Fatigue Resistance Research Articles

Effects of key parameters of hybrid cable-stayed suspension bridge on the fatigue resistance of side hangers in bonding zones

Hybrid cable-stayed suspension bridges (HCSBs) have excellent mechanical and spanning capacities. However, their hangers in the bonding zones experience high cyclic stresses, and their fatigue failure jeopardize HCSB safety. This study adopted a HCSB finite element model and applied the influence line method to assess the effects of the following six factors on the stress amplitudes of side hangers: ratio of hanger force to vertical force in the stay cable in the bonding zone, relative stiffness of the main beam in the bonding zone, number of hangers in the bonding zone, arrangement pattern of stay cables and hangers, suspension-to-span ratio, and number of auxiliary piers. The exemplary HCSB had asymmetric span arrangement and a main span of 1400 m. Nodal loads were imposed on the main beam in the entire bridge to obtain the influence lines of each hanger and identify the most unfavorable hanger. The values of the above six parameters of the model varied to obtain the new influence lines of each hanger and identify the most unfavorable hanger in the new reasonably completed state. The stress amplitudes of the hangers thus obtained ware compared with those in the original model, reflecting the effects of these six factors on the stress amplitudes in the most unfavorable hanger. Based on the above, the cyclic stress amplitudes of the side hangers were reduced by installing additional stay cables with optimization of their arrangement pattern and relative cross-sectional areas. The results are considered instrumental in improving the fatigue resistance of HCSBs hangers under realistic bridge engineering scenarios.

Underwater fatigue behavior of cementitious mortar and a countermeasure using a water-dispersible polyurethane ether–Portland cement composite

The underwater fatigue behaviors of cementitious mortars are investigated with a focus on the mechanisms contributing to the deterioration of bridge foundations and possible countermeasures. This study analyzes the effects of water penetration during repetitive loading and unloading on the underwater fatigue lives of cementitious materials, highlighting the importance of considering dynamic water motion, in addition to the surface energy mechanism. Water penetration during underwater fatigue occurs even in water-saturated specimens, and water in- and egress within a few millimeters of the specimen surface can significantly reduce the fatigue life. This study proposes a novel approach using a water-dispersible polyurethane ether–Portland cement composite as a countermeasure to improve underwater fatigue resistance. Based on experimental analysis of fatigue in water and microstructural investigations of the composite, the presence of polyurethane compounds within the nano-to-micron-sized microstructures may mitigate water penetration and contribute to a corresponding increase in underwater fatigue life.

Significantly enhanced joint strength and fatigue life of aerospace composite joints by using novel PEEK/PPS woven meshes as joining agent

The development of advanced joining methods for carbon fibre/epoxy composites is critical for aerospace applications. Herein, advanced PEEK/PPS meshes were used to join carbon fibre/epoxy composites upon a novel co-curing process. The meshes were surface-activated upon an atmospheric plasma technique, which largely promoted the chemical interactions between the meshes and the composite epoxy matrix. The quasi-static joint strengths at both of 22 °C and 130 °C and fatigue loading resistance of the composite joints were evaluated using a single-lap joint test and the corresponding failure mechanisms were investigated. Encouragingly, the lap-shear strengths and fatigue resistance of the PEEK/PPS mesh-joined composites were larger than that of their counterparts bonded by an aerospace adhesive. Subsequent analysis on the failure surfaces proved PEEK/PPS fibre damage and epoxy crack were the main failure mechanisms of the PEEK/PPS co-cured joints. This work proposed a novel method for the production of robust aerospace joints by co-cure joining the composites using advanced PEEK/PPS meshes.

Improving the fatigue property of diffusive film cooling holes in nickel-based single crystal superalloy via ultrashort pulse laser drilling coupled with abrasive flow machining

A novel hole-making technique, which combines ultrashort pulse (UP) laser drilling with abrasive flow machining (AFM), has been developed to enhance the surface quality and fatigue resistance of diffusive holes in nickel-based single crystal (NBSC) superalloys. This study conducted a comprehensive analysis of the surface morphology and metallurgical characteristics of the hole wall, evaluated by fatigue testing at elevated temperatures and fractography analysis. The findings demonstrate that AFM can effectively eliminate the solidified debris generated during UP laser drilling, significantly reducing surface roughness and inducing a rounded effect at the outlet acute zone of the diffusive hole. Such improvements have been shown to increase the fatigue life of the holes by up to 50.6 % compared to those without polish. Furthermore, the crystal plasticity finite element method (CPFEM) was employed to investigate the localized stress concentration and the accumulation of plastic slip around the diffusive hole, elucidating the mechanisms behind fatigue failure in NBSC superalloys. The study also discusses the influence of the different hole-making technology on the fatigue properties of diffusive holes, integrating CPFEM results with analyses of surface quality and fatigue fractography. The study conducted in this study can provide valuable guidance for the fabrication of diffusive holes in turbine blades. Furthermore, it can also improve our understanding of the fatigue failure mechanism of diffusive holes in NBSC superalloy.

Regulatory and enhancement mechanism of a micro-nano material of sodium polystyrene sulfonate on the electro-responsive fatigue resistance of ion gel artificial muscles by cross-linking reaction

The Ion Gel Artificial Muscle (IGAM) is an emerging electro-responsive chemical actuator that holds significant potential for flexible actuation. This paper investigates the regulatory and enhancement mechanism of Sodium Polystyrene Sulfonate (PSS) on the electro-responsive fatigue resistance of IGAM by cross-linking with varying amounts of PSS, aiming to achieve high performance and durability. The study is conducted based on control variables and proposes four primary evaluation metrics. The results indicated that the optimal PSS amount of 0.75 mL yielded the highest values for peak difference (32.4 mN/g), response speed of output force density (0.07818 mN/g·s), and operating life of the IGAM (10,800 s). Additionally, under the excitation of short time reversible voltage, the quasi-impulse density of IGAM reached 10915 mN·s/g. Furthermore, the elastic modulus and internal resistance were found to be the lowest at 27.2 MPa and 1.55 Ω, respectively, while the area-specific capacitance increased to 400 nF/cm2. This phenomenon occurred due to the protonation of the sulfonyl groups, which were highly electronegative, on the PSS molecule. As a result, cross-linking hydrogen bonds were formed with the sodium alginate (SA) molecule, leading to the wrapping of its network and the formation of a regular three-dimensional ion gel network inside the IGAM. Both PSS and SA were polyanionic electrolytes, capable of increasing the number of anions present. This, in turn, facilitates the migration of hydrated cations within the IGAM, thereby enhancing its electro-responsive performance to suit various circumstances.

A multifunctional conductive organohydrogel as a flexible sensor for synchronous real-time monitoring of traumatic wounds and pro-healing process

Ion-conductive hydrogels (ICHs) offer promising applications in the realm of flexible bioelectronics due to their attributes, such as transparency, flexibility, tissue adhesion, and self-healing. However, it remains a challenge to integrate excellent material properties and bio-properties into ICHs-based smart wearables for intricate biomedical applications. Herein, a multifunctional conductive C-CST/PMC organohydrogel with stretchability, anti-freezing, self-healing, antibacterial, antioxidant, and biocompatibility was synthesized by catechol-conjugated chitosan (C-CS), silk nanofibrils (SNFs), tannic acid (TA), MgCl2/CaCl2, and polyvinyl alcohol (PVA), with a binary solvent system composed of ethylene glycol (EG) and H2O. The C-CST/PMC exhibited excellent mechanical properties (175 kPa stress and 330 % strain), anti-freezing capacity, moisture retention, self-healing performance, and high conductivity (1.76 mS/cm at 25 °C and 1.43 mS/cm at –18 °C). Moreover, we have employed this organohydrogel as a flexible strain sensor for monitoring human movements, achieving a high sensitivity (gauge factor (GF) = 1.20 at 100 % strain) and glorious fatigue resistance (>1000 cycles). Furthermore, the C-CST/PMC displayed good antibacterial, antioxidant, hemo-, and cytocompatibility. In vivo tests on diabetic rats have demonstrated that the C-CST/PMC organohydrogel effectively reduces inflammatory responses and promotes angiogenesis, consequently expediting wound healing. Notably, trials involving the use of this flexible sensor in a medical silicone bionic shoulder have indicated its potential as a wearable electronic device for real-time monitoring of human wound healing progress. Overall, this research contributes novel insights to the design and fabrication of multifunctional flexible sensors that synchronous pro-healing and real-time monitoring of traumatic skin.

Hydrogen evolution and thermal treatments for removal of plating induced impurities from Ni to extend life of solder joints

As operating temperatures and times continue to increase for solder joints with Ni pads, large voids inevitably form at the intermetallic compound (IMC) – solder interface when the IMC thickness nears 5 μm. This leads to a significant reduction of strength, thermal conductivity, and fatigue resistance, and in small joints, also to electromigration life degradation. We have identified the voiding root cause as the breakdown and subsequent outgassing of deposition-related Ni-oxo-hydroxide impurities, impractical to avoid during Ni commercial electroplating. This work takes as the starting point a successful post-deposition voiding-mitigation treatment with 24-hour annealing at 450 °C. It then explores the possibility of rendering this routine more practical by lowering the annealing temperature down to 250 °C. While that approach shows relatively limited success, an alternative strategy, involving treatment with hydrogen evolution reaction for one hour at room temperature, alone or in combination with thermal, leads to a substantial reduction in the voiding propensity.

Spider silk inspired strong yet tough composite hydrogels

Nanofillers are frequently used to improve the mechanical properties of hydrogels, but it is still difficult to develop composite hydrogels with simultaneously enhanced strength, fracture strain, toughness and fatigue threshold at a high water content. Inspired from the unique structure of spider silk, we propose a “macromolecular chain engineered aramid nanofiber (ANF) reinforcement” strategy to prepare strong and fatigue resistance hydrogels with a relatively high water content. ANFs can decrease the crystallinity while increase the stretchability of PVA hydrogels. The stress can be effectively transferred from the soft and ductile polymer chains to the hard and strong ANFs through strong interfacial hydrogen bonding. The tensile strength, fracture strain, toughness and fracture energy of ANF reinforced hydrogels (ARHs) can reach as high as 2.07 ± 0.15 MPa, 1084 ± 116 %, 12.66 ± 1.1 MJ m−3, and 3196 ± 219 J m−2, respectively, at a water content of ∼80 %. Also, ARHs show great crack propagation resistance with the fatigue threshold up to ∼157 J m−2. The bioinspired ARHs with outstanding mechanical properties and anti-swelling performance have promising applications in the field of soft underwater robots, artificial muscles, and so on.

Assessment and mechanism of antioxidant activity of phenols as asphalt antioxidants

ABSTRACT The interest in the antioxidant properties of phenolic compounds in asphalt has grown significantly. This study primarily focuses on the comprehensive assessment and elucidation of the antioxidant activity mechanisms of various phenolic compounds as asphalt antioxidants. The suitability of different antioxidant indices for evaluating the effectiveness of phenolic anti-aging agents in base and SBS modified asphalt is assessed by experimental outcomes. Through this analysis, the anti-aging mechanism of phenolic antioxidants in asphalt is unveiled. Phenolic antioxidants were observed to diminish the G* value of aged asphalt, elevate the δ values of aged asphalt binder, augment the non-recoverable creep compliance of aged asphalt binder, decrease the percent recovery of aged asphalt binder, enhance the fatigue performance of aged asphalt binder, and lower the Ic = o and Ic = c of aged asphalt binder. This indicates that phenolic antioxidants improve the aging and fatigue resistance of asphalt binders, with antioxidant activities in the following order: p-Coumaric acid < resveratrol < tea polyphenol. Moreover, incorporating phenolic antioxidants into SBS modified asphalt not only restrains asphalt oxidation but also mitigates SBS degradation.

Fatigue-induced fracture assessment for duplex stainless steel protruding tube-to-tubesheet welded joints in multiple-tube test specimen

A novel fatigue testing result and its numerical discussions are presented for protruding tube-to-tubesheet welded joints. The welded joint specimen is made up of an innovative stainless steel grade for pressure vessels of urea production plant, i.e., duplex stainless steel (DSS). Multiple-tube test specimen in which a bundle of tubes fabricated to the tubesheet by welding, is employed. During the fatigue test, several initial cracking positions were observed from the central tube and surrounding tubes. The crack propagation (CP) phenomena, i.e., CP from the central tube to surrounding tube and from the surrounding tube to central tube, were characterized for the multiple-tube problem. Two types of fracture pattern were occurred. These distinctive cracking behaviors are studied using numerical analyses, i.e., FEM stress analysis and X-FEM CP simulation. The possible crack initiating positions as well as the fracture patterns are identified based on the stress distributions. The CP behaviors between the tubes are carefully examined using stress intensity factor (SIF). Additionally, fatigue life cycles and fatigue resistance of the specimen are discussed. The fatigue-induced fracture of the DSS specimen is rigorously investigated and presented using the experimental and numerical methods.

Ammonia-responsive chitosan/polymethacrylamide double network hydrogels with high-stretchability, fatigue resistance and anti-freezing for real-time chicken breast spoilage monitoring

Hydrogels are a promising option for detecting food spoilage in humid conditions, but current indicators are prone to mechanical flaws, posing a concern for packaging systems that require strong mechanical properties. Herein, a double network hydrogel was prepared by polymerizing methacrylamide in a chitosan system with aluminum chloride and glycerol. The resulting hydrogel demonstrated high stretchability (strain >1500 %), notch insensitivity, excellent fatigue resistance, and exceptional anti-freezing capabilities even at −21 °C. When incorporating bromothymol blue (BB) or methyl red (MR), or mixtures of these dyes into the hydrogels as indicators, they exhibited sensitive colorimetric responses to pH and NH3 levels at different temperatures. Hydrogels immobilizing BB to MR ratios of 1:1 and 1:2 displayed clearer and more sensitive color responses when packed into chicken breast, with a sensitivity level of 1.5 ppm of total volatile basic nitrogen (TVB-N). This color response correlated positively with the accumulation of TVB-N on the packaging during storage at both 25 °C and 4 °C, providing sensitive indications of chicken breast deterioration. Overall, the developed hydrogels and indicators demonstrate enhanced performance characteristics, including excellent mechanical strength and highly NH3-sensitive color responses, making significant contributions to the food spoilage detection and intelligent packaging systems field.

RANKL signaling drives skeletal muscle into the oxidative profile.

The bone-muscle unit refers to the reciprocal regulation between bone and muscle by mechanical interaction and tissue communication via soluble factors. The receptor activator of NF-κB ligand (RANKL) stimulation induces mitochondrial biogenesis and increases the oxidative capacity in osteoclasts and adipocytes. RANKL may bind to the membrane bound receptor activator of NF-κB (RANK) or to osteoprotegerin (OPG), a decoy receptor that inhibits RANK-RANKL activation. RANK is highly expressed in skeletal muscle, but the contribution of RANKL to healthy skeletal muscle fiber remains elusive. Here we show that RANKL stimulation in C2C12-derived myotubes induced activation of mitochondrial biogenesis pathways as detected by RNA-seq and western blot. RANKL expanded the mitochondrial reticulum, as shown by mitochondrial DNA quantification and MitoTracker staining, and boosted the spare respiratory capacity. Using MEK and MAPK inhibitors, we found that RANKL signals via ERK and p38 to induce mitochondrial biogenesis. The soleus from OPG-/- and OPG+/- mice showed higher respiratory rates compared to C57BL6/J wild-type (WT) mice, which correlates with high serum RANKL levels. RANKL infusion using a mini-osmotic pump in WT mice increased the number of mitochondria, boosted the respiratory rate, increased succinate dehydrogenase (SDH) activity in skeletal muscle, and improved the fatigue resistance of gastrocnemius. Therefore, our findings reveal a new role of RANKL as an osteokine-like protein that impacts muscle fiber metabolism.

Comparison in microstructure, mechanical properties, and fatigue behavior of 9Cr3W3Co turbine rotor steel and its welded joint

AbstractThe microstructure, mechanical properties, and high/very‐high cycle fatigue behaviors of 9Cr3W3Co turbine rotor steel and its welded joint were investigated. The welded nugget zone (WNZ) exhibited the highest microhardness value, followed by the heat‐affected zone (HAZ), while the base material (BM) showed the lowest hardness value. The WNZ exhibited a higher yield strength compared to the BM with reduced elongation, while maintaining similar ultimate tensile strength. The fatigue strength of the BM at 5 × 107 cycles is roughly 53.7% of the yield strength, whereas that of the WNZ increases to approximately 55.0%. In comparison to the fatigue strength of the BM at 5 × 107 cycles, the WNZ exhibits a notable increase of 10.5%. Smaller grain size and higher dislocation density were observed in the WNZ, which contributed to improved microhardness, yield strength, and fatigue resistance through grain boundary strengthening and dislocation hardening.

Transformation pathway of NiTi shape memory alloy and its modulation based on grain size gradient: A molecular dynamics study

The reduction of grain size to nanoscale is a common approach to enhance material properties, such as fatigue resistance and overall strength of Shape Memory Alloys (SMAs), but it normally decreases the volume fraction of the material undergoing reversible martensitic transformation (MT), leading to a reduction in the reversible strain amplitude. This paper explores the atomic relationship between specific transformation pathways and grain size/gradient of NiTi polycrystal based on molecular dynamics simulation. It is found that during a tensile loading-unloading cycle, four different transformation pathways can be observed, with their volume fractions being dependent on grain size. Importantly, these transformation pathways serve as the medium of the grain size effects on the global mechanical properties of NiTi. The gradient nanograined (GNG) NiTi with large gradients can maintain a high stress level while reducing the residual strain by adjusting the fraction of different transformation pathways, thus achieving both advantages of small residual strain and high strength during cyclic loading with large strain amplitude. The results provide atomic insights into the grain size effects on the transformation pathways of SMAs and the development of high-performance SMAs based on the grain size gradient.

Home-based resistance training performed at either fast or slow speeds improves power output in older adults

PurposeWe investigated the effect of an unsupervised, body mass- home-based resistance training program in older adults performed at either a fast or slow contractile speed on changes to muscle-power, -volume, -architecture, and fatigue resistance of the knee extensors. MethodsThirty-two male older adults (age 65–88 years) were separated into 1) fast-speed exercise (Fast-group), 2) slow-speed exercise (Slow-group), and 3) no exercise (Control-group) groups. Participants in the exercise groups performed 30–45 repetitions of knee-extension and sit-to-stand exercises 3 times a week for 8 weeks with different exercise speed between the groups. Before and after the intervention period, the following variables were measured: Isotonic power, isometric strength, twitch contractile properties, muscle-activity, -architecture, and -quality, neuromuscular fatigue resistance of the knee extensors, and thigh muscle volume. ResultsPeak power was increased in both the Fast-group (+24 %, P < 0.01, d = 0.65) and Slow-group (+12 %, P < 0.05, d = 0.33) but not in the Control-group. Training increased pennation angle of the vastus lateralis in both the Fast-group (+8 %, P < 0.01, d = 0.42) and Slow-group (+8 %, P < 0.01, d = 0.42), while only the Fast-group showed increase in pennation angle of the rectus femoris (+12 %, P < 0.01, d = 0.64) and thigh muscle volume (+16 %, P < 0.01, d = 0.52). There was no time × group interaction effect for the other neuromuscular measures. ConclusionsUnsupervised, body mass- and home-based resistance training performed at either fast or slow speeds can improve muscle power in older adults, while fast-speed exercise may be preferable over slow-speed owing to the relatively greater improvement of muscle-power, −volume, −architecture, and better time efficiency.

Xanthan gum‐toughened hydrogel for durable strain and pressure sensors

AbstractIonic conductive hydrogels have caught much attention for serving as wearable sensors, however, most hydrogel sensors are difficult to be used for repeated sensing due to low toughness and poor self‐recovery. Here, tough and self‐recovery hydrogels are constructed by introducing xanthan gum into poly(acrylamide‐co‐trimethyl ammonium chloride) (P(Am‐DMC)) hydrogel network. Xanthan gum can bind with P(Am‐DMC) chains via hydrogen bonds and the carboxyl group from xanthan gum generates electrostatic interaction with DMC, thus improving the mechanical toughness and fatigue resistance. Furthermore, the hydrogel is highly sensitive to mechanical deformation and presents durability for repeated stretching or compression, which can be designed as a wearable sensor for human motion detection. It is anticipated that the successful design of xanthan gum‐toughened hydrogel can broaden the application range of hydrogels.

Artificial neural network and ANFIS approaches for mechanical properties prediction and optimization of a turbine blade treated by laser shock peening

This research article covers a comprehensive investigation on the prediction and optimization of residual stresses, plastic deformations and damage induced by laser shock peening (LSP) on a thin Ti-6Al-4 V leading edge sample of a turbine blade. The study is based on a two-part approach: the first part involves a numerical simulation of the LSP process and the characterization of its effects on the material. The finite element analysis (FEA) is used to simulate the complex interactions between laser parameters, material properties and mechanical response. This numerical analysis generates valuable insights into the residual stress distribution, the plastic deformation and the potential damage within the sample being treated. Based on the numerical results, the second part highlights the novel application of the artificial neural network (ANN) and the adaptive neuro-fuzzy inference system (ANFIS) methods. By taking advantage of the strength of ANN and ANFIS, the models are able to learn accurately from all the data generated by numerical simulations, allowing them to precisely predict and optimize the effects induced by LSP: residual stresses, plastic strains and damage. This paper provides a significant contribution to the field of materials science and engineering, by offering an in-depth comprehension of the LSP process impact on Ti-6Al-4 V leading-edge turbine blade specimens. The combined utilization of numerical analysis and artificial intelligence AI-driven techniques offers a comprehensive approach to optimize the LSP process, leading to the fatigue resistance improvement and service life extension of turbine blades and other critical aerospace components. The proven capability of ANN and ANFIS enables innovative solutions in aerospace engineering and surface treatment technologies, promoting the AI implementation in materials processing and design.

Applicability of normalized resistance rate model for predicting fatigue life and resistance evolution in composite conductive inks

The electrical resistance of metal-polymer conductive inks increases as they undergo cyclic loading, posing a major challenge to their reliability as interconnect materials for flexible electronic devices. To characterize an ink’s fatigue performance, extensive electro-mechanical testing is usually performed. Phenomenological models that can accurately predict the resistance increase with cyclic loading can save time and be useful in flexible conductor design against fatigue failure. One such model was recently developed for only one composite ink type. The model is based on experiments monitoring resistance under monotonic stretch data and multiple experiments measuring the rate of increase of the resistance under different strain amplitudes and mean strains. The current work examines whether such resistance rate model could be generalized to apply for more types of composite inks. Two composite inks with different binder material, metal flake sizes and shapes, and substrate material were experimentally tested under monotonic and cyclic loading. It was found that the two new inks are also more sensitive to strain amplitude than mean strain. The resistance rate model accurately predicts early/catastrophic failure (<1000 cycles) in all inks and conservatively estimates high fatigue life for low strain amplitudes. A protocol detailing the procedures for applying the resistance model to new inks is outlined.

Rhodamine 6G salicylaldehyde hydrazone-Zn complex: Synthesis, characterization, photochromism, understanding, and application

Zinc complex-based photochromic materials are gaining significant interest from their applications in various consumer products such as toys, cosmetic products, photochromic inks (for security marking), data storage, erasable optical memory media, etc. Here, the rhodamine 6G salicylaldehyde hydrazone-Zn complex was synthesized, characterized, and understood to develop a molecular device. The crystal structure analysis (CCDC number 2260405) confirms the synthesis of a new photo-ligand, rhodamine 6G salicylaldehyde hydrazone. The NMR, ESI-Mass, FT-IR, UV–Vis spectra (Job's and Benesi Hildebrand plots), chelation-enhanced fluorescence spectra (CHEF), and geometry optimization confirm the synthesis of the Zn-complex. We have studied photochromic behavior with absorption, emission, fatigue resistance, role of solvents, thermal bleaching kinetics, FMO computation, and vibrational analysis. The vibrational analysis has rationalized excited state intramolecular proton transfer (ESIPT) involved in photochromism. An attempt has been made to construct photochromic devices.

Prediction of fretting fatigue lifetime of aluminium alloy treated with Friction Stir Processing using continuum damage mechanics

In the aviation industry, fretting fatigue represents a significantly hazardous form of fatigue failure, thus warranting substantial research attention. A key area of focus lies in enhancing the fretting fatigue resistance of aluminium alloys. This study investigates the feasibility of using Friction Stir Processing (FSP) to enhance fretting fatigue lifetime. After FSP, large-size inclusions in the Base Material (BM) are shattered, clustered inclusions are dispersed, and the preferred orientation of inclusions is disrupted. Consequently, areas of stress concentration, damage concentration and merging of hazardous zones become less, and the peak values of stress, stress triaxiality, and damage are decreased with 6–10 %. Compared to BM, the results show that the fretting fatigue lifetime of Friction Stir Processed (FSPed) material is increased by 50.1 %. Hence, FSP could emerges as a highly promising surface treatment technique for enhancing fretting fatigue performance.