Stochastic meshless analysis of elastic–plastic cracked structures

This paper presents a stochastic mesh-free method for probabilistic fracture-mechanics analysis of nonlinear cracked structures. The method involves enriched element-free Galerkin formulation for calculating the J-integral; statistical models of uncertainties in load, material properties, and crack geometry; and the first-order reliability method (FORM) for predicting probabilistic fracture response and reliability of cracked structures. The sensitivity of fracture parameters with respect to crack size, required for probabilistic analysis, is calculated using a virtual crack extension technique. Numerical examples based on mode-I fracture problems have been presented to illustrate the proposed method. The results from sensitivity analysis indicate that the maximum difference between sensitivity of the J-integral calculated using the proposed method and reference solutions obtained by the finite-difference method is about three percent. The results from reliability analysis show that the probability of fracture initiation using the proposed sensitivity and meshless-based FORM are very accurate when compared with either the finite-element-based Monte Carlo simulation or finite-element-based FORM. Since all gradients are calculated analytically, the reliability analysis of cracks can be performed efficiently using meshless methods. The authors would like to acknowledge the financial support of the U.S. National Science Foundation (NSF) under Award No. CMS-9900196. The NSF program director was Dr. Ken Chong.     

Assessment of alkali–silica reaction damage through quantification of concrete nonlinearity

A newly developed nondestructive evaluation technique, Nonlinear Impact Resonance Acoustic Spectroscopy (NIRAS), is applied to concrete specimens in an ongoing assessment of aggregate alkali reactivity during standard concrete prism testing. NIRAS measures the nonlinearity in a specimen caused by the inception and growth of microcracks throughout the sample and debonding at the aggregate/cement interface. NIRAS is used to exploit the nonlinear effect of excitation amplitude dependent resonance frequency changes, which are related to nonlinearity measurements of concrete samples cast with aggregates of varying reactivity. To relate microstructural changes to changes in nonlinearity and expansion, sample characterization is performed with uranyl-acetate staining. The results demonstrate the utility of NIRAS for not only assessing the potential for ASR under standardized test conditions, but for more general damage characterization in concrete and assessment of “job mixtures.” NIRAS can distinguish reactive from nonreactive aggregates without ambiguity, as supported by sample characterization results.     

Effect of nickel on the microstructure and mechanical property of die-cast Al–Mg–Si–Mn alloy

The effect of nickel on the microstructure and mechanical properties of a die-cast Al–Mg–Si–Mn alloy has been investigated. The results show that the presence of Ni in the alloy promotes the formation of Ni-rich intermetallics. These occur consistently during solidification in the die-cast Al–Mg–Si–Mn alloy across different levels of Ni content. The Ni-rich intermetallics exhibit dendritic morphology during the primary solidification and lamellar morphology during the eutectic solidification stage. Ni was found to be always associated with iron forming AlFeMnSiNi intermetallics, and no Al₃Ni intermetallic was observed when Ni concentrations were up to 2.06 wt% in the alloy. Although with different morphologies, the Ni-rich intermetallics were identified as the same AlFeMnSiNi phase bearing a typical composition of Al_[100–140](Fe,Mn)_[2–7]SiNi_[4–9]. With increasing Ni content, the spacing of the α-Al–Mg₂Si eutectic phase was enlarged in the Al–Mg–Si–Mn alloy. The addition of Ni to the alloy resulted in a slight increase in the yield strength, but a significant decrease in the elongation. The ultimate tensile strength (UTS) increased slightly from 300 to 320 MPa when a small amount (e.g. 0.16 wt%) of Ni was added to the alloy, but further increase of the Ni content resulted in a decrease of the UTS.     

Effects of Zn on the microstructure,mechanical property and bio-corrosion property of Mg–3Ca alloys for biomedical application

The effects of the addition of Zn element on the microstructures, mechanical properties and bio-corrosion properties of Mg–3Ca alloys are investigated. The microstructure and X-ray diffraction topography indicate that as-cast Mg–3Ca alloys are composed of primary Mg and eutectic (α-Mg + Mg₂Ca) phases, while Mg–3Ca–2Zn alloys are constituted of primary Mg and eutectic (α-Mg + Mg₂Ca + Ca₂Mg₆Zn₃) phases. Mechanical properties results show that the element Zn could improve both tensile strength and elongation of Mg–3Ca alloys. The ultimate tensile strength is enhanced by 22%. Meanwhile, the corrosion resistance is increased by the addition of Zn element. It is thought that the presence of Ca₂Mg₆Zn₃ phase mainly contributes to these improvements. Mg–3Ca–2Zn alloy provides moderate strength and excellent corrosion resistance for biomedical application.     

Influence of Sc and Zr additions on microstructure and properties evolution of Al–Zn–Mg alloy

Journal of Materials Science - In this study, the effects of different Sc?+?Zr compound addition on the tensile properties, impact toughness, stress corrosion cracking (SCC) properties,...     

Thermal postbuckling analysis of fiber–metal laminated plates including interfacial damage

Considering the effects of interfacial damage, geometric nonlinearity and transverse shear deformation, thermal postbuckling of fiber–metal laminated plates including interfacial damage is analyzed in detail. Firstly, the Heaviside step function and higher order shear deformation functions are introduced into displacement field so that the damage degree can be characterized. Then, the shape functions can be determined by using the stress continuity conditions between interfaces and the stress boundaries on surfaces. By using the generalized variational principle, the thermal postbuckling equilibrium equations of fiber–metal laminated plates including interfacial damage are established. Finally, the thermal postbuckling problem is solved by adopting finite difference method and iteration method. In numerical examples, the effects of interfacial damage, width-to-thickness ratio and thermal load on the thermal postbuckling of fiber–metal laminated plates including interfacial damage are investigated.     

An analysis of microstructure and impression creep response of squeeze-cast AZ91–xBi–ySr alloys

ABSTRACT

The present investigation deals with the microstructural modification following the Bi?+?Sr additions to the squeeze-cast AZ91 alloy and its effect on impression creep response. The Bi?+?Sr additions form the Al₄Sr and Sr₂Bi phases besides the α-Mg and β-Mg₁₇Al₁₂ phases, and improves creep resistance of the AZ91 alloy. The AZ91?+?1.0Bi?+?0.5Sr alloy reveals the best creep resistance among the alloys. The stress exponent and the activation energy values of all the alloys are in the range of 4–7 and 100.2–112.7?kJ?mol^?1, respectively, depicting the pipe diffusion-controlled dislocation creep is the governing creep mechanism. The post-creep microstructural study confirms several dislocations pile-ups around the Al₄Sr and Sr₂Bi phases resulting in improved creep resistance of the modified AZ91 alloys.     

Random vibration analysis for train–track interaction from the aspect of uncertainty quantification

This work is devoted to constructing a stochastic analysis model for train–track​ interaction. The fundamentals of the modelling framework in the establishment of the dynamic model, simulation of system uncertainties and randomness propagation process have been properly illustrated and unified in detail. For modelling train–track interaction, a matrix representation method is developed to depict the displacement compatibility and force equilibrium between the train and tracks. This dynamic model possesses advantages in computational stability and accuracy. Using uncertainty quantification approaches, the randomness of system geometries and longitudinal inhomogeneity of system properties can be simulated properly. Finally, the probabilistic transmission between the system inputs and response outputs are investigated from physical concepts, and a family of probability density evolution methods is introduced. Following the fundamental framework of train–track stochastic analysis, numerical examples are presented in detail to show the efficiency and accurateness of the proposed model. Moreover, the applications and advancements of this model in reliability assessment, response and frequency analysis, derailment, etc., are illustrated.     

The effect of Si and microstructure evolution on the thermal expansion properties of Fe–42Ni–Si alloy strips

An alloying element of 0–1.5 wt.% Si was added to an Fe–42%Ni system, and alloy strips were fabricated using a melt drag casting process. The effects of the Si and annealing treatments on the thermal expansion properties of Fe–42Ni alloy were investigated. The addition of Si enlarged the coexisting temperature region of the solid–liquid phase and reduced the melting point, which improved the formability of the alloy strip. An alloy containing 0.6 wt.% Si had a lower thermal expansion coefficient than any other alloy in the temperature range from 20 to 350 °C. The grain size increased with the rolling reduction ratio and annealing temperature, which caused an increase in magnetostriction and consequently a decrease in the thermal expansion coefficient of the strip. The alloy strip containing 1.5 wt.% Si had a higher thermal expansion coefficient than the alloy containing 0.6 wt.% Si because of grain refining caused by the precipitation of Ni₃Fe.     

Maximisation of the ratio of microhardness to the Young's modulus of Ti–12Mo–13Nb alloy through microstructure changes

Alloys for orthopaedic and dentistry applications require high mechanical strength and a low Young's modulus to avoid stress shielding. Metastable β titanium alloys appear to fulfil these requirements. This study investigated the correlation of phases precipitated in a Ti–12Mo–13Nb alloy with changes in hardness and the Young's modulus. The alloy was produced by arc melting under an argon atmosphere, after which, it was heat treated and cold forged. Two different routes of heat treatment were employed. Phase transformations were studied by employing X-ray diffraction and transmission electron microscopy. Property characterisation was based on Vickers microhardness tests and Young's modulus measurements. The highest ratio of microhardness to the Young's modulus was obtained using thermomechanical treatment, which consists of heating at 1000 °C for 24 h, water quenching, cold forging to reduce 80% of the area, and ageing at 500 °C for 24 h, where the final microstructure consisted of an α phase dispersed in a β matrix. The α phase appeared in two different forms: as fine lamellas (with 240 ± 100 nm length) and massive particles of 200–500 nm size.     

Influence of wood flour moisture content on the degree of silane-crosslinking and its relationship to structure–property relations of wood–thermoplastic composites

The objective of this work was to examine how the moisture content of wood flour affects the degree of crosslinking when producing silane-crosslinked wood–thermoplastic composites. Crosslinked composites were produced by adding a silane solution to the compounding process of wood flour and polyethylene. Crosslinked composites of pre-dried as well as non-dried wood flour were prepared and their degree of crosslinking at various storage conditions was determined. Mechanical properties and the creep response of the crosslinked composites were tested in order to establish structure–properties relations. The results showed that all crosslinked composites displayed higher strengths and lower creep responses compared with non-crosslinked control samples. However, the degree and rate of crosslinking proved to be lower when a larger amount of moisture was present in the compounding process. It was concluded that the silane-grafting yield was lower when wood flour of a higher moisture content was used.     

Comparison between the process–structure–property relationships of silica foams prepared through two different processing routes

Cellular silica with improved framework, crosslinking, and stability properties are desirable for applications in thermal insulation. A process for the preparation of cellular silica foam with interconnected cells with tailored porosity and pore size distribution has been attempted. The silica foams have been prepared through two different methods; surfactant- and particle-based stabilization. The silica foams prepared through two different processes namely surfactant-stabilized foams (SSF) and particle-stabilized foams (PSF) have exhibited a wide range of differences in their structure which in turn have shown to affect the final properties of the foam. The cell size distributions in SSF (89 vol% porosity) and PSF (85 vol% porosity) have been found in the range of 50–250 μm (monomodal) and 4–10 μm and 50–100 μm (bimodal), respectively, whereas the cell counts of both have been found in close proximity. The microstructure of both the sintered SSF as well as PSF samples foams have shown an open and interconnected porosity with the permeability of both in the region of ~10⁻⁸ m². The mechanical (compressive) strength and Young’s modulus of the PSF are a third of that in SSF. The structure–property relationship of both the SSF and PSF and their comparison have been discussed.     

High speed rolling of Mg–3Al–1Zn alloy: texture and microstructure analysis

Abstract

High speed rolling (HSR) of 1000 m min^?1 was employed to successfully roll AZ31 alloy in one pass with 65% reduction in thickness at 300 and 450°C. The rollability, texture and microstructure after HSR, in comparison with low speed rolling (15 m min^?1), improved significantly. It is suggested that the double peak and weaker basal texture obtained after HSR are attributed to the activation of compression and double twins. After annealing, the double peak basal texture is replaced by a single peak one, which may be due to preferential grain growth of basal grains.     

Magnetic M–H loops family characteristics in the microstructure evolution of BaFe12O19

A polycrystalline magnetic materials series produced from the same batch of high-reactivity powder by multi-sample sintering in a range of increasing temperatures can supposedly reveal its property and microstructural evolution through magnetic property measurement and clear morphological changes. Hence, in our work, we attempt to find out the variation of magnetic properties of BaFe₁₂O₁₉ against its evolving microstructure. Hexagonal BaFe₁₂O₁₉ nanometer-sized powder with the M-type structure was synthesized by the mechanical alloying method. The crystal structure, grain size and magnetic properties were studied by means of XRD, field emission scanning electron microscopy and a vibrating sample magnetometer. The ferrite materials were obtained from a mixture of barium carbonate and iron oxide by mixing them using conventional ball milling (12 h) and then assisted by high energy ball milling for 6 h. The comminuted powder was divided into several batches, moulded in pellet shape and sintered at different temperatures ranging from 400 to 1400 °C for a constant sintering time of 10 h in a static air atmosphere. Effects of sintering temperature on the formation, crystallite size, morphology and magnetic properties were systematically studied. In the sintering temperature range from 800 to 1200 °C, the coercivity (Hc) gradually increased due to the effect of rapid grain growth. This is because the fine starting powder was so reactive that rapid grain growth easily occurred. In the grains, strong interaction of magnetic moments within domains due to exchange forces led to strong anisotropy. It was also observed that three M–H-loop groups each belonging to a particular range of grain sizes exhibited different strengths of magnetism. It is believe of similar results have never been reported before and should be useful to the permanent-magnet industry.     

Influence of the microstructure evolution of ZrO2 fiber on the fracture toughness of ZrB2–SiC nanocomposite ceramics

ZrB₂–SiC nanocomposite ceramics toughened by ZrO₂ fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO₂ fiber incorporated into the ZrB₂–SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO₂ fiber exhibited remarkable effects on the fracture toughness of the ZrO_2(f)/ZrB₂–SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m^1/2 ± 0.3 MPa m^1/2 by the addition of 15 mass% of ZrO₂ fiber. The microstructure of the ZrO₂ fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO₂ to m-ZrO₂, which was also an important reason behind the improvement in toughness.     

Influence of Co addition on microstructure evolution and mechanical strength of solder joints bonded with solid–liquid electromigration

Journal of Materials Science: Materials in Electronics - The influence of solid–liquid electromigration on Cu-xCo/Sn-3.0Ag-0.5Cu/Cu-xCo (x?=?0, 30 and 50 wt.%) joints bonded at...     

Review on microstructure evolution in Sn–Ag–Cu solders and its effect on mechanical integrity of solder joints

The use of Pb-bearing solders in electronic assemblies is avoided in many countries due to the inherent toxicity and environmental risks associated with lead. Although a number of “Pb-free” alloys have been invented, none of them meet all the standards generally satisfied by a conventional Pb–Sn alloy. A large number of reliability problems still exist with lead free solder joints. Solder joint reliability depends on mechanical strength, fatigue resistance, hardness, coefficient of thermal expansion which are influenced by the microstructure, type and morphology of inter metallic compounds (IMC). In recent years, Sn rich solders have been considered as suitable replacement for Pb bearing solders. The objective of this review is to study the evolution of microstructural phases in commonly used lead free xSn–yAg–zCu solders and the various factors such as substrate, minor alloying, mechanical and thermo-mechanical strains which affect the microstructure. A complete understanding of the mechanisms that determine the formation and growth of interfacial IMCs is essential for developing solder joints with high reliability. The data available in the open literature have been reviewed and discussed.     

Effect of pre-deformation anneal on the microstructure and texture evolution of Mg–3Al–1Zn–0.7Sr alloy during hot extrusion

The effect of pre-deformation annealing on the microstructure and texture of an AZ31 + 0.74 wt% Sr alloy has been investigated. As-cast samples as well as three samples that have been annealed at 400 °C for 10, 30, and 120 min were extruded at 300 °C. Results indicate that annealing transforms the bulky non-equilibrium Al–Mg–Sr precipitates to stable Al₄Sr spheroids. As the extent of this transformation increases before extrusion, there is seen an increase in the amount of uniformly dispersed intermetallic stringers in the extruded material. Texture measurements reveal the alignment of basal poles with the compression axis (perpendicular to the circular cross section of the extruded bar) and the formation of the basal ring texture in all the samples. However, an increase in the duration of the pre-deformation anneal switches the plane facing the extrusion direction from first order prismatic (10-10) to second order prismatic planes (11-20). Annealing decreases the Al solute concentration in Mg and lowers the lattice resistance against dislocation movement. Consequently, the more favorable (0002)[11-20] slip system is activated in grains that see low basal resolved shear stress (τ). As a result, those grains work harden and are consumed by dynamic recrystallization (DRX). However, the (0002)[-1100] slip system with high τ still avoids basal dislocation movement. Hence, the grains with high τ_{(0002)[-1100]}, which need to move dislocations in the (0002)[-1100] system to fulfill the strain compatibility conditions across the microstructure would be prevented from work hardening and DRX. This specific orientation has a (11-20) plane facing the extrusion direction.     

Effect of polyethylene glycol on the mechanical property, microstructure, thermal stability, and flame resistance of phenol–urea–formaldehyde foams

In this study, polyethylene glycol (PEG) was added to phenol–urea–formaldehyde foam to improve its toughness, and the effects of PEG, with different molecular weights and dosages, on the mechanical property, microstructure, thermal stability, and flame resistance of phenol–urea–formaldehyde foam were studied. The addition of PEG significantly increased the toughness and impact strength and decreased the pulverization rate of the foam. The compression strength of the foam first increased and then decreased with increasing amounts of PEG. When 2 wt% PEGs were added, the compression strength of foams was the highest. The addition of PEG significantly influenced the microstructure of phenol–urea–formaldehyde foams, in which the cell diameter decreased and wall thickness increased with increasing amount and molecular weight of PEG. The addition of PEG also slightly decreased the thermal stability of phenol–urea–formaldehyde foams, and increased the heat release rate, total heat release, and total smoke release of the foams.     

Strain path and microstructure evolution during severe deformation processing of an as-cast hypoeutectic Al–Si alloy

Microstructure evolution in an as-cast Na modified Al–7%Si (wt. pct.) alloy was examined during redundant and monotonic straining by repetitive equi-channel angular pressing (ECAP) under ambient temperature conditions, and during friction stir processing (FSP). Redundant straining during repetitive ECAP was accomplished by processing following route B_C while monotonic straining employed route A. Single- and multi-pass FSP was conducted on this same as-cast material using an FSP tool having a threaded pin. The as-cast microstructure comprises equiaxed primary α dendrite cells embedded in the Al–Si eutectic constituent. The evolution of this microstructure during repetitive ECAP can be described by idealized models of this process. The primary and eutectic constituents can still be discerned and the Si particle distribution is not homogenized even during ambient temperature processing involving von Mises strains >9.0. In contrast, the primary and eutectic constituents cannot be distinguished in the stir zone after even a single FSP pass. Strain estimates based on the shape change of the primary α constituent indicate that the Si particle distribution has become homogeneous at local von Mises strains of 2.5–3.0 during the FSP thermomechanical cycle. Mechanical property data are consistent with strain path during SPD processing by repetitive ECAP and FSP.