Peak Tensile Stress Research Articles

Chemo-Mechanical Analysis of Lithiation/Delithiation of Ni-rich Single Crystals

Single crystal, Ni-rich layered lithium metal oxides are promising candidates for next-generation cathodes in lithium-ion batteries. However, these Ni-rich materials display anisotropic swelling and contraction during cycling, and this may lead to the generation of internal stresses and thereby to fracture and capacity loss. In this work, the spatio-temporal evolution of lithium concentration and stress state within a (NMC811) single crystal are predicted using a fully coupled chemo-mechanical model. The stress state in the crystal arises from a spatially non-uniform distribution of Li concentration, and from a non-linear dependence of intercalation strain upon lithium concentration. The peak tensile stress is greatest near top-of-charge, due to the high sensitivity of intercalation strain upon lithium occupancy at low concentrations, and the peak tensile stress increases with both cycling rate and particle dimension. Significantly, the predicted peak tensile stress is insufficient to cause basal plane fracture of single crystals when their diameter is below 2.5 μm and the charging and discharging rates are below 5C. This suggests that intraparticle fracture is not a significant degradation mode for well-designed NMC811 single crystals.

Numerical Investigation on the Dynamic Response of Fault-Crossing Tunnels under Strike-Slip Fault Creep-Slip and Subsequent Seismic Shaking

Tunnels built in geologically active areas are prone to severe damage due to fault dislocation and subsequent earthquakes. Using the Ngong tunnel in the East African Rift Valley as an example, the dynamic response of a fault-crossing tunnel and the corresponding sensitivity are numerically simulated by considering four factors, i.e., tunnel joint stiffness, isolation layer elastic modulus, strike-slip fault creep-slip and earthquakes. The results show that a valley-shaped propagation of peak displacement at the tunnel invert occurs in the longitudinal axis direction under an earthquake alone. Then, it transforms into an S-shaped under strike-slip fault creep-slip and subsequent seismic shaking. The tunnel invert in the fault zone is susceptible to tensile and shear failures under strike-slip fault creep-slip movements of less than 15 cm and subsequent seismic shaking. Furthermore, the peak tensile and shear stress responses of the tunnel invert in the fault zone are more sensitive to fault creep-slip than earthquakes. They are also more sensitive to the isolation layer elastic modulus compared to the joint stiffness of a segmental tunnel with two segments. The stress responses can be effectively reduced when the isolation layer elastic modulus logarithmic ratio equals −4. Therefore, the isolation layer is more suitable to mitigate the potential failure under small strike-slip fault creep-slip and subsequent seismic shaking than segmental tunnels with two segments. The results of this study can provide some reference for the disaster mitigation of fault-crossing tunnels in terms of dynamic damage in active fault zones.

Simultaneous prediction the strain and energy absorption capacity of ultra-high performance fiber reinforced concretes by using multi-output regression model

Although a few machine learning (ML) models have successfully been developed for Ultra-high performance fiber reinforced concrete (UHPFRC), they only limited to one output per model and do not explain the effects of its ingredients. This paper proposes an novel approach to tackle the multiple outputs problems by using multi-output regression model (MORM) and providing some insights into the relations between dosages and their outputs via partial dependence plots. A UHPFRC database with 980 mixtures designs with 34 features and two outputs, namely the strain at peak tensile stress and energy absorption capacity is used to verify the propose approach. The MORM with three estimators, namely XGBoost Regressor, Decision Tree Regressor and Gradient Boosting Regressor is trained and tested. Three quantitative measures (R2, MAE and RMSE) are employed to evaluate the accuracy and the obtained results are superior to previous study. Among the results found, the greater feasibility of deformed high-strength steel macrofibers to achieve their exceptional values can be highlighted. On the one hand, the energy absorption capacity directly depends on the cementitious matrix’s quality. On the other hand, the strain at peak tensile stress decreases with the improvement of the matrix’s compressive strength to around 200 MPa. From this resistance, the trend is reversed due to an exponential improvement caused by the improvement in the bond strength between fiber and matrix. The proposed method could be properly used for the optimization of new UHPFRC dosages.

Seismic performance analysis of shallow-buried large-scale three-box-section segment-connected pipeline structure under multiple actions

To ensure the safety operation of inter-basin water transfer project, it is important to understand the seismic performance of shallow-buried large-scale segment-connected pipeline structure under earthquake and other multiple actions. In this paper, a 3D FE model is built in details for the seismic assessment of large-scale prestressed concrete inverted-siphon shallow-buried under riverbed, and a method is proposed to realize multiple interactions of the viscoelastic boundary elements to finite elements, the compressible fluid to structure and the sealed joints made of rubber belt between adjacent segments. The inverted-siphon is longitudinally composited by 58 segments in total length of 858.64 m, and transversally consisits of three rectanguler-section boxes with net dimension of each box of 6.5 m width and 6.6 m height. The main influencing factors are set as the thickness of overburden soil, the flow condition and the engineering geological condition. The time-history analysis is carried out on the 3D FE model under Elcentro seismic wave excitation. The numerical analyses figures out the main vibration modes and vibrational frequency of this inverted-siphon, reveals the regions of peak tensile stress appeared on the concrete surface and the risk sealed joints with extremum relative displacement. This provides a scientific reference to take specific prevention measures of anti-earthquake, based on a deep understanding to the seismic cracking of concrete and the tearing broken of sealed joints for this kind of segment-connected pipeline structure.

Tensile and compressive mechanical properties of nanocrystalline calcite with grain size effect

Abstract Calcite is one of the most main components of microbially induced calcium carbonate precipitation (MICP). With the in-depth research of MICP, the mechanical properties of nanocrystalline calcite attract much attention. In this paper, the deformation and failure behaviors of nanocrystalline calcite under a uniaxial tensile or compressive condition are studied by molecular dynamics simulation, and then the dominant deformation and failure mechanisms, as well as their grain size effect, are analyzed. The results show that the grain boundary densification dominates the elastic deformation, while the intragranular phase transition dominates the plastic deformation. Compared with single-crystal calcite, the elastic modulus of nanocrystalline calcite is significantly reduced and its ultimate strength is decreased by more than 50%. Nanocrystalline calcite has stronger plastic deformation ability in compression than in tension. Its tensile and compressive elastic moduli and peak stresses all increase with the average grain size. The effects of grain size on the limit stresses can be described by the inverse Hall–Petch equation. This study is helpful for tailoring the mechanical properties of MICP by the morphology of nanocrystalline calcite.

Manufacture and mechanical characterization of composite material of carbon fiber with matrix of PEEK

In this study, a low-cost simple manufacturing process for a composite material of thermoplastic matrix PolyEtherEtherKetone (PEEK) and carbon fiber (CF) was developed. The composite material CF/PEEK was mechanically evaluated by three-point bending and tension tests, obtaining the elastic modulus and the maximum tensile stress respectively. Also, threads were made in the composite material and the strength of threads machined on the composite material was evaluated with tension tests. A comparison of the composite material CF/PEEK and the composite material of carbon fiber and epoxy resin (CF/EP) was performed, the elastic modules, peak tensile stresses and the strength of threads were compared. It was found that it was possible to produce the CF/PEEK composite at low cost by hot molding. The elastic modulus and tensile strength of the CF/PEEK were lower than those obtained in the CF/EP. However, the performance of the thread in tension was better for CF/PEEK compared to CF/EP.

Rapid calculation of part scale residual stress – Powder bed fusion of stainless steel, and aluminum, titanium, nickel alloys

A novel analytical model is developed to compute the part scale through-thickness longitudinal residual stress distributions and applied for laser powder bed fusion of four commonly used powder alloys. An important input for the analytical modeling calculations is the peak residual stress for a deposited layer, which is estimated using a unique functional relationship and presented as a function of important process conditions for laser powder bed fusion of different powder alloys. The analytically calculated results of longitudinal residual stress distributions through the part and baseplate thickness are tested rigorously with the corresponding numerically computed and experimentally measured results in the literature for laser powder bed fusion of small and large parts involving the deposition of several thousands of layers. It is shown further that the analytical model can serve as a fast and practical design tool to estimate the through-thickness longitudinal residual stress distribution, which is along the length of the part, for part scale laser powder bed fusion using inexpensive computational resources and with appreciable accuracy. The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations. • An analytical model for residual stresses in 3D printed parts is extensively tested. • The model is validated with experimental data and numerically computed results. • Residual stresses in large 3D printed parts of 5 alloy compositions were examined. • Powder-specific process maps of peak tensile residual stresses are presented.

Study on residual stress evolution of laser cladding low chromium carbon alloy on low-pressure rotor

The in-service behavior of low-pressure rotor components can potentially be impacted by the residual stress distribution of cladding layers. To predict the evolution of residual stress in a multi-pass multi-layer cladding, a thermal-metallurgical-mechanical model that incorporates the effect of solid-state phase transformation (SSPT) is proposed. In the multi-pass case, high longitudinal tensile stress peaks were generated in the former pass, reaching 588 MPa. With multi-pass deposition, the maximum transverse and longitudinal stresses in the substrate increased to 282 and 523 MPa, respectively, which were adjacent to the heat-affected zone (HAZ). In the multi-layer case, multiple thermal cycles had a significant effect on increasing the longitudinal stress of the substrate plate. The maximum of longitudinal stress reached 634 MPa. In the case of the multi-pass multi-layer, the fluctuation of residual stress was generated inside and on the top surface of the cladding layers. The transverse tensile stress changed from 110 to 10 MPa, and the longitudinal tensile stress decreased from 385 to 173 MPa. The interval of high tensile stress was related to the hatch space of the laser beam. In the simulation of the cladding process on a low-pressure rotor, the residual-stress distributions were not uniform along the cladding sequence. Owing to the heat accumulation and scanning strategy, a compressive stress zone adjacent to the endpoint of the cladding sequence, emerged in the cladding layers and substrate. From the simulation results, to estimate the high residual stress and ease the non-uniform residual stress, the laser scanning strategy should be changed in the actual manufacturing process.

Simplified Procedure for Capacity Check of Historic Monolithic Glass Windows under Soft-Body Collision/Bird-Strike

Differing from present structural design procedures, most of the existing glass windows and even historic components in traditional/old buildings are not specifically designed to resist possible accidental loads. Rather thin monolithic ordinary annealed glass panels can be found in vertical non-structural envelopes, where they are often arranged to cover large surfaces. As such, an accidental glass fracture could originate even from rather common and moderate impact events and result in severe risk for people, due to propagation of dangerous shards from these vulnerable and fragile building components. To assess potential risks and support possible mitigation strategies, the present study is focused on the bird-strike analysis of existing/historic linearly restrained non-structural glass windows, based on a parametric Smoothed-Particle Hydrodynamics (SPH)–Finite Element (FE) model. Starting from a 1 m–wide and 1.5 m–high configuration, the attention is first given to various influencing parameters, such as impactor features (mass, 0.35–1.81 kg; impact speed, 0–40 m/s; and, thus, impact energy) and the target window (glass thickness, 4–6 mm; impact point; and, thus, glass stiffness). Local and global effects due to parametric localized bird-strikes are discussed based on non-linear dynamic numerical analyses and in terms of expected deflections, tensile stress peaks, and damage extension/severity (i.e., D1 to D3 damage levels). Scale effects are also examined for a case-study historic envelope (≈7 m in total size, 5 mm in thickness), and one of its 2.58 m × 3.3 m large glass components. Furthermore, a simplified empirical approach based on analytical formulations and normalized charts is proposed for a preliminary vulnerability assessment of historic monolithic glass envelopes, including parameters to account for impactor features and glass panel size/thickness, based on vibration-frequency considerations.

Stress Concentration Analysis of the Corroded Steel Plate Strengthened with Carbon Fiber Reinforced Polymer (CFRP) Plates

The purpose of this study is to investigate the stress concentration of a corroded steel plate strengthened with carbon fiber reinforced polymer (CFRP) plates. An accelerated corrosion experiment was first executed to acquire corroded steel plates, and then surface profile measurements were conducted to obtain 3D coordinate data of the corroded steel surface. Finite element models considering the surface morphology of the corroded steel plate and the interfacial bonding properties between the CFRP plate and the corroded steel plate were established to investigate the stress concentration of the corroded steel plate strengthened with and without CFRP plates. The reliability of the numerical modeling method was verified based on a mesh convergence analysis and a comparison of the fatigue test, 3D morphology scanning, and numerical analysis results. Specimens with five levels of corrosion damage, six kinds of CFRP-strengthening stiffness, five kinds of adhesive thickness, and five levels of CFRP prestress were numerically modeled. The primary indications consist of features of stress distribution, and the stress concentration factors Kt and Ktg were analyzed. Results showed that the features of stress distribution and the stress concentration factor Kt of the corroded steel plate strengthened with and without CFRP plates are only related to the shape, size, and position of rust pits, but not to the degree of uniform corrosion or the reinforcement parameters. The value of Kt for the corroded steel plate with a corrosion duration of 6~18 months and a weight loss rate of 9.16~21.78% was approximately 1.199~1.345. The converted stress concentration factor Ktg has more practical significance than the stress concentration factor Kt in describing the influence of corrosion and CFRP reinforcement on the peak tensile stress of the corroded steel plate. The value of Ktg increased linearly with the increase of the weight loss rate of the corroded steel plate and decreased appreciably with the increase of the strengthening stiffness and prestress level of the CFRP plates, and it presented a very small increasing trend with the increase of the adhesive thickness.

Effects of strain rate on the tensile and creep-fatigue properties of 316H stainless steel

Since the unstable start-ups and shutdown processes of nuclear power plants, the effect of strain rate on tensile and creep-fatigue (CF) properties has attracted attention. This paper aims to study the strain rate dependent tensile and CF behavior of 316H stainless steel. The tensile and CF tests were independently performed with different strain rates of 0.0001, 0.0002, 0.0005, 0.001 and 0.005 s−1 at 600 °C. Due to the dynamic strain aging (DSA), the tensile stress-strain curve appears serrated flow phenomenon and shows negative strain rate sensitivity (SRS), which means the tensile strength of the material does not increase significantly with an increase of the strain rate, and even decreases. Meanwhile, it was found that peak tensile stress and inelastic strain increase in CF tests under low strain rates. The CF life of 316H steel was predicted using the hysteresis energy method. At the same time, based on microstructure and fracture observation, different dominant damage mechanisms were discussed.

Finite element analysis and biomimetic optimal design of full-crown restoration

Objective: To improve the mechanical properties of full-crown restorations and illuminate the optimal elastic modulus distribution through bionic optimization design and finite element analysis (FEA). Methods: Seven 3D models of mandibular first premolars with different full-crown restorations were constructed: (A) zirconia monolithic crown, (B) lithium disilicate monolithic crown, (C) zirconia bilayer crown, (D) lithium disilicate bilayer crown, (E) 8-layer crown referred to the elastic modulus distribution of human enamel, (F) and (G) were 8-layer crowns with elastic modulus distribution calculated by a genetic algorithm (GA) to minimize the tensile stresses in the crown and the shear stresses at the cementing line, respectively. Results in the crowns and cementing lines were obtained with maximum principal stress after applying a static load of 600 N. Results: The principal tensile stresses in the full-crown restorations were mainly concentrated in the cervical margins and the crown-cementing interface. Among them, G exhibited a peak tensile stress of 25.79 MPa, which decreased to 17.72 MPa in E and 16.25 MPa in F; The principal shear stresses in cementing lines were concentrated along the margins and low on the axial wall. The peak shear stress of the cementing line of E and F was 11.81 MPa and 11.79 MPa, respectively. While G was found to has the lowest peak shear stress of 6.14 MPa. Conclusion: The elastic modulus distribution optimized by GA to reduce the peak shear stress of the cementing line can better improve the mechanical properties of the full-crown restoration.

Coupled Eulerian-Lagrangian model for residual stress prediction in orthogonal cutting of Waspaloy

Residual stress is one of the most critical factors of machined surface integrity, which directly determines corrosion resistance and fatigue life of components. Therefore, it is necessary to accurately predict the machining-induced residual stress, especially for critical aerospace parts. In this paper, a finite element model based on the Coupled Eulerian-Lagrangian method is employed to predict the residual stress in orthogonal cutting of Waspaloy material under different cutting conditions. The depth profiles of residual stress along the cutting speed direction are determined using the X-ray diffraction and electropolishing techniques. The results show tensile residual stresses at the cutting speeds of 64 m/min and 86 m/min with 100 microns uncut chip thickness. The depth at the machined surface influenced by the residual stress is identified and compared from both simulations and experiments. The simulated cutting forces and residual stress distributions from the finite element model are compared with the experimental results to validate the model. In addition, the microstructure property at the machined surface is examined to determine the depth of plastic deformation layers. White layers are identified at the higher cutting speed, and it is shown that the formation of white layers is directly related to the location of the peak tensile residual stress.

P12. The influence of ligament biomechanics on proximal junctional kyphosis and failure in patients with adult spinal deformity

<h3>BACKGROUND CONTEXT</h3> It is unknown whether the biomechanics of the posterior ligamentous complex (PLC) are impaired in individuals undergoing surgery for adult spinal deformity (ASD). Characterizing these properties may improve our understanding of proximial junctional kyphosis (PJK, defined as proximal junctional angle [PJA] of >10deg from UIV-1 to UIV+2), as well as proximal junctional failure (PJF, symptomatic PJK requiring revision). <h3>PURPOSE</h3> Ligament biomechanical properties will be associated with PJK and failure PJF. <h3>STUDY DESIGN/SETTING</h3> A prospective observational study. <h3>METHODS</h3> Intraoperative biopsies of PLC were obtained from 32 consecutive spinal fusions for ASD (>4 levels). Ligament peak force, tensile stress, tensile strain and elastic modulus (EM) were measured with a materials testing system. Biomechanical properties and tissue dimensions were correlated with age, gender, BMI, vitamin D level, osteoporosis, sagittal alignment, PJA and change in PJA preoperatively (PRE), within 3 months (3MO), and at one-year (1YR) postoperatively. <h3>RESULTS</h3> Mean (SD) PLC peak force was 207.8(100.6) N, tensile stress was 3.3(1.5) MPa, tensile strain was 1.9(1.2) mm/mm, and EM was 7.1(5.1) MPa. Mean PLC length was 5.9(2.6) mm, thickness was 3.8(1.5) mm, and width was 17.7(4.7) mm. Longer and thinner ligaments were associated with greater PJA change at 3MO (r=0.38, p=0.04; r=0.34, p=0.08 respectively), and thinner ligaments were associated with greater PJA change at 1YR (r=0.57, p=0.01). Greater EM was associated with greater 3MO (r=0.43, p=0.03) and 1YR (r=0.54, p=0.03) PJA. Age was associated with 3MO PJA (r=0.41, p=0.03). EM remained related to PJA when correcting for age (p=0.01). Five had a change in PJA of >10° from PRE to 1YR while PJA change from 3MO to 1YR was 0.6°(3.6). EM was significantly higher in individuals who required revision surgery within 1YR (16.2 (9.7) MPa) versus those who did not (6.5(3.6) MPa, p=0.003). Neither PRE sagittal alignment nor change at 3MO were related to change in PJA or need for revision surgery (p>0.10). At 1YR, three had PJF (2 for PJK, 1 discitis). <h3>CONCLUSIONS</h3> Biomechanical properties of the PLC may be associated with higher risk for proximal failure. Ligaments that are longer, thinner and less elastic are associated with higher postoperative PJA. Furthermore, stiffer EM of the ligament is associated with the need for revision surgery. <h3>FDA DEVICE/DRUG STATUS</h3> This abstract does not discuss or include any applicable devices or drugs.

Multifunctional MEN-Doped Adhesives: Strengthening, Bond Quality Evaluation, and Variations in Magnetic Signal with Environmental Exposure

Adhesive bonding of polymer matrix composites offers various advantages over traditional fasteners, such as a uniform stress state, reduced weight, and delay of composite delamination. However, adhesive bonding has limited implementation due to challenges in the prediction of durability. This work introduces a new method to monitor an adhesively bonded composite joint by dispersing magneto-electric nanoparticles (MENs) into the polymer precursor and monitoring changes in their surface charge density by evaluating the output magnetic signal under an applied magnetic field. Real-time monitoring of the curing process of a polymer adhesive was performed and corroborated via thermal analysis and mechanical testing. Lap shear and end notch flexure testing showed that adding 1 vol% MENs led to a ~23% increase in shear strength and a ~12% increase in mode II critical energy release rates compared to the undoped adhesive. Adding 5 vol% MENs also increased the adhesive’s peak tensile stress by ~8%. Strengthening mechanisms of the doped adhesive were monitored using in situ electron microscopy. A correlation between water ingression and a change in the magnetic moment was observed. Results show the MENs’ potential as a structural health-monitoring tool for a wide range of materials and applications.

A micro–macro method for evaluating progressive and direct tensile fractures in brittle rocks

A micro–macro method for evaluating the behaviors of direct tensile fractures during progressive loadings in brittle rocks is proposed in this study. The method consists of the suggested equation of the stress intensity factor of the mode-I crack that considers crack initiation, growth, and coalescence subjected to triaxial tensile loadings and the expression of axial strain relating to the extended length of the wing crack. The direct tensile correlation of stress and strain for depicting the complete initial elasticity, strain hardening, strain softening, and fracture stages is also studied. The reasonability of the presented method is proved by contrasting published results of experiment. Furthermore, the sensitivities of the density, inclination angle and size of the initial crack on the axial stress–strain curve, axial stress–crack length curve, tensile strength, crack initiation stress, and elastic modulus are determined. The tensile peak stress, tensile stress at crack initiation, and tensile elastic modulus descend with the increment of the inclination angle or size of the initial crack. The tensile peak stress initially descends and then remains constant, finally reaching a critical value with the increment of the density of the initial crack. The tensile elastic modulus descends with the increment of the density of the initial crack. The calculated results have a great significance for the safety evaluation of surrounding rocks in deep-buried underground engineering.

Evaluation of Tensile properties of FRP Composite Laminates under Varying Strain Rates and Temperatures

The present investigation deals with the characterization of tensile behavior of various Fiber Reinforced Polymer composites under Thermo-Mechanical loading. Five different types of Uni-Directional (UD) composites of Carbon, Glass, Carbon-Glass hybrid and Metal Laminates of Carbon &amp; Glass were tested for tensile behavior. Tensile tests were performed at strain rates of 10-3, 10-2, &amp; 10-1 s-1 at Room Temperature,250 0C and 450 0C. Stress-strain relations reveal the strain rate and temperature sensitive behavior of composites. Glass, Glass-Carbon, Glass-metal epoxy composites showed higher peak tensile stress under room temperature with varying strain rates as compared to neat carbon epoxy composites. Also, high strain rate tensile properties such as peak stress and peak strain of Glass-Carbon-Epoxy specimens were 26%, and 60% higher than that of the neat carbon epoxy composite. The failure mechanisms of both the composites were analyzed through scanning electron microscopy. The composites mainly failed due to matrix crack within elastic range under room temperature and failed with significant plastic deformation of matrix and fibers under test temperatures 250 0C and 450 0C. Finally, this study reveals that the continuous phase of metal layer embedded between Uni-Directional Glass and Carbon fiber, based composite system can be tailored to act as an energy-absorbing material system under both elastic and plastic stress strain regimes.

Effects of Steel Fiber and Specimen Geometric Dimensions on the Mechanical Properties of Ultra-High-Performance Concrete.

Ultra-high-performance concrete (UHPC) is an advanced concrete with superior mechanical strength, ductility and durability properties. However, the influence of steel fiber on its constitutive laws and the specimen geometric dimension effect on its strength had not been paid enough attention. To investigate the effect of steel fibers on the properties of UHPC, specimens with different fiber volume contents and fiber types were tested. Meanwhile, the mechanical properties of UHPC at different ages from 3 days to 28 days were conducted. Moreover, specimens with various geometric dimensions were also prepared to study the effect of specimen geometric dimensions (dog-bone-shaped, prism and cylinder specimens) on the properties of UHPC. The results indicated that elastic modulus, tensile peak stress and the corresponding strain increased as the fiber volume content and curing age increased. Specimens with hooked-end fibers exhibited better tensile performance than those with straight fibers. Furthermore, different geometric dimensions of specimens significantly influenced the tensile properties of UHPC. Based on the experimental results, conversion factors were suggested for the transformation of strength obtained from specimens with different geometric dimensions to reference specimens. In addition, both compressive and tensile constitutive laws were proposed to generate the stress–strain relationship of UHPC.

Microstructure and high-temperature mechanical properties of TP310HCbN welded joint

The microstructure of TP310HCbN welded joints and its effect on the tensile and low-cycle fatigue properties were investigated in the temperature range 20–700 °C, particularly in the operating range of 500–700 °C. The results showed that the welded joint was fully austenitic; however, a significantly different microstructure was formed in each welding zone, inducing material inhomogeneity. The material inhomogeneity was most pronounced in the weld metal, wherein an austenitic columnar grain structure composed of dendrite cores and interdendritic regions was developed. A combined analysis of the nanoindentation, failure mechanism, and kernel average misorientation using electron backscatter diffraction revealed that the dendrite core was the softest region in the welded joint and the deformation was localized here, thereby serving as a crack nucleation site and propagation path. This promoted premature failure of the welded joint, resulting in a reduction in the tensile strength, ductility, and the fatigue resistance, in comparison to the base metal. As both the welded joint and base metal showed significant cyclic hardening behavior (more than two-fold increase in the tensile peak stress) during fatigue deformation, the plastic strain energy density was found to be a suitable fatigue parameter, which was invariant throughout the fatigue life. An energy-based unified fatigue life prediction model using material toughness was developed, and its validity was demonstrated by successfully predicting the temperature dependence of the fatigue life as well as the fatigue life reduction of the welded joint.

Plastic Softening Induced by High-Frequency Vibrations Accompanying Uniaxial Tension in Aluminum.

We have investigated the influences of high-frequency vibration (HFV) superimposed onto the monotonic uniaxial tension in single-crystal aluminum (Al) specimens by molecular dynamics simulations. It was found that HFV induces softening, i.e., reduction in peak stress. Similar to previous experimental results, the softening increases with the increasing HFV amplitude. Dependences on lattice orientation, tensile strain rate, and a preset notch are considered. Lattice orientation plays an important role in peak stress and plasticity. The evolution of the atomic structure reveals that dislocations have enough time to annihilate under a lower tensile strain rate, resulting in strong ups and downs in the strain–stress curves. Under a higher strain rate, newly appearing dislocations interact with previous ones before the latter annihilate, densifying the dislocation network. As a result, further dislocation motions and annihilations are considerably impeded, leading to a relatively smooth flow stage. Furthermore, by modifying the propagation direction of shear bands, a preset notch can strengthen the peak tensile stress under low-level amplitude HFVs.