particles morphology during the screw tightening processes could be very useful in the design of the cleaning system in hard disc drive production.The morphology of particles generated during screw tightening process in hard disc drive assembly was studied using a media installing tool kit under a class 100 clean room condition. The screws were made of martensitic 410 stainless steel and the ‘bit’ was made of S2 tool steel. The ‘bit’ speeds used during the screw tightening process can be divided into two steps: the beginning and the final speeds. The effect of both speeds on the morphology of particles generated was investigated. The studied parameters were the aspect ratio and the appearance cross-sectional area of particles. Particles with different sizes were found suggesting that there were different wear mechanisms. Small particles were caused by adhesive wear, while the larger particles were generated by fatigue wear. The appearance cross-sectional area of particles was found to decrease with increase in both speeds within the speed of 250 r/min, after which the appearance crosssectional area appeared to be constant. The effect of cold-weld at asperities was obvious resulting in an increase in aspect ratio at a higher speed. The understanding of the effect of bit speed on the particles morphology during the screw tightening processes could be very useful in the design of the cleaning system in hard disc drive production

Experimental and modelling study of fatigue crack initiation in an aluminium beam with a hole under 4-point bending

Slip band formation and crack initiation during cyclic fatigue were investigated by in-situ experiments and non-local CPFEM simulations systematically. Experimental techniques including EBSD, digital image correlation (DIC) and SEM have been used to obtain consistent grain orientations, local strains, as well as the locations where slip bands and micro-cracks form on the sample surface. The realistic microstructure based on the EBSD map has been generated and used for finite element modelling. An advanced non-local crystal plasticity model, which considers the isotropic and kinematic hardening of the plastic strain gradient, has been adopted. The simulation results match well the corresponding experimental results. It was found that total strain and averaged slip on all slip systems, combined with accumulated slip on specific slip planes help predict the location and orientation of slip bands and micro-crack initiation correctly. Furthermore, a fatigue indicating parameter based on competition between maximum slip and the total slip has been proposed to reproduce the experimental observations

Material characterization and finite element modelling of cyclic plasticity behavior for 304 stainless steel using a crystal plasticity model

Low cycle fatigue tests were carried out for a 304 stainless steel at room temperature. A series of experimental characterisations, including SEM, TEM, and XRD were conducted for the 304 stainless steel to facilitate the understanding of the mechanical responses and microstructural behaviour of the material under cyclic loading including nanostructure, crystal structure and the fractured surface. The crystal plasticity finite element method (CPFEM) is a powerful tool for studying the microstructure influence on the cyclic plasticity behaviour. This method was incorporated into the commercially available software ABAQUS by coding a UMAT user subroutine. Based on the results of fatigue tests and material characterisation, the full set of material constants for the crystal plasticity model was determined. The CPFEM framework used in this paper can be used to predict the crack initiation sites based on the local accumulated plastic deformation and local plastic dissipation energy criterion, but with limitation in predicting the crack initiation caused by precipitates

Microstructure-sensitive estimation of small fatigue crack growth in bridge steel welds

A probabilistic finite element model is implemented to estimate microstructurally small fatigue crack growth in bridge steel welds. Simulations are based on a microstructure-sensitive crystal plasticity model to quantify fatigue indicator parameters (FIPs) at the slip system level and a fatigue model that relates FIPs to fatigue lives of individual grains. Microstructures from three weld zones, namely, fusion zone (FZ), heat affected zone (HAZ), and base metal (BM), are constructed based on their microstructural attributes such as grain morphology, size, and orientation. Statistical volume elements (SVEs) are generated and meshed independently for the three welding zones. Each grain within the SVEs is divided into several slip bands parallel to crystallographic planes. During the loading process, cracks nucleate at the slip bands (SBs) with the largest FIP next to the free surface. The crack extension path is assumed to be transgranular along SBs and the number of cycles required to crack the neighbor grain is calculated by the corresponding FIP-based crack growth rate equation. The simulation process is carried out using ABAQUS with a user defined subroutine UMAT for crystal plasticity. After the calibration of the constitutive model and irreversibility parameters, numerical simulations for small crack growth in three zones are presented. The crack length vs. the predicted fatigue resistance shows significant differences in the mean values and variability among the three weld zones

Effects of heat treatment on microstructure and creep properties of a laser powder bed fused nickel superalloy

Nickel-based superalloy C263 has been consolidated with Laser Powder Bed Fusion (LPBF) with two perpendicular build orientations and exposed to either of two heat treatment programmes. This study analyses the effects of build orientation and heat treatment on the resulting microstructures produced in LPBF C263 variants, evaluated against a cast equivalent. Results show that although a strongly anisotropic microstructure was present in standard heat-treated (HT1) LPBF material, this was eradicated following an alternate heat treatment regime (HT2) through recrystallisation, aided by high local strain. Subsequently, their mechanical properties have been assessed by means of the Small Punch (SP) creep test. A contrasting presence of Σ3 formations was observed between the two LPBF heat treatment programmes with the resulting random grain boundary network (RGBN) revealing shorter potential intergranular crack paths in the HT2 material, although grain boundary carbides were found to be the dominant strengthening mechanism for improved creep resistance. Adapted Wilshire equations have been implemented to predict the long-term creep lives of the C263 variants and their apparent activation energies have been determined

Micromechanical finite element modelling of thermo-mechanical fatigue for P91 steels

In this paper, the cyclic plasticity and fatigue crack initiation behaviour of a tempered martensite ferritic steel under thermo-mechanical fatigue conditions is examined by means of micromechanical finite element modelling. The crystal plasticity-based model explicitly reflects the microstructure of the material, measured by electronic backscatter diffraction. The predicted cyclic thermo-mechanical response agrees well with experiments under both in-phase and out-of-phase conditions. A thermo-mechanical fatigue indicator parameter, with stress triaxiality and temperature taken into account, is developed to predict fatigue crack initiation. In the fatigue crack initiation simulation, the out-of-phase thermo-mechanical response is identified to be more dangerous than in-phase response, which is consistent with experimental failure data. It is shown that the behaviour of thermo-mechanical fatigue can be effectively predicted at the microstructural level and this can lead to a more accurate assessment procedure for power plant components

A multi-scale crystal plasticity model for cyclic plasticity and low-cycle fatigue in a precipitate-strengthened steel at elevated temperature

peer-reviewedIn this paper, a multi-scale crystal plasticity model is presented for cyclic plasticity and low-cycle fatigue in a tempered martensite ferritic steel at elevated temperature. The model explicitly represents the geometry of grains, sub-grains and precipitates in the material, with strain gradient effects and kinematic hardening included in the crystal plasticity formulation. With the multiscale model, the cyclic behaviour at the sub-grain level is predicted with the effect of lath and precipitate sizes examined. A crystallographic, accumulated slip (strain) parameter, modulated by triaxiality, is implemented at the micro scale, to predict crack initiation in precipitate-strengthened laths. The predicted numbers of cycles to crack initiation agree well with experimental data. A strong dependence on the precipitate size is demonstrated, indicating a detrimental effect of coarsening of precipitates on fatigue at elevated temperature. (C) 2016 Elsevier Ltd. All rights reserved.ACCEPTEDpeer-reviewe

Fatigue crack initiation in AA2024: A coupled micromechanical testing and crystal plasticity study

A new combined experimental and modelling approach has been developed in order to understand the physical mechanisms that lead to crack nucleation in a polycrystalline aluminium alloy AA2024 undergoing cyclic loading. Four-point bending low-cycle fatigue tests were performed inside the chamber of a scanning electron microscope on specimens with a through-thickness central hole, introduced to localize stresses and strains in a small region on the top surface of the sample. Fatigue crack initiation and small crack growth mechanisms were analyzed through high-resolution scanning electron microscope images, local orientation measurements using electron-back-scattered-diffraction, and local strain measurements using digital image correlation. A crystal plasticity finite element model was developed to simulate the cyclic deformation behaviour of AA2024. Two-dimensional Voronoi-based microstructures were generated, and the material parameters for the constitutive equations (including both isotropic and kinematic hardening) were identified using monotonic and fully reversed cyclic tests. A commonly used fatigue crack initiation criterion found in the literature, the maximum accumulated plastic slip, was evaluated in the crystal plasticity finite element model but could not predict the formation of cracks away from the edge of the hole in the deformed specimens. A new criterion combining 2 parameters: The maximum accumulated slip over each individual (critical) slip system and the maximum accumulated slip over all slip systems were formulated to reproduce the experimental locations of crack nucleation in the microstructure