Cyclic Fatigue Loading Research Articles

Fatigue lifetime predictions of notched specimens based on the critical distance and stress concentration factors

The theory of critical distances is used for evaluation of effects of notches on load-bearing capacity of notched components under static or cyclic fatigue loading. The theory can also be used to predict the fatigue life of notched components. One of the assumptions for a reliable prediction is that the critical distance is independent of the notch geometry and its stress concentration level. The study shows that the critical distance depends not only on the number of cycles to failure (under fatigue loading), but also on the notch radius. In this case, direct use of the critical distance leads to unreliable predictions. The study quantifies the relation between the model notch (from which the critical distance is determined) and the predicted notch by means of the ratio of their stress concentration factors. The predictions modified by the ratio of stress concentration factors provide results that are in satisfactory agreement with the experimental data. The predictions were made and verified on the basis of fatigue data measured on two stainless steels 1.4306 and 1.4307, and high strength structural steel S690QL. Smooth and notched specimens were tested on an ultrasonic fatigue machine in a symmetrical tension–compression mode. The results were evaluated in the regions of high cycle and very high cycle fatigue.

Optimization of Design Parameters for Improved Buoy Reliability in Wave Energy Converter Systems

Wave energy converters are frequently subjected to cyclic fatigue loads, making them prone to structural failure. This study presents a comprehensive design for reliability analysis of buoy structures used in ocean energy converters. A finite element model (FEM) was developed using ABAQUS to evaluate the effects of different materials—linear low-density polyethylene (LLDPE) versus high-density polyethylene (HDPE)—as well as variations in rib spacing and structural thickness under uniform pressure conditions. The analysis considered configurations with 3, 5, and 7 ribs, and wall thicknesses of 0.5, 0.7, and 1 inch. Results indicated that increasing the number of ribs and wall thickness significantly reduces deflection and von Mises stress, enhancing structural stability. HDPE demonstrated superior strength and lower deflection compared to LLDPE, although with reduced ductility. This study provides critical insights into optimizing buoy design parameters to improve the structural performance and durability of wave energy converter buoys, ensuring their reliability and longevity in harsh marine environments.

Strain distribution of a fir‐tree tenon/mortise structure under combined high and low cycle fatigue loads

AbstractThe fir‐tree tenon/mortise structure in areo‐engine suffers from the combined high and low cycle fatigue (CCF) loads during service. The structural integrity of the tenon/mortise structure is significantly affected by the local strain distribution, while the strain value under the CCF loads is difficult to acquire. In this study, a simulated specimen of tenon/mortise structure is designed, and the CCF test is carried out using a divided‐path loading fixture to avoid interference between the high cycle fatigue (HCF) loads and the low cycle fatigue (LCF) loads. The high speed digital image correlation (DIC) method is adopted to acquire the real‐time strain distribution during CCF test. According to the strain results, the critical position of the mortise and its strain variation are determined. The high cycle strain of the mortise is proportional to the vibration amplitude of the tenon. The experimental results can provide basis for strength and life assessment.

Plastic Shakedown Behavior and Deformation Mechanisms of Ti17 Alloy under Long Term Creep–Fatigue Loading

Ti17 alloy is mainly used to manufacture aero-engine discs due to its excellent properties such as high strength, toughness and hardenability. It is often subjected to creep–fatigue cyclic loading in service environments. Shakedown theory describes the state in which the accumulated plastic strain of the material stabilizes after several cycles of cyclic loading, without affecting its initial function and leading to failure. This theory includes three behaviors: elastic shakedown, plastic shakedown and ratcheting. In this paper, the creep–fatigue tests (CF) were conducted on Ti17 alloy at 300 °C to study its shakedown behavior under creep–fatigue cyclic loading. Based on the plasticity–creep superposition model, a theory model that accurately describes the shakedown behavior of Ti17 alloy was constructed, and ABAQUS finite element software was used to validate the accuracy of the model. TEM analysis was performed to observe the micro-mechanisms of shakedown in Ti17 alloy. The results reveal that the Ti17 alloy specimens exhibit plastic shakedown behavior after three cycles of creep–fatigue loading. The established finite element model can effectively predict the plastic shakedown process of Ti17 alloy, with a relative error between the experimental and simulation results within 4%. TEM results reveal that anelastic recovery controlled by dislocation bending and back stress hardening caused by inhomogeneous deformation are the main mechanisms for the plastic shakedown behavior of Ti17 alloy.

Fatigue life prediction of metal materials under random loads based on load spectrum extrapolation

The impact of random loading on the fatigue life of mechanical components contains numerous uncertainties. This study aims to enhance the precision of life prediction by developing mean and amplitude distribution models based on the road load spectrum and extrapolating the operational load beyond the threshold value in the time domain. Additionally, it considers the influence of high cycle fatigue (HCF) and low cycle fatigue (LCF) loads on the initiation life of metal cracks. To address this, a load correction factor is introduced, and critical stresses for high and low cycle loads are proposed. Furthermore, a two-dimensional programmed load spectrum is prepared, taking into account the mean and amplitude. The accuracy and rationality of the fatigue life model are validated through examples using 45# steel and 316L, while experiments on engine support are conducted to confirm the effectiveness and accuracy of the over-threshold extrapolation life prediction method under random loading.

Experimental investigation of fracture mechanism in organically modified silica-based coating applied on austenitic stainless steel AISI 904L under monotonic and very high cycle fatigue loading

Surface morphology significantly influences the fatigue life of the metallic materials. Therefore, improving surface performance is important for economic and safety applications. Silica-based coatings are anticipated to protect the metallic materials under synergistic operating conditions, such as fatigue and corrosion. However, pure inorganic silica-based coatings are brittle and fail to protect metallic materials. The properties of the sol–gel coatings can be determined by optimizing synthesis factors, such as synthesis steps, precursors, solvents, and catalysts. Organically modified silica-based sol–gel coatings were synthesized using 3-glycidoxypropyl trimethoxysilane and 3-aminopropyltriethoxysilane as precursors, with ethanol as the solvent. The resulting coating was named EtOH. This coating was applied on the surface of stable austenitic stainless steel AISI 904L with the dip coating method. The fracture behavior of the coating during monotonic loading has been studied by interrupted tensile tests at different load steps and examined closely with scanning electron microscopy. On the other hand, very high cycle fatigue experiments were conducted using an ultrasonic fatigue testing system at load frequency of 20 kHz on coated specimens to investigate the fracture behavior of the coating during cyclic loading. Three different kinds of fracture due to stress relaxation in the coating and at its interface with substrate were observed in monotonic loading. The same kinds of stress relaxation were observed in cyclic loading with the difference being that the range of experienced elongation in cyclic tests was significantly smaller. Therefore, the fracture in cyclic loading were concentrated in the vicinity of macro crack. The application of organically modified coatings could suppress the persistent slip bands and result in increased fatigue life for coated specimens.

Fatigue behavior of RC beams strengthened with iron-based shape memory alloy rebars

This paper presents the experimental research results on the fatigue behavior of reinforced concrete (RC) beams integrated with prestress using iron-based shape memory alloys (Fe-SMAs). Four RC beams reinforced with Fe-SMA rebars as tensile members were fabricated for the fatigue experiments. The experimental variables included the type of load (static and fatigue) and the size of the fatigue load (40 %, 50 %, and 60 % of the maximum load). Before the experiments, prestress was introduced into the Fe-SMA rebars through electric resistance heating. The minimum load for the fatigue test was set at 20 % of the maximum load of the control beam, and the tests were conducted at a frequency of 2 Hz. As the number of fatigue load cycles increased, all specimens exhibited an increasing trend in deflection and strain. The specimens subjected to fatigue loads of 50 % and 60 % of the maximum load failed due to the crushing of the compression concrete and the fracture of the Fe-SMA rebars, respectively. In contrast, the specimen subjected to 40 % of the maximum load showed a similar load resistance capacity to the control beam even after enduring 2 million cycles of fatigue load.

Strength degradation and damage mechanism of TA1 titanium alloy clinched joints under fatigue loading

A digital model for the strength degradation of titanium alloy clinched joints was established using strength degradation tests and a fatigue cumulative damage model. This model aims to investigate the strength degradation law and fatigue damage failure mechanism of titanium alloy clinched joints under fatigue loading. Fractured test specimens were scanned to analyse the fatigue damage failure mechanism by examining the evolution of fracture morphology under varying fatigue cyclic loading times. The results demonstrate that the power index degradation model accurately predicts the strength degradation of titanium alloy clinched joints. As the number of fatigue loading cycles increases, cleavage features emerge alongside fatigue cracks, leading to a gradual reduction in joint strength. Ultimately, the strength degradation power index model for titanium alloy clinched joints is verified by the test to have high accuracy in predicting residual strength.

Feasibility biomechanical study of injectable Biphasic Calcium Phosphate bone cement augmentation of the proximal femoral nail antirotation (PFNA) for the treatment of two intertrochanteric fractures using cadaveric femur

This study evaluated the feasibility of the femoral bone after fixation using biphasic calcium phosphate cement-augmentation of the proximal femoral nail antirotation (PFNA) compared with PFNA without cement. This study presented to compare the stiffness, fatigue testing, and compressive strength between stable (AO31-A2.1) and unstable (AO31-A3.3) intertrochanteric fractures treated by cement augmented PFNA of the cadaveric femoral. Biphasic calcium phosphate cement was injected to align and compatible with PFNA and the reconstructive procedure was monitored the cement placement using x-ray imaging during operation. The testing demonstrated that the cement could be injected through a small needle (13 G, 16 cm length, 1.8 mm inner diameter) within a suitable operating time. The feasibility study of the biomechanical testing was divided into three tests: stiffness test, fatigue cyclic load, and compression test. The results showed that the cement-augmented specimens exhibited higher stiffness than the control specimens without cement. The cement-augmented specimens also showed lower strain energy during the fatigue test, resulting in higher compressive strength (4730.7 N) compared to the control specimens (3857.4 N). There is a correlation between BMD and fracture load and the increase in compression load of the cement-augmented femoral compared to the controls as well as an increase in strain energy of fatigue cyclic testing was found. Biphasic calcium phosphate cement-augmented of the PFNA biomechanically enhanced the cut-out resistance in intertrochanteric fracture. This procedure is especially efficient for unstable intertrochanteric fracture suggesting the potential benefits of using biphasic calcium phosphate cement in medical applications.

Fatigue and failure mode analyses of glass infiltrated 5Y-PSZ bonded onto dentin analogues

The purpose of this study was to evaluate the fatigue survival of 5Y-PSZ zirconia infiltrated with an experimental glass and bonded onto dentin analogues. Disc-shaped specimens of a 5Y-PSZ (Katana UTML Kuraray Noritake) were cemented onto dentin analogs (NEMA G10) and divided into four groups (n = 15): Zctrl Group (control, without infiltration); Zglz Group (Glaze, compression surface); Zinf-comp Group (Experimental Glass, compression surface); Zinf-tens Group (Experimental Glass, tension surface). Surface treatments were varied. Cyclic fatigue loading, oblique transillumination, stereomicroscope examination, and scanning electron microscopy were performed. Fatigue data were analyzed (failure load and number of cycles) using survival analysis (Kaplan–Meier and Log-Rank Mantel–Cox). There was no statistically significant difference in fatigue survival between the Zglz, Zctrl, and Zinf-comp groups. The Zinf-tens group presented a significantly higher failure load when compared to the other groups and exhibited a different failure mode. The experimental glass effectively infiltrated the zirconia, enhancing structural reliability, altering the failure mode, and improving load-bearing capacity over more cycles, particularly in the group where the glass was infiltrated into the tensile surface of the zirconia. Glass infiltration into 5Y-PSZ zirconia significantly enhanced structural reliability and the ability to withstand loads over an increased number of cycles. This approach has the potential to increase the durability of zirconia restorations, reducing the need for replacements and save time and resources, promoting efficiency in clinical practice.

Mechanism of concrete damage under the coupled action of freeze-thaw cycle and low-stress impact fatigue load：From pore structure to energy dissipation

The interaction mechanism behind concrete damage induced by the combined effects of freeze-thaw cycles and fatigue load (FTF) remains insufficiently understood. This study aims to shed light on this mechanism by employing three sets of specimens, each subjected to different conditions: freeze-thaw cycles alone, low-stress impact fatigue (LIF) load alone, and FTF coupled action. Macro performance and microstructural changes of these specimens were measured after each testing round to analyze the evolution of concrete damage. Additionally, the influence of load duration on the damage under the coupled action was also explored. Results indicated that under FTF action, when LIF loading is applied for a short duration, freeze-thaw cycles play a dominant role in concrete damage, while the influence of LIF loading is minimal. However, when LIF loading is sustained for an extended period coupled with freeze-thaw cycles, it can accelerate concrete degradation. This can be attributed to the accumulation of micro-cracks during longer loading duration, eventually manifesting as visible cracks. These cracks link adjacent pores and increase the number of interconnected pores. This, coupled with freeze-thaw damage increasing the probability of crack forming, results in accelerated deterioration of concrete. The mechanism underlying this concrete deterioration was also analyzed from an energy dissipation prospect.

Effect of Fe-Al intermetallics on fatigue properties of aluminum to steel dissimilar spot welds

Lightweight structures made of aluminum alloys and high strength steels are increasingly used for improving vehicle fuel efficiency. One barrier for joining aluminum and steel is the formation of brittle Fe-Al intermetallics that can have deleterious effects on joint ductility and strength under static loading conditions. However, it is unclear how the intermetallics affect the joint fatigue properties. In this study, the effect of intermetallics on fatigue properties of dissimilar metal joints between a 6xxx aluminum alloy and a high-strength steel is investigated on welds created by ultrasonic interlayered resistance spot welding (Ulti-RSW). Joint efficiency calculations are used to normalize the peak load so the fatigue properties of Ulti-RSW are compared with a wide range of joining processes in the literature. During fatigue testing the failure mode transitions from interfacial or button pull-out failure during low cycle fatigue loading (below 100,000 cycles) to through-thickness failure during high cycle (above 100,000 cycles). Based on the stress intensity factors calculated using a simple 2D fracture mechanics model, the transition in failure mode is attributed to the plastic deformation near the notch tip. Overall, this study shows that the intermetallics have little effect on the high-cycle fatigue properties of dissimilar spot joints.

Visualization of fatigue load cycle numbers using a glass/carbon hybrid composite sensor

A sensor for visualizing the fatigue load cycles was designed, fabricated, and tested. The sensor is made of a glass/carbon hybrid composite and utilizes the delamination length at the glass/carbon interface as an indicator for fatigue cycles. Appropriate design parameters were obtained by performing finite element analysis on the delamination development at the interface between the glass and carbon layers. Hybrid sensors with different carbon layer thicknesses were manufactured, attached to glass/epoxy substrates, and tested under fatigue loading. The predicted results based on the Paris law for crack extensions in one configuration are compared with the experiments for a different configuration to illustrate the efficacy of the approach.

Numerical and experimental study of new fully prefabricated ballastless track

Reducing the workload of cast-in-place construction of ballastless tracks can effectively reduce diseases. This study designed the limiting structures of the prefabricated slabs and base plates. A new fully prefabricated ballastless track was proposed. And the track limiting capacity tests and fatigue tests were carried out to verify its service performance. Results indicated that it is necessary to insert two limiting steel bars into the grouting holes for slab limitation, and double piles with a diameter of 0.2 m for base plate limitation. The field tests demonstrated it could withstand large loads from freight trains, and the limiting capabilities of limiting structures were satisfactory. The track structure showed no fatigue damage after 3 million fatigue load cycles. This research can provide insights for low-damage and high-efficiency construction of ballastless tracks for new lines.

Dose-dependent effects of gamma radiation sterilization on the collagen matrix of human cortical bone allograft and its influence on fatigue crack propagation resistance

Fatigue crack propagation resistance and high-cycle S–N fatigue life of cortical bone allograft tissue are both negatively impacted in a radiation dose-dependent manner from 0 to 25 kGy. The standard radiation sterilization dose of 25–35 kGy has been shown to induce cleavage of collagen molecules into smaller peptides and accumulation of stable crosslinks within the collagen matrix, suggesting that these mechanisms may influence radiation-induced losses in cyclic fracture resistance. The objective of this study was to determine the radiation dose-dependency of collagen chain fragmentation and crosslink accumulation within the dose range of 0–25 kGy. Previously, cortical bone compact tension specimens from two donor femoral pairs were divided into four treatment groups (0 kGy, 10 kGy, 17.5 kGy, and 25 kGy) and underwent cyclic loading fatigue crack propagation testing. Following fatigue testing, collagen was isolated from one compact tension specimen in each treatment group from both donors. Radiation-induced collagen chain fragmentation was assessed using SDS-PAGE (n = 5), and accumulation of pentosidine, pyridinoline, and non-specific advanced glycation end products were assessed using a fluorometric assay (n = 4). Collagen chain fragmentation increased progressively in a dose-dependent manner (p < 0.001). Crosslink accumulation at all radiation dose levels increased relative to the 0 kGy control but did not demonstrate dose-dependency (p < 0.001). Taken together with our previous findings on fatigue crack propagation behavior, these data suggest that while collagen crosslink accumulation may contribute to reduced notched fatigue behavior with irradiation, dose-dependent losses in fatigue crack propagation resistance are mainly influenced by radiation-induced chain fragmentation.

Design, simulation and energy dissipation evaluating of a new lead shear damper

This study presents a novel design for a lead shear damper (LSD) that features lead blocks encased in laminated steel and rubber plates, intersected by X-shaped steel sticks. It combines the excellent energy dissipation and fatigue resistance of lead with the uniform yielding capabilities of X-shaped steel sticks along their height. Cyclic loading testing and fatigue tests are conducted in order to determine the mechanical properties of the LSD. The test results reveal that the LSD has a variable yielding force, and exhibits stiffness hardening when the damper's deformation direction deviates from the origin. However, when the deformation direction returns to the origin, there is no stiffness hardening observed, and the resistant force remains constant. Furthermore, this LSD can work effectively despite the failure of energy dissipation parts, which improves hysteretic energy dissipation under large deformation. In order to replicate the hysteretic behaviour, a uniaxial constitutive model is formulated and its parameters are determined by the utilization of the differential evolution approach, based on test data. The observed strong concurrence between the experimental and simulated hysteresis curves serves as compelling evidence for the efficacy of the differential evolution method. Meanwhile, a three-story building from the third-generation benchmark problem is employed as the testbed structure to examine the energy dissipation capacity of the developed dampers. Nonlinear seismic analysis on the benchmark model equipped with the proposed dampers is also conducted, and the results are compared to those fitted with conventional dampers, such as mild dampers and friction dampers. The findings of this study indicate that the performance of the LSD in reducing structural displacements is superior to that of mild dampers and friction dampers.

An approach to estimate the low cycle fatigue probabilistic curves of PBF-LB/M 316L steel from small size datasets using the remora optimization algorithm

The significant dispersion in fatigue properties is a prevalent attribute observed in metals manufactured by the Additive Manufacturing (AM). To ascertain the safety of components subjected to low cycle fatigue (LCF) loadings at a temperature of 550 °C, it is necessary to carry out a fatigue reliability assessment of AM metals. Nevertheless, the number of LCF life tests is limited owing to time-consuming nature of test and high total production cost of AM materials. Such a small size dataset cannot satisfy the criteria of the traditional reliability assessment method. This work aims to develop a novel method to evaluate the LCF reliability of 316L stainless steel manufactured by laser-based powder bed fusion of metals (PBF-LB/M) technology. Firstly, the Weibull model is coupled with the Coffin-Manson-Basquin relationship in order to characterize the dispersion of life data points at various strain levels. Secondly, reliability results from a small size dataset show a maximum relative error in fatigue life of less than 25 %, which is similar to results from a large size dataset and the traditional method using fatigue data from public sources. Among this step, the remora optimization algorithm is firstly applied to calculate the characteristic parameters of newly-proposed method. Thirdly, the LCF reliability of PBF-LB/M 316L based on the small size dataset is evaluated. The N95% is equivalent to 44 % of N50%. Finally, the PBF-LB/M 316L is compared with traditional 316L considering the reliability. The LCF properties of PBF-LB/M 316L is similar to the traditional 316L at 50 % reliability, however it’s slightly worse at 95 % reliability.

Low-cycle fatigue behaviour of ribbed 1.4362 duplex stainless steel reinforcement

To investigate the fatigue behaviour of stainless steel (SS) reinforcement, monotonic tensile and cyclic fatigue loading tests on 1.4362 duplex SS bars with diameters of 12 mm, 16 mm, and 20 mm are conducted. To keep the characteristics of reinforcement used in engineering construction, the specimens remain unprocessed. Two loading schemes for cyclic tests are considered in this work: (i) constant strain-amplitude, and (ii) variable strain-amplitude. The Ramberg-Osgood model was used to fit the experimental results. The strain-based and energy-based fatigue life equations are obtained. The strain-based fatigue life equation is compared with the present fatigue life estimation equations. Moreover, the ReinforcingSteel material model in OpenSees is calibrated with the experimental data. The results show that SS bars under cyclic experience hardening followed by softening. The bar diameter does not significantly affect the fatigue life prediction. The modified fatigue life equation suggests that SS bars have better fatigue resistance than conventional steel (CS) bars. The numerical analysis indicates that the calibrated steel material model (i.e. ReinforcingSteel) can produce a good simulation of SS bars under cyclic loading. It can improve the accuracy of numerical models of concrete components reinforced by SS bars.

Fatigue life and behaviour of ribbed GFRP reinforced concrete beams

Despite the growing adoption of glass-fibre reinforced polymer (GFRP) as an alternative to steel reinforcement, there is limited research addressing the fatigue performance of GFRP bars in reinforced concrete structures. Existing literature presents contradictory experimental data and conclusions regarding the fatigue behaviour of GFRP rebars embedded in concrete. This study investigates the fatigue life of ribbed GFRP bars embedded in concrete beams through an experimental program, considering factors such as concrete strength and fatigue stress levels. Additionally, it introduces a testing protocol utilizing a displacement-controlled scheme to conduct fatigue testing, addressing many issues associated with force-controlled fatigue testing. Furthermore, the paper discusses cracking behaviour, deflection, and slippage, providing insights into the interaction between GFRP rebars and concrete under fatigue loading. The results of this study demonstrated that ribbed GFRP bars can withstand 2 million cycles of fatigue loading at a 40% stress ratio. The obtained fatigue life exceeds findings in existing literature, emphasizing the impact of stress ratio and bar surface profile on fatigue performance.

Mechanical Behavior of Dental Restorations: A Finite Element Pilot Study of Implant-Supported vs. Multiunit-Supported Restorations

Implant-supported-screw-retained prostheses are highly popular. Some of the most frequent complications are connected with the mechanical properties of the fixing elements. These include abutment screw loosening or even screw fracture. Using an intermediate abutment can offer several advantages. However, few studies detail how this affects the mechanical behavior of dental restorations. This study focuses on understanding the mechanical behavior of implant-supported restorations with a transepithelial component compared to direct implant-supported restoration. It was carried out using the finite element method (FEM) and was experimentally validated. The results showed that in the case of transepithelial-supported restoration, the prosthetic screw mounted over the transepithelial component suffered higher stress than the one screwed directly into the implant. After applying a cyclic fatigue load, it was experimentally proven that, in the transepithelial-supported restorations, the fuse changed from being the screw that went into the implant to being the upper one. In conclusion, we can state that the use of an intermediate abutment in dental restoration not only provides better protection for the rest of the dental restoration but also allows for easier repair in the event of a fracture. This can potentially lead to more efficient procedures and improved patient outcomes.