Strain Evolution Research Articles

Biaxial Thermo–Mechanical Response of ABS Sheet Above Tg Using DIC–Based Bubble Inflation Technique

ABSTRACTIn this study, we examine the biaxial, large, and rapid deformation characteristics of heavy‐gauge ABS sheets within the temperature range of 120°C to 160°C, relevant to the industrial forming process. A custom‐built bubble inflation apparatus with a 300 mm diameter was designed for this purpose. Temperature uniformity across the sheet was verified using a thermal camera, while the temperature profile through the material's thickness was monitored during the heating using thermocouples near the top and bottom surfaces of the sheet. The bubble inflation test, combined with displacement, strain fields, and deformation rates at different locations, enables detailed characterization of the bubble's shape under large deformation. To address early‐stage geometric instabilities due to sagging from thermal expansion, airflow was regulated with a precise flow control valve before inflation, while 3D‐DIC monitored the real‐time displacements to correct for sag deformation. The inflation pressure profile was recorded and synchronized with 3D‐DIC strain evolution data for each test, allowing for analysis of the characteristic stress–strain curve at the pole. By evaluating the material's equi‐biaxial properties at the pole and biaxial strain measurements away from the pole, the collected data provides a strong basis for future calibration of constitutive material models through an inverse approach, enabling accurate integration into process simulations.

Exploring mechanical properties and cracking behavior of AC-13 and OGFC-16 aggregate-segregated asphalt mixtures

Aggregate segregation compromises the homogeneity of asphalt pavement, which diminishes the road performance of the asphalt mixture. The segregation behavior and damage mechanism of asphalt mixture are complex. To investigate the mechanical properties and cracking behavior of asphalt mixtures with varying degrees of aggregate segregation, the dense-graded asphalt mixture with the nominal maximum size of 13.2mm (AC-13) and Open Graded Friction Course with the nominal maximum size of 16.0mm (OGFC-16) with varying degrees of segregation were designed. Indirect tensile test and the digital image correlation (DIC) technology was employed to analyze the strain distribution and strain evolution at the crack initiation point within the asphalt mixtures. The parameters were also proposed to evaluate the resistance to cracking for the segregated asphalt mixtures at a mesoscopic scale. The results show that increased segregation of coarse aggregates in AC-13 mixtures leads to decreased indirect tensile strength, lower maximum tensile strain along the horizontal direction (E11), and reduced failure displacement. Conversely, segregation of fine aggregates in OGFC-16 mixtures results in increased tensile strength, maximum strain, and failure displacement. Higher amount of coarse aggregates in the asphalt mixture contributes to a more intricate strain distribution and a greater number of strain peaks. Dense-graded mixtures show larger strain concentration zones prior to cracking, while smaller stress concentration zones are observed in open-graded mixtures. The horizontal strain-time curve displays distinct stages of stable growth, rapid growth, and rapid decline. Aggregate segregation results in more intricate crack patterns with larger inclination angles, influenced by the random distribution and shapes of coarse aggregates. Novel indexes such as strain uniformity (P1,2) and strain density during crack development (DW) effectively evaluate mesoscopic cracking resistance of asphalt mixtures. The results offer valuable insights into cracking behavior and improve evaluation methods for the mechanical performance of aggregate-segregated asphalt mixtures.

Damage model of NC-UHPC interface zone in UHPC wet joint based on DIC

The bond interface of the ultra-high performance concrete (UHPC) wet joint composed of normal concrete (NC) and UHPC is a weak part, which can easily damage the interface zone and affects the structure's safety. However, the damage model of the NC-UHPC interface zone in UHPC wet joint has not been carried out yet. In this study, firstly, the effects of different interfacial moisture content and interfacial roughness on the interfacial bond strengths were investigated by splitting tensile tests, NC-UHPC specimens with different interfacial bond strength were obtained, and digital image correlation (DIC) technique was used to obtain the strain evolution regulation of the NC-UHPC interface zone; Secondly, the extended interface zone damage factor Df was used to investigate the damage differences of NC-UHPC interface zone with different bond strengths; Finally, the damage model of NC-UHPC interface zone with different interfacial bond strengths was established. This research will be of great significance in improving the theoretical system of UHPC wet joint structure design.

An X-ray diffraction study of the influence of linear and changing strain paths on strain and texture evolution in AA6111-T4 aluminium alloy sheets

Multi-stage automotive stamping processes involve both linear and changing strain paths. While extensive research exists on linear strain paths and discontinuous strain path change, the study of continuous strain path change is limited due to the need for sophisticated experimental procedures. In this paper, the effect of continuous strain path change on strain, strain hardening behaviour, microstructure, and texture evolution was compared with that of discontinuous strain path change in AA6111-T4 aluminium alloy using a novel experimental setup comprising an in-situ mechanical rig and cruciform sample. An X-ray source was used to obtain diffraction patterns, which were analysed to measure diffraction intensities and lattice strains at the {111}, {200}, {220} and {311} lattice planes to study strain hardening behaviour, microstructure, and texture evolution during the loading paths. The experiments were repeated outside the X-ray diffraction chamber to study macroscopic strain evolution and hardening behaviour of the samples using a digital image correlation system. It was found that the absence of unloading and reloading in the continuous strain path change posed challenges for plastic deformation in the next deformation stage, leading to strain softening, premature failure, and relatively weaker textural development. Conversely, the presence of unloading and reloading in the discontinuous strain path change facilitated increased plastic deformation in the next deformation stage, resulting in strain hardening, delayed failure, and stronger textural development.

Genomic surveillance and evolutionary dynamics of influenza a virus in Sri Lanka

BackgroundInfluenza A has been named as a priority pathogen by the WHO due to the potential to cause pandemics. Genomic sequencing of influenza strains is important to understand the evolution of the influenza strains and also to select the appropriate influenza vaccines to be used in the different influenza seasons in Sri Lanka. Therefore, we sought to understand the molecular epidemiology of the influenza viruses in the Western Province of Sri Lanka, including mutational analysis to investigate the evolutionary dynamics.MethodologyA total of 349 individuals presenting with fever and respiratory symptoms were enrolled in this study from November 2022 to May 2024. Nasopharyngeal and oropharyngeal specimens were collected and screened using quantitative PCR to detect Influenza A, Influenza B, and SARS-CoV-2. Subtyping and genomic sequencing was carried out on influenza A strains using Oxford Nanopore Technology.ResultsInfluenza A was detected in 49 (14%) patients, influenza B in 20 (5.7%) and SARS-CoV-2 in 41 (11.7%). Co-infections were observed in five participants. The phylogenetic analysis assigned the H1N1 HA gene sequences within the 6B.1 A.5a.2a clade. The HA gene of the H1N1 sequences in 2023 were assigned as belonging to the subclades C.1, C.1.2, and C.1.8, while the 2024 sequences were assigned to subclades C.1.8 and C.1.9. The H3N2 sequences from 2023 were assigned to the 3 C.2a1b.2a.2a.1b clade and subclade G.1.1.2, while the 2024 sequences were assigned to the 3 C.2a1b.2a.2a.3a.1 clade and subclade J.2. The K54Q, A186T, Q189E, E224A, R259K, K308R, I418V, and X215A amino acid substitutions were seen in the H1N1 in the 2023 and 2024 sequences. The 2024 H1N1 sequences additionally exhibited further substitutions, such as V47I, I96T, T120A, A139D, G339X, K156X, and T278S.ConclusionIn this first study using genomic sequencing to characterize the influenza A strains in Sri Lanka, which showed different influenza A viruses circulating in an 18-month period. As the Sri Lankan strains also had certain mutations of unknown significance, it would be important to continue detailed surveillance of the influenza strains in Sri Lanka to choose the most suitable vaccines for the population and the timing of vaccine administration.

Investigation of full-field strain evolution behavior of Cu/Ni clad foils by interpretable machine learning

Void characteristics are fundamentally correlated with the macroscopic deformation responses of materials, yet traditional modeling methods exhibit inherent limitations in data mining. In this study, a machine learning (ML) framework is proposed to predict the full-field strain evolution of Cu/Ni clad foils, and the impact of intrinsic voids is quantitatively assessed using interpretative analysis methods. The local strain and void data are extracted and integrated through digital image correlation and computed tomography. To accommodate the nature of the constructed dataset, a ML model is established with reference to the concept of time series forecasting. Subsequently, the influence of microstructural features such as volume fraction (VVF), area, and size of voids are investigated, alongside their role in driving local strain evolution. This approach successfully predicts strain localization, and accurately pinpoints the onset of plastic instability and the location of crack initiation. The VVF is identified as the most predominant factor, followed by void size along the tensile direction and grain size. The strongest association is observed between the VVF and grain size, which intensifies over extended time scales. Moreover, as void coalescence is almost completed, the promoting effect of the concentrated void distribution on macroscopic strain concentration will become increasingly pronounced. These findings provide novel perspectives for exploring the intricate relationship between deformation and damage.

UHPC girder multi-modal deformation measurements: Photogrammetry, physical sensing, and FEA

Ultra-high performance concrete (UHPC) has become increasingly popular in flexural design of structural members that require high performance. Although numerous experimental studies on passively reinforced UHPC flexural members exist, studies on monolithic members with larger cross sections are limited, and no information exists on full-field deformation measurements of such specimens. An experimental program consisting of 8 large-scale UHPC doubly-reinforced specimens with continuous longitudinal rebars was conducted under 4-point monotonic loading. The experimental objectives were to investigate rebar slip from one side of the specimens (longitudinal rebar with hooks only on one side), track crack propagation, and capture the full-field displacement and strain measurements during the loading. The noncontact measurements from the multi-camera computer vision system using 3D digital image correlation (3D-DIC) and AprilTag-based photogrammetry were compared with the physical (contact) displacement measurement system. Strain fields were obtained using dual-camera 3D-DIC and finite element (FE) analysis. Results showed a close agreement of the point-wise displacements obtained from the physical and computer vision monitoring systems. The asymmetric structural design caused slip that delayed rebar fracture and reduced the peak load below that predicted by sectional analysis. The measured global force-deflection curve was predicted by the FE model when using a calibrated bond-slip model between rebar and UHPC. Comparison between the full-field measurements using 3D-DIC and FE numerical models showed that the evolution of principal strains and cracking were consistent. 3D-DIC proved to be a promising measurement method for monitoring strain/displacement and calibrating/confirming FE model that conventional methods without full-field measurements cannot provide.

Study on the tensile failure mechanisms of CFRP‐Al flat‐joggle‐flat bonded joints

AbstractThis paper focuses on CFRP‐Al bonded joints, experimentally investigating the effects of joint geometries, different types of adhesives, and overlap lengths on the mechanical properties of bonded joints. Digital Image Correlation (DIC) technology was used to monitor the entire tensile processes, and the deformations and fracture processes of the bonded joints were captured. The strain evolution processes of the joints were obtained through analysis, and quantitative analyses of the joint fracture surfaces were conducted. The results indicate that the special geometric shape of the Flat‐Joggle‐Flat (FJF) bonded joint reduce secondary bending effects. Additionally, the joggle section participates in load transfer, significantly enhancing the joint's mechanical performance and altering its failure mode. Joints using ductile adhesives exhibit higher tensile mechanical properties compared to those using brittle adhesives. Compared to the bearing capacity of single‐lap bonded joints with the same overlap length and adhesive, the mechanical performance improvement of FJF bonded joints using ductile adhesives is less pronounced than that of FJF bonded joints using brittle adhesives. Increasing the overlap length enhances the mechanical properties of bonded joints. This study is expected to provide a reference for the engineering application of FJF bonded joints.Highlights FJF joints have higher bearing capacities compared to single lap joints. Ductile adhesives can effectively increase the bearing capacities of joints. Increasing the overlap length can improve the bearing capacities of joints. The end and middle of the FJF joints jointly bear the load effect. The failure modes of FJF joints are different from that of single lap joints.

Finite Element Combined Design and Material Optimization Addressing the Wear in Removable Implant Prosthodontics.

Wear at the male-female interface of retentive elements in implant-supported removable prostheses is the most frequent complication in such applications. The lack of an ideal/optimal insertion path, as well as the fabrication inaccuracies, are the primary contributors to this issue. A male attachment with a common ball anchor enhanced by lateral flexibility was investigated as a solution, compared to the widely used rigid ball anchor design. A parametric finite element analysis was performed to compare the wear-inducing maximum strain at the female polymer counterpart by various attachment designs made from titanium and Nitinol. The evolution of mechanical strains causing wear in the female part, as well as the contribution of stresses and martensitic transformation in the implant's flexible shaft, were evaluated under several insertion misfit scenarios. Results indicate that introducing a long flexible shaft in the titanium implant reduced maximum strains in the female attachment part by up to 61% as compared to the solid ball anchor. Further improvement was observed by using the shape memory alloy Nitinol as shaft material, leading to a minor reduction in stress and strain at the contact surface but allowing for a shorter abutment. Finally, the optimized Nitinol implant design with a short, necked flexible shaft promoting martensitic transformation at low plateau stress resulted in an approximate 90% reduction in maximum strains at the inner surface of the female part during manual insertion, which indicates a significantly reduced wear phenomenon at the contact.

Orientation-dependent strain and dislocation in HVPE-grown α -Ga2O3 epilayers on sapphire substrates

Orientation-dependent substrates provide effective platforms for achieving α-Ga2O3 with low dislocation densities, whereas the associated strain and dislocation dynamics have not been fully explored. Herein, we investigated the evolution of growth mode, interfacial strain, and dislocation propagation in the α-Ga2O3 epitaxial layer with various orientations, grown by the halide vapor-phase epitaxy. Strain tensor theory and geometric phase analysis indicate that the m-plane α-Ga2O3 epitaxial layer exhibits the lowest misfit tensile strain, measured at εxx = 1.46% and εyy = 1.81%, resulting in the lowest edge dislocation density. The m-plane lattice exhibits an inclination of 33.60°, while the c-plane lattice is horizontally aligned and the a-plane lattice oriented perpendicularly. The orientation-dependent growth significantly influences stress relaxation through the generation of misfit dislocations, originating from either basal or prismatic slip. Edge dislocations, induced by misfit dislocations, favor the c-axis, remaining well confined within the in-plane interfacial layer of the m-plane α-Ga2O3, leading to reduced low edge dislocation density in the subsequent thick epitaxial layer. These findings shed light on the epitaxial dynamics of α-Ga2O3 heteroepitaxy, paving the way for the development of high-performance power devices.

Microstructure and mechanical properties of Mg-Nd-Zn-Zr alloy fabricated by TIG-based wire – Arc directed energy deposition with pulsed current

In this study, the Mg-Nd-Zn-Zr alloy thin-wall specimens were prepared by tungsten inert gas (TIG) based wire-arc directed energy deposition (WA-DED) with different pulsed currents. The arc characteristics of different pulsed current frequents were observed. The microstructures, and mechanical properties of as-deposited (AD), 5 Hz, 10 Hz, 15 Hz, and 20 Hz specimens were systematically analyzed and evaluated. The uniformly equiaxed crystals with random grain orientations and intergranular network Mg-Nd-Zn eutectics were found in the WA-DED fabricated Mg-Nd-Zn-Zr alloy thin-wall specimens. No significant voids were found. The microstructures were regulated, and the mechanical properties were improved by adjusting the pulsed current frequency. The 10 Hz specimen had the optimal microstructure with an average grain size of 10.35 μm. Concurrently, the 10 Hz specimen exhibits excellent strength-ductility synergy and isotropic, benefiting from the finely equiaxed crystals. The average microhardness of the 10 Hz specimen was 68.51 HV0.2, and the ultimate tensile strengths in the building and traveling directions were 223 MPa and 229.7 MPa, respectively, and the yield strengths in the building and traveling directions were 138.3 MPa and 145.3 MPa, respectively. Notably, the elongations in the building and traveling directions of the 10 Hz specimen were 16.8 % and 17.4 %, respectively. The local strain evolution and fracture surfaces of AD and 10 Hz specimens in the building and traveling directions were observed. The mechanisms of grain refinement and mechanical properties improvement were revealed.

A survey on the occurrence of papaya ringspot virus-P (PRSV-P) in Malaysia and genetic diversity assessment of the coat protein region.

Papaya ringspot virus (PRSV) is a plant virus transmitted by aphids that has spread throughout many countries, including Malaysia, causing yield losses and economic impacts to the papaya industry worldwide. PRSV infection in papaya-distinctive ring-shaped patterns on papaya leaves resulted in stunted growth and reduced fruit quality. Management strategies such as the use of resistant varieties, cultural practices, and vector control are employed to mitigate the spread of PRSV. However, the evolution of new virus strains and the uncertainties posed by climate change pose ongoing challenges for the management of PRSV worldwide. Therefore, in this present study, we aim to confirm the presence of PRSV in symptomatic papaya leaves, to depict the current status of PRSV in Malaysia. Using reverse-transcription PCR (RT-PCR) targeting PRSV partial nuclear inclusion b protein (NIb) and coat protein (CP), 13 out of 40 papaya leaves collected were found positive for the PRSV strain-P (PRSV-P). Nucleotide analysis revealed a high similarity with strains from Taiwan and India, showing 96.83%, 97.03%, and 97.03% identity with the Taiwan strains (DQ340771, AY027810) and the India strain (KJ755852), respectively. Compared to the CP gene of Malaysian isolates reported in 2016 (EU082207), several nonsynonymous mutations have been discovered suggesting genetic diversity within the PRSV population in Malaysia. Overall, this study confirms the current circulation of PRSV infection in Malaysia since it was first identified in Johore in 1991. The re-occurrence of PRSV-P in this study highlights the need for continuous monitoring and targeted management strategies to prevent the further spread of PRSV-P in Malaysia.

Simulation of thermal-mechanical coupling in Al alloy/steel inertia friction welding

Inertia friction welding (IFW) is a complicated thermal-mechanical coupling process. In-depth understanding of the thermal-mechanical process can improve the joint quality. In this study, a three-dimensional (3D) thermal-mechanical coupling model was successfully established to simulate the evolution of temperature, energy density, stress, strain, and flow behavior of the IFW process between 2219-O Al alloy and 304 stainless steel. The developed model was validated by comparing it with the measured temperature and deformation. Results show a transforming distribution of interface temperature from a M-shape to a squished V-shape during the IFW process. The characterization is attributed to the transformation of the accumulated friction energy density at the interface from a M-shape to an approximate V-shape due to the combined effect of varying sliding velocity and contact pressure. The Mises stress at the region near the interface is high and low at the steel and Al alloy sides, respectively. A significant portion of the kinetic energy stored in the flywheel is dissipated in the form of plastic deformation energy and cannot be neglected. The tracer particles show that most Al alloy at the initial interface is extruded to form flash, while a small amount of Al alloy is sticky and remains at the interface center. Increasing friction pressure and initial rotation speed result in higher interface temperature, axial shortening distance, and proportion of plastic work in total energy input.

A critical discussion of elasto-visco-plastic self-consistent (EVPSC) models

Two recently proposed elasto-visco-plastic self-consistent (EVPSC) models, based on solving for stress or stress increment, are discussed, and their results are compared with the ones of the visco-plastic self-consistent (VPSC) approach. The advantages and shortcomings of the models are demonstrated, with emphasis on how the grain responses are affected by the time increment and inclusion-medium interaction strength used. Experimental data obtained for an FCC stainless steel is used as the benchmark system for performing the simulations and assessing the reliability of the results. Predictions of stress-strain response of the aggregate, standard deviations in intergranular stress and strain rate, along with the evolution of internal lattice strain are presented and compared, with special focus on their dependence on the incremental approach chosen. We find that the chosen time increment may significantly affect predicted grain responses. To avoid such effect a method based on using two separate time increments is proposed, one for approximating the stress rate and another for actual integration of stress, strain, and state variables. The proposed method successfully eliminates the dependence of results on the time increment used, thus allowing more reliable computations while preserving the computational efficiency of the elasto-visco-plastic model.

Characteristics of vhvp-2 gene distribution and diversity within the Vibrio causing translucent post-larvae disease (TPD)

Translucent post-larva vibriosis (VTPD) in Penaeus vannamei, caused by the virulence factor Vibrio high virulent protein 2 (vhvp-2) gene, poses a significant threat to the shrimp aquaculture industry in China. This study conducted a comprehensive analysis of the vhvp-2 gene distribution and genetic diversity among Vibrio, which play a key role in the pathogenesis of translucent post-larvae disease (TPD). A total of 6281 Vibrio genomes from the NCBI database were screened, revealing the presence of the vhvp-2 gene in 26 strains, predominantly in V. parahaemolyticus, with diverse locations on both plasmids and chromosomes. Comparative analysis of the genomes, plasmids, and the vhvp-2 gene successfully differentiated three distinct clusters of Vibrio species and identified at least two major plasmid groups. However, these plasmid groups did not exhibit the same evolutionary relationships as the host bacteria themselves. The analysis also revealed the conserved presence of the vhvp-2 gene along with genetic variations. Moreover, the widespread presence of antimicrobial resistance genes (ARGs) and virulence factors in Vibrio strains enhances their pathogenicity and poses increased public health risks. The findings underscore the importance of understanding the adaptability, evolution, and ecological characteristics of Vibrio strains harboring vhvp-2 gene. This research provides a scientific foundation for the development of effective disease prevention and control strategies, contributing to the mitigation of TPD’s impact on the shrimp aquaculture industry and supporting its sustainable and healthy growth.

Evolution of axial cumulative strain and micro-pore structure characteristics of solidified mud under cyclic loading

Embankments and foundation geotechnical structures are frequently subjected to long-term cyclic loading due to traffic during their service life. Excessive cumulative deformation can lead to pavement cracking and uneven settlement of the subgrade. This study conducts a series of dynamic triaxial tests to analyze the effects of the number of cycles (N), effective confining pressure (σc), and dynamic stress amplitude (σd) on the axial cumulative strain (εd) characteristics of solidified mud samples. Additionally, it investigates the evolution model of εd of solidified mud and establishes a predictive model for this strain. In conjunction with the NMR tests, this research further investigates the effects of σc and σd on the pore distribution of solidified mud after loading. Ultimately, the correlation between microscopic pore structure indicators and εd is elucidated. The results indicate that the εd behavior of solidified mud under cyclic loading exhibits characteristics of plastic shakedown. Furthermore, the exponential hyperbolic function model more accurately characterizes the relationship between εd of the samples and N. Before and after cyclic loading, the micropores of the samples accounted for over 95 % of the total pore volume, predominantly concentrated in the radius range of r < 0.3 μm. A correlation exists between the average pore size of the sample and εd, which is primarily influenced by σd and σc.

Visualization test and numerical simulations of 2D blasting crack propagation

Drilling and blasting, characterized by their efficiency, ubiquity, and cost-effectiveness, have emerged as predominant techniques in rock excavation; however, they are accompanied by enormous destructive power. Accurately controlling the blasting energy and achieving the directional fracture of a rock mass have become common problems in the field. A two-dimensional blasting (2D blasting) technique was proposed that utilizes the characteristic that the tensile strength of a rock mass is significantly lower than its compressive strength. After blasting, only a 2D crack surface is generated along the predetermined direction, eliminating the damage to the reserved rock mass caused by conventional blasting. However, the interior of a natural rock mass is a "black box”, and the process of crack propagation is difficult to capture, resulting in an unclear 2D blasting mechanism. To this end, a single-hole polymethyl methacrylate (PMMA) test piece was used to conduct a 2D blasting experiment with the help of a high-speed camera to capture the dynamic crack propagation process and the digital image correlation (DIC) method to analyze the evolution law of surface strain on the test piece. On this basis, a three-dimensional (3D) finite element model was established based on the progressive failure theory to simulate the stress, strain, damage, and displacement evolution process of the model under 2D blasting. The simulation results were consistent with the experimental results. The research results reveal the 2D blasting mechanism and provide theoretical support for the application of 2D blasting technology in the field of rock excavation.

Residual Strain Development in Rapid Frontally Curing Polymers.

Frontal polymerization (FP) has emerged as a rapid and energy-efficient process for fabricating thermoset polymers and composites. In this process, a self-propagating reaction front cures the polymer rapidly by the exothermic heat of polymerization reaction instead of an external heat source. Design for FP-based manufacturing in commercial applications requires more comprehensive characterization and prediction of material evolution and residual deformation throughout the process. Here, we report experimental and numerical studies in response to this need. The experimental study focuses on measuring the temperature and cure-dependent properties of mono/poly dicyclopentadiene to capture the strain evolution during the frontal polymerization process. The experimentally measured elastic moduli, Poisson's ratios, and coefficients of thermal expansion and chemical shrinkage show strong dependence on the degree of cure. Based on the experimental output, a coupled thermo-chemo-mechanical model has been developed to capture the measured residual strains. The chemical shrinkage is closely related to the curing rate, leading to strong localization of residual strains in accelerated reaction regions, especially where two fronts merge. Preheating of the monomer (or gel) at the fronts merging area is suggested as an effective method to mitigate residual deformations.

Integrated Control of Thermal Residual Stress and Mechanical Properties by Adjusting Pulse-Wave Direct Energy Deposition.

Directed energy deposition with laser beam (DED-LB) components experience significant residual stress due to rapid heating and cooling cycles. Excessive residual tensile stress can lead to cracking in the deposited sample, resulting in service failure. This study utilized digital image correlation (DIC) and thermal imaging to observe the in situ temporal evolution of strain and temperature gradients across all layers of a deposited 316 L stainless steel thin wall during DED-LB. Both continuous-wave (CW) and pulsed-wave (PW) laser modes were employed. Additionally, the characteristics of thermal cracks and geometric dislocation density were examined. The results reveal that PW mode generates a lower temperature gradient, which in turn reduces thermal strain. In CW mode, the temperature-stress relationship curve of the additive manufacturing sample enters the "brittleness temperature zone", leading to the formation of numerous hot cracks. In contrast, PW mode samples are almost free of cracks, as the metal avoids crack-sensitive regions during solidification, thereby minimizing hot crack formation. Overall, these factors collectively contribute to reduced residual stress and improved mechanical properties through the adjustment of pulsed-wave laser deposition.

Study on the tensile behaviour and nano deformation mechanism of Nickel-based single crystal superalloys with γ/γ' structure at atomic scale

To study the deformation mechanism of Nickel-based single crystal superalloys (NBSC) during tensile process, tensile experiments on NBSC are carried out by molecular dynamics to systematically analyze stress-strain, shear strain, atomic migration and defect evolution. The effects of the structure of the two phases and the strengthening elements on the tensile behavior are analyzed by comparing the pure Ni phase, the pure Ni3Al phase and the Ni phase containing the strengthening elements Cr and Co from a microscopic view. The results show that the interaction between the two-phase structural dislocations during tensile process balances the stress concentration and reduces the degree of plastic deformation that occurs within the matrix. The γ/γ' interface separates the strain morphology and inhibits the slip of the shear band between the two phases, resulting in enhanced deformation resistance of the matrix. By comparing the three single-phase structures, the γ/γ' structure has the highest yield point. From the shear strain distribution pattern, the portion of the two-phase structure with a larger degree of shear strain mainly occurs at the interface as well as at the edge of the matrix.