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The transportation and disposal of tunnel muck have detrimental environmental impacts. For this reason, the practice of reusing such a resource in engineering materials has substantial environmental and economic benefits. Seasonal frozen regions are extensively distributed worldwide, and freeze–thaw (F-T) cycles can largely degrade the performance of engineering materials, underscoring the importance of knowledge on the F-T resistance of tunnel muck. This study investigates the impact of F-T cycles on the mechanical and microscopic behavior of slurry shield tunnel muck stabilized with basalt fiber (BF) and inorganic curing agent (ICA). The qualities of muck discharged from slurry shield tunneling were improved in the composite formation of fine sand and muddy silty clay. Then tests were conducted to obtain the stress-strain characteristics, uniaxial compressive strength (UCS), and microstructure of the improved tunnel muck with different numbers of F-T cycles (N) and BF contents. Based on the Weibull probability distribution function, the damage evolution model of ICA-BF stabilized tunnel muck was proposed. The results show that the UCS decreases parabolically when the F-T cycle increases, and the F-T damage degree is 24.5%–40.6% after 12 cycles. Increasing BF content can effectively improve tunnel muck's F-T resistance (an optimal ratio of 0.5%). The failure strain of the reinforced tunnel muck is linearly correlated to the BF content and has a weak correlation with the number of F-T cycles. The F-T damage to material strength can be divided into rapid (N=0–3), slow (N=3–9), and stable (N>9) stages. The established F-T damage model can accurately predict the evolution of the F-T damage degree. [ABSTRACT FROM AUTHOR]

Large-scale, high-strength aluminum alloy forgings are essential components in the aerospace industry, with benefits including increasing strength and decreasing weight. Accurate shape-property control is the secret to forging quality. This study uses the alloy 7050 to experimentally evaluate the parametric influence of cold compression on residual stress and mechanical characteristics. The evolutions of mechanical properties, microstructure and residual stress are theoretically studied using various cold compression strains from 1% to 5% on an equivalent part, of which the results are further applied on a complicated rib-structured die forging. It is demonstrated that increasing the compression strain reduces the tensile strength of the material, but has little impact on conductivity and fracture toughness. According to the TEM results, compression also encourages the precipitation and growth of precipitated phases, particularly in positions with high dislocation densities after aging. Cold compression significantly reduces residual stress; nevertheless, as compression strain increases, residual stress first decreases and then increases. With the use of rib-structured forging, it is observed that the compression strain for 7050 aluminum alloy ranges from 2% to 4%, and the combined pressing method of the rib and web improves the uniformity of residual stress. [ABSTRACT FROM AUTHOR]

The wide range of gypsum facies observed all over the world and the strong heterogeneity that may be present even within a single facies often cause an inhomogeneous mechanical response that, if neglected, may be particularly dangerous in the framework of underground excavations. In addition, gypsum is particularly sensible to the presence of water. The high relative humidity conditions often registered in underground gypsum quarries may imply an additional worsening of mechanical properties. In the present study, the strength and the creep response of a natural gypsum rock facies are investigated, considering the influence of material heterogeneity and relative humidity conditions. The heterogeneity of the material, quantified with MIP and SEM analyses, is observed to strongly affect the mechanical response. To this intrinsic mechanical variability, the influence of an external parameter as the relative humidity is observed to generate an additional reduction of material strength and to increase the creep strain rate in the long-term tests. The effect of all these elements in the underground quarry framework is discussed and a constitutive model of these experimental results is provided. [ABSTRACT FROM AUTHOR]

The effect of temperature on the damage mechanisms occurring in a toughened acrylic resin was investigated by studying volume changes during tensile tests, analyzing the reversibility of the damage after applying proper thermal histories and by direct transmission electron microscope observations. It was found that an increase of temperature, from 0°C to 60°C, shifts the predominant mechanism from crazing to cavitation and shear yielding. When the latter occur, the material response is very susceptible to previous thermal treatments; and in particular to the cooling rate from above glass transition temperature. Crazing on the other hand is not influenced by the thermal history of the material. POLYM. ENG. SCI., 59:566–572, 2019. © 2018 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

Abstract Selective laser melting, due to the high energy density input and the small interaction time (106 K/s), can result in an ultrafine microstructure and excellent mechanical properties. However, due to the nonuniform nature of the temperature distribution and the transition from liquid to solid, high residual stresses exist. High residual stresses in the parts can increase the risk of material distortion and cause many problems, such as dimensional inaccuracy or cracks. This work systematically investigated the influence of annealing on the microstructure and mechanical properties of SLM-processed AlSi7Mg alloy parts. The residual stresses relaxed significantly after annealing. The Vickers hardness and the tensile stress greatly reduced, while the elongation increased. The fracture mode of the as-fabricated sample was ductile and brittle mixed fracture, whereas the fracture morphology of the annealed sample presented many more dimples, and the elongation also increased, which indicated the ductile mode. [ABSTRACT FROM AUTHOR]

Abstract The present article presents the effect of Sc addition on the secondary dendritic arm spacing, eutectic Si modification, intermetallic phase modification, aging behavior and subsequently the mechanical behavior in A356-5TiB 2 in-situ composite. The A356-5TiB 2 in-situ composite showed a 40% reduction in SDAS compared with A356 alloy, whereas addition of 0.4 wt% Sc and 5 wt% TiB 2 in A356 alloy resulted in a 65% reduction in SDAS. The eutectic Si in A356-5TiB 2 composite had a plate like morphology similar to A356 alloy but showed a reduction in length, whereas the addition of Sc resulted in a fibrous and particulate morphology of eutectic Si. The needle like β-Al 5 FeSi phase in A356-5TiB 2 composite changed to Al 5 Fe(Si,Sc) phase having smaller size and irregular morphology with the addition of Sc. The intermetallic phase formation and phase modification was confirmed and characterized using X-ray diffraction, Transmission electron microscopy - Energy dispersive spectroscopy and Scanning electron microscopy. Addition of Sc to A356-5TiB 2 in-situ composite resulted in improved mechanical properties. Further, T6 heat treatment revealed a significant improvement in mechanical properties for A356-5TiB 2 in-situ composite with and without Sc addition. [ABSTRACT FROM AUTHOR]

Abstract Stir tools with different pin eccentricities were applied to the friction stir processing of Al-5052 alloy. Results show that pin eccentricity enhances material flow and refines grains of stir zone. The stir zone produced by a stir tool with 0.4 mm pin eccentricity performs the highest hardness and yield strength, attributing to the enhanced grain-boundary strengthening. [ABSTRACT FROM AUTHOR]

Abstract Laser powder bed fusion additive manufacturing (L-PBF AM) offers great potential for local microstructure control. During this process, solidification occurs in conditions that are far from equilibrium and possesses – in the majority of cases – a strong directionality. In general, the size and morphology of the resulting microstructure is a function of two well-known parameters: the temperature gradient within the liquid phase (G) and the velocity of the solidification front (R). To provide guidance in selecting appropriate, systematically defined, process parameters for L-PBF of 316L stainless steel square pillars, we developed an intentionally simple thermal model to express these two parameters, G and R, as a function of selected process variables (laser scan speed, laser power) and material properties (thermal diffusivity). Results from both microstructural and mechanical characterization of the pillars indicate that high-strength, fully-dense parts with a highly oriented cellular microstructure can be obtained when using significantly different sets of process parameters. Furthermore, despite its simplicity, the numerical model correlates well with experimental evidence and confirms that rather than creating variable microstructures, the process parameter constraints actually lead to a stable cellular microstructure regardless of the wide process window studied. [ABSTRACT FROM AUTHOR]

Abstract Thermomechanical processing determines the practical applications of metal–matrix composites (MMCs) by contributing to the interrelationship between their microstructure and properties. In this study, we investigate the changes in Al-3.0Cu-1.2Mg/SiC p (40 vol%) caused by the tension applied at high temperatures and strain rates. The study shows the microstructural changes and mechanical characteristics resulting from the cold/warm and hot rolling of the extruded material. It is observed that hot rolling has a favorable effect on the restoration of the defects formed in parts of the extruded bar. The microstructural characteristics of the composite related to the deformation-induced redistribution of the secondary inclusions in the matrix and fracture of the reinforcement particles are shown. High-temperature uniaxial tensile testing of the studied composite conducted under high strain rates (10−1 and 10−2 s−1) indicates an increase in the ductility. The decrease in the stress of the tested samples is attributed to the refined grains in the matrix after the thermomechanical processing. The testing of the given material provides the stress–strain relationships, which indirectly assist in ranking the relative formabilities of the differently processed sheets of the MMCs with a high-volume fraction of the reinforcement particles. The proposed approach may be considered for the near net-shaped manufacturing of MMCs with high wear resistance and low thermal expansion coefficients. [ABSTRACT FROM AUTHOR]

Abstract Revealing the microstructural parameters-mechanical properties relationship is very critical for the microstructure tailoring and properties optimization of titanium alloy. In this work, the dependence of mechanical properties on microstructural parameters of TA15 titanium alloy with tri-modal microstructure was investigated. The results indicate that at a certain lamellar α (α l) content (28%), both of yield strength (σ 0.2) and ultimate tensile strength (σ b) decrease continuously with the increase of equiaxed α (α p) content, while the decrease speed slows down at higher α p content. At a certain content of α p of 20%, σ 0.2 and σ b both increase first and then decrease as the content of α l increases. The above effect laws result from the competition between the softening effect caused by increasing the softer α p and α l contents and the strengthening effect relating to α l /β t interfaces and β t hardening. As for the elongation (δ) and reduction of area (Φ), they show an increase trend with α p content increasing at a certain α l content of 28%. However, they decrease first and then increase with α l content increasing at a certain α p content of 20%. The early plasticity decrease at lower α l content is due to the popular rapid void nucleation and cleavage fracture along α l /β t interfaces. When α p or α l content increases to higher level, the deformation compatibility and homogeneity improves, which suppress the rapid void nucleation and cleavage fracture along α l /β t interfaces. Meanwhile, the fracture mechanism changes from the coexistence of void coalescence and cleavage fracture to the void coalescence dominated fracture, thus the plasticity increases. Moreover, a back-propagation neural network model was developed to correlate the mechanical properties with microstructural parameters of tri-modal microstructure. The prediction results suggest that better combination of strength and plasticity can be achieved by controlling the contents of α p and α l in the range of 10–15% and 22–27%, respectively. [ABSTRACT FROM AUTHOR]

Abstract The effects of hydrogen charging on the evolution of strain-induced lattice defects and phase transformation were investigated in 316L stainless steel. The initial material was obtained by cold rolling to a thickness reduction of 20%. The surfaces of some of the initial samples were cathodically charged with hydrogen. Afterward, both the charged and uncharged samples were subjected to tension until failure. The dislocation density and twin-fault probability in the charged and uncharged specimens during tension were compared. We found that hydrogen charging reduced the degree of increase in the dislocation density and twin-fault probability during the application of tension to the same strain level. Significant martensitic phase transformation was observed in the uncharged samples tested to the strain of 10% or higher. In the hydrogen-charged samples, only a slight increase in the martensite phase fraction was detected. A correlation between the α′-martensite fraction and the dislocation density was found for the studied samples, suggesting that the lower degree of martensitic phase transformation in the charged 316L steel was caused by the smaller amount of stress developed due to the lower dislocation density. In accordance with the differences observed in the phase composition and defect densities, the hydrogen-charged material exhibited lower surface hardness. [ABSTRACT FROM AUTHOR]

We report a method, named as T & A treatment, to improve the high-cycle fatigue (HCF) property of twinning-induced plasticity (TWIP) steel without sacrificing its ductility and work-hardening ability. A long-range gradient in grain size was obtained in the TWIP steel, whose HCF resistance is better than both coarse-grain and fine-grain steels, breaking the rule of mixture. The surprising HCF property of the graded grain structure is believed to originate from the large generation of geometrically necessary dislocations and the formation of hard core and soft shell construction during cyclic loading. A new strategy, refers to long-range gradient in grain size, is proposed to improve the HCF property of TWIP steel. The graded structure exhibits better fatigue resistance than single unit. [ABSTRACT FROM AUTHOR]

Abstract Dwell fatigue facet nucleation has been investigated in isothermal rig disc spin tests and under anisothermal in-service engine conditions in titanium alloy IMI834 using α-HCP homogenised and faithful α-β lamellar microstructure crystal plasticity representations. The empirically observed facet nucleation and disc failure at low stress in the isothermal spin tests has been explained and originates from the material rate sensitivity giving rise to soft grain creep accumulation and hard grain basal stresses which increase with fatigue cycling until facet nucleation. The α-HCP homogenised model is not able to capture this observed behaviour at sensible applied stresses. In contrast to the isothermal spin tests, anisothermal in-service disc loading conditions generate soft grain slip accumulation predominantly in the first loading cycle after which no further load shedding nor soft grain creep accumulation is observed, such that the behaviour is stable, with no further increase in hard grain basal stress so that facet nucleation does not occur, as observed empirically. The thermal alleviation , which derives from in-service loading conditions and gives the insensitivity to dwell fatigue dependent on the temperature excursions, has been explained. A stress-temperature map for IMI834 alloy has been established to demarcate the ranges for which the propensity for dwell fatigue facet nucleation is high, threatening or low. Highlights • It is essential to include microstructural detail to capture local slip activities. • Load shedding has been observed in isothermal rig disc spin test models. • Creep accumulation does not occur under in-service engine conditions. • Temperature plays a key role in cold dwell fatigue, in addition to stress. • A dwell sensitivity map has been constructed for alloy IMI834. [ABSTRACT FROM AUTHOR]

This paper introduces a new model for prediction of tensile properties of Al A357 castings in the form of UTS (MPa) = a + b ln (G/√Ṫ) and El (%) = c + d ln (G/√Ṫ). While previous models use only SDAS for prediction of tensile properties of the castings, the present model uses G (temperature gradient at solidification front) to evaluate the difficulty of liquid metal feeding and formation of microshrinkages and Ṫ (cooling rate of solidifying alloy) to provide information about SDAS. The proposed model has been developed based on the data from a series of defect free plates and later was applied for prediction of tensile properties of new castings from A357 alloy. It was found that the model successfully predicts the tensile properties of cast Al A357 alloys. [ABSTRACT FROM AUTHOR]

Highlights • Activation volume in the presence of semi-coherent β'' precipitates is significantly lower compared to other microstructural features. • Effective stress is almost two times higher in the presence of semi-coherent β'' precipitates compared to other microstructural features. • Exhaustion rate of mobile dislocation density is negligible in the over-aged sample compared to solutionized and peak-aged sample. • Relaxation strain is higher in the presence of semi-coherent β'' precipitates compared to other microstructural features. Abstract In the present investigation, stress relaxation tests were carried out on an aluminium magnesium silicon alloy to determine rate controlling mechanism in different temper conditions. Single and multiple relaxation tests were carried out to determine apparent and physical activation volume. It was found that in the presence of solutes and incoherent precipitates, dislocation-dislocation interaction is the rate controlling mechanism, whereas dislocation-precipitate interaction is the rate controlling mechanism in the presence of semi-coherent precipitates. Moreover, the nature of deformation was found to be more viscous (higher strain rate sensitivity) in the presence of semi-coherent precipitates compared to other microstructural features (solutes, incoherent precipitates). Special emphasis is placed upon understanding exhaustion rate of mobile dislocation density during relaxation period. It was found that the exhaustion rate of mobile dislocation density is negligible in the presence of incoherent precipitates, whereas a significant reduction in mobile dislocation density was observed in the presence of solutes and semi-coherent precipitates. Furthermore, thermal and athermal components of the flow stress were determined to develop a comprehensive understanding regarding deformation behavior in different temper conditions. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

The paper analyzes the effect of ageing on the variations in the mechanical and technological properties of steel wire. The process of drawing 5.5 mm-diameter wire rod into 1.70 mm wire was carried out in 12 draws on a Koch KGT multi-stage drawing machine in the drawing velocity range of 5-25 m/s. Finished 1.7 mm-diameter wires after, respectively, 1, 24, 720 and 8760 hours of the completion of the drawing process were subjected to testing to determine their mechanical and technological properties. The yield strength, YS; tensile strength, UTS; uniform elongation, Ar ; total elongation, Ac; reduction of area, Z; number of twists, Nt; and the number of bends, Nb, have been determined. It has been demonstrated that variations in mechanical properties occur after the multi-stage drawing process due to ageing, with their degree and mode being dependent on the drawing speed. [ABSTRACT FROM AUTHOR]

Commercially pure titanium (CP-Ti) has been recently used as metallic biomaterials due to excellent biocompatibility and specific strength. CP-Ti has less static and dynamic strength as compared to other metallic biomaterials. Processing by the equal channel angular pressing (ECAP) as one of the most effective severe plastic deformation (SPD) method could lead to an increase in the mechanical strength of materials, significantly. In this study, Grade 2 CP-Ti billet is inserted into Al-7075 casing, and is then deformed by ECAP, with the channel angle of 135°, through 3 passes at route B C and room temperature. The purpose of using casing is to attain higher deformation homogeneity and more material ductility in the billet. The microstructural analysis shows that the coarse grain (CG) CP-Ti is developed to ultra-fine grain (UFG) structures after ECAP. In order to investigate the static and dynamic strength of CG and UFG CP-Ti, the tensile and axial fatigue tests are conducted. The results represent that UFG CP-Ti has much more tensile and fatigue strength than CG CP-Ti, and it could be utilized as biomaterials for production of implants. Surface features of fatigue fracture are also investigated. It should be noted that the investigation of fatigue strength of UFG CP-Ti produced by ECAP at RT utilizing casing, has not been conducted so far. [ABSTRACT FROM AUTHOR]

The objective of this research is to study the evolution of piezomagnetic fields of various ferromagnetic steels during the fatigue process. A series of strain-controlled fatigue tests were carried out on Q235, U71Mn and GCr15 Chinese steels. The analysed results show a consistent evolution of the piezomagnetic hysteresis traces with these steels throughout the fatigue degradation process. However, the shapes and positions of the magnetic traces were found to vary conspicuously in different steels. The differences in the magnetic field responses corresponding to the different steels are discussed in terms of the chemical composition and microstructure of the steel. [ABSTRACT FROM AUTHOR]

In a unidirectional composite under static tensile loading, breaking of a fiber is shown to be a locally dynamic process, leading to stress concentrations in the matrix and neighboring fibers and debonding of the interface, which can propagate at high speed over long distances. In our previous work, a fiber break within a two-dimensional fiber array embedded in elastic epoxy matrix (with cohesive interface) was modeled to quantify the effects of these dynamic stresses. The results indicated that the elastic limit of the polymer matrix can be exceeded. In this study, the effects of matrix plasticity on dynamic stress concentrations due to a single fiber break are investigated. For the range of matrix yield stresses considered, the dynamic stress concentrations are significantly higher than corresponding values predicted by a quasi-static model with a pre-broken fiber. Based on the ratio of shear yield strength of the matrix and mode II peak traction of the interface cohesive law, two distinct regimes of damage are shown to exist. Only matrix yielding occurs when this ratio is less than 1.0, while both interfacial debonding and matrix yielding occur when it is greater than 1.0. At higher fiber break strengths, where the elastic matrix model predicts unstable interfacial debonding, reduction in matrix yield strength leads to a transition to stable debonding and arrest. Reducing the matrix yield strength also leads to a lowering of the peak dynamic stress concentrations in adjacent fibers, while spreading the stress concentrations over a larger volume of the composite microstructure. [ABSTRACT FROM AUTHOR]

The prospect of introducing natural biological secrets to improve the crack resistance of gray cast iron is presented. We demonstrated that the mechanism of resisting crack owned by plant leaf can be referred to fabricate an alternating structure of hard and soft with low cost, high strength, and an exceptional ability to resist crack growth. Laser alloying was introduced to fabricate the hard phase which associated with the original matrix constituted the alternating structure. A homogeneous microstructure and a uniform distribution of chemical composites were achieved in hard phase. Nanoindentation test showed the resistance to plastic deformation of hard phase was improved. Crack propagation tests verified the crack resistance was enhanced by the alternating structure which could change and prolong crack path, and the effectiveness of resisting crack propagation was proportional to the size of hard phase. [ABSTRACT FROM AUTHOR]

A novel method is proposed for monitoring the plastic flow localization during in-situ scanning electron microscopy (SEM) testing of small-scale AISI 316 L stainless steel. Stress-strain behavior of the material was obtained using a hybrid numerical-experimental (HNE) approach. By repeatedly illustrating each pair of sequentially taken SEM surface images throughout the deformation history in alternating order in form of a video, location of the material points which are not moving during the deformation can be detected. At the initial stages of deformation these points are located on the geometrical symmetry line of the test sample, however; when uniform straining limit of the material is reached, the locations of the stationary material points reveal the plastic localization regions. The current results clearly prove the feasibility of the presented method in monitoring primary plastic localization events through in-situ SEM tensile testing. [ABSTRACT FROM AUTHOR]

Microstructure evolution during long-term thermal exposure and stress rupture in Waspaloy was investigated in detail. During thermal and stress exposure, γ′ phase and grain boundary M 23 C 6 carbides kept microstructure stability. The primary MC carbides only experienced long time thermal exposure didn't decompose significantly while those experienced long time thermal and stress exposure degenerate severely. Decomposition of primary MC carbides is detrimental to the stress rupture properties of the alloy and it can be expressed as: MC + γ → M 23 C 6 + γ′. After long term aging (650°C/5000h), the stress rupture specimen (815°C/325MPa) showed a mixed mode of deformation mechanism : particle shearing and precipitate bypassing. [ABSTRACT FROM AUTHOR]

This paper describes experimental and numerical investigations conducted to characterize the basic material properties and design of YSt–310 cold–formed structural steel sections at Indian Institute of Technology, Guwahati. Square (SHS) and rectangular (RHS) hollow sections with minimum yield strength of 310 MPa manufactured by Tata Steel India, were considered in the study. Initially, results from elemental analysis via optical emission spectrometer (OES) investigation and metallographic examination using optical microscope are presented. Key stress–strain parameters viz., Young's modulus, proof stress, ultimate strength, percentage elongation, strain hardening exponent etc. were generated based on flat, corner and weld coupon tests data. Extent of corner strength enhancement due to cold – forming determined using Vickers's microhardness test are reported. Additionally, cross – section capacity of the stub columns were also investigated experimentally and numerically. The column capacities generated from test and finite element study are compared with the existing design code – EN 1993–1–1 (EN 1993-1-1, 2005) [1] and design rules – continuous strength method (CSM) (Zhao et al., 2017) [2] and direct strength method (DSM) (North American Specifification for the Design of Cold-Formed Steel Structural Members, 2016; Arrayago et al., 2017) [3,4]. Based on the comparison, modifications on the existing design code and rules are suggested to provide a more accurate and reliable compressive design prediction. [ABSTRACT FROM AUTHOR]

The tensile flow behaviour of mechanically milled iron and a low-alloy steel are analysed using power law models that describe the evolution of stress-strain during a tensile test. A new analytical power law equation is presented that is shown to be superior to the Hollomon and Ludwigson equation to describe the room temperature flow properties of the low-alloy steel over a wide strain rate range. It not only provides a better fit to the homogenous flow curve of the low-alloy steel but also predicts the maximum strength without the prior knowledge of the uniform strain. The new equation also fitted the flow curves of mechanically milled iron with grain sizes in nano to micrometer range. [ABSTRACT FROM AUTHOR]

A multiscale modeling framework for polycrystal materials is established, using a combination of an extended vertex model and a crystal plasticity formulation. The 2D vertex model is cast to incorporate a range of mesoscale processes such as grain structure evolution and the influence of second-phase particles. It is combined with a finite strain crystal plasticity formulation whereby also texture development and stored energy accumulation is traced. Computational efficiency is enhanced by GPU-parallelization. The full model captures a wide range of microstructure processes such as dynamic recrystallization, grain growth, texture evolution, anisotropic grain boundary properties as well as particle pinning effects. The macroscale material behavior is directly coupled to the evolving microstructure, for example in terms of a grain size dependent flow stress behavior. Illustrative numerical examples are provided to show the capabilities of the model. For example, the interplay between particle strengthening and grain size influence on macroscopic flow stress behavior is shown, as well as effects due to dynamic recrystallization. Special attention is given to the formulation of the vertex model as the combination of stored energy, particle pinning and anisotropic grain boundary properties give rise to intricate topological transformations which have not been previously addressed. [ABSTRACT FROM AUTHOR]

This study investigates the microstructure, surface mechanical properties, and fatigue life of 56SiCr7 leaf specimens produced under serial conditions. The investigation occurs at different stages of the manufacturing process of the leaf springs; mainly heat treatment and surface treatment by shot peening. Macro-hardness and micro-hardness measurements, at the Rockwell C and Vickers scales, respectively, are taken following the thermal treatment and surface shot peening treatment steps of the manufacturing process. Residual stress measurements accurately show the induction depth of the residual stresses in the material. The above measurements are used to determine how heat treatment and shot peening affect the mechanical properties of the surface and core of the spring material. Though the performed micro-hardness measurements clearly indicate surface decarburization effects and quantify the decrease in strength, they do not reveal the insufficient martensitic transformation during the thermal treatment of selected specimens (protocol 1). The macro-hardness measurements are completely insensitive in revealing the surface decarburization and phase transformation phenomena that take place in the investigated case. Solely the microstructural investigations with the aid of a metallographic optical microscope revealed the degree of phase transformation achieved by the thermal treatment. The effect of the individual manufacturing processes, of thermal treatment and shot peening, on the fatigue life of the leaf springs is demonstrated by experimental Wöhler curves at stress amplitudes between 250 MPa and 600 MPa. Two stress ratios are examined, R = 0 and R = 0.4, which reflect the stress ratios recommended by the automotive industry. The corresponding Haigh diagrams and mean stress sensitivity factors can be applied for fatigue life assessments of new leaf spring products produced using the suggested protocols. [ABSTRACT FROM AUTHOR]

Progressive deterioration in the mechanical properties with increasing service period is commonly observed and hence needs to be addressed. Awareness about the rate of degradation of the properties can circumvent a lot of problems faced during service. Hence the present work aims to understand the degradation of tensile properties due to prior cyclic loading. Systematic comparative evaluation study for the evolution of tensile properties with prior fatigue loading and the deformation and damage evolution process in modified 9Cr-1Mo steel during low cycle fatigue (LCF) and creep-fatigue interaction (CFI) loadings has been carried out. For this, LCF and CFI experiments were carried out at constant strain amplitude of ±0.6%, strain rate of 3 × 10 −3 s −1 and temperature of 600 °C. Experiments interrupted after 5, 10, 30, and 50% of the total fatigue life (N f ) under both LCF and CFI conditions were followed by monotonic loading at the same strain rate and temperature until fracture. Drastic reduction in the yield strength value due to both LCF and CFI prior fatigue loading was revealed and the extent of decrease in the YS values increased with the extent of prior fatigue damage imparted on the steel. Substantial difference in the YS evolution with prior fatigue damage under LCF and CFI conditions occurred only after 10% life fraction due to the predominance of creep recovery process. Factors such as microstructural changes due to creep alone or fatigue alone or both and surface oxidation changed the tensile properties under LCF and CFI. [ABSTRACT FROM AUTHOR]

In this article, the effect of twist extrusion (TE) on the fatigue behavior of the AA1050 aluminum alloy is investigated. For this purpose, samples of AA1050 are TE processed for 1, 2 and 4 passes. Tensile testing and rotational bending fatigue testing were performed on samples extracted from the surface of the TE specimens. As well, the correlation between microstructure, tensile properties and fatigue response are discussed. The un-deformed AA1050 sample showed a yield strength (YS) of 26.6 MPa and ultimate tensile strength (UTS) of 46.1 MPa and resisted the 38 MPa bending stress for 373,000 cycles. While the YS and UTS increased after TE, the fatigue life of TE processed specimens reduced compared with the un-deformed sample. Indeed, the fatigue life reduced to 335,168, 299,798 and 259,260 cycles after 1, 2 and 4 passes of TE, respectively. Reducing fatigue life was attributed to smaller strength ratio, i.e., UTS/YS, achieved after TE processing. Analyzing the fracture surface of the TE samples showed that the cracks were initiated from preferential locations and moved towards center leading to ultimate ductile fracture at the center. The area of ductile fracture was found to be reduced in TE processed specimens while the widths of striations were increased. [ABSTRACT FROM AUTHOR]

Heat-to-heat variation of creep strength is significant in grade 91 (Gr.91) steel, but the causes of the variation have not been well understood yet. In the present paper, creep rupture data of 14 heats of Gr.91 steel were analyzed paying attention to their chemical compositions and microstructures. The longest creep rupture lives analyzed are 2 × 10 5 h at 500 and 550 °C and 10 5 h at 600 °C. The causes of the heat-to-heat variation are different, depending on creep test conditions. At low temperature and high stress (creep rupture life of 10 4 h at 500 and 550 °C), creep rupture strength increases with increase of hardness after tempering. This suggests strengthening by a fine subgrain microstructure developed during normalizing and subsequent tempering. At higher temperature and intermediate time range (10 4 h at 600 °C), creep rupture strength depends on Cr concentration of the heats in addition to the hardness. This finding suggests an important contribution of recovery process of the subgrain microstructures to creep strength of the steel. In long-term creep (2×10 5 h at 550 °C and 10 5 h at 600 °C) creep rupture strength primarily increases with increasing grain size of the heats. This suggests that grain boundary sliding is an important deformation mode at low strain rate because of fine grain size usual with Gr.91 steel. Specifications on Ni concentration and N% / Al% ratio are newly introduced in the type II version of Gr.91 steel. They are not effective to eliminate a heat with low creep strength. [ABSTRACT FROM AUTHOR]

During the Selective Laser Melting (SLM) process large temperature gradients can form, generating a mismatch in elastic deformation that can lead to high levels of residual stress within the additively manufactured metallic structure. Rapid melt pool solidification causes SLM processed Ti6Al4V to form a martensitic microstructure with a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. The use of high temperature pre-heating during an SLM build can assist in reducing thermal gradients, enable a more controlled cooling with the possibility to control/tailor as-built mechanical properties. In this work a high temperature SLM powder bed capable of pre-heating to 800 °C is used during processing of Ti6Al4V feedstock. The effect of powder bed temperature on residual stress formation, microstructure and mechanical properties was investigated. It was found that increasing the bed temperature to 570 °C significantly reduced residual stress formation within components and enhanced yield strength and ductility. This pre-heating temperature enabled the decomposition of α ′ martensitic microstructure into an equilibrium α+β microstructure. At 570 °C the yield strength and elongation of components was improved by 3.2% and 66.2% respectively. [ABSTRACT FROM AUTHOR]

This paper investigates the mechanical properties of hyper deformed wires of Ti-26 at.%Nb (Ti-26Nb) alloy that can be equally expressed as Ti-40.5 wt.%Nb. These wires obtained by hot drawing process have a diameter of 85 μm and 285 μm, respectively. The effect of a short heat treatment at 748 K for 600 s was also investigated. The influence of the heat treatment and the diameter of the wires evaluated by mechanical tests, such as nano indentation, quasi-static tensile, and fatigue tests, and microstructure investigations. The quasi-static tensile tests show that the mechanical properties, like elasticity modulus and ultimate tensile strength, are improved with the decrease of the diameter. An increase of 55% and 26% in strength is obtained for the non-treated state and the heat treated state, respectively. The improvements in the mechanical properties for the 285 μm diameter wire are due to the refinement of grains to submicron scale between 100 and 200 nm. The short heat treatment induces an increase of the Young's modulus and a decrease of the ultimate tensile strength by the emergence of a dual phase ( α and β ) microstructure. For fatigue tests, the increase of frequency from 4Hz to 30Hz leads to an increase of 45% of the fatigue life time. [ABSTRACT FROM AUTHOR]

The tensile and high-cycle fatigue tests of Fe-30Mn-0.9C twinning-induced plasticity (TWIP) steel after 30%, 60% and 70% pre-straining were performed. Meanwhile, the surface damage morphologies of post-fatigue specimens and microstructure evolutions of pre-strained and post-fatigue pre-strained specimens were also investigated. It is found that the fatigue properties of the TWIP steel can be effectively improved through pre-straining, because the pre-straining can change the fatigue strength coefficient and exponent, respectively. The improvement of fatigue strength coefficient may be attributed to the strengthening mechanisms induced by both twin boundaries and dislocations; while the variation of fatigue strength exponent should be resulted from the combined effects of deformation homogeneity and slip reversibility, as well as the internal damages. Furthermore, the detailed mechanisms associated with the variations of fatigue strength coefficient and exponent were discussed. This study may enrich the fundamental knowledge about how to improve the high-cycle fatigue properties of TWIP steels. [ABSTRACT FROM AUTHOR]

The paper present the evaluate the bending load capacity of a composite beam made of titanium Grade 2 and titanium alloy Grade 5. The beam was made using the Resistance Spot Welding technology. All elements of the construction were cut from sheet thickness of 0.8 mm. Web and flanges of beams made from titanium alloy Grade 5 sheet. Flanges were joined with the web using the cold-formed brackets made of titanium Grade 2. The welding parameters of RSW were selected on the basic of strength tests and analysis of the microstructure. Grade 2 Grade 5 and Grade 5-Grade 5 sheets titanium were combined. Optimized parameters were also used to analyze joints with two welds: arranged parallel and perpendicular to the shear direction. The load capacity of joints, plastic strain distribution and the method of destruction was assessed. Results from the experimental research of joints with single welds are compared with the results of the numerical analysis. The numerical analysis was carried out using the ADINA system based on the Finite Element Method. Static tensile test was carried for titanium Grade 2 and Grade 5 with a thickness of 0.8 mm to numerical analysis. The composite beam made at paper was subjected to three-point bending. [ABSTRACT FROM AUTHOR]

At low homologous temperatures (down to cryogenic temperatures), the CrMnFeCoNi high-entropy alloy possesses good combination of strength, work hardening rate (WHR), ductility, and fracture toughness. To improve understanding of the deformation mechanisms responsible for its mechanical properties, tensile tests were performed at liquid nitrogen and room temperature (77 K and 293 K) and interrupted at different strains to quantify the evolution of microstructure by transmission electron microscopy. Dislocation densities, and twin widths, their spacings, and volume fractions were determined. Nanotwins were first observed after true strains of ∼7.4% at 77 K and ∼25% at 293 K; at lower strains, deformation occurs by dislocation plasticity. The tensile stress at which twinning occurs is 720 ± 30 MPa, roughly independent of temperature, from which we deduce a critical resolved shear stress for twinning of 235 ± 10 MPa. In the regime where deformation occurs by dislocation plasticity, the shear modulus normalized WHR decreases with increasing strain at both 77 K and 293 K. Beyond ∼7.4% true strain, the WHR at 77 K remains constant at a high value of G /30 because twinning is activated, which progressively introduces new interfaces in the microstructure. In contrast, the WHR at room temperature continues to decrease with increasing strain because twinning is not activated until much later (close to fracture). Thus, the enhanced strength-ductility combination at 77 K compared to 293 K is primarily due to twinning starting earlier in the deformation process and providing additional work hardening. Consistent with this, when tensile specimens were pre-strained at 77 K to introduce nanotwins, and subsequently tested at 293 K, flow stress and ductility both increased compared to specimens that were not pre-strained. [ABSTRACT FROM AUTHOR]

In this paper we develop a finite deformation micromechanical framework for modeling rate-dependent failure in unidirectional composites under off-axis loading. The model performance is compared with original experiments on thermoplastic carbon/PEEK composites tested at different strain-rates and off-axis angles. To achieve quantitative agreement with the experiments, a microcrack initiation criterion based on the local stress and the local rate of deformation state in the polymer matrix is proposed. Microcracking is represented by a cohesive zone model, with special attention to the inclusion of geometric nonlinearity in the formulation. In this regard, the cohesive geometric nonlinearity is based on extension of an existing formulation to three-dimensional space. Beside microcracking, the Representative Volume Element (RVE) also accounts for viscoplasticity in the polymer matrix. A recently introduced dedicated arclength control method is utilized to impose a strain-rate on the micromodel. Accordingly, kinematic relations governing the RVE deformation allow for the change in orientation of the micromodel in the loading process. This change in orientation of the microstructure has an important implication on the apparent material strength. [Display omitted] • An RVE model to study failure in UD composites undergoing finite strains is developed. • The model is compared with original experiments on UD carbon/PEEK composite system. • Microcrack initiation based on the polymer deformation rate is proposed. • An arclength model imposes strain-rate on the RVE under an arbitrary off-axis angle. [ABSTRACT FROM AUTHOR]

The hot flat rolling process of twin-roll cast (TRC) AZ31B magnesium alloy was investigated and carried out under conditions of four initial temperatures, four-pass successive reductions and four rolling routes. Qualitative and quantitative analysis on the microstructure, micro-Vickers hardness, 0.2% proof stress and tensile strength was performed. Cooperative effect of grain refinement, basal texture, dispersion of microstructural distribution and anisotropy of plate on 0.2% proof stress was considered to determine the optimum fitting scheme for Hall–Petch equation (H-P for short). The relationships between 0.2% proof stress, tensile strength and micro-Vickers hardness were defined through regression analysis and mathematical modeling. Then the micro-Vickers hardness parameter in equations was eliminated using an elimination method to establish the mathematic relationship between 0.2% proof stress and tensile strength (T-Y for short). And the relationship between grain size and tensile strength was established using the mathematical elimination again to eliminate the yield strength parameter in H-P equation and T-Y equation. The analysis shows that the relationship between tensile strength and d m − 1/2 is approximately characterized as a quadratic function where coefficients are influenced greatly by texture and the dispersion of microstructural distribution. [ABSTRACT FROM AUTHOR]

In this investigation, wear and tensile properties of 6061-Al/Al2O3 and 6061-Al/SiC metal matrix composites were studied with respect to their microstructural features and residual stresses. Variation in the residual stress values in the composites differed from each other due to their particulate size and thermal expansion coefficient. Tribological behavior of the composites studied with the aid of nanoscratch tester showed higher amount of debris in 6061-Al/SiC composite than that in 6061-Al/Al2O3 composite due to the presence of higher residual stresses and shorter interparticle distance in the former. Variation of wear volume, coefficient of friction and wear rate at different load were analyzed and correlated with the variation of microstructure along the scratch tracks. It is seen that wear resistance of 6061-Al/Al2O3 is better than that of 6061-Al/SiC composite, where as tensile property revealed that ultimate tensile stress and yield stress is higher in case of 6061-Al/SiC composite compared to 6061-Al/Al2O3. Fractographic studies shows bimodal fracture failure where cleavage type brittle fracture failure is dominant in 6061-Al/Al2O3 whilst ductile failure is dominant in 6061-Al/SiC composite. [ABSTRACT FROM AUTHOR]

Monzonitic granite is a low-permeability rock. Monzonitic granite formations are ideal for underground storage of oil due to their low permeability and high mechanical strength. In this study, a series of coupled hydromechanical triaxial tests are carried out using monzonitic granite specimens. The influence of confining and fluid pressures on stress, strain, and permeability is investigated. Failure characteristics under different confining and fluid pressures are discussed based on the analysis of macro fracture planes and micro scanning electron microscopy (SEM). The test results show that the change of permeability with stress and strain reflects the deformation stages of compaction, compression, crack propagation, coalesce, and failure of cracks. Due to the low porosity, the change of permeability is small in the initial phases of compaction and compression, whereas there is a significant increase in permeability when new cracks start to develop and coalesce. Confining pressures have a significant impact on the strength and permeability, particularly the crack damage stress of the rock. Compared with confining pressure, the effect of fluid pressure on rock strength and crack damage stress is small. For the monzonitic granite specimens tested, changing the confining pressure results in different failure modes, whereas the fluid pressure has a relatively small effect on the failure modes. [ABSTRACT FROM AUTHOR]

Coarse-grained (CG) metals strengthened by nanotwinned (NT) regions have an excellent combination of high strength and good ductility. In this paper, numerical simulations based on the mechanism-based strain gradient plasticity and the Johnson–Cook failure criterion are carried out to investigate the direct and indirect effects of volume fraction of the NT regions and several other key attributes on the strength and ductility of the CG metal strengthened by NT regions. Our results indicate that twin spacing, shape, distribution, and volume fraction of NT regions all have significant effects, and that, as the volume fraction increases, the roles played by the twin spacing and shape of NT regions also change. At low volume fraction of NT regions, their shape is found to have a significant influence on the overall ductility. As the volume fraction increases, the shape effect on the ductility is found to diminish even though the effect on the strength is noted to continue to enhance. While twin spacing and volume fraction of NT regions have shown different effects on the ductility, the effect of twin spacing always decreases as the volume fraction increases. During the simulations we have also discovered several intrinsic deformation and failure mechanisms through the analysis of microcrack initiation sites and microcrack propagation paths. It is believed that this simulation has revealed significant insights into the roles of volume fraction, twin spacing, shape, and distribution of NT regions on the strength and ductility of this novel class of materials. [ABSTRACT FROM AUTHOR]

Mechanical properties of short-fiber reinforced composites are crucially influenced by their microstructure. The microstructure itself is mainly governed by the manufacturing process like injection or compression molding. The main contribution of this paper lies in the homogenization of linear elastic properties using experimental microstructural information. For this purpose, the microstructure of injection-molded specimens made of polypropylene reinforced with 30wt.% of short glass fibers are analyzed through micro-computer tomography (μCT) measurements. Applying a recently developed segmentation algorithm, the spatial position, the orientation distribution and the length of the fibers are determined. This data is evaluated in terms of orientation tensors and length distribution, and is used within three mean field approaches: a self-consistent homogenization method, the interaction direct derivative estimate, which is based on the three-phase model, and a two-step bounding method. All methods account for the orientation, the length and the diameter distribution. The numerical results are compared to experimental tensile tests. [ABSTRACT FROM AUTHOR]

The purpose of this study was to explore consequences of accelerated weathering on the behavior of polylactide/halloysite nanotube composites. Nanocomposites were compounded by melt mixing method via a twin-screw extruder while specimens for testing and analyses were shaped by injection molding. Behaviors of the specimens were compared before weathering and after exposure to UV irradiation and humidity steps according to Cycle-C of ISO 4892-3 standards for a total duration of 300 h. IR studies revealed that photolysis and partly hydrolysis were the main degradation mechanisms leading to random main chain scission of the PLA macromolecular structure, which was reflected with drastic reductions both in the molecular weight of PLA and the mechanical properties of flexural strength and fracture toughness. On the other hand, no significant reductions were observed in the values of thermal degradation temperatures, storage modulus and flexural modulus of the specimens, which was due to the compensating effect of higher amounts of crystallinity formed during UV irradiation cycles at 70℃. [ABSTRACT FROM AUTHOR]

Composite structures are found in modern aircraft designs ranging from air transport to general aviation. Maintenance repair technology varies for each original equipment manufacturer and aircraft type. This research reports on two different composite repair methods commonly used within the composite aviation industry and how they compare when inspected with transient infrared thermography non-destructive evaluation technique. Composite sandwich test coupons made with carbon fiber laminate, nomex honeycomb, and glass fiber laminate were used for this work. Impact damages were generated in the sandwich test coupons and repairs were conducted by following repair procedures of two leading general aviation composite aircraft manufactures. During the repair process, controlled flaws were also induced to simulate bad repairs with weak bond areas, disbonds, and excessive porosity. During transient infrared thermography, several patches were identified that contained wrinkles, porosity, and disbond defects in the repaired panels. The indications were evaluated utilizing the time vs. temperature plot curves and profile data. The porosity indications displayed significant variations compared to the surrounding areas and were subsequently deemed defects as a result of the data. [ABSTRACT FROM AUTHOR]

Translation of tensile properties from high-performance fibers to end-use fabrics is sensitive to weaving-induced filament defects and environmental exposure. In this effort, isolated and combined effects of hygrothermal exposure and curvature-induced kink bands on tensile strength of Vectran™ HT (polyarylate liquid crystal polyester fiber) and Kevlar® KM2 are studied. Hygrothermal conditioning was conducted at temperatures ranging from 40℃ to 100℃ in water for 30 days. Curvature-induced defects were created by wrapping tows around stainless steel rods of different diameters (0.25 mm to 5 mm) to create kink bands. Combined effects were evaluated by conditioning tows with kink bands at 100℃ for 30 days. All conditioned samples were dried and tested at room temperature. Hygrothermal aging showed that tensile properties for Vectran fibers were not appreciably affected below 100℃ (∼12% reduction), while KM2 fibers dropped continuously with increasing temperatures (∼48% at 100℃). The influence of curvature on kink band density was established for each fiber type. The isolated effect of kink band density on residual strength was approximately 15% for both Vectran 1670/600 and KM2-600. Combined effects of curvature-induced kink bands followed by hygrothermal exposure showed significant reductions in tenacity up to ∼96% for KM2 and 60% for Vectran HT1670/600. Inspection of the microstructure within the kink bands reveals extensive micro-cracking and fibril failure due to accelerated moisture ingress. [ABSTRACT FROM AUTHOR]

In this study, flake graphite was dispersed in immiscible polyamide 6/polypropylene (PA6/PP) blends to improve their thermal conductivity. Blends of PA6/PP were prepared in different relative fractions and thermal conductivity of the graphite-containing blends showed enhanced values for immiscible blends when compared with those of mono-PA6-based composites at the same graphite loading fractions. Moreover, a PA6/PP mass ratio of 8/2 showed better dispersion of graphite and less interfacial defects, resulting in better heat conduction ability when compared with other immiscible blends. At graphite loading of 40 wt%, thermal conductivity of PA6/PP/graphite composite reached 2.896 W/mK, which is nearly 40% higher than that of mono-PA6/graphite composite. Morphology and melt rheology measurements confirmed that flake graphite began to form thermally conductive network at 30 wt% in immiscible blends which is crucial to the improvement of thermal conductivity. [ABSTRACT FROM AUTHOR]

Montmorillonite is intercalated with different amounts of tetradecyltrihexylphosphonium chloride in supercritical carbon dioxide and poly(butylene succinate)-based nanocomposites are prepared with the organically modified or commercial organoclay via melt-mixing. The effect of amount of tetradecyltrihexylphosphonium chloride on the structure of organically modified montmorillonite tactoids, organically modified montmorillonite dispersion and finally on properties of nanocomposites is systematically investigated. Different from the compact spherical structure for pristine montmorillonite tactoids, flexible expanded structure with large cavities on the surface can be obtained for organically modified montmorillonites, and their structure show slight difference with the tetradecyltrihexylphosphonium chloride amount. It is interesting to find that, despite almost no polymer molecule being intercalated into the interlayer, significant dispersion can be achieved for the scCO2-processed organically modified montmorillonite in polymer matrix, due to the easily broken tactoid structure. It is noteworthy, montmorillonite-tetradecyltrihexylphosphonium chloride 08, which is modified with 0.8 CEC tetradecyltrihexylphosphonium chloride, less than in most of the literature, shows almost the same degree of dispersion in poly(butylene succinate) matrix as commercial montmorillonite, which has a stronger interaction with the polymer. Differential scanning calorimetry and dynamic modulus analysis results demonstrate that the crystallization temperature and storage modulus of poly(butylene succinate)-based nanocomposites increase with the amount of tetradecyltrihexylphosphonium chloride, and montmorillonite-tetradecyltrihexylphosphonium chloride08/poly(butylene succinate) shows even higher crystallization temperature and low-temperature storage modulus compared to commercial montmorillonite/poly(butylene succinate). [ABSTRACT FROM AUTHOR]