Fatigue Test Method Research Articles

Assessing the Very High Cycle Fatigue Behavior and Frequency Effect of Structural Steel Welds

ABSTRACTThe very high cycle fatigue behavior of S275J2+N flux‐core arc welded joints was investigated using the ultrasonic fatigue testing method at a loading frequency of 20 kHz. A bespoke specimen design featuring the weld toe was employed to more suitably represent in‐service welded joints than typical ultrasonic fatigue testing specimens. This revealed that fracture occurs above 10 million cycles, beyond the classically accepted fatigue limit. Additionally, comparative fatigue tests were performed at 10 Hz to investigate the frequency effect on the fatigue behavior. A 35% increase in fatigue strength was measured at 20 kHz when compared to 10 Hz, indicating that a correction must be considered in the use of ultrasonic fatigue testing data for design purposes. Fracture surfaces for both test frequencies showed similar morphologies and typical characteristics for fatigue failures of ductile metals.

Investigation of indoor and field tests on asphalt pavement with inverted asphalt layers based on the vertical vibration compaction method

An inverted asphalt pavement is created by reversing the sequence of the lower and middle layers in a conventional asphalt pavement. The lower layer is composed of material with larger particle size and lower asphalt content, which improves its ability to withstand deformation caused by rutting. On the other hand, the middle surface has a higher asphalt content, specifically designed to resist fatigue cracking. This paper examines the mechanical response of two pavement structures and investigates the potential of two measures, inverted asphalt pavement and asphalt mixture design by vertical vibration compaction method (VVCM), in reducing stresses and stress levels in asphalt pavements. Additionally, a large thickness rutting and fatigue test method was developed to study the rutting resistance and fatigue life of the pavement structures, and to construct rutting deformation and fatigue life prediction models. Finally, test sections were paved to verify the feasibility of the inverted pavement and VVCM materials. The findings show that inverted pavement and VVCM materials have a minimal impact on pavement stress, but can reduce pavement shear and tensile stress levels by up to 18% to 25%. Furthermore, inverted pavement and VVCM materials have positive effects on improving the rutting resistance and fatigue life of asphalt pavements.

Characteristics evaluation of TFTs method for fatigue testing of ultra-large offshore wind turbine blade

Characteristics evaluation of TFTs method for fatigue testing of ultra-large offshore wind turbine blade

A novel fatigue test method for bitumen-stone combinations under cyclic tension-compression loading

A novel fatigue test method for bitumen-stone combinations under cyclic tension-compression loading

Study of Vibration‐Ultrasonic Combined Fatigue on 7075‐T6 Aluminum Alloy

ABSTRACTComponents in aero‐engine are likely excited in multiple resonance models in dynamic loads. This paper proposes an accelerated fatigue testing method that combines the vibration test with ultrasonic loading to develop a feasible experimental system for combined cycle fatigue (CCF) and explore the CCF characteristics in the high cycle fatigue (HCF) associated with very high cycle fatigue (VHCF) conditions. A thin plate specimen of 7075‐T6 aluminum alloy with two natural frequencies of 2 and 20 kHz is designed for the experiment. The stress waveform and distribution during real‐time monitoring provide validation for the method. Finally, the influences of composite loads are demonstrated by exploring the characteristics of the S–N curves and fracture morphology. The results suggest that the increase in both axial and bending stress markedly diminishes fatigue life, while the difference in stress levels under combined loading is revealed through variations in fatigue striation morphology.

Fatigue Scattering Analytics and Prediction of SS 316L Fabricated by Laser Powder Bed Fusion

Abstract Laser powder bed fusion (LPBF) is an enabling process manufacture of complex metal components. However, LPBF is prone to generate geometrical defects (e.g., porosity, lack of fusion), which causes a significant fatigue scattering. However, LPBF fatigue scattering data and analysis in the literature are not only sparse and limited to tension-compression mode but also inconsistent. This article presents a robust high-frequency fatigue testing method to construct stress-cycle curves of SS 316L to understand the scattering nature and predict the scattering pattern. A series of bending fatigue tests are performed at different stress amplitudes. Two different runout criteria are used to investigate fatigue life, fatigue limits, and scattering. The endurance limit reaches around 300 MPa for the defect size distribution at the selected process space. The defect size-based fatigue limit model is found to underestimate the endurance limit by about 30 MPa when comparing with the experimental data. Fatigue scattering is further calculated by using 95% prediction intervals, showing that low fatigue scattering is present at high stresses while a large variation of fatigue life occurs at stresses near the knee point.

Fatigue properties of non-press-fitted part of full-scale induction-hardened axles of medium-carbon steel for high-speed railway vehicles

This study aimed to examine the stress–number of cycles (S–N) curve and fatigue limit of non-press-fitted parts to improve the sophistication of the design method of induction-hardened axles for medium-carbon steel high-speed railway cars. Macroscopic cracks were generated in the non-press-fitted parts of the axles by selecting appropriate fatigue test methods. Thus, the S–N curve was approximated, and the fatigue limit was obtained. The value of an index in the power-law expression of the S–N curve was 11, which was proposed to fatigue damage evaluation standard. Moreover, we constructed a prediction model for high-and very-high-cycle fatigue limits based on the local fatigue-limit approach and fatigue test results of cut-out specimens from several depth regions of induction-hardened axles. The fatigue limit predicted by the model agrees with the experimental high-cycle fatigue limit. The model estimated that the fatigue limit did not decrease in the very-high-cycle regime.

Development of the small punch fatigue test method based on the finite element method

The Small Punch Test (SPT) is a method of evaluating the mechanical properties of metallic materials that overcomes the limitations of the Uniaxial Test (UT), a traditional method of testing. Unlike UT, which provides strain data for each stress, SPT provides displacement data for each load. Therefore, SPT must be converted to UT to evaluate the mechanical properties of materials. However, SPT and UT employ disparate loading mechanisms. The difficulty in converting SPT to UT, which stems from the disparate loading mechanisms, has thus far limited SPT to mechanical property evaluation areas such as tensile and creep. This paper, therefore, aims to extend SPT to the fatigue domain, which is currently limited to the tensile and creep domains. The fatigue properties of metallic materials were evaluated based on the Finite Element Method (FEM) for the Small Punch Fatigue Test (SPFT). Moreover, the fatigue properties derived from the FEM for SPFT were converted to Uniaxial Fatigue Test (UFT) by employing the equivalent equation. Finally, an S-N curve was constructed based on SPFT and was validated by comparison with the same curve constructed based on UFT.

A novel bidirectional LSTM network model for very high cycle random fatigue performance of CFRP composite thin plates

This study proposes a novel Double-layer Bidirectional Long Short-Term Memory (BiLSTM) neural network model, shorted as TDA-BiLSTM, which integrates Transfer learning and Attention mechanisms. The model aims to predict the fatigue life of carbon fiber reinforced polymer (CFRP) thin plate structures subjected to very high cycle random vibration fatigue loads. Distinct from conventional servo-hydraulic and ultrasonic fatigue testing methods, this research pioneers the use of a vibration table for very high cycle fatigue (VHCF) testing to fill the gap in the field. By training and validating data across various life ranges, the TDA-BiLSTM model demonstrates significant advantages in training speed and predicting accuracy. Its bidirectional structure and attention mechanism effectively capture complex patterns and long-term dependencies in sequence data. Experimental results indicate that the TDA-BiLSTM model achieves significantly lower Mean Absolute Error (MAE) and Root Mean Square Error (RMSE) compared to Long Short-Term Memory (LSTM) and Long Short-Term Memory with Transfer learning (Tr-LSTM) models across different life ranges on the ε-N curve, indicating higher accuracy and stability in strain life prediction tasks. Additionally, an analysis of typical damage areas using Scanning Electron Microscopy (SEM) reveals the failure mechanisms of CFRP plates under very high cycle vibration fatigue loads.

Utilization of High-Temperature miniaturized testing to Assist in life management for gas turbine Blades: An Overview

Safely operating gas turbine blades in aero-engines or industrial gas turbines under severe thermo-mechanical conditions always poses a continuous challenge. There is an urgent need for more reliable life prediction tools, necessitating earlier and more rigorous assessments of in-service damage. This paper introduces a systematic examination of high-temperature miniature specimen testing method within a practical life management framework. To improve current practices in assessing the condition of service-aged gas turbine blades, a proactive utilization of miniature specimen testing is recommended for early in-service assessment within the component lifecycle and integrated into a comprehensive life assessment/management framework. The paper outlines a structured approach to life management, combining miniature specimen sampling and creep and fatigue testing methods. This forms the basis for a novel, holistic life assessment methodology, which is proposed in this paper, incorporating the innovative use of high-temperature miniature specimen testing.

Accelerated Fatigue Test for Electric Vehicle Reducer Based on the SVR-FDS Method.

The reducer serves as a pivotal component within the power transmission system of electric vehicles. On one hand, it bears the torque load within the power transmission system. On the other hand, it also endures the vibration load transmitted from other vehicle components. Over extended periods, these dynamic loads can cause fatigue damage to the reducer. Therefore, the reliability and durability of the reducer during use are very important for electric vehicles. In order to save time and economic costs, the durability of the reducer is often evaluated through accelerated fatigue testing. However, traditional approaches to accelerated fatigue tests typically only consider the time-domain characteristics of the load, which limits precision and reliability. In this study, an accelerated fatigue test method for electric vehicle reducers based on the SVR-FDS method is proposed to enhance the testing process and ensure the reliability of the results. By utilizing the support vector regression (SVR) model in conjunction with the fatigue damage spectrum (FDS) approach, this method offers a more accurate and efficient way to evaluate the durability of reducers. It has been proved that this method significantly reduces the testing period while maintaining the necessary level of test reliability. The accelerated fatigue test based on the SVR-FDS method represents a valuable approach for assessing the durability of electric vehicle reducers and offering insights into their long-term performance.

Accounting for defect position in ultrasonic fatigue test specimen with heterogeneous stress distribution

To account for or neglect the defect position, i.e. the fatigue initiating defect location, both radially and axially, is evaluated for hourglass-shaped ultrasonic fatigue specimen. The commonly used analytical equations to calculate the stress is compared against a finite element (FE) based approach, which is able to fully considering the stress state at the defect position. Notably, the effects on several common fatigue analyses are evaluated: the fatigue strength distribution, the stress–life and the stress–defect relationships. Fracture mechanical assessment is also performed, for a comprehensive VHCF characterization of the EN-GJS-500-7 ductile cast iron used in the study. The VHCF properties are characterized up to 3⋅108 cycles, using the Step-Stress fatigue testing method under fully reversed loading. The FE-model and Weibull distribution as the choice of fatigue strength distribution, enables size effect evaluation by Weakest-link effective volume with the highly stressed volume method as benchmark. The work shows that it is imperative to use the local stress state at the defect position, as the distribution of failures can diverge largely from the center of the specimen, and that neglecting this causes systematic error and flawed potentially results.

Study on fatigue parameters of high modulus asphalt pavement

High Modulus Asphalt Mixtures (HMAM) have gained widespread utilization in asphalt pavement construction. It is great critical to study the fatigue characteristics of HMAM. At present, various fatigue test methods are employed in indoor testing to assess HMAM under different mechanical states. Therefore, there is a critical problem that the value of fatigue parameters is not unique in the structural design of HMAM. In this study, three fatigue testing methods were initially used to address the above issues: direct tensile testing, indirect tensile testing, and uniaxial compression testing. These tests help to determine the strength yield surface states at different loading rates. And to reveal the effect of loading rate on the mechanical behavior of HMAM. The results show a power function fit and a good correlation between strength and loading rate. The relationship models of strength with speed for indirect tensile, direct tensile and uniaxial compression tests are SI=3.22(v)0.14, SD=3.22(v)0.15 and SU=3.22(v)0.12, respectively.The higher the loading rate for the three fatigue test results, the greater the expansion of the strength yield surface.The fatigue life (Nf) of HMAM underwent testing, yielding its fatigue effective stress ratio. The relationship between the two was and then established. It is found that the point (1, 1) appears on the backextension line, thus effectively complementing the conventional S-N equation. In addition, the fatigue equations for various fatigue test methods were unified based on effective stress strength ratio and the fatigue life. This paper introduces a novel method for the antifatigue design of HMAM.

Design of a continuously repeated impact method with constant amplitude based on Hopkinson bar: Principle and impact fatigue life testing

The lack of reliable testing methods and standards hinders systematic research on the impact fatigue performance of materials. The aim of developing an impact fatigue test method is to generate repeated impact loads with a constant amplitude, which is difficult to achieve using various existing impact test devices. This study investigated the principles of cyclic loading-resetting and constant-amplitude loading based on the split Hopkinson pressure bar (SHPB) system, established a novel impact fatigue test device, and verified the developed method by testing the impact fatigue life of a Ti-6Al-4V alloy. Electromagnetic valves, laser sensors, and a vacuum pump were used in the developed device to launch and reset the striker bar; a servo motor and laser sensors were used to reset the loading bars and specimen, and the entire system was controlled by a programmable controller to achieve continuously repeated loading. To achieve constant-amplitude loading, the conditions required to ensure that the specimen moves away from the incident bar before the returning stress wave reloads the specimen was derived. It was found that a constant-amplitude repeated impact on the specimen could only be achieved through the precise design of the geometric configuration and material of the loading bars. This method is considerably simpler than various existing methods based on energy absorption and is more applicable to impact fatigue tests. The verification tests showed that the highest loading frequency of the device was 0.5 Hz, the loading rate exceeded 105 kN/s, and the amplitude error of the repeated impact loads did not exceed 2.45 %. Under loads with the same amplitude, the impact fatigue life of a Ti-6Al-4V alloy was considerably shorter than the non-impact (low strain rate) fatigue life, which indicates the necessity of investigating the strain rate effect on the fatigue performance of materials.

Fatigue Test Method to Evaluate the 50 Year Durability of Venous Stents

Iliofemoral venous obstructive disease can result in significant, potentially debilitating symptoms that can negatively affect quality of life. Unlike arterial disease, patients with deep venous disease have a significantly lower median age, therefore the need for long term stent patency becomes a matter of decades rather than years. Furthermore, iliofemoral lesions frequently require stent placement across the inguinal ligament. Such stents are subject to dynamic stress from leg movement and associated concerns for device fatigue, resulting in stent fracture. The aim of this study was to describe an invitro 50 year stent fatigue test method designed to assess durability against dynamic stress induced device fracture. Through literature review, cadaver studies, and computer modelling, the most challenging loading was confirmed to be hip flexion across the inguinal ligament. This occurs when the patient adjusts between a seated and standing position. Sit to stand hip flexion at the inguinal ligament was effectively simulated on the bench in this invitro experimental study. When tested under challenge parameters, hip flexion was reliably found to cause fractures in non-venous nitinol stents. However, a dedicated self expanding nitinol venous stent, engineered for improved durability, underwent up to 50 years of simulated loading on the bench with 15% (3/20) of stents experiencing fractures at 50 years, compared with fractures in 35% (14/40) of non-venous stents tested to 1.4 years; no statistical testing was performed as durations do not match and the objective was to demonstrate the test method. The presented fatigue test method is a suitable approach for evaluating the durability of stents intended for venous use. Venous stents demonstrated superior fatigue resistance compared with non-venous stents via invitro hip flexion testing.

Mechanisms affecting the difference in fatigue resistance between welds and base materials were investigated based on in-situ fatigue method

Coiled tubing (CT) is manufactured using plastic processing and weld molding techniques, rendering the weld zone (WZ) susceptible to fatigue fractures under the continuous action of complex alternating loads during field applications. While this risk has garnered significant attention, the underlying mechanisms of its fatigue failure remain elusive. To address this knowledge gap, this study employs an in-situ SEM fatigue test method, coupled with metallographic analysis, hardness testing, SEM observation, and EBSD characterization, to deeply analyze the fatigue microdamage mechanism in the WZ by comparing the fatigue performance of the base material (BM) of the CT with that of WZ. It is found that the microstructure of the BM consists of different scales and shapes of ferrite and a small amount of pearlite nested and intertwined with each other, which has a strong coordinated deformation ability and can effectively relieve stress concentration, delay crack initiation and form a single fatigue source. In contrast, the WZ’s ferrite organization is large, uniform, and independent, making it prone to early-stage plastic deformation with weak inter-grain coordination, resulting in the formation of numerous slip bands and multiple crack sources. During the crack propagation stage, the abundant grain boundaries in the BM effectively deflect the crack expansion direction and slow down its progression; while multiple secondary cracks in the weld expand simultaneously and connect with each other, resulting in a higher crack expansion rate than the BM. The research findings provide crucial insights and guidance for enhancing the overall fatigue performance of CT.

Effect of Wear on Alternating Bending Fatigue Life of 20CrNi2Mo Martensitic Steel

Bending fatigue failures are commonly related to the wear behavior in an active system. The surface wear and plastic deformation of the tribolayer play crucial roles in the wear–bending fatigue behaviors of steels. In particular, the lamellar structure of martensitic steel leads to its unique wear–bending fatigue behavior. In this work, the wear–bending fatigue testing method and device were introduced to explore the wear–bending fatigue behavior of the martensitic steel. The effect of wear on the alternating bending fatigue life of 20CrNi2Mo martensitic steel was studied under low and high fatigue stress. The influence of wear debris on the fatigue life at two different sliding speeds was also analyzed. The results show that the fatigue life decreased with the wear load increased under high bending stress. Moreover, for systems with nanoscale wear debris on the steel surface, the wear–bending fatigue lifetimes are significantly enhanced compared with large wear debris.

A Review of the Studies on the Effect of Different Additives on the Fatigue Behavior of Asphalt Mixtures

The fatigue phenomenon significantly weakens road pavement due to repeated reloading. To enhance fatigue resistance, numerous studies have explored various additives in asphalt mixtures. This review focuses on key variables influencing the effectiveness of additives, including fibers, polymers, nanomaterials, waste materials, and biomaterials, in improving the fatigue performance of asphalt mixtures. The study initially identifies different additives and fatigue testing methods used for asphalt mixtures. It evaluates the impact of factors such as modifier content and size, base asphalt binder type, mixing processes, dispersion behavior, and testing conditions on the fatigue behavior of modified asphalt mixtures. The cost-effectiveness and environmental impact of additive application have also been assessed. Additionally, research gaps and future prospects for modified asphalt mixes are outlined. Existing studies demonstrate the benefits of additives like basalt fiber, polyester fiber, styrene–butadiene–styrene (SBS), nanosilica, crumb rubber, and biooils in enhancing the fatigue life of pavement constructions. However, challenges exist in the application of modifiers due to limited practical implications and insufficient knowledge. Further research is needed on factors such as additives’ dispersity, compatibility, aging resistance, economic viability, and modifying mechanisms in morphological and micromechanical aspects to enhance the fatigue performance of the modified asphalt mixture.

Research on Laboratory Testing Method of Fatigue Performance of Semi-Rigid Base Considering Spatial Stress State

In order to accurately assess the fatigue performance of semi-rigid base layers, this paper proposes a novel fatigue testing method for semi-rigid base layers that takes into account the spatial stress state. Based on this method, the fatigue performance of three types of reinforced semi-rigid base-layer materials (steel wire mesh, plastic geogrid, and fiberglass) was tested and compared with unreinforced materials. The fatigue strain evolution patterns of these materials were analyzed, and a fatigue strain characteristic value at the limit state was proposed as an evaluation index for the fatigue failure of base layer materials. The results showed that in terms of fatigue performance, plastic geogrid > steel wire mesh > fiberglass > unreinforced specimens. The development of tensile strain can be approximately classified into a three-stage growth pattern, consisting of “curve + straight line + curve”. For the unreinforced specimens, the three stages of bending strain accounted for 10%, 70%, and 20% of the total fatigue life, respectively. The fatigue failure stages of the three types of reinforced materials had similar proportional ranges, representing 5%, 75%, and 20% of the total fatigue life, respectively. The fatigue strain characteristic values for plastic geogrid, steel wire mesh, fiberglass, and unreinforced specimens were 371 με, 280 με, 280 με, and 195 με, respectively. In summary, the use of reinforced materials within semi-rigid base layers enhances their fatigue performance, providing new insights and methods for extending the service life of road surfaces and offering scientific guidance for the practical application of reinforced materials in semi-rigid base layer road surfaces’ fatigue performance.

Infrared thermography applied to accelerated evaluation of the fatigue strength of structural elements made of composites

When designing structures made of composite materials, it is necessary to ensure their fatigue strength. This requires experimental data on fatigue resistance characteristics. The standard approach to determining these characteristics involves lengthy and expensive experimental work. In the case of composite materials, it is necessary to carry out fatigue tests of expensive full-scale structures or standard elements. This paper proposes an accelerated method of fatigue testing based on infrared thermography. It is based on the effect of self-heating of the material under cyclic loads exceeding the fatigue limit. In the study, this method is applied to a full-scale structural element, namely a shell made of laminated carbon fiber with a flange. A sample cut from the shell is subjected to block cyclic loading on a vibration stand. Self-heating is recorded during loading by means of an infrared camera. The obtained fatigue limit estimates are consistent with the results of standard tests.