Search Results (752)

Background: Nickel–titanium (NiTi) endodontic instruments have undergone significant improvements in heat treatment processing and geometric design, aimed at enhancing flexibility, cutting efficiency, and fatigue strength. Reciprocating motion was introduced to increase cyclic fatigue resistance, which remains the predominant mode of failure in NiTi endodontic file systems. Although these instruments are widely used in both clinical practice and research, few comparative studies have integrated geometric, metallurgical and mechanical evaluations of the most commonly used reciprocating systems. Methods: In the present study, four single-file reciprocating NiTi systems (Reciproc Blue, WaveOne Gold, EdgeOne Fire, and Easy-File Flex) were evaluated for their geometric design, metallurgical composition, and cyclic fatigue strength. Stereomicroscopy and scanning electron microscopy were employed to assess active blade length, spiral configuration, and surface finish, while elemental composition and phase transformation temperatures were analyzed using energy-dispersive X-ray spectroscopy and differential scanning calorimetry. Ten instruments from each group were tested for cyclic fatigue using a standardized curved stainless-steel canal at room temperature, and the time to fracture was recorded. Fatigue data were statistically analyzed using Mood’s median test, with significance set at p < 0.05. Results: Reciproc Blue exhibited the longest active blade length, highest spiral density, and superior surface finish. R-phase start and finish temperatures were highest in WaveOne Gold and lowest in Easy-File Flex. Reciproc Blue demonstrated the higher cyclic fatigue strength, whereas Easy-File Flex showed the lowest. Conclusions: These findings suggest that the metallurgical and geometric characteristics of the Reciproc Blue file significantly enhance its strength to cyclic fatigue compared with the other instruments evaluated. Full article

This study investigated a 460 MPa marine engineering steel’s microstructure and low-cycle fatigue (LCF) behavior along the thickness direction. The results showed that the low-cycle fatigue life was reduced from 9681, 4395, 2107, 1020, 829 to 7222, 1832, 1015, 630, 242 with the specimen taken from the surface to the middle of steel plate, increasing grain size and decreasing the content of high-angle grain boundaries (HAGBs). All specimens showed notable cyclic hardening and softening. This was related to the dislocation movement, interaction, accumulation, annihilation, and dynamic recovery during fatigue tests. Furthermore, the crack propagation paths in the fatigue specimens were also observed and discussed. Finally, the Basquin and Coffin–Manson relationships were used to suggest a prediction model for the LCF life at strain amplitudes ranging from 0.4% to 1.2%, and the anticipated outcomes agreed well with the test results. Full article

Friction stir spot welding (FSSW) is a novel variant of Friction Stir welding (FSW), developed by Mazda Motors and Kawasaki Heavy Industries to join similar and dissimilar materials in a solid state. It is an economic and environmentally friendly alternative to resistance spot welding (RSW). The FSSW technique, however, includes some structural defects imbedded within the weld joint, such as keyhole formation, hook crack, and bond line oxidation challenging the joint strength. The unique properties of nanomaterials in the reinforcement of metal matrices motivated researchers to enhance the FSSW joints’ strength. Previous studies successfully fabricated nano-reinforced FSSW joints. At different volumetric ratios of nano-reinforcement, nanoparticles may agglomerate due to inefficient stirring of the welding tool pin, forming stress concentration sites and brittle phases, affecting tensile and fatigue strength under static and cyclic loading conditions, respectively. This work investigated how the welding tool pin affects stirring efficiency by controlling the distribution of a nano-reinforcing material within the joint stir zone (SZ), and thus the tensile and fatigue strength of the FSSW joints. Sheets of AA6061-T6 of 1.8 mm thickness were used as a base material. In addition, graphene nanoplatelets (GNPs) with lateral sizes of 1–10 µm and thicknesses of 3–9 nm were used as nano-reinforcements. GNP-reinforced FSSW specimens were prepared and successfully fabricated. Optical microscope (OM) and field emission scanning electron microscope (FE-SEM) methods were employed to visualize the GNPs’ incorporation into the SZs of the FSSW joints. Micrographs of as-welded specimens showed lower formations of scattered, clustered GNPs achieved by the threaded pin tool compared to continuous agglomerations observed when the cylindrical pin tool was used. Tensile test results revealed a significant improvement of about 30% exhibited by the threaded pin tool compared to the cylindrical pin tool, while fatigue test showed an improvement of 46–24% for the low- and high-cycle fatigue, respectively. Full article

The mechanical response of geomembranes in hydraulic structures is strongly influenced by temperature variations, which alter both material stiffness and interface shear strength behavior. This study develops a non-linear, temperature-dependent tensile behavior constitutive model for a polyvinyl chloride (PVC) geomembrane and evaluates its implications for the stability of geomembrane-lined reservoir slopes. The empirical relationship was calibrated using tensile tests reported in literature for temperatures between 10 °C and 60 °C, reproducing the observed non-linear softening and modulus reduction with increasing temperature. A classical thermal dilation formulation was incorporated to simulate cyclic thermal expansion and contraction. The constitutive and thermal formulations were implemented in FLAC2D and applied to a 2H:1V covered geomembrane slope representative of dam lining systems. The results show that temperature-induced softening significantly increases tensile strain within the geomembrane. The model also shows that the lower surface interface friction angle of the geomembrane plays a significant role in the slope stability. Thermal cycle analysis demonstrates the accumulation of efforts resulting from the fatigue of the geomembrane. The proposed model provides a practical framework for incorporating thermo-mechanical coupling in design analyses and highlights the necessity of accounting for realistic thermal conditions in assessing the long-term stability of geomembrane-lined reservoirs. Full article

Additive manufacturing has been widely adopted in industry as an alternative to traditional manufacturing processes for complex component production. In fact, a diverse range of materials, particularly polymers, can be processed using 3D printing for biomechanical applications (e.g., prosthetics). However, in-depth evaluation of these materials is necessary to determine their suitability for demanding applications, such as those involving cyclic loading. Following previous work that studied Polylactic Acid (PLA) and Polyethylene Terephthalate Glycol-modified (PETG) under experimental fatigue testing, this study examines the fatigue behaviour of other current 3D-printed polymeric materials, namely Acrylonitrile Styrene Acrylate (ASA), Polycarbonate (PC), Polyamide 12 (Nylon 12), and Polycarbonate–Acrylonitrile Butadiene Styrene (blend) (PC-ABS), for which fatigue data remain limited or even non-existent. The findings revealed performance differences on Tensile Strength (σ_R), Young’s Modulus and Ultimate Strain among tensile specimens made from these materials and characterised S-N curves for both high-cycle (HCF) and low-cycle (LCF) fatigue regimes at room temperature, with a tensile load ratio (R = 0.05). These results establish relationships among fatigue limit and quasi-static mechanical properties, namely 25% × σ_r for ASA (8 MPa), 7% × σ_r for PC (3.6 MPa), 17% × σ_r for Nylon 12 (7.4 MPa), and 15% × σ_r for PC-ABS (4.7 MPa), as well as between mechanical properties and preliminary potential biomechanical applications. Main conclusions were further supported by micro-computed tomography (micro-CT), which revealed levels of porosity in between 4% and 11%, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). Full article

This study presents an approach for predicting the hysteresis behavior of shape memory alloys (SMAs) based on automated machine learning (AutoML) integrated with explainable artificial intelligence (XAI). Experimental data from cyclic tests of NiTi wire under loading frequencies of 0.3, 0.5, 1, and 5 Hz were used for model development. The AutoML framework PyCaret enabled automated model selection, hyperparameter optimization, and performance comparison of regression algorithms. The highest prediction accuracy was achieved by the LightGBM model (for 0.3 Hz and 1 Hz) and the CatBoost model (for 0.5 Hz and 5 Hz), both demonstrating a coefficient of determination R² > 0.997 and low MSE, MAE, and MAPE values. Integration of XAI through the SHAP method provided both global and local interpretability of the model’s behavior. The analysis revealed the dominant influence of the Stress parameter, the physically meaningful role of the loading or unloading stage (UpDown), and a gradual increase in the contribution of the Cycle parameter in later cycles, reflecting fatigue accumulation effects. The obtained results confirm the high accuracy, interpretability, and physical consistency of the developed models. Full article

This study evaluated the effect of clockwise reciprocation motion used in the original Optimum Torque Reverse kinematics, compared with clockwise continuous rotation, on the cyclic fatigue strength of nickel–titanium rotary instruments (NiTi) with different metallurgical characteristics. A total of 120 instruments, ProFile and EndoSequence in sizes 25/.04, 30/.04, and 35/.04, were tested under continuous rotation or reciprocation motions (n = 10 per subgroup). Instruments were examined by optical and scanning electron microscopy to exclude manufacturing defects. Phase transformation temperatures were determined by differential scanning calorimetry, and cyclic fatigue testing was conducted using a custom device simulating a curved canal with a 6 mm radius and an 86° curvature. The time to fracture was recorded, and the number of cycles to fracture was calculated. Statistical comparisons were performed using the Mann–Whitney U test with a significance level of p < 0.05. Differential scanning calorimetry showed that ProFile instruments were fully austenitic at the test temperature, while EndoSequence instruments exhibited a mixed R-phase and austenitic structure. Clockwise reciprocation motion significantly increased cyclic fatigue resistance in all groups compared with clockwise continuous rotation. Time to fracture increased by 241.3% to 337.5%, and EndoSequence instruments consistently demonstrated higher fatigue resistance. The greatest relative improvement was observed in ProFile 35/.04, with a 422.4% increase in the number of cycles to fracture. Overall, the reciprocation motion markedly enhanced cyclic fatigue strength irrespective of metallurgical phase composition, indicating a practical mechanical benefit that may reduce the risk of instrument separation during endodontic procedures. Full article

This paper reviews the fracture mechanics parameters associated with the variability in the crack growth curves associated with forty-two different tests that range from additively manufactured (AM) steels to cold spray additively manufactured (CSAM) 316L steel. As a result of this review, it is found that, to a first approximation, the effects of different building processes and R-ratios on the relationship between ΔK and the crack growth rate (da/dN) can be captured by allowing for changes in the fatigue threshold and the apparent cyclic toughness in the Schwalbe crack driving force (Δκ). Whilst this observation, when taken in conjunction with similar findings for AM Ti-6Al-4V, Inconel 718, Inconel 625, and Boeing Space Intelligence and Weapon Systems (BSI&WS) laser powder bed (LPBF)-built Scalmalloy^®, as well as for a range of CSAM pure metals, go a long way in making a point; it is NOT a mathematical proof. It is merely empirical evidence. As a result, this review highlights that for AM and CSAM materials, it is advisable to plot the crack growth rate (da/dN) against both ΔK and Δκ. The observation that, for the AM and CSAM steels examined in this study, the da/dN versus Δκ curves are similar, when coupled with similar observation for a range of other AM materials, supports a prior study that suggested using fracture toughness measurements in conjunction with the flight load spectrum and the operational life requirement to guide the choice of the building process for AM Ti-6Al-4V parts. The observations outlined in this study, when taken together with related findings given in the open literature for AM Ti-6Al-4V, AM Inconel 718, AM Inconel 625, and BSI&WS LPFB-built Scalmalloy^®, as well as for a range of CSAM-built pure metals, have implications for the implementation and certification of limited-life AM parts. Full article

Loess subgrades are prone to significant strength reduction and deformation under cyclic traffic loads and moisture ingress. Permeable polyurethane polymer grouting has emerged as a promising non-excavation technique for rapid subgrade reinforcement. This study systematically investigated the fatigue behavior of polymer-grouted loess using laboratory fatigue tests and numerical simulations. A series of stress-controlled cyclic tests were conducted on grouted loess specimens under varying moisture contents and stress levels, revealing that fatigue life decreased with increasing moisture and stress levels, with a maximum life of 200,000 cycles achieved under optimal conditions. The failure process was categorized into three distinct stages, culminating in a “multiple-crack” mode, indicating improved stress distribution and ductility. Statistical analysis confirmed that fatigue life followed a two-parameter Weibull distribution, enabling the development of a probabilistic fatigue life prediction model. Furthermore, a 3D finite element model of the road structure was established in Abaqus and integrated with Fe-safe for fatigue life assessment. The results demonstrated that polymer grouting reduced subgrade stress by nearly one order of magnitude and increased fatigue life by approximately tenfold. The consistency between the simulation outcomes and experimentally derived fatigue equations underscores the reliability of the proposed numerical approach. This research provides a theoretical and practical foundation for the fatigue-resistant design and maintenance of loess subgrades reinforced with permeable polyurethane polymer grouting, contributing to the development of sustainable infrastructure in loess-rich regions. Full article

Objectives: This study compared the cyclic fatigue resistance and the mechanical, thermal, and metallurgical characteristics of SlimShaper^® and SlimShaper PRO^® instruments. Both sequences include three instruments (ZS1–ZS3) with identical geometries, although SlimShaper PRO features an apically modified thermal treatment. Methods: Cyclic fatigue tests were performed using a standardized metallic guide with a 45° curvature, on six instruments of each sequence type. Fractured segments were measured, and fracture surfaces were analyzed using optical microscopy and Scanning Electron Microscopy (SEM). Elemental composition was assessed by Energy-dispersive Spectroscopy (EDS), while Differential Scanning Calorimetry (DSC) was used to determine transformation temperatures and enthalpy. Results: ZS1 and ZS1 PRO exhibited comparable cyclic fatigue resistance, whereas ZS2 and ZS3 showed significantly higher resistance than their PRO counterparts. SlimShaper PRO^® instruments fractured with segments approximately 0.5–1 mm longer. EDS confirmed that both instruments were made of NiTi, with minor differences in surface composition, while DSC demonstrated similar enthalpy values but distinct transformation ranges. At room temperature (27 °C), SlimShaper^® ZS2 remained predominantly martensitic, whereas ZS2 PRO could be partially austenitic, explaining its observed reduced fatigue resistance. The results of the DSC allowed to deduce the microstructure and thus the fatigue behavior at the temperature of the oral cavity. Conclusions: SlimShaper ZS2 and ZS3 showed significantly greater cyclic fatigue resistance than their PRO counterparts, while DSC analysis revealed distinct differences in phase transformation behavior that explain their mechanical performance. Full article

The consumption of energy and sports drinks is on the rise globally, exposing dental restorations to more frequent low-pH challenges, which affect degradation. This in vitro study simulated the combined effect of energy drink exposure and cyclic fatigue loading on the fatigue survival rate and flexural strength of three direct dental resin restorative materials with distinct chemistries: a bioactive ionic resin (Activa Presto), a giomer (Beautifil Flow Plus F00) and a conventional nano-hybrid composite (Tetric Ceram). Bar-shaped specimens (25 × 2 × 2 mm) were fabricated according to ISO 4049 and stored for 24 h in either distilled water or 0.2 M citric acid (pH ≈ 2.5), simulating an energy drink (n = 10/group). The samples then underwent chewing simulation (40 N, 100,000 cycles, 1.6 Hz) using a steel antagonist; surviving specimens were tested via three-point bending to determine their flexural strength. All the materials were affected by storage conditions: Activa Presto showed the lowest fatigue survival (20% in water; 0% in citric acid), Tetric N-Ceram moderate survival (40% in both solutions) and Beautifil Flow Plus F00 the highest and most stable survival (90% in water; 40% in citric acid). Among the surviving specimens, Tetric Ceram exhibited the highest flexural strength, followed by Beautifil Flow Plus F00 and then Activa Presto. Citric acid exposure and cyclic loading adversely affected the mechanical performance of all the materials within the limitations of this study. Full article

Nickel superalloy Inconel 718 (IN718) is widely employed in harsh environments with prolonged cyclic stresses in the aerospace and energy sectors, due to its corrosion/oxidation resistance and mechanical strength obtained by precipitation hardening. This work investigates the mechanical behavior in fatigue of IN718 manufactured by Additive Manufacturing (AM), specifically by Laser Powder Bed Fusion (PBF-LB), and compares its results with the material produced by forging and rolling. Samples from both processes were subjected to heat treatments of solution and double aging to increase their mechanical strength. Then, tensile, microhardness, microstructural characterization, and uniaxial fatigue tests were performed (with loading ratio R = −1). The results showed that, although the IN718 produced by AM had higher microhardness and a higher tensile strength limit than the forged and rolled material, its fatigue performance was lower. The S–N curve (stress vs. number of cycles) for the material obtained by PBF-LB demonstrated shorter fatigue life, especially under low and medium stresses. The analysis of the fracture surfaces revealed differences in the regions where the crack initiated and propagated. The shorter fatigue life of the material obtained by PBF-LB was attributed to typical process defects and microstructural differences, such as the shape of the grains, which act as points of crack nucleation. Full article

This study thoroughly investigates the fatigue behavior and properties of additively manufactured (AM) 18Ni300 maraging steel, crucial for the design of load-carrying engineering components. Thin-walled flat specimens, produced via Selective Laser Melting (SLM), underwent a specific thermal process of annealing and aging. Extensive monotonic and cyclic tests (R = 0.1) were conducted to determine static and fatigue material properties. Optical microscopy revealed the thermal process’s effect on microstructure. Experimentally determined design S-N curves, covering low-cycle to engineering endurance limits, allowed estimation of fundamental elastoplastic properties. This work provides valuable new insights and experimentally verified data, addressing a significant literature gap and supporting advanced fatigue-based design for AM components. Full article

Background/Objectives: The long-term clinical success of dental luting cements largely depends on their mechanical performance. This study systematically compared six commonly used definitive dental cements by assessing key mechanical characteristics such as compressive strength and fatigue resistance. Methods: The tested materials included Adhesor Zinc Phosphate (AphC), Harvard Zinc Phosphate (HphC), polycarboxylate cement (CaC), glass ionomer cement (GIC), resin-modified glass ionomer cement (RMGIC), and resin cement (ReC). Both static and dynamic compressive load tests were performed using an Instron ElectroPuls E3000 dynamic testing instrument. During static testing, 77 samples were subjected to an increasing load up to 1500 N. Dynamic tests on 78 samples involved cyclic loading over seven phases from 50 N to 1600 N, with 1500 cycles per phase at 10 Hz. Results: Static load results indicated that GIC, CaC, and phosphate cements exhibited similar performance and were significantly weaker compared to RMGIC and ReC. In the dynamic fatigue tests, most ReC and RMGIC samples maintained integrity throughout the entire protocol, demonstrating markedly superior mechanical reliability. Finite element analysis (FEA) further confirmed the experimental observations, revealing more homogenous stress distribution and lower peak stresses in ReC and RMGIC compared with the conventional cements. Conclusions: Overall, the resin-based and resin-modified glass ionomer cements showed the highest compressive strength and fatigue resistance, indicating superior long-term mechanical stability compared to the conventional cements. These findings support the clinical use of resin-based cements as reliable luting agents for definitive fixation in high-load prosthodontic applications. Full article

Steel girders with corrugated webs are increasingly used in bridge and building structures subjected to cyclic variable loads, where the geometry of the corrugation plays an important role in fatigue performance. This paper investigates the fatigue behaviour and failure mechanisms of full-scale steel girders with sinusoidal corrugated webs subjected to static and cyclic four-point bending. Five simply supported girders were tested: one reference beam under monotonic static loading, two beams under long-term cyclic loading with different load ranges ΔF and numbers of cycles N, and two beams subjected to cyclic loading followed by a static test to failure. The experimental programme focused on the influence of the load range ΔF and the number of cycles N on damage development, stiffness degradation and residual load-bearing capacity, as well as on the interaction between local web instability and global lateral–torsional buckling. The test results show that two main failure mechanisms may occur: (I) local buckling of the corrugated web combined with yielding of the flanges, and (II) a combined mechanism involving local web buckling and lateral–torsional buckling of the girder. For the investigated configurations and within the range of load ranges and numbers of cycles considered, the load range ΔF was found to be the dominant parameter governing fatigue damage, whereas the number of cycles had a secondary influence. The global stiffness of the girders in the elastic range remained almost unchanged until the late stages of loading, and even after pre-fatigue loading, the girders were able to carry a significant portion of their original ultimate load. The results provide experimental data and insight that are relevant for the fatigue assessment and design of steel girders with sinusoidal corrugated webs in bridge and building applications. Full article