Anisotropic Low Cycle Behavior of the Extruded 7075 Al Alloy

The quasi-static and low cycle fatigue tests of extruded 7075 Al alloy (Φ200 mm) were investigated in three directions: the extrusion direction (ED), the radial direction (RD), and 45° with ED (45°). Grain morphology analysis, texture measurement, and fatigue fracture characterization were conducted to discuss the relationship between microstructure and mechanical properties. The experimental results showed that the ED specimen had higher ultimate tensile strength (UTS) and low cycle fatigue (LCF) properties, which were mainly attributed to the following three causes. First, the grain boundaries (GBs) had an obvious effect on the crack growth. The number of GBs in the three directions was different due to the shape of the grains elongated along the ED. Second, the sharp <111> texture and the small Schmidt factor along the ED explained the higher ultimate tensile strength (UTS) of the ED specimens. Third, fatigue fracture observation showed that the ED specimen had a narrow fatigue striation spacing, which indicated that the plastic deformation of the ED specimen was the smallest in each cycle. In addition, two fatigue prediction models were established to predict the LCF life of extruded 7075 Al alloy, based on the life response behavior of the three directions under different strains.     

Low Cycle Fatigue Behavior of Plastically Pre-Strained HSLA S355MC and S460MC Steels

Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel.     

Low-Cycle Fatigue Behavior of the Novel Steel and 30SiMn2MoV Steel at 700 °C

As a newly developed gun barrel steel, the novel steel has shown excellent high-temperature strength, high resistance to wear and erosion, contributing to the superior ballistic life of gun barrels. As ballistic life increases, the fatigue life becomes essential for the safety and reliability of gun barrels. This paper presents a comparison of the low cycle fatigue (LCF) behaviors between a novel steel and 30SiMn2MoV steel at 700 °C. A strain-controlled fatigue test was carried out on the novel steel and 30SiMn2MoV steel in the strain range from 0.2 to 0.6%. The cyclic stress response behaviors of the novel steel and 30SiMn2MoV steel show cyclic softening behavior. In addition, the shape of the hysteresis rings of the novel steel and 30SiMn2MoV steel exhibit no-Masing model behavior. Energy–life relationships results show that the novel steel has higher fatigue resistance than the 30SiMn2MoV steel at 700 °C. The results of fatigue fracture analysis show that the failure mode of the 30SiMn2MoV steel is a mixed mode of intergranular fracture and transgranular fracture, while the failure mode of the novel steel is intergranular fracture. The cyclic softening of the two materials can be attributed to the lath structure with a high density of dislocations gradually transforms into low energy subcrystalline and cellular structures at 700 °C. The novel steel has a better fatigue life than the 30SiMn2MoV steel at 700 °C and different strain amplitudes, which is mainly related to the carbides and lath martensite in the materials.     

Usability of Ultrasonic Frequency Testing for Rapid Generation of High and Very High Cycle Fatigue Data

Ultrasonic fatigue testing is an increasingly used method to study the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of materials. Specimens are cycled at an ultrasonic frequency, which leads to a drastic reduction of testing times. This work focused on summarising the current understanding, based on literature data and original work, whether and how fatigue properties measured with ultrasonic and conventional equipment are comparable. Aluminium alloys are not strain-rate sensitive. A weaker influence of air humidity at ultrasonic frequencies may lead to prolonged lifetimes in some alloys, and tests in high humidity or distilled water can better approximate environmental conditions at low frequencies. High-strength steels are insensitive to the cycling frequency. Strain rate sensitivity of ferrite causes prolonged lifetimes in those steels that show crack initiation in the ferritic phase. Austenitic stainless steels are less prone to frequency effects. Fatigue properties of titanium alloys and nickel alloys are insensitive to testing frequency. Limited data for magnesium alloys and graphite suggest no frequency influence. Ultrasonic fatigue tests of a glass fibre-reinforced polymer delivered comparable lifetimes to servo-hydraulic tests, suggesting that high-frequency testing is, in principle, applicable to fibre-reinforced polymer composites. The use of equipment with closed-loop control of vibration amplitude and resonance frequency is strongly advised since this guarantees high accuracy and reproducibility of ultrasonic tests. Pulsed loading and appropriate cooling serve to avoid specimen heating.     

About the Role of Interfaces on the Fatigue Crack Propagation in Laminated Metallic Composites

The influence of gradients in hardness and elastic properties at interfaces of dissimilar materials in laminated metallic composites (LMCs) on fatigue crack propagation is investigated experimentally for three different LMC systems: Al/Al-LMCs with dissimilar yield stress and Al/Steel-LMCs as well as Al/Ti/Steel-LMCs with dissimilar yield stress and Young’s modulus, respectively. The damage tolerant fatigue behavior in Al/Al-LMCs with an alternating layer structure is enhanced significantly compared to constituent monolithic materials. The prevalent toughening mechanisms at the interfaces are identified by microscopical methods and synchrotron X-ray computed tomography. For the soft/hard transition, crack deflection mechanisms at the vicinity of the interface are observed, whereas crack bifurcation mechanisms can be seen for the hard/soft transition. The crack propagation in Al/Steel-LMCs was studied conducting in-situ scanning electron microscope (SEM) experiments in the respective low cycle fatigue (LCF) and high cycle fatigue (HCF) regimes of the laminate. The enhanced resistance against crack propagation in the LCF regime is attributed to the prevalent stress redistribution, crack deflection, and crack bridging mechanisms. The fatigue properties of different Al/Ti/Steel-LMC systems show the potential of LMCs in terms of an appropriate selection of constituents in combination with an optimized architecture. The results are also discussed under the aspect of tailored lightweight applications subjected to cyclic loading.     

Low Cycle Fatigue Characteristics of Oxygen-Free Copper for Electric Power Equipment

The effect of heat treatment on tensile and low cycle fatigue properties of the oxygen-free copper for electric power equipment was investigated. The heat treatment at 850 °C for 20 min, which corresponds to the vacuum brazing process, caused the grain growth and relaxation of strain by recrystallization, and thus, the residual stress in the oxygen-free copper was reduced. The tensile strength and 0.2% proof stress were decreased, and elongation was increased by the heat treatment accompanying recrystallization. The plastic strain in the heat-treated specimen was increased compared with that in the untreated specimen under the same stress amplitude condition, and thus, the low cycle fatigue life of the oxygen-free copper was degraded by the heat treatment. Striation was observed in the crack initiation area of the fractured surface in the case of the stress amplitude less than 100 MPa regardless of the presence of the heat treatment. With an increase in the stress amplitude, the river pattern and the quasicleavage fracture were mainly observed in the fracture surfaces of the untreated specimens, and they were observed with striations in the fracture surfaces of the heat-treated ones. The result of the electron backscattered diffraction (EBSD) analysis showed that the grain reference orientation deviation (GROD) map was confirmed to be effective to investigate the fatigue damage degree in the grain by low cycle fatigue. In addition, the EBSD analysis revealed that the grains were deformed, and the GROD value reached approximately 28° in the fractured areas of heat-treated specimens after the low cycle fatigue test.     

Influence of Corrosion on Fatigue of the Fastening Bolts

The aim of the present work was to evaluate high-strength bolt corrosion fatigue based on metallographic examinations. The conducted tests were focused on the analysis of damaged martensitic bolts. It was found that the combined presence of cyclic loads and a corrosive environment was the cause of the accelerated fatigue of the fastening bolts. The tests carried out indicate that the actual operating conditions were different than expected. The corrosion contributed to the loosening of the bolts and initiation of fatigue cracks in the bolt threads. Further damage of the galvanized bolts was caused by fatigue crack growth in their threaded part that propagated towards the centre of the material. Cracks in the zinc coating were transferred to the steel substrate. The corrosion was favored by the oxygen concentration cell and numerous radial cracks appear in the zinc coating. The vibrations accompanying the operation of the wind tower led to their further propagation and the formation of the fatigue fracture in one of the bolts.     

Fatigue Assessment of Twin Wire Arc Sprayed and Machine Hammer-Peened ZnAl4 Coatings on S355 JRC+C Substrate

Structural elements for applications in maritime environments, especially offshore installations, are subjected to various stresses, such as mechanical loads caused by wind or waves and corrosive attacks, e.g., by seawater, mist and weather. Thermally sprayed ZnAl coatings are often used for maritime applications, mainly due to good corrosion protection properties. Machine hammer peening (MHP) has the potential to increase fatigue and corrosion fatigue resistance of ZnAl coatings by adjusting various material properties such as hardness, porosity and roughness. This study investigates the fatigue behavior of twin wire arc sprayed and MHP post-treated ZnAl4 coatings. Unalloyed steel (S355 JRC+C) was selected as substrate material and tested as a reference. MHP achieved the desired improvements in material properties with increased hardness, decreased roughness and uniform coating thickness. Multiple and constant amplitude tests have been carried out to evaluate the fatigue capability of coating systems. In the high cycle fatigue regime, the additional MHP post-treatment led to an improvement of the lifetime in comparison to pure sandblasted specimens. The surface was identified as a crack initiation point. ZnAl coating and MHP post-treatment are suitable to improve the fatigue behavior in the high cycle fatigue regime compared to uncoated specimens.     

Fatigue Testing of Wearable Sensing Technologies: Issues and Opportunities

Standards for the fatigue testing of wearable sensing technologies are lacking. The majority of published fatigue tests for wearable sensors are performed on proof-of-concept stretch sensors fabricated from a variety of materials. Due to their flexibility and stretchability, polymers are often used in the fabrication of wearable sensors. Other materials, including textiles, carbon nanotubes, graphene, and conductive metals or inks, may be used in conjunction with polymers to fabricate wearable sensors. Depending on the combination of the materials used, the fatigue behaviors of wearable sensors can vary. Additionally, fatigue testing methodologies for the sensors also vary, with most tests focusing only on the low-cycle fatigue (LCF) regime, and few sensors are cycled until failure or runout are achieved. Fatigue life predictions of wearable sensors are also lacking. These issues make direct comparisons of wearable sensors difficult. To facilitate direct comparisons of wearable sensors and to move proof-of-concept sensors from “bench to bedside”, fatigue testing standards should be established. Further, both high-cycle fatigue (HCF) and failure data are needed to determine the appropriateness in the use, modification, development, and validation of fatigue life prediction models and to further the understanding of how cracks initiate and propagate in wearable sensing technologies.     

Material Behavior and Fatigue Assessment of Old Steel Bridges of the Spanish Conventional Rail Network

This work presented salient features of the steel behavior of seven metallic bridges close to, or over, 100 years old, among the Spanish conventional rail network as well as the results of a fatigue life expectancy study. A preliminary study of the properties of the constituent materials obtained from the bridges samples was carried out followed by dynamic fatigue tests under service representative loads. Due to the steelmaking techniques in the late 19th and early 20th centuries, disperse fatigue behavior results were obtained. However, the wide safety margins with which these bridges were designed, as well as the mechanical properties of the steel (relatively good mechanical resistance but with low ductility), seem to guarantee a long fatigue life. This estimate decreases sharply with increasing loads.     

Influence of Creep Damage on the Fatigue Life of P91 Steel

The following paper presents the results of tests on samples made of P91 steel under the conditions of simultaneously occurring fatigue and creep at a temperature of 600 °C. The load program consisted of symmetrical fatigue cycles with tensile dwell times to introduce creep. Static load (creep) was carried out by stopping the alternating load at the maximum value of the alternating stress. The tests were carried out for two load dwell times, 5 s and 30 s. A comparative analysis of the test results of fatigue load with a dwell time on each cycle confirmed that creep accompanying the variable load causes a significant reduction in sample durability. It was shown in the paper that regarding the creep influence in the linear fatigue damage summation approach, it is possible to improve the compliance of the fatigue life predictions with the experimental results.     

A Simplified Ductile Fracture Model for Predicting Ultra-Low Cycle Fatigue of Structural Steels

Under strong earthquakes, steel structures are prone to undergoing ultra-low cycle fatigue (ULCF) fracture after sustaining cyclic large-strain loading, leading to severe earthquake-induced damage. Thus, establishing a prediction method for ULCF plays a significant role in the seismic design of steel structures. However, a simple and feasible model for predicting the ULCF life of steel structures has not been recognized yet. Among existing models, the ductile fracture model based on ductility capacity consumption has the advantage of strong adaptability, while the loading history effect in the damage process can also be considered. Nevertheless, such models have too many parameters and are inconvenient for calibration and application. To this end, focusing on the prediction methods for ULCF damage in steel structures, with the fragile parts being in moderate and high stress triaxiality, this paper proposes a simplified uncoupled prediction model that considers the effect of stress triaxiality on damage and introduces a new historical-effect related variable function reducing the calibration work of model parameters. Finally, cyclic loading test results of circular notched specimens verify that the proposed model has the advantages of a small dispersion of parameters for calibration, being handy for application, and possessing reliable results, providing a prediction method for ULCF damage of structural steels.     

Oxidation Damage Evolution in Low-Cycle Fatigue Life of Niobium-Stabilized Austenitic Stainless Steel

Austenitic stainless steel is a vital material in various industries, with excellent heat and corrosion resistance, and is widely used in high-temperature environments as a component for internal combustion engines of transportation vehicles or power plant piping. These components or structures are required to be durable against severe load conditions and oxidation damage in high-temperature environments during their service life. In this regard, in particular, oxidation damage and fatigue life are very important influencing factors, while existing studies have focused on materials and fracture behavior. In order to ensure the fatigue life of austenitic stainless steel, therefore, it is necessary to understand the characteristics of the fracture process with microstructural change including oxidation damage according to the temperature condition. In this work, low-cycle fatigue tests were performed at various temperatures to determine the oxidation damage together with the fatigue life of austenitic stainless steel containing niobium. The characteristics of oxidation damage were analyzed through microstructure observations including scanning electron microscope, energy-dispersive X-ray spectroscopy, and the X-ray diffraction patterns. In addition, a unified low-cycle fatigue life model coupled with the fracture mechanism-based lifetime and the Neu-Sehitoglu model for considering the influence of damage by oxidation was proposed. After the low-cycle fatigue tests at temperatures of 200–800 °C and strain amplitudes of 0.4% and 0.5%, the accuracy of the proposed model was verified by comparing the test results with the predicted fatigue life, and the validity by using the oxidation damage parameters for Mar-M247 was confirmed through sensitivity analysis of the parameters applied in the oxidation damage model. As a result, the average thickness of the oxide layer and the penetration length of the oxide intrusion were predicted with a mean error range of 14.7% and 13%, respectively, and the low-cycle fatigue life was predicted with a ±2 factor accuracy at the measurement temperatures under all experimental conditions.     

Fatigue Resistance and Cracking Mechanism of Semi-Flexible Pavement Mixture

Semi-flexible pavement (SFP) is widely used in recent years because of its good rutting resistance, but it is easy to crack under traffic loads. A large number of studies are aimed at improving its crack resistance. However, the understanding of its fatigue resistance and fatigue-cracking mechanism is limited. Therefore, the semi-circular bending (SCB) fatigue test is used to evaluate the fatigue resistance of the SFP mixture. SCB fatigue tests under different temperature values and stress ratio were used to characterize the fatigue life of the SFP mixture, and its laboratory fatigue prediction model was established. The distribution of various phases of the SFP mixture in the fracture surface was analyzed by digital image processing technology, and its fatigue cracking mechanism was analyzed. The results show that the SFP mixture has better fatigue resistance under low temperature and low stress ratio, while its fatigue resistance under other environmental and load conditions is worse than that of asphalt mixture. The main reason for the poor fatigue resistance of the SFP mixture is the poor deformation capacity and low strength of grouting materials. Furthermore, the performance difference between grouting material and the asphalt binder is large, which leads to the difference of fatigue cracking mechanism of the SFP mixture under different conditions. Under the fatigue load, the weak position of the SFP mixture at a low temperature is asphalt binder and its interface with other materials, while at medium and high temperatures, the weak position of the SFP mixture is inside the grouting material. The research provides a basis for the calculation of the service life of the SFP structure, provides a reference for the improvement direction of the SFP mixture composition and internal structure.     

A Novel Model of Ultrasonic Fatigue Test in Pure Bending

The very high cycle fatigue (VHCF) failure of in-service components is mainly caused by the vibration of thin-wall elements at a high frequency. In this work, a novel model of ultrasonic fatigue test was developed to test thin-wall material in bending up to VHCF with an accelerated frequency. The theoretical principle and finite element analysis were introduced for designing a sample that resonated at the frequency of 20 kHz in flexural vibration. In the advantage of the second-order flexural vibration, the gauge section of the sample was in the pure bending condition which prevented the intricate stress condition for thin-wall material as in the root of cantilever or the contact point of three points bending. Moreover, combining the constraint and the loading contact in one small section significantly reduced heating that originated from the friction at an ultrasonic frequency. Both strain gauge and deflection angle methods were applied to verify the controlling of stress amplitude. The fractography observation on Ti6Al4V samples indicated that the characterized fracture obtained from the novel model was the same as that from the conventional bending test.     

Torsional Fatigue Life Prediction of 30CrMnSiNi2A Based on Meso-Inhomogeneous Deformation

In this paper, torsional fatigue failure of 30CrMnSiNi2A steel which exhibited non-Masing behavior was studied under different constant shear strain amplitudes, using thin-walled tubular specimens. The relationship between shear fatigue and the evolution of meso-deformation inhomogeneity and the prediction method of the torsional fatigue life curve were investigated. Shear fatigue of the material under constant amplitude was researched by numerical simulation with reference to tests, by using crystal plasticity of polycrystalline representative volume element (RVE) as the material model. Considering the non-Masing behavior of material, when determining the parameter values of the crystal plasticity model the correlation between these parameters and strain amplitude was taken into account. The meso-deformation inhomogeneity with increments in the number of cycles was characterized by using the statistical shear strain standard deviation of RVE as the basic parameter. Considering the effect of strain amplitude on fatigue damage, ratio cycle peak stress/yield stress was taken as the weight to measure the torsional fatigue damage and an improved fatigue indicator parameter (FIP) to measure the inhomogeneous deformation of the material was proposed. The torsional fatigue life curve of 30CrMnSiNi2A steel was predicted by the critical value of the FIP and then the result was confirmed.     

Crack Initiation Mechanism and Life Prediction of Ti60 Titanium Alloy Considering Stress Ratios Effect in Very High Cycle Fatigue Regime

Ultrasonic fatigue tests were performed on Ti60 titanium alloy up to a very high cycle fatigue (VHCF) regime at various stress ratios to investigate the characteristics. The S-N curves showed continuous declining trends with fatigue limits of 400, 144 and 130 MPa at 10⁹ cycles corresponding to stress ratios of R = −1, 0.1 and 0.3, respectively. Fatigue cracks found to be initiated from the subsurface of the specimens in the VHCF regime, especially at high stress ratios. Two modified fatigue life prediction models based on fatigue crack initiation mechanisms for Ti60 titanium alloy in the VHCF regime were developed which showed good agreement with the experimental data.     

Assessment of the Post-Cracking Fatigue Behavior of Steel and Polyolefin Fiber-Reinforced Concrete

Some types of fiber-reinforced concrete (FRC) such as steel fiber-reinforced concrete (SFRC) or polyolefin fiber-reinforced concrete (PFRC) are suitable for structural uses but there is still scarce knowledge regarding their flexural fatigue behavior. This study aimed to provide some insight into the matter by carrying out flexural fatigue tests in pre-cracked notched specimens that previously reached the Service Limit State (SLS) or the Ultimate Limit State (ULS). The fatigue cycles applied between 30% and 70% of the pre-crack load at 5 Hz until the collapse of the material or until 1,000,000 cycles were reached. The results showed that the fatigue life of PFRC both at SLS or ULS was remarkably higher than the correspondent of SFRC. The fracture surface analysis carried out found a linear relation between the fibers present in the fracture surface and the number of cycles that both SFRC and PFRC could bear.     

Static and Flexural Fatigue Behavior of GFRP Pultruded Rebars

This paper presents the experimental results of composite rebars based on GFRP manufactured by a pultrusion system. The bending and radial compression strength of rods was determined. The elastic modulus of GFRP rebars is significantly lower than for steel rebars, while the static flexural properties are higher. The microstructure of the selected rebars was studied and discussed in light of the obtained results—failure processes such as the delamination and fibers fracture can be observed. The bending fatigue test was performed under a constant load amplitude sinusoidal waveform. All rebars were subjected to fatigue tests under the R = 0.1 condition. As a result, the S-N curve was obtained, and basic fatigue characteristics were determined. The fatigue mechanism of bar failure under bending was further analyzed using SEM microscopy. It is worth noting that the failure and fracture mechanism plays a crucial role as a material quality indicator in the manufacturing process. The main mechanism of failure under static and cyclic loading during the bending test is widely discussed in this paper. The results obtained from fatigue tests encourage further analysis. The diametral compression test reflects the weakest nature of the composite materials based on the interlaminar compressive strength. The proposed methodology allows us to invariantly describe the experimental transversal strength of the composite materials. Considering the expected durability of the structure, the failure mechanism is likely to significantly improve their fatigue behavior under the influence of cyclic bending. The reasonable direction of searching for reinforcements of composite structures should be the improvement of the bearing capacity of the outer layers. In comparison with steel rebars (fatigue tensile test), the obtained results for GFRP are comparable in the HCF regime. It is worth noting that in the near fatigue endurance regime (2–5 × 10⁶ cycles) both rebars exhibit similar behavior.