Abstract This study presents the fracture mechanics based synergistic effect of load ratio (R = 0.1 to 0.7) and temperature (25 to 200°C) on fatigue crack growth in aerospace grade AA7075-T651. Quarter compact tension specimens under constant amplitude fatigue loading revealed that fatigue life decreased significantly (by 65%) for a temperature increase from 25 to 200°C at R = 0.1. However, increasing R from 0.1 to 0.7 at 200°C extended life several fold (26 times). A transition in the crack growth mechanism was observed from ΔK dominated behavior at ambient conditions to Kmax controlled propagation due to synergistic effect of elevated temperatures and high R values. Succinct and close-spaced striations were observed on fractographic examination of fractured surfaces for specimens tested at 25°C. However, the striations were blurred and widely spaced (a 25-fold increase in striation spacing compared to ambient temperature) for specimens tested at elevated temperatures. While ΔK was observed as the primary crack driving force for combinations of room temperature and low values of R, however, ΔKeff and Kmax dominated the crack growth behavior for high R and elevated temperatures. The use of a single-parameter model accurately predicted crack growth rates at low R (R = 0.5) across all temperature values used in this investigation. The two-parameter model improved the correlation of high R Paris-regime crack-growth, while deviations observed in the near-threshold region. This modeling approach provides with quantitative framework for predicting da/dN under complex thermomechanical conditions.
Abstract This study characterizes the microstructural evolution and coupled mechanical response along the build height of the SS316L wire arc additive manufacturing (WAAM). Correlation between elemental segregation, solidification mode, ferrite content, microhardness, and residual stress at top, middle, and bottom regions. A progressively Creq/Nieq ratio (1.31–1.75) toward the bottom region altered the solidification pathway from primarily austenitic in the top to an austenitic-ferrite mode at the middle and ferrite-stabilized solidification in the bottom region. This shift corresponds with ferrite number increases from ∼1 to ∼9 and a residual stress transition from tensile stress (80 ± 2 MPa) and lower hardness (216 ± 1.0 HV0.5) at the top to high compressive stress (220 ± 3 MPa) and high hardness (270 ± 2.5 HV0.5) near the substrate. Microstructural observations confirmed this gradient with finer cellular austenite evolving into a mixed dendritic columnar structure containing lathy, skeletal, and vermicular δ-ferrite. The results demonstrate that thermal history and solute redistribution inherently produce a functionally graded microstructure and property profile in SS316L WAAM.
In this article, a series of high-cycle fatigue tests were carried out on S280, S350, and LQ550 cold-rolled steel sheets considering different rolling directions and specimen geometries. The S-N curves with a 95% survival probability were derived by fitting the test data using the least squares method. Furthermore, the test results were analyzed from fatigue strength, fatigue damage, and microscopic perspective. We can conclude that the fatigue performance of the base metal along the rolling direction (L) is superior to that transverse to the rolling direction (T). S280 and S350 cold-rolled steel sheets exhibit better fatigue performance than LQ550 cold-rolled steel sheet. The aforementioned results are attributed to the grain size of the base metal. Besides, the fatigue performance of cold-rolled steel sheets is better than Q690 high-strength steel and common hot-rolled steel. Under 95% survival probability, the fatigue limit values obtained from test fitting curves are 44-67% higher than theoretical calculation values based on AISI 360-2016, AISI S100-2016, and GB50017-2017.
Abstract Thermoplastic polymers are increasingly adopted in automotive applications due to their compatibility with high-volume manufacturing, reduced weight, and improved processing efficiency. However, their mechanical response at intermediate strain rates (1–100 s−1), representative of crash conditions, remains insufficiently understood. In particular, large pre-yield deformations and post-yield instabilities limit the applicability of conventional test methods, resulting in a lack of reliable tensile data and constitutive models for accurate impact simulations. To address this gap, a symmetric, double-acting drop-weight impact apparatus was employed to characterize the intermediate strain-rate tensile behavior of five automotive-grade thermoplastics. Baseline rate sensitivity was established through quasi-static testing at four strain rates, followed by dynamic tests conducted at drop heights of 10, 20, and 25 in. Drop height was selected as the control parameter, as the achieved strain rate varied with polymer morphology. Digital image correlation confirmed a homogeneous strain field across the gauge length for all materials, demonstrating the reliability of the test configuration. The results indicate increasing tensile strength and decreasing failure strain with increasing strain rate, with material-specific trends governed by microstructural differences. Strain-rate sensitivity was quantified using strain-averaged power-law relationships calibrated directly from experimental data. Together, the experimental methodology and constitutive modeling provide a robust framework for incorporating intermediate strain-rate effects of thermoplastics into impact-critical design and simulation tools.
Abstract Materials subjected to cyclic loading accumulate thermodynamic entropy until a critical value known as the fracture fatigue entropy (FFE) is reached, whereupon they experience final fracture. Research into FFE values commonly relies upon deformation entropy generation (DEG) theory to derive a model that is then validated with constant amplitude fatigue tests under low-cycle fatigue (LCF) conditions. Yet many engineering structures experience variable amplitude fatigue and/or high-cycle fatigue (HCF) conditions. In addition, the traditional DEG-based approach requires an adjustment to account for internal friction, which requires empirical relations with parameters whose values are known only through material-specific testing. This article presents an alternative approach to FFE estimation for aluminum alloys that accommodates variable stress amplitudes under either LCF or HCF conditions without the need to know the initial temperature rise in the test specimen or to account for internal friction. This alternative approach estimates FFE within the same bounds of variability as the FFE values provided by other researchers using the traditional DEG-based approach by utilizing an empirical correction factor (kload). Empirical fatigue test data validate the alternative approach for all test specimens except those with only two loading blocks in the LCF region, which produced results displaying the most variability.
The feasibility of laser surface melting (LSM) as a surface modification of wire arc (WA)-DED deposited AZ31 Magnesium alloy was experimentally investigated. Emphasis was placed on identifying the effects of independent critical LSM process parameters, such as laser power, scan speed, and hatch distance, and their corresponding laser energy densities on surface integrity, including roughness, wettability, microstructure, phase formation, microhardness, and electrochemical corrosion of WA-DED-deposited AZ31 Mg alloy. The optimized LSM process parameters were a laser power of 50 W with a scan speed of 700 mm/min, a hatch distance of 33%, and a calculated laser energy density (LED) of 7.14 J/mm(2). The surface roughness of the WA-DED specimen increased to 5.5 & micro;m after LSM at an LED of 7.14 J/mm(2); however, this poor surface roughness exhibited increased surface wettability. Moreover, at the same LED, the microstructure of the surface is refined to 4 & micro;m, up to a depth of 297 & micro;m from the surface, and HAGB dominates the grains, with an orientation toward the 0001 basal plane. Therefore, the surface hardness increased to 142 HV. The intermetallic phases of Al12Mg17 and Al8Mn5 were refined up to 50 nm with uniform distribution in the alpha-Mg matrix. The refined phases reduced the area ratio of the cathode (Al12Mg17) and anode (alpha-Mg), which slowed down the initiation of corrosion pits, resulting in increased corrosion resistance to 0.435 mm/year.
This study systematically investigates the static tensile and cyclic tension-tension fatigue behavior of 3D-printed polylactic acid (PLA) to understand the effects of key printing parameters using fused deposition modeling (FDM). ASTM D638 Type IV dog-bone specimens were fabricated with varying infill orientations (0 deg/90 deg and +45 deg/-45 deg), infill densities (25%, 50%, 75%, and 100%), and build orientations (flat and edge). Uniaxial tensile tests and tension-tension fatigue tests (R = 0.1, 1 Hz) were conducted, with fatigue loading set at 90% of each configuration's ultimate tensile strength (UTS). Statistical analysis via three-way analysis of variance (ANOVA) revealed that while infill density was the dominant factor for static properties (UTS and Young's modulus), fatigue life was primarily governed by the interaction between build orientation and raster angle. The highest UTS (28.99 MPa) was achieved with a flat, 100% infill, +45 deg/-45 deg configuration. In contrast, the highest fatigue life (637.5 cycles) was observed in the flat, 25% infill, +45 deg/-45 deg configuration, which was over 14 times more durable than the worst-performing specimen (edge, 100% infill, 0 deg/90 deg). These findings demonstrate a critical trade-off between designing for static strength and designing for fatigue durability, providing essential guidelines for optimizing FDM-printed PLA in mechanically demanding applications and establishing a baseline for predictive fatigue modeling.
High-temperature components such as boiler tubes and heat-exchange surfaces often face premature degradation because conventional alloys offer limited resistance to cyclic oxidation. To mitigate this problem, the behavior of high-velocity oxy-fuel (HVOF)-deposited 304L stainless-steel coatings on T11 and Superfer800 substrates was examined under repeated thermal exposure at 700 degrees C. The coated layers exhibited a compact, uniform morphology and a significant increase in surface hardness, rising to 975 HV0.3 for T11 and 1075 HV0.3 for Superfer800. Cyclic mass-change measurements over 50 heating-cooling cycles highlighted the protective role of the coating. Bare T11 showed the greatest mass gain, whereas the coated T11 specimens observed roughly two-thirds lower oxidation, yielding performance comparable to uncoated Superfer800. EDS analysis indicated the development of a stable chromium-rich oxide scale on the coated surfaces, acting as an effective diffusion barrier. The comparative results reveal that a single 304L HVOF coating can provide a cost-effective means of strengthening oxidation resistance across alloys of differing baseline performance. Overall, the study demonstrates the practicality of employing stainless-steel thermal-spray coatings to extend the service life of materials used in boiler and heat-exchanger environments.
The joint strength of extruded 6xxx series aluminum alloys deteriorates after MIG welding. A process combining localized deformation with stable annealing has been specifically designed to enhance the strength of 6061-T6 MIG-welded joints. The weld zone (WZ) undergoes varying degrees of localized deformation using a mechanical press at room temperature, followed by distinct stable annealing treatments. Posttreatment analyses reveal microstructural evolution and changes in mechanical properties. Results indicate that localized deformation in the WZ induces the formation of high-density dislocations within a microstructure exhibiting < 111 >(Al)//WD (welding direction) orientation, leading to the disappearance of the original as-welded microstructure. Subsequent stable annealing develops a plate texture in the WZ, characterized by {111}(Al)//WD and < 110 >(Al)//PD(pressing direction). Localized deformation levels between 48% and 58% in the WZ combined with annealing at 220-250 degrees C yield the best overall mechanical performance, enhancing tensile strength by 29.1% and restoring joint efficiency to 80%. This deformation shifts the joint's weakest point from the WZ to the heat-affected zone (HAZ). Stable annealing promotes static recovery through dislocation annihilation and rearrangement rather than recrystallization. Although strengthening particles precipitate during stable annealing, the primary strengthening mechanism in this process is attributed to strain hardening.
Measuring residual stress is critical for assessing the structural integrity and performance of fiber-reinforced plastic (FRP) laminates. The incremental hole-drilling (IHD) method, a common technique for this purpose, relies on calibration constants that are typically determined through computationally intensive Finite Element (FE) analysis for each unique laminate configuration. While Deep Operator Networks (DeepONet) can serve as efficient surrogate models, the optimal strategy for composing the necessary training data has not been fully established. In this work, a simple yet effective strategy for training data selection is proposed, which is shown to reduce error in predicted stress by 12.4–15.2% over uniform random selection. Further analysis of training data length with the proposed selection strategy shows that acceptable test error, within the inherent uncertainty of the IHD method, can be achieved using just a small fraction of the possible laminate configurations. In this study, strategically selecting only 15 laminate configurations for training, out of the 70 possible laminate configurations, provided acceptable accuracy for complex residual stress profiles, including steep gradients. These findings provide a practical framework for developing reliable surrogate models, making computationally demanding residual stress analysis more accessible for the design and validation of composite structures.
The current work uses powder-mixed electrical discharge machining (PMEDM) to modify Ti–6Al–4V extra low interstitial (ELI) surface, with boric acid powder suspended in de-ionized (DI) water. The study characterizes surfaces machined by electrical discharge machining (EDM) with pure dielectric and PMEDM (15 g/L) under identical discharge conditions. Voltage–current waveforms were analyzed to evaluate spark behavior, while optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used for analyzing surface morphology. Cross-sectional microscopy was used to assess the thickness of the recast layer, and X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) analyses were utilized to comprehend the surface constituents. The formation of TiB and TiO2 phases was confirmed in the PMEDM sample, demonstrating chemical surface modification due to boron incorporation. TEM showed needle-shaped TiB whiskers with 0.217 nm interplanar spacing, closely matching XRD's 0.210 nm spacing. XPS validated the bonding states between Ti–B and Ti–O, supporting the formation of TiB and TiO2. Vickers microhardness (MH) testing indicated a considerable increase from 435 HV in the unmachined sample to 1125 HV in PMEDM. Dry sliding wear testing showed a significant decrease in the coefficient of friction from 0.549 to 0.292. Wettability analysis indicated a reduction in contact angle from 71.5 deg (unmachined) to 38.1 deg (PMEDM), suggesting a hydrophilic surface. TiB and TiO2 phases increase wettability, which is particularly useful in biomedical applications that need better fluid interaction. Thus, boric acid PMEDM improved Ti–6Al–4V ELI surface integrity, MH, and tribology, with DI water supporting its biomedical and eco-friendly applications.
Auxetic materials, characterized by their unique negative Poisson's ratio, exhibit exceptional tensile and impact strength, outperforming traditional bulk materials. This makes them ideal for high-performance applications in aerospace, automotive, and biomedical industries, where energy absorption is vital. This study optimizes the topology of reentrant chiral auxetic (RCA) structures. An analytical model was developed to analyze energy absorption with specific unit-cell variables. The mechanical behavior of these metamaterials was parameterized based on the length and height of struts and internal angles between the cell struts. A Taguchi design (L27) was employed to evaluate the impact of six geometric factors on the energy absorbed per unit volume (EAV) and the specific energy absorption (SEA) of the RCA structure. Additionally, an analysis of variance was conducted to statistically assess the comparative significance and contribution percentage of each factor. The Taguchi results have shown that the height of the struts significantly affects both the EAV and SEA. Machine learning models, namely polynomial regression and support vector regression, were developed to enhance predictive capability and optimize within the Taguchi design space. These models captured the nonlinear relationships between geometric parameters and energy-absorption metrics, and were subsequently used to optimize RCA structures with close agreement to finite element analysis results.
Abstract In this study, as an alternative to typical ceramics, crab shell particles (CSP) are reinforced in Mg-AZ91 alloy in varying wt% (6, 12, and 18) via a vacuum hot press machine at the sintering temperature of 455 °C. The X-ray diffractometer (XRD) result proves that the important phase composition of CSP is chitin and Mg-calcite. Due to its low thermal stability, the chitin polymer decomposed at a sintering temperature of 455 °C and became bio-carbonaceous char. It has a hydrophilic nature. The decomposition of chitin increases the pores and reduces the density and contact angle from 76.1 deg to 59.8 deg in the specimen. There is a uniform distribution of magnesium calcite in the composite, but the distance between the particles is greater, as confirmed through scanning electron microscope (SEM) images. The presence of bio-carbonaceous char and the greater distance between magnesium calcite have reduced the hardness values from 92 to 66 HV0.3.
Rocks used in historical buildings exhibit a wide range of chemical compositions due to the diversity of volcanic activities and the complexity of their formation processes. The structural integrity and chemical form of these rocks can be significantly deformed when exposed to various environmental factors (wind, rain, earthquakes, fires, etc.). In particular, fires can have destructive effects on the mineral composition of these rocks. When building stones used in historical structures are exposed to high temperatures, chemical changes occur. These chemical changes can cause immediate damage to the structures and also reduce their resistance to environmental factors. To pre-emptively identify the chemical changes occurring in structures after a fire and to enhance the fire resistance of the rocks, preliminary laboratory studies are necessary. In this context, pyroclastic rocks collected from the Ni & gbreve;de region (Turkey) were subjected to thermal treatment at 450 degrees C, 650 degrees C, and 1000 degrees C based on the chemical changes observed through thermogravimetric analysis (TGA). Phase changes in the samples (as-received, 450, 650, and 1000 degrees C) were detected using X-ray diffraction analysis, microstructural changes were examined with scanning electron microscopic (SEM) analysis, and chemical changes were identified using SEM-energy-dispersive X-ray spectroscopy (EDS) and X-ray fluorescence (XRF) analyses. Significant changes were observed in the chemical composition, mineralogical structure, and texture of the rocks when subjected to thermal treatment. TGA analysis showed that chemical changes occurred at 450 degrees C and 650 degrees C. It has been determined that silicon and iron-containing minerals undergo phase changes depending on the increase in temperature (transformation of jarosite into Opal-CT and magnetite into hematite).
This study focuses on the corrosion-wear (tribocorrosion) performance of selective laser melted (as-built SLM) Inconel 718 in natural seawater and quantifies the benefit of a solution heat treatment and double aging (SHT + DA). Potentiodynamic polarization and potentiostatic tribocorrosion tests were conducted while continuously monitoring friction response, and the damaged surfaces were analyzed to clarify the dominant degradation mechanisms. Compared with the as-built condition, the SHT + DA condition exhibited a substantial reduction in corrosion current density (approximate to 30-65%), together with a similar to 50% lower friction coefficient and markedly reduced material loss, indicating a significantly mitigated corrosion-wear synergy under sliding. Microstructural characterization shows that SHT + DA promotes a more homogeneous microstructure with strengthened gamma '/gamma '' precipitation and a higher hardness (from 280.3 HV0.3 to 503.6 HV0.3), which provides improved load-bearing capacity during contact and supports more stable passivation during tribocorrosion. Overall, the results demonstrate that solution heat treatment and double aging reduce the sensitivity of SLM Inconel 718 to coupled mechanical-electrochemical degradation and improve surface durability for chloride-containing service.
Under the complex downhole conditions of torque and pressure, the friction-welded joint of a drill pipe becomes the critical region for fracture failure due to inherent stress concentration and welding residual stress. Traditional homogeneous material models fail to accurately predict the crack propagation behavior of friction-welded drill pipe joints. This study investigates the crack-tip stress-strain fields and propagation paths at various locations of a friction-welded drill pipe joint through a computational model that accounts for its inherent mechanical inhomogeneity across the tool joint, pipe body, and heat-affected zone. First, the mechanical properties of the friction-welded drill pipe joint were characterized in detail using a microhardness tester. Then, a numerical model with continuously varying material properties along spatial positions was developed using the USDFLD user subroutine, ensuring the continuous transition of mechanical properties across the joint region. Finally, the extended finite element method was employed to analyze the influence of inhomogeneous mechanical properties on the crack propagation paths in the tool joint, pipe body, and heat-affected zone. The results show that the inhomogeneous mechanical properties of the friction-welded joint cause the crack propagation path to deflect toward the region with lower material strength, and the total crack propagation length is affected by the mechanical strength near the crack tip.
Hydroxyapatite (HA) is a bioactive ceramic widely applied in dental, bone replacement, and implant therapies because of its excellent biocompatibility and chemical similarity to natural bone. Despite these advantages, its poor strength, limited thermal stability, low crystallinity, and unfavorable surface morphology restrict its use in demanding, load-bearing implants. Improving these properties without compromising biocompatibility remains a key challenge in developing reliable biomedical materials. In this study, HA-titanium (Ti) composites were synthesized by the wet precipitation method and examined to understand how Ti content and processing temperature influence their structural and functional characteristics. Scanning electron microscopy (SEM) revealed distinct morphological changes: low Ti concentrations promoted finer, more uniform particles, whereas higher Ti content and elevated temperatures led to agglomeration, grain coarsening, and the appearance of rod-like TiO2 structures. X-ray diffraction (XRD) showed that crystalline HA was preserved at lower temperatures, with partial Ti addition into the HA lattice. At higher temperatures, reflections of rutile and anatase TiO2 phases emerged, indicating limited Ti solubility and the onset of phase segregation. Fourier-transform infrared (FTIR) spectroscopy confirmed these trends, with characteristic HA phosphate bands present across all samples, alongside band shifts, broadening, and diminished OH peaks that reflected lattice distortion. The growth of Ti- O bands at higher calcination temperatures further supported TiO2 formation. Collectively, these findings highlight that carefully controlled Ti incorporation enhances the crystallinity, thermal stability, and morphology of HA, while excessive Ti or high temperatures promote TiO2 segregation. Optimizing these parameters can pave the way for stronger, more reliable HA-Ti composites tailored for orthopedic, dental, and load-bearing implants that better serve patient needs.
A study was conducted on high-temperature hardness of multifunctional polyphenylene sulfide (PPS)-matrix composite containing carbon fibers (CFs), graphite flakes (Gr), and polytetrafluoroethylene (PTFE). The PPS matrix offers excellent mechanical properties, thermal stability, and chemical resistance at high temperatures. Carbon fibers are added as reinforcement to improve strength and stiffness. Graphite flakes and carbon fibers are good thermal conductors for thermal management. PTFE and graphite are solid-state lubricants for tribological improvements. Thus, CF/Gr/PTFE/PPS material is truly multifunctional due to the attractive combined mechanical, thermal, and tribological properties of its constituents. The high temperature referred to the temperature near, at, and above the composite glass transition. The Rockwell hardness (M scale) was determined from room temperature to 155 degrees C. Hardness of the composite was found to decrease monotonically with increasing temperature, particularly above its glass transition temperature. At high temperature, rigid carbon fibers and graphite carried significant indentation load and constrained segmental chain mobility, thus preserving composite hardness. The injection-molded PPS-matrix composite had flow-induced carbon fiber orientation. The composite with transverse fibers had the highest transverse hardness, while the composite with randomly oriented fibers saw an increase in hardness. The composite hardness was directionally dependent and had a linear relationship with its yield strength at high temperature, but its proportionality changed above and below its glass transition. The relationship enables subsequent prediction of composite tribological behavior based on basic yield property and provides insight into the roles of high temperature and plastic yielding on friction and wear.
Measuring residual stress is critical for assessing the structural integrity and performance of fiber-reinforced plastic (FRP) laminates. The incremental hole-drilling (IHD) method, a common technique for this purpose, relies on calibration constants that are typically determined through computationally intensive finite element (FE) analyses for each unique laminate configuration. While deep operator networks (DeepONet) can serve as efficient surrogate models, the optimal strategy for composing the necessary training data has not been fully established. In this work, a simple yet effective strategy for training data selection is proposed, which is shown to reduce error in predicted stress by 12.4-15.2% over uniform random selection. Further analysis of training data length with the proposed selection strategy shows that acceptable test error, within the inherent uncertainty of the IHD method, can be achieved using just a small fraction of the possible laminate configurations. In this study, strategically selecting only 15 laminate configurations for training, out of the 70 possible laminate configurations, provided acceptable accuracy for complex residual stress profiles, including steep gradients. These findings provide a practical framework for developing reliable surrogate models, making computationally demanding residual stress analyses more accessible for the design and validation of composite structures.
The present study investigated the various aspects of mechanical behavior, microstructure evolution, and formability of a SS430/AA1050 clad sheet at three different elevated temperatures with or without lubrication. Tensile experiments revealed a reduction in tensile strength and ductility of the SS430 layer at elevated temperatures, while the AA1050 layer showed a decrease in strength but an increase in its ductility when compared to that at room temperature. The average normal anisotropy was also increased for both layers of the sheet, enhancing overall clad sheet formability. At elevated temperatures, recovery and recrystallization influenced texture intensity significantly, especially in AA1050, resulting in a more random distribution of grain orientation; however, the texture of SS430 became stronger after deformation. With MoS2 as lubrication, friction values decreased notably for both layers, reducing sticking friction during forming operations. Forming limit diagrams demonstrated improved limit strains under lubrication, with major strain at the plane strain condition increasing more than 65% at 300 degrees C compared to the dry conditions. Deep drawing experiments of the clad sheet revealed that the lubricant significantly improved drawability at all temperatures. Also, the use of lubrication increased the limiting draw ratio, with a maximum draw ratio of 2.0 achieved at 300 degrees C. An analytical model was also developed to predict the required punch force in the deep drawing of the clad sheet based on the Barlat-89 anisotropic yield criterion. The results achieved by numerically and analytically were observed to be in good agreement with the experimental results.
