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The Journal of Materials Science awards the annual Robert W. Cahn Prize to recognize the best paper published in the journal each year. The winner of the 2023 Cahn Prize is a research paper titled "Molecular modelling of graphene nanoribbons on the effect of porosity and oxidation on the mechanical and thermal properties." The paper presents molecular simulation models of graphene nanoribbons and examines the impact of porosity and oxidation on their mechanical and thermal conductivity properties. The paper is well-written and provides valuable insights for researchers in this field. Two other papers were selected as runners-up for the prize. All the monthly finalists for the Cahn Prize can be downloaded for free on the journal's website. [Extracted from the article]

In this paper, the effects of biaxial mean stress, mainly contributed by the weld residual stress, and thermal loading conditions on cracking directions and damage in high cycle thermal fatigue crazing subjected to variable amplitude loadings are investigated by a combined analytical and computational approach. The cracking directions are related to the orientation of the critical plane defined by the maximum damage. Analytical solutions of the critical plane orientation under constant amplitude biaxial tension/compression loadings are first derived and then employed to study the effects of biaxial mean stress on the critical plane orientation. The critical plane orientation appears to strongly depend on the dominant direction defined by the larger maximum stress. The developed analytical solution of the critical plane orientation and the analytical solution of the thermal stress from the literature are employed to study the effects of thermal loading conditions on the critical plane orientation. The critical plane orientation does not seem to significantly depend on the frequency, the amplitude and the mean value of the fluid temperature fluctuations, and the heat transfer film coefficient between the fluid and the pipe wall. The critical plane orientation under variable amplitude loadings is also studied, and an approximate solution is proposed for convenient engineering applications. The critical plane orientations are used to partially explain the observed cracking directions in the high cycle thermal fatigue crazing in the old residual heat removal system of a nuclear power plant. Finally, the effects of biaxial mean stress and thermal loading conditions on the fatigue crack initiation life are discussed. [ABSTRACT FROM AUTHOR]

This paper reviews the current understanding of the mechanisms of stress corrosion cracking of pipeline steels. The similarities, the differences and the influencing factors are considered for the "high pH" stress corrosion cracking caused by a concentrated bicarbonate-carbonate solution, and for the "low pH" stress corrosion cracking due to a diluter solution. For high pH stress corrosion cracking, it is well accepted that the mechanism involves anodic dissolution for crack initiation and propagation. In contrast it has been suggested that the low pH stress corrosion cracking is associated with the dissolution of the crack tip and sides, accompanied by the ingress of hydrogen into the pipeline steel. But the precise influence of hydrogen on the mechanism needs to be further studied. [ABSTRACT FROM AUTHOR]

Tensile tests were carried out at both constant stress and constant extension rate on polyamide 6.6 blends containing 0–20 wt% of maleinated ethylene-propylene rubber (EPR-g-ma), using specimens conditioned at 50% RH. Longitudinal and lateral strain measurements on creep specimens showed that blends containing EPR-g-ma exhibited dilatation at tensile stresses above 27 MPa. This observation is consistent with the predictions of the particle cavitation model proposed in an earlier paper (A. Lazzeri and C. B. Bucknall, J. Mater. Sci. 28 (1993) 6799). In standard tensile tests on PA6.6/EPR-g-ma blends, cavitation preceded yielding (defined by the 1% offset strain). Creep and yield data obtained over a range of temperatures and strain rates were correlated using the Eyring equation. The onset of dilatation in these PA6.6/EPR blends was marked by a change in deformation kinetics, which is analysed using a modified version of Gurson's equation. [ABSTRACT FROM AUTHOR]

In this paper, the plastic deformation behavior and strain homogeneity of the ZK60 Mg alloy during the multi-pass cyclic extrusion and compression (CEC) was simulated using the finite-element method (FEM) with a view to provide an insight into the mechanics of the process. Physical modeling (PM) experiment with same alloy was carried out to verify the results of the numerical simulations. The results show that two vortex flow regions with opposite flow direction are formed inside the cylindrical workpiece during CEC deformation. Although the deformation is inhomogeneous in both end regions of workpiece, a uniform region of equivalent strain exists, and the extent of uniform deformation increased with the increase in workpiece length. [ABSTRACT FROM AUTHOR]

This paper has clarified the anelasticity of 8Y-FSZ/α-alumina composites wherein the 8Y-FSZ phases are dispersed like islands. The amount of anelastic strain generated and the manner of anelastic deformation were compared to those of monolithic 8Y-FSZ. The anelastic strains of six kinds of 8Y-FSZ/α-alumina, as well as of monolithic 8Y-FSZ and monolithic α-alumina, were measured. The results showed that the anelastic strain was produced even in the composite where 8Y-FSZ phases existed as islands, and that the more the anelastic strain produced, the higher the volume fraction of 8Y-FSZ. In addition, the composition with a fully densified alumina phase had the effect of inhibiting anelastic strain in the 8Y-FSZ phase. [ABSTRACT FROM AUTHOR]

Some basic aspects of electromagnetic radiation (EMR) emission during quasi-static plastic deformation and crack propagation in ASTM B 265 grade 2 titanium sheets are reported in this paper. It has been observed that titanium emits several intermittent EMR signals starting near the yield and continuing up to fracture. The EMR emissions are sensitive to the applied strain rates. The number of EMR emissions shows a parabolic variation with strain rate, exhibiting a maxima. It appears that the temporary pinnings of the dislocations injected from the crack tip, which is essential for the emission of EMR as described in a recent model by the authors, do not adequately occur at high as well as low strain rates. At elevated temperatures, the square of the EMR amplitude is observed to be proportional to $$ \exp {\left( { - {mU} \mathord{\left/ {\vphantom {{mU} {kT}}} \right. \kern-\nulldelimiterspace} {kT}} \right)}, $$ where U is the activation energy for the movement of jogs, k the Boltzmann constant, T the temperature and m ≤ 1.0. [ABSTRACT FROM AUTHOR]

A general approach to simulate the mechanical behaviour of entangled materials submitted to large deformations is described in this paper. The main part of this approach is the automatic creation of contact elements, with appropriate constitutive laws, to take into account the interactions between fibres. The construction of these elements at each increment, is based on the determination of intermediate geometries in each region where two parts of beams are sufficiently close to be likely to enter into contact. Numerical tests simulating a 90% compression of nine randomly generated samples of entangled materials are given. They allow the identification of power laws to represent the evolutions of the compressive load and of the number of contacts. [ABSTRACT FROM AUTHOR]

In recent years, important efforts have been focused on rapid production of tools using Rapid Prototyping and Manufacturing (RP&M) technologies such as the Stereo-Lithography Apparatus (SLA). One of the applications is the development of rapid polymer tooling such as dies for injection molding. For these applications, optimal thermal as well as mechanical properties of final tools are of significance. In order to characterize the mechanical response of materials made by SLA, a standard set of material tests, including uniaxial tension and compression tests under different strain rates and different temperatures, was conducted for both silica filled and non-filled resin. In this paper, the mechanical response of the non-filled SL5170 cured resin is discussed in terms of an elastic-viscoplastic material model. Further, a new model for silica filled SL5170 cured resin was developed to estimate the stress-strain relationship of the composite. This composite model is an extension of the elastic-viscoplastic model for non-filled resin to include the elastic deformation of the silica particles. The stress-strain curves predicted by the models under homogeneous deformation show good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

The strength of a continuous-fiber-reinforced ceramic composite is directly related to the strength distribution of the reinforcements. Therefore, it is essential to understand how processing and service conditions affect the filament strength distribution. The objective of this paper was to determine the strength distribution of Nextel™ 610 alumina filaments, a potential reinforcement in oxide-oxide ceramic composites, and to characterize the defect population as a function of exposure temperature. The filament strength was measured in single filament tests and decreased as the exposure temperature increased. The defect population was quantified via fractography, and eight distinct defect types were identified. Grain growth and filament-to-filament sintering were identified as critical microstructural changes during the heat treatments, and both affected the development of grain boundary grooves, either from thermal etching or from the breaking of the filament-to-filament sintering bonds. The initial crack path was intergranular but quickly changed to a mixture of transgranular and intergranular fracture. During catastrophic crack growth, the crack path changed back to intergranular. The critical crack size, as calculated from linear elastic fracture mechanics, was found in many cases to be greater than the size of the strength limiting defect suggesting the presence of subcritical cracking. [ABSTRACT FROM AUTHOR]

This paper presents an overview of fatigue fracture modes in selected structural alloys employed in gas turbine engines. These include the mechanisms of fatigue crack growth in the near-threshold, Paris and high-δK regimes obtained from Ti-6A1-4V, Inconel 718 and PWA 1472 (a single crystal nickel-based superalloy of similar chemical composition to Inconel 718). Fatigue fracture modes in these materials are shown to be strong functions of the stress intensity factor range, AK, and the maximum stress intensity factor, Kmax. Fatigue mechanism maps are also presented to show the parametric ranges of ΔK and Kmax corresponding to the different fatigue fracture modes. [ABSTRACT FROM AUTHOR]

The applicability of instrumented falling weight impact techniques in characterizing mechanically thermoplastic foams at relatively high strain rates is presented in this paper. In order to try simulating impact loading of foams against sharp elements, an instrumented dart having a hemispherical headstock was employed in the tests. Failure strength and toughness values were obtained from high-energy impact experiments, and the elastic modulus could be measured from both flexed plate and indentation low-energy impact tests. The results indicate a dependence of the failure strength, toughness, and the elastic modulus on the foam density, the foaming process, and the chemical composition. This influence was found to be similar to that of pure nonfoamed materials and also to that observed from low-rate compression tests. The results also indicate that the indentation low-energy impact tests were more accurate in obtaining right values of the elastic modulus than the flexed plate low-energy impact tests usually used to characterize rigid plastics. The foam indentation observed with this test configuration contributes to obtaining erroneous values of the elastic modulus if only a simple flexural analysis of plates is applied. [ABSTRACT FROM AUTHOR]

Discusses the tensile behavior of silicon carbide/aluminum composites subjected to quasi-static and high strain-rate loading. Metal matrix composites; Dynamic tension tests; Powder metallurgy aluminum alloy; Tensile impact loading-unloading-reloading stress-strain results at high strain rate; Isothermal stress-strain curve; Adiabatic stress-strain curve; Phenomenologically-based constitutive model.

Ultrafine-grained AA7020 (AlZnMg) and AA7075 (AlZnMgCu) were produced by accumulative roll bonding (ARB) with up to 6 cycles. Different pre-heating treatments and their effect on the mechanical performance of the materials were investigated by means of hardness measurements and uniaxial tensile testing. It was found for AA7020 that by pre-heating at 230 °C for 2.5 min prior to each rolling step, an UTS of 550 MPa can be achieved, which is 51 % higher compared to the peak-aged T6 reference. For AA7075, pre-heating at 280 °C for 2.5 min leads to a very high UTS of 720 MPa after four ARB passes. A negative strain rate sensitivity was found for both alloys, which shifts toward zero with increasing number of ARB cycles. Post-ARB heat treatment was performed in order to overcome the reduced ductility after ARB. This leads to an enhanced strain hardening capacity after 4 cycles, resulting in an increase of the uniform elongation from 1.4 to 2.0 % for AA7020 and from 0.9 to 2.1 % for AA7075. [ABSTRACT FROM AUTHOR]

A new way of generating large plastic strain in a billet by splitting it plastically by a reciprocating punch is investigated. The concept of incremental angular splitting (I-AS) is explained by referring to the severe plastic deformation process of incremental equal channel angular pressing (I-ECAP). Results of laboratory trials of I-ECAP and I-AS, using a purposely designed AA1070 billet, are presented and both processes are compared. Further comparison is based on Vickers hardness distribution in the billet subjected to I-ECAP and I-AS. FE simulation results give an indication of strain distribution in both processes. The main advantages of I-AS appear to be more flexibility in the billet choice and more uniform strain distribution in the first pass of the process. A possibility of creating a gradient material by I-AS with a flat punch is considered. [ABSTRACT FROM AUTHOR]

The review focuses on the current studies of the deformation response and accompanying structural transformations of thermoplastic semicrystalline polymers subjected to uniaxial tension prior to the yield point. The mechanisms of strain-induced cavitation of amorphous layers and damages of crystalline lamellae are analyzed in line with novel results on the deformation behavior of solid polymers at temperatures exceeding the glass transition point. The coupling of viscoelastic and plastic deformation mechanisms with the small-strain structural transformations is critically discussed on the basis of the advanced theoretical modeling of mechanical properties of semicrystalline polymers. [ABSTRACT FROM AUTHOR]

study was carried out into the possibility of employing ECAP processing in lieu of mechanical milling for the purpose of developing powder-based hydrogen storage alloys. Mg and Mg-Ti powder compacts were encapsulated in a copper block and were subjected to ECAP deformation to an apparent strain of ε = 4. This resulted in the consolidation of the compacts as well as in the refinement of their structures. The values of coherently diffracting volume size were as small as 70-80 nm, quite comparable to those achieved with mechanical milling. It is, therefore, concluded that ECAP processing can be employed successfully for the purpose of structural refinement. As for material synthesis, however, the ECAP is less efficient in expanding the interfacial area. Therefore, it is necessary to impose relatively heavy strains to able to achieve comparable expansion in the interfacial area. It appears that an advantage of ECAP deformation is the development of structures which have improved ability for milling. It is, therefore, recommended that in the processing of hydrogen storage alloys, the powder mixtures may be first processed with ECAP in open atmosphere and then by mechanical milling of a short duration carried out under protective atmosphere. [ABSTRACT FROM AUTHOR]

In this study, equal channel angular pressed (ECAP) 5052Al alloy sheet was friction stir processed (FSP). This was carried out to understand the effect of FSP on the microstructure and mechanical properties of the ECAP sheet. FSP led to further grain refinement and a tighter distribution of grains. Fraction of high-angle grain boundaries changed from 15% in ECAP condition to more than 70% after FSP. Although FSP caused lowering of yield strength (YS) and ultimate tensile strength (UTS), it resulted into a substantial improvement in uniform deformation region of the tensile sample (from 1.3% in as-received condition to 12.9% in FSP condition). Strain hardening rate (SHR) analysis showed lowering of recovery rate on FSP. A static grain growth model correctly predicted the average grain size obtained after FSP. Existing grain boundary, solid solution, and dislocation strengthening models were used to estimate the YS of 5052Al alloy in both the conditions. The strengthening model was able to predict the YS of the alloy in as-received and FSP conditions very well. [ABSTRACT FROM AUTHOR]

The creep resistance of SP700 (Ti–4.5Al–3V–2Mo–2Fe) is superior to Ti–6–4 (Ti–6Al–4V) at 500 °C under a constant load corresponding to an initial stress of 100 MPa. The β phase grains in the SP700 alloy prefer to orient along the loading axis in contrast to the Ti–6–4 alloy. The grain growth occurs during the stress drop incubation period. The observation of different amounts of negative creep/anelasticity upon loading is closely associated with the difference in the amount of grain/subgrain coarsening. [ABSTRACT FROM AUTHOR]

We have investigated the influence of drawing on orientation, crystallinity, and structural properties of polyamide 6 films using X-ray diffraction. The samples were uniaxially and biaxially stretched resulting in the formation of monoclinic crystallites (α-form) in the size range of 8–10 nm. Depending on the drawing ratio, a degree of crystallinity of up to 60% is obtained. The average orientation of the crystallite axes was evaluated using the pole figure technique. The b*-axis, which corresponds to the chain direction of the polyamide molecules, lies in the film plane and shows a preferred orientation upon drawing. For uniaxial drawing, b* aligns with the drawing direction. For biaxially drawn films, which were prepared using the sequential stretching method, the second drawing determines the orientation of b*, at least at the center of the films. At the sides, b* is located between the two drawing directions reflecting the inhomogeneous distribution of mechanical stress during stretching. [ABSTRACT FROM AUTHOR]

Equal channel angular pressing (ECAP) was conducted at room temperature to impose high strain into high purity copper. Differential Scanning Calorimeter (DSC) was used to estimate the stored energy from ECAP and recrystallization temperature. It was found that the stored energy increases upon ECAP processing until a peak is reached at 12 passes of ECAP, and a slight decrease in stored energy was observed at higher ECAP passes. The recrystallization temperature decreases upon the increase of the stored energy up to ~50 J/mol, and reaches a stable valve of ~210 °C. Partial annealing of an ECAP processed (8 passes) sample by heating to ~185 °C at a heating rate of 20 °C/min released the stored energy from ~55 to ~18 J/mol, without substantial change on the recrystallization temperature of the sample. A two parameters model was used to help calculate stored energy of ultrafine-grained copper after high strain level processing. [ABSTRACT FROM AUTHOR]

Excellent superplastic elongations (in excess of 1,200%) were achieved in a commercial cast AZ31 alloy processed by low temperature equal-channel angular pressing (ECAP) with a back-pressure to produce a bimodal grain structure. In contrast, AZ31 alloy processed by ECAP at temperatures higher than 200 °C showed a reasonably uniform grain structure and relatively low ductility. It is suggested that a bimodal grain structure is advantageous because the larger grains contribute to strain hardening thus delaying the onset of necking, while grain boundary sliding associated with small grains provides a stabilizing effect due to enhanced strain rate sensitivity. [ABSTRACT FROM AUTHOR]

Severe plastic deformation is generally achieved using novel techniques such as Equal Channel Angular Pressing (ECAP) or High Pressure Torsion (HPT), but may also be achieved by more conventional methods such as very heavy rolling. Microstructure evolution is examined in an iron aluminide intermetallic rolled to strains up to 3.3 using Transmission Electron Microscopy (TEM) and orientation determinations by Kikuchi line analysis. After the highest strains the microstructure is still characterized as a recovered submicron-scale dislocation structure, with generally low angles across the various boundaries, and a high density of dislocations inside these boundaries. The structures observed show a dependence on orientation of the underlying parent grain, with [001] orientations showing poorer rearrangement to cellular structures than grains with [113–111] orientations. [ABSTRACT FROM AUTHOR]

Texture development and substructure evolution are described for pure beryllium given two-pass equal channel angular extrusion (ECAE) processing following routes A and C. These routes impose different strain paths between the first and second passes—the former cross-shearing and the latter reversal. Polycrystal calculations that are in good agreement with the texture measurements suggest that basal slip and, secondly, prismatic slip are operative in both passes of both routes. Multi scale polycrystal modeling is shown to effectively predict texture evolution to strains of two caused by both ECAE processing routes. Shear-like deformation textures observed in the second pass of route C are explained by differences in deformation characteristics between the first and second passes. [ABSTRACT FROM AUTHOR]

High-quality ultra-fine grained (ufg) and nanocrystalline (nc) bulk Cu samples of proper sizes reliable for mechanical testing, with grain sizes ( d) ranging from 720 down to 22 nm were prepared by means of room temperature ball-milling and consolidation processes. The specimens were subjected to compressive loading at the quasi-static strain rate of 10−4 s−1 to large strains ( ε = 50%). The specimens prepared from the 10-h-milled powder ( d = 32 nm) were tested at a wide range of strain rates (10−4 to 1,860 s−1), and the strain rate sensitivity (SRS) of the material was determined as a function of strain. The strength and work-hardening behavior were dramatically influenced by change in the grain size; the strength approached ∼900 MPa for the 30-h-milled Cu ( d = 22 nm) at the strain level of ∼50%. The SRS increased several fold as the grain size was reduced to 32 nm. Further, the results obtained in this study were compared with those of other investigators on ufg and nc Cu, to gain insights into the effect of different processing routes on the investigated material properties. [ABSTRACT FROM AUTHOR]

The splitting behavior and structural transformation process of K2Ti6O13 whiskers in various hydrothermal solutions were investigated by the X-ray diffraction technique, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. TiO2 (B) particle aggregates and rutile twinned crystals were produced respectively in diluted and concentrated HCl solutions via “dissolution-precipitation” mechanism, while no changes were observed in deionized water. In contrary to the chemical inertia of K2Ti6O13 whiskers in KOH solution, trititanate nanowires were synthesized by splitting the bulk K2Ti6O13 whiskers in NaOH solution. The driving force for the formation of nanowires originated from the intrinsic strain induced by the phase transition from K2Ti6O13 with a tunnel structure to layered trititanate. [ABSTRACT FROM AUTHOR]

A theory is provided for piezoresistivity in carbon fiber reinforced cement (with and without embedded steel reinforcing bars) under flexure (three-point bending). The phenomenon, which involves the reversible increase of the tension surface electrical resistance and the reversible decrease of the compression surface electrical resistance upon flexure, allows strain sensing. The theory is based on the concept that the piezoresistivity is due to the slight pull-out of crack-bridging fibers during crack opening and the consequent increase in the contact electrical resistivity of the fiber-matrix interface. This work is an extension of prior theory, which concerns the effect of uniaxial loading on the volume resistance. The extension requires modeling the surface resistance and its change under flexure. The theoretical results on the piezoresistivity, both with and without rebar, are in good agreement with prior experimental results. Differences between theoretical and experimental results are probably due to minor damage and rebar debonding during flexure. [ABSTRACT FROM AUTHOR]

Diametral compression is usually used to determine the mechanical resistance of cylindrical ceramic specimens. This work deals with the possibility of employing diametral compression in order to evaluate the mechanical response of ceramic rings, by testing two sets of rings (used as pump seals) that had two different sizes. The rings were characterized by different techniques: qualitative X-ray diffraction, apparent density and porosity measurements, determination of Vickers hardness, surface roughness, and microstructural analysis. α-alumina was identified as the majority crystalline phase in both types of rings. The porosities were rather similar, even though the observed mean grain size of the large rings was slightly larger. Significant differences were observed in the average roughness. Diametral compression tests at room temperature were carried out on a statistical number of each ring set. The fracture features were analyzed by ocular inspection and SEM observation of the fracture surfaces. Several approaches were used to estimate the fracture strengths: three analytical formulae with and without an empirical constant, and a finite element calculation. The simplest approach, an analytical formula that only requires the knowledge of the geometrical magnitudes of rings besides de fracture loads, gave a conservative estimation of the mechanical strength of rings and a limited explanation of fracture features. On the other hand, the numerical model being the most complex and informative of the approaches, gave the complete stress distributions. [ABSTRACT FROM AUTHOR]

It has been acknowledged that hydrogen plays a critical role in near-neutral pH stress corrosion cracking (SCC) of pipelines. However, the accurate mechanism for hydrogen involvement remains unknown. This work reviewed the applications of various electrochemical techniques towards understanding near-neutral pH SCC. The techniques reviewed include electrochemical hydrogen permeation, cyclic voltammetry, electrochemical impedance spectroscopy, electrochemical noise and scanning photo-electrochemical microscopy. The manner by which these techniques allow for the investigation of the hydrogen evolution mechanism, adsorption/desorption and permeation kinetics and hydrogen diffusion and accumulation in steel as well as the interactions between hydrogen, anodic dissolution and stress at crack tip in near-neutral pH environmental condition is described. It is anticipated that the advanced electrochemical measurement techniques provide essential tools to investigate the mechanistic aspects on hydrogen involvement in near-neutral pH stress corrosion cracking in pipelines. [ABSTRACT FROM AUTHOR]

Quantitative estimation of failure of a LiTaO3 single crystal due to thermal stress was investigated. Cylindrical test slabs were heated in a silicone oil bath, then subjected to large thermal stress by pouring silicone oil with room temperature. Cracking occurred during cooling. A transient heat conduction analysis was performed to obtain a temperature distribution in a test slab at the time of cracking, using the surface temperatures measured in the test. Then thermal stress was calculated using a temperature profile of the test slab obtained from the heat conduction analysis. It is found from the results of thermal stress analyses and the observation of the cracking in the test slabs that the cracking induced by thermal stress occurs mainly in the cleavage planes due to the stress component normal to the plane. As for a size effect of failure stress, large-sized cylindrical test slabs show lower failure stress than small-sized ones. Four-point bending tests were also performed to examine the relationship between the critical stress for cracking induced by thermal stress and the four-point bending strength. A useful relation was derived for predicting the critical stress for cracking induced by thermal stress from the four-point bending strength. [ABSTRACT FROM AUTHOR]

Flow stress, Young’s Modulus, energy and strain of fracture of poly(methyl methacrylate) (PMMA) and polystyrene (PS) were studied under compressive loading at strain rates of 10−4–10 s−1 and temperatures from 293 K to temperatures ∼20 K below T g. It was found that the energy of fracture shows an increase in the quasi-static strain rate (10−4–10−3 s−1) region and becomes constant in the low strain rate (10−2–10 s−1) region, while the strain of fracture shows a slow decrease with rate over the strain rate range tested. The activation energies and volumes of PMMA and PS at yield stress, 20% and 30% strain were evaluated using Eyring’s theory of viscous flow. Δ G was found to be constant for all strain rates and strains for both PMMA and PS. The activation volume for both materials increased as a function of strain. [ABSTRACT FROM AUTHOR]

The tensile behaviors of the ultrafine-grained (UFG) pure copper prepared by cold rolling have been investigated. The UFG-Cu exhibited high strengths and low ductility. The ductile dimple-like fracture surfaces and persistent-slip-bands (PSBs)-like local shear bands in the tension-deformed UFG-Cu were observed, both of which typically spanned tens of grain sizes. This observation indicated that the fracture mechanisms operated at a larger scale than the grain size and eventually involved collective grain activities. Moreover, localized shear bands provided the experimental evidence to the localized plastic deformation, which was the one of the dominant reasons, causing the low ductility in the UFG-Cu. [ABSTRACT FROM AUTHOR]

The formation of In-rich quantum dot structures will induce strain energy in the quantum well layer, forming the clusters and stacking faults influencing the optical properties. Our results showed different QW widths with the formation of various In-rich quantum dot structures and different levels of strain energy. Upon thermal annealing, energy relaxation resulted in the reshaping of quantum dots and hence the changes of optical properties. The results of temperature variations of PL spectral peak, integrated PL intensity and PL decay time showed consistent trends in varying strain energy distribution. [ABSTRACT FROM AUTHOR]

A SiCw/6061Al composite was fabricated through a squeeze-casting route and hot extruded to obtain a composite with directionally aligned whiskers. Based on observed changes in whisker orientation and length before and after deformation, compressive deformation behaviour of the directionally aligned SiCw/Al composite was investigated. It is found that when the compressive temperature is much lower than the solidus of the matrix alloy, the compressive flow stress of the directionally aligned composite is increased with compressive strain first and then decreased. When the compressive temperature equals the solidus of the matrix, however, the compressive flow stress of the directionally aligned composite is increased monotonously with compression strain. During compression, whisker rotation and breakage occurred, and the higher the compressive temperature, the easier the whisker rotation and hence the smaller the degree of whisker breakage. When the compressive strain was quite high, the degree of whisker breakage was serious even at the temperature as high as the solidus of the matrix. Analyzing changes in whisker orientation and breakage before and after compression indicates that the decreased compressive flow stress with compressive strain is the result of the decreased load carrying ability of whiskers caused by whisker rotation and breakage. Compared with whisker rotation, whisker breakage has a bigger contribution to the decreased compressive flow stress. No strain softening in the composite compressed at 580°C can be thought to be a result of the very low strengthening effect of whiskers at such a high temperature. From the point of view of whisker breakage, to get higher properties of SiCw/Al composite parts made by means of plastic forming, too high plastic strain should not be suffered by SiCw/Al composites during the plastic forming. [ABSTRACT FROM AUTHOR]

This study outlines the mechanical properties of an explosive material tested at AWE Plc, Aldermaston, Reading, Berkshire, UK. The material is a composite polymer bonded explosive of energetic binder and HMX crystals, referred to as EDC37. An understanding of material deformation under a range of conditions and age, along with damage quantification, is required to predict mechanical behaviour throughout service life. The mechanical testing of EDC37 was performed alongside simultaneous recording of Acoustic Emission (AE) data at static strain rates and temperatures. The simultaneous testing was intended to focus on the link between mechanical properties and damage evolution. Forty tests were performed in various configurations, consisting of live explosive material and an inert simulant. Evaluation of the Acoustic Emissions was performed and data grouped into categories of activity. It was concluded that AE confirms mechanical behaviour associated with damage accumulation in compression (crack initiation and growth, crack closure and/or HMX twinning) and crack damage (binder HMX debonding and coalescence) above a threshold stress or time in tension. The activity seen is different between tensile and compressive modes and needs to be taken into consideration in Finite Element Models. [ABSTRACT FROM AUTHOR]

Austenitic stainless steel specimens have been examined by positron-lifetime measurements at various stages until failure during fatigue tests at constant stress or plastic strain amplitudes. A positron-source-detector assembly has been mounted on the servohydraulic testing machines that allowed truly non-destructive positron annihilation studies without removing the specimens from the load train. Positrons were generated by a 72Se/72 As source with a maximum activity of 0.9 MBq (25 μCi). The average positron lifetime has been determined by a β+ − γ-coincidence applying a simplified data evaluation procedure. Under constant stress or plastic strain amplitudes early stages of fatigue damage could be detected. The strong increase of the average positron lifetime already during the first 10% of fatigue life could be related to the fatigue life of the specimens. Issues of lifetime prediction by positron annihilation measurements are discussed. [ABSTRACT FROM AUTHOR]

The mechanical behavior of open-cell foams may be modeled either on a microscopic or a macroscopic scale. In the first case, the behavior of the individual cell walls is described by beam models, while in the second case a continuum mechanical approach is applied. Both approaches have different advantages: On the one hand, the microscopic approach allows for a simple formulation of the constitutive equations but requires detailed knowledge of the heterogeneous microstructure, e.g. geometrical data of the beams and of the topology, and becomes numerically expensive for large structures. On the other hand, the macroscopic approach leads to efficient computations but requires more complicated constitutive equations, if e.g. anisotropy is taken into account. In the present contribution the advantages of the microscopic and macroscopic descriptions are combined by a numerical so-called second order homogenization scheme. Therefore, a small but representative element of the microstructure consisting of a few beam elements is chosen and attached to the quadrature points of the macroscopic finite element model. The macroscopic model is formulated in the framework of a Cosserat continuum, which allows to take care of size effects. The macroscopic strain and curvature tensors are projected onto the microstructure leading to a deformation mode of the beam ensemble. The resulting forces and moments in the beams are homogenized by an appropriate averaging procedure defining the corresponding stresses and couple stresses on the macroscale. In this approach, anisotropy is included in a natural way choosing an anisotropic distribution of the beams in the testing volume element (TVE). In addition, damage is described on the microscopic level of the individual beams. [ABSTRACT FROM AUTHOR]

The determination of the mechanical properties of fabrics in biaxial tension and in-plane shearing is made from 3D finite element analyses of the unit woven cell. Compared to experimental tests these virtual tests have several advantages. They can easily be carried out for sets of varied parameters, they provide local information inside the woven material and above all they can be performed on woven materials that have not yet been manufactured. The 3D computations are not classical analyses because the yarns are made up of several thousands of fibres and their mechanical behaviour is very special. Several specific aspects of the analysis are detailed, especially the use of a hypoelastic law based on an objective derivative using the rotation of the fibre which allows a strict evolution of the directions of orthotropy according to the fibre direction. Examples of analyses are presented in biaxial tension and in-plane shear for woven reinforcements and in the case of the biaxial tension of a knitted fabric. The results obtained are in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

The defects appearing during the process of martensitic transformation from bcc austenite to a microtextured hcp martensite are examined, with reference to molecular dynamics simulations in zirconium. The simulations involve cooling from the high temperature austenitic bcc phase. A variety of geometric defects are identified, with the structure and dynamics being understood in terms of vicinal twin boundaries and dislocations. These defects are classified as either geometrically necessary or nanoscale effects, and the evolution of the material in response to annealing and external stress is described. The observed mechanism for twin boundary motion under applied strees involves a combination of stress concentration by preexisting sessile dislocation, causing nucleation, motion and absorption of defects in the boundary plane. [ABSTRACT FROM AUTHOR]

The formation of stress-induced α ″ martensite in a metastable β Ti-Mo-based alloy has been studied using X-ray diffraction and transmission electron microscopy. The martensite nucleates heterogeneously at pre-existing sub-grain boundaries by dissociation of the boundary dislocations which bow out on inclined planes. The growth of martensite laths from the resultant stacking faults occurs by the motion of interfacial disconnections: the steps at the cores of these defects account for the apparent discrepancy between the 112 glide plane for the initial partial dislocation and the 334 habit plane for the martensite laths. [ABSTRACT FROM AUTHOR]

The first stages of the mechanical alloying process, fragmentation and welding, are analysed. In order to clarify the mechanisms of milling-induced diffusion, different interstitial solid solutions are examined. It is shown that diffusion of interstitial is mainly controlled by the stress field. In case of a substitutional solid solution, the mechanism of atomic transport is controlled by shear deformation. The consequence of the corresponding diffusion models are discussed. [ABSTRACT FROM AUTHOR]

The small strain (below yielding) tensile loading-unloading tests were carried out on the low-density polyethylene (LDPE) and polypropylene (PP) at low strain rate and room temperature. The experiments unambiguously indicate to a remarkable decrease in residual strains in comparison with those predicted by conventional viscoelastic models. These deviations cannot be explained without taking into account structural transformations of semi-crystalline polymers. As long as small deformations cannot result in significant change in content and texture of crystalline and amorphous components, it was assumed that such transformations should include disintegration of connectivity in crystallite clusters. This structural rearrangement is supposed to be caused by the strain-induced decrystallization of narrow (and thus highly stressed) "bridges" connecting domains of conjugated crystallites or inside crystallites. A simple 1D modelling of the deformation processes supports this expectation. The disconnection in polymer morphology is simulated by small portions of amorphous ligaments appearing between neighbouring crystallites in the course of deformation. In spite of simplicity of the model a precise fitting of the stress-strain diagram is obtained along with small variations in structural and material characteristics (crystallinity degree, effective rigidity and plastic ability) of the concerned polymers. [ABSTRACT FROM AUTHOR]

Studies the microstructures and mechanical properties of three-dimensional C[sub f]/silicon-oxygen-carbon composites fabricated by polysiloxane pyrolysis. Interfacial structure and density; Flexural strength; Fracture toughness.

Low cycle fatigue tests for a hot extruded Nickel base alloy tube material have been performed at room temperature and at 204°C. The alloy shows a normal hardening and softening cyclic stress-strain response at room temperature. At 204°C, however, the cyclic stress-strain response shows a strain hardening first followed by a relatively stable stress and finally a secondary cyclic strain hardening. This stable stress disappears with increasing strain amplitude. The mechanisms of the secondary cyclic strain hardening have also been investigated by transmission electron microscopy (TEM). Besides dislocation multiplication, interactions between stacking faults and moving dislocations and between interstitial atoms and moving dislocations could also contributed to this secondary cyclic strain hardening. The formation of micro-twins during cyclic loading at 204°C and its influence on the cyclic stress-strain response were also discussed. [ABSTRACT FROM AUTHOR]

The bipolar electric fatigue behaviour of antiferroelectric ceramics with the composition of Pb0.88Ba0.10La0.02 (Zr0.55Sn0.35Ti0.10)O3 was investigated under various cycling fields. The material exhibits a degradation in the maximum field induced strain, a diffuse AFE-FE phase transition and an enhancement in the diffusion character of the FE-AFE phase transition due to electric cycling. Those variations increase with cycle number, indicating a logarithmic fatigue up to 108 cycles. There is no indication for the variations to be recovered, and the symmetry of the negative and positive parts of the strain hysteresis loops still remains. Higher cycling field results in a stronger deterioration of the maximum field induced strain and a larger extent of diffusion in AFE-FE and FE-AFE phase transitions. The normalized maximum strain shows nearly the same scale of degradation when the materials are cycled at various electric fields. After a heat treatment at 500°C for 1 h, the variations in the AFE-FE and FE-AFE phase transition due to cycling disappear, whereas the maximum strain resumes almost to its original value. Electrochemical variations are considered to contribute to the main fatigue mechanism for the material under investigation. [ABSTRACT FROM AUTHOR]

The tensile behaviour of different geothermal well cement formulations was investigated in order to determine how these materials perform under such loading and how tensile capacity can be improved. The influence of latex, perlite and fibres on the load-displacement relationship of the cements was measured on notched cylinders. The fracture surfaces were examined to further elucidate failure mechanisms. Unreinforced cements exhibited linear elastic behaviour to different degrees and failed in a brittle manner. Cements reinforced with either carbon (150 μm) or steel (1–3 mm) microfibres required greater loads for failure. However, the microfibres did not provide any major improvement in ductility. Addition of 13 mm steel fibres to the cements resulted in both strengthening and transition to ductile behaviour. Inclusion of the types of fibres studied in this work in cements offers potential benefits in maintaining the integrity of geothermal wells when tensile stresses are involved. [ABSTRACT FROM AUTHOR]

The potential of electrophoretic deposition as a shaping process of free-standing objects with a gradient in composition is investigated. The capability to produce relatively thick continuously graded ZrO2-Al2O3 and WC-Co-Ti(C,N) discs was explored. After electrophoretic deposition, the dried deposits were cold isostatically pressed and pressureless sintered. In order to create a symmetric gradient profile along the thickness of the plates, a mathematical model for the EPD process was developed in order to predict the composition and slope of the gradient profile in the discs from the starting composition of the suspension, the EPD operating parameters and the powder-specific EPD characteristics. The sintered graded materials have a continuous variation in composition, microstructure and mechanical properties. In the illustrated ZrO2-Al2O3 example, the ZrO2 content increases from 0 vol% on the outer surface up to 25 vol% in the core. For the WC-Co-Ti(C,N) discs, the Ti(C,N) content decreases from 5 wt% on the outer edge to 0 wt% in the bulk of the material. In both materials, a hard outer rim and a tough core are obtained. Finally, the residual surface stresses of the densified graded ZrO2-Al2O3 discs were investigated. [ABSTRACT FROM AUTHOR]

Focuses on the role of stability of the passive film formed on the surface of the material in determining the stress corrosion cracking (SCC) resistance of a material in a given environment. Consideration of the semiconducting behavior of passive film as important in determining the corrosion resistance of the material; Importance of characterizing the electronic properties of the passive film in-situ in order to have a better understanding of the material degradation.

Joining of ceramics to metals is of great interest from both technical and economical points of view. SHS welding of SiC ceramic to Ni-based superalloy has been achieved with TiC-Ni functionally gradient materials as filler and a tungsten sheet as an intermediate layer. The obtained welded strength of the welded sample is 60% of the strength of the SiC ceramic measured under the identical conditions. This value is 25% greater than that without the tungsten intermediate layer. Within the range tested in this study, the strength of welded samples increases with increasing thickness of the tungsten layer. The presence of the tungsten intermediate layer can effectively improve the distribution of process-induced thermal stresses in the joints and reduce the concentration of residual thermal stresses in the ceramic near the welding seam. Microstructural study reveals that diffusion takes place at the interfaces, which promotes bonding at the interfaces. [ABSTRACT FROM AUTHOR]

The effect of low dosage plasma ion implantation on hydrogen embrittlement was studied for an HSLA steel using notched tensile samples. The plasma treatment caused an enhancement in the linear strain to failure under embrittling conditions. This was however not reflected in the fracture surfaces of the treated samples which had similar fractographic features as those of untreated samples. The plasma treatment delayed the process of embrittlement without causing any alteration in the basic mechanism of embrittlement. This was due to introduction of residual compressive stresses as well as reduction in the hydrogen permeation flux. Implantation in pure nitrogen seemed most beneficial while implantation in pure argon caused very little improvement. [ABSTRACT FROM AUTHOR]