Structural Fatigue Research Articles

Full-scale resonant bending fatigue testing of casing joints under bending moment load

The quality and reliability of oil casings play a pivotal role in the safety and efficiency of offshore oil and gas exploration. This study conducted research on resonant bending fatigue of 9-5/8-inch casing joints and established a systematic theoretical model for resonant bending fatigue based on the Euler–Bernoulli beam equation. The relationships between parameters affected by frequency and mode shape functions were deduced and solved to analyze the influence of wall thicknesses, length and weights at both ends of the specimen on the natural frequency of the resonant bending fatigue system. A full-scale fatigue test was then designed and conducted to investigate the circumferential stress distribution, reveal the stress amplitude–frequency response characteristics of the specimen, and explore resonance response behaviors under different excitation forces. This was done to establish a method for evaluating the oil casing life and obtain fatigue life characteristic curves of the specimen. A finite element model was employed to analyze the internal stress distribution of the coupling under bending moment load, considering the influence of material properties, thread lead angle and non-linear contact on casing joints. Finally, the fatigue life algorithm was summarized and compared by integrating the stress simulation results with the material properties of the specimen, elucidating the fatigue damage evolution process of casing joints. This research provides a significant engineering reference value for structural design optimization, fatigue testing and life analysis of oil casing threaded joints.

Predicting degraded lifting capacity of aging tower cranes: A digital twin-driven approach

Aging tower cranes face an elevated risk of failure, primarily due to structural fatigue and deterioration. Surprisingly, the degradation of aging-induced lifting capacity (LC) remains an unexplored domain. In response to this research gap, this paper introduces a digital twin-driven (DTD) framework and model to predict the degraded LC of aging tower cranes. This framework combines theoretical and numerical analysis of fatigue and degradation behavior in tower cranes with real-time vibration data obtained during cyclic load scenarios on the actual cranes. Machine learning (ML) techniques are employed to develop a model that accurately predicts the degraded LC caused by aging. A scaled-down tower crane prototype is adopted as a demonstrative case to illustrate the feasibility and effectiveness of the DTD framework. The DTD model predicts the degraded LC of the prototype with high accuracy, achieving a mean-square error (MSE) of 0.2253 and a coefficient of determination (R2) of 0.9973. The predicted degraded load charts of the tested tower crane for each decade of usage from 0 to 70 years are also presented to assist crane operators in applying safe loads, preventing unexpected failures and damages, and enhancing workplace monitoring and safety. This study helps monitor the safety conditions of tower cranes that are aging and susceptible to structural fatigue and deterioration, facilitates the prediction of the deterioration of complex machines and systems in the construction industry with real-time data, and highlights the potential of DTD approaches in improving efficiency, safety, and decision-making.

Durability and Damage Tolerance Analysis Approaches for Wind Turbine Blade Trailing Edge Life Prediction: A Technical Review

The size of wind turbine blades is increasing rapidly, and they are being installed in remote offshore locations. Consequently, it is essential to focus on improving the design and maintenance procedures in the blade industry to meet the growing demand. Of particular concern is the long-term operational performance of the wind turbine blade trailing edge. In this paper, we discuss the application of durability and damage tolerance analysis (DADTA) approaches to trailing edge service life prediction. DADTA is mandated in the aerospace sector to support airworthiness certification and to provide an updated life prediction of the structure based on the different stages of their service life. The DADTA framework has two main parts: durability and damage tolerance analysis. The durability part uses a structural fatigue approach based on a damage accumulation method during the initial design phase to predict the lifespan of a structure without defects. On the other hand, the damage tolerance analysis part uses a fracture mechanics approach and a damage growth method to update the lifespan prediction of a structure during the operation stages. This is achieved by utilizing sensors and inspection data as inputs while the structure is in service. Both these methods are comprehensive and have merits; however, their broad adoption in the wind turbine blade industry is still lacking. The current paper provides an extensive review of these methods and shows how these can be applied to the wind turbine blade industry, specifically for predicting the structural design life of the trailing edge of composite wind turbine blades. The review includes (a) defining wind turbine trailing edge failure modes, (b) trailing edge design procedures, and (c) a detailed discussion of the application of durability and damage tolerance analysis for trailing edge life prediction. Overall, this review paper would be of special interest to blade designers and would guide researchers and engineers interested in life prediction methodologies based on DADTA approaches for wind turbine blades.

Numerical and experimental investigations on shape optimization of submerged floating tunnels with a discrete adjoint method

The shape of the tube in submerged floating tunnels (SFTs) plays a critical role in determining their performance and safety in marine environments. In this study, a gradient optimization procedure based on the discrete adjoint method is performed to minimize the drag force under uniform currents action. The free form deformation approach is employed to parameterize the design variables. The physical mechanisms of the optimization process are revealed via unsteady numerical simulations and experimental tests. Moreover, the hydrodynamic performance of the tube with the optimal shapes subjected to a wave–current combination is also evaluated. The results show that the drag coefficient is significantly reduced through optimization by reducing the pressure amplitude in the negative-pressure area. Additionally, the lift oscillation is also suppressed to delay structural fatigue, owing to the strength reduction and increased distance of the wake vortex. The experimental analysis indicates the advantage of the optimal shape in reducing the loads under wave–current actions, accompanied by changes in the frequency distribution of the force and vortex structure. The elliptical shape of the SFT's tube provides significant advantages in drag reduction at high Reynold number. Suggestions on the shape design of the section of SFT are given according to different types of constraints.

Detection of cracks in aircraft structures using digital image processing technology

Full-scale aircraft structural fatigue testing exposes structural weaknesses by simulating the use of the aircraft in service and monitoring the development and expansion of fatigue cracks to guide the optimisation of aircraft structural design, the determination of service life and the development of maintenance and repair programmes. If crack damage is not detected in time, the uncontrolled expansion of the crack may lead to structural damage and prolong the test cycle. Therefore, it is crucial to achieve accurate detection of early cracks in fatigue testing of aircraft structures. Digital image technology is a new technology supported by a number of technical and theoretical achievements. In practical application, it relies on the support of computer functions, by converting image information into two-dimensional digital signals, thus realising various operations. In this paper, digital image processing techniques are used to deepen the backbone feature extraction network structure and improve the prediction scale of small targets, thus improving the performance of the model in terms of feature extraction and small defect detection. Experimental results show that the method can achieve accurate detection of small cracks, providing an effective technical approach for the automatic detection of cracks in aircraft structural fatigue tests. The technique of detecting cracks in aircraft structures using digital image processing also provides a new idea for research into improving aircraft endurance testing, which can be further developed with the aid of image technology.

An Equivalent Structural Stress-Based Frequency-Domain Fatigue Assessment Approach for Welded Structures under Random Loading

Welded structures under random loadings are usually susceptible to fatigue-induced failures that lead to significant economic and safety effects. However, accurately predicting these structures’ fatigue damage and life in the frequency domain remains challenging due to the limitations associated with using traditional weld stress extrapolation methods, such as nominal, hotspot, and notch stress methods. These methods struggle with precisely defining and characterizing the stresses at the weld toe and root as they vary depending on factors like weld stress concentration effects, joint geometry, and loading modes. This research introduces an Equilibrium Equivalent Structural Stress (EESS)-based frequency-domain fatigue analysis approach for welded structures subjected to random loading. The proposed method utilizes the EESS formulations, which are based on the decomposition and characterization of weld toe stresses with a single stress parameter, together with incorporating structural dynamic properties’ effects on the stresses acting on the weld joints and the corresponding accumulated fatigue damage of the structure. The numerical demonstration and validation of the proposed method have been performed using a welded Rectangular Hollow Section (RHS) T-joint structure subjected to stationary random fatigue loading. The proposed method’s fatigue damage and life results are compared with the fatigue test data and the equivalent hotspot stress extrapolation-based technique results.

A kind of numerical model combined with genetic algorithm and back propagation neural network for creep-fatigue life prediction and optimization of double-layered annulus metal hydride reactor and verification of ASME-NH code

The safety and stability of metal hydride reactor (MHR) structure are significantly important for normal operations of hydrogen isotope storage and supply system. Moreover, considering that the experiments of studying the fatigue and creep properties of MHR structure are always time, money, difficulty and energy consuming. To further improve the service life of MHR as well as lower the failure risk, this paper proposes a system integration method based on ASME code validation, which combines genetic algorithm (GA) and back propagation neural network (BPNN) for rapid assessment of creep-fatigue life (CFL) and optimal design of structural parameters. First, numerical simulation is applied to perform thermal-mechanical coupled finite element analysis and verified by ASME code. Then, parametric sensitivity analysis and experimental design of parameter-based training set based on Taguchi method were performed, and response values are obtained from physical models combined with numerical simulations. Finally, a parametric optimization procedure combining BPNN and GA is developed based on the training data. In the workflow, parametric data flow is passed through the experimental design, numerical simulation, prediction and optimization processes. The results show that the proposed BPNN proxy model based on ASME code validation has good performance in predicting the life of MHR. Based on this, the life of the MHR is improved by 8% compared to the reference sample.

Assessing the fatigue damage of concrete structures using automatically classified crack severity level information

The aim of this paper is to assess the fatigue damage of concrete structures using automatically classified crack severity level information. To solve the problem that the label data used for the automatic classification of regional complex crack severity levels can only be labeled by intuition, this paper proposes a quantitative index of regional complex crack severity levels that can consider the regional crack width, height, and density information. The testing set F1 score of the Swin Transformer classification model trained by the label data reaches 0.9903. To investigate the feasibility of assessing the concrete structure fatigue damage based on the crack change information, the paper adopted the scanning shooting method to acquire the concrete beam panoramas at different cyclic loading times in the fatigue test, and automatically classified the crack severity levels in different regions of the panoramas using the Swin transformer. The test results show that the information on the change in the number of regions with the same crack severity level on the concrete structure surface can be used to estimate approximately the fatigue life percentage interval that the concrete structure is currently in, which provides a reference for the development of maintenance measures and safety warnings.

Meso-scopically homogeneous superelastic transformation and related elastocaloric effect in a textured Ti-based shape memory alloy

Homogeneous superelastic behavior in shape memory alloys (SMAs) has been confirmed to be crucial for functional and structural fatigue properties, and more importantly, for stable elastocaloric effect (eCE). Here, the Ti–22Nb–4Zr–2Ta plate was prepared by successive cold rolling at 98.5%, followed by annealing at 1023 K for 30 min, forming a strong recrystallized texture, denoted as {112}β < 110>β. Such favorable texture contributed to a completely recoverable superelastic strain of ∼2.4% with narrow hysteresis of 35/45 MPa under isothermal/quasi-adiabatic conditions, which resulted in homogeneous martensitic transformation (MT) and adiabatic temperature changes (ΔTadi) of 4.8 K upon loading and 4.4 K upon unloading. The observation on the evolution of space-resolved strain and temperature throughout the sample revealed that the intrinsic feature of diffuse transformation along with favorable texture contributed to meso-scopically homogeneous transformation and related eCE. A standard deviation of strain was determined to be only 0.05%, corresponding to a small temperature deviation of 0.2–0.3 K, which quantitatively demonstrated the homogeneity, potentially high solid-state cooling performance efficiency, and long structural and functional fatigue life in Ti-based SMAs.

A Novel Individual Aircraft Life Monitoring Method Based on Reliable Life Consumption Assessment

Individual life monitoring is crucial for ensuring aircraft flight safety. Conventional life-consumption-based monitoring methods ignore reliability, thus disjoining them from the aircraft’s reliable life determination and extension, where high confidence and reliability are required. Therefore, this paper proposes a reliable life consumption and individual life monitoring method for aircraft structure fatigue. In the paper, the P-S-N curve, i.e., the relationship between the aircraft structure’s life (N) and fatigue load (S) under a certain probability (P), is established, by which the lower confidence limit of the aircraft structure’s reliable life can be evaluated under any fatigue loads. Based on that and the aircraft’s monitored fatigue loads, the indexes of reliable life consumption and remaining reliable life percentages are proposed and assessed in real time for individual aircraft life monitoring and online life management. Case studies indicate that the proposed method can guarantee high confidence and reliability requirements in individual life monitoring, consistent with the aircraft’s life determination and extension, which are widely accepted nowadays in engineering practice.

Study on wind-induced fatigue performance of large-span transmission tower-line system considering the combined distribution probability of wind direction and speed

To accurately evaluate the fatigue life of transmission line structures under wind loads, this study proposes a multivariate member fatigue damage analysis method for investigating the wind-induced fatigue performance of a large-span transmission tower-line with full consideration of the combined distribution effects of wind direction and speed in the actual wind field. With the background of in-service transmission line project, a refined finite element model of the large-span transmission tower-line system is established, and the wind load is simulated and generated based on the linear filtering method. According to the meteorological statistics of the project site in the past 50 years, the wind-induced fatigue sensitivity analysis of the tower-line system is performed through considering the combined distribution probability of wind direction and speed. The wind-induced fatigue damage of the tower-line system is calculated based on the rain-fall counting method and Miner's fatigue cumulative damage theory, and its wind-induced fatigue life is further analyzed. The analysis results show that the sensitive members of the tower-line system are mainly distributed in the 2nd, 6th, 8th and 9th segments by combining the effects of wind attack angle and wind speed. The fatigue damage of the sensitive members decreases gradually with the increase of wind attack angle (0° wind attack angle damage is the largest), and the effect of wind speed on the vertical main member is larger than that of the crossarm upper chord member. Moreover, the fatigue damage at the end of the member is significantly larger than that in the center, and even at small wind speeds, the member's end can produce a stress amplitude larger than the fatigue cut-off limit. The fatigue life of crossarm upper chord members is 72.89 years, and that of the vertical main member is 77.60 years. In particular, the vertical main member should be employed as the control member for the structural fatigue life when considering extreme wind loads.

The effect of welding deformation on the equivalent structural stress of dissimilar girth welded joint

Weld distortion refers to the phenomenon of shrinkage around the weld seam and warping of the outer weldment edges. This leads to a deviation from the original straight force transfer path in the joint, resulting in higher stress concentration. The weld root's stress concentrations can cause significant fatigue issues in dissimilar girth welded joints. However, previous evaluation methods have generally overlooked the impact of the welding process on fatigue assessment. To address this gap, this paper proposes integrating welding deformation into the fatigue life calculation process for welded joints made of G20Mn5 cast steel. In this study, a welding calculation program was developed to accurately simulate joint deformation using a two-curvature conical heat source model. Subsequently, high-frequency fatigue test data from scaled specimens of four types of joints were analyzed. The original equivalent structural stress fatigue data were corrected by taking into account the initial condition of weld deformation. This correction resulted in a reduction in the scatter of the fatigue data. Furthermore, the study also analyzed the welding deformation differences and the dispersion of stress concentration in scaled specimens caused by welding inhomogeneity.

Statistical learning prediction of fatigue crack growth via path slicing and re-weighting

Predicting potential risks associated with the fatigue of key structural components is crucial in engineering design. However, fatigue often involves entangled complexities of material microstructures and service conditions, making diagnosis and prognosis of fatigue damage challenging. We report a statistical learning framework to predict the growth of fatigue cracks and the life-to-failure of the components under loading conditions with uncertainties. Digital libraries of fatigue crack patterns and the remaining life are constructed by high-fidelity physical simulations. Dimensionality reduction and neural network architectures are then used to learn the history dependence and nonlinearity of fatigue crack growth. Path-slicing and re-weighting techniques are introduced to handle the statistical noises and rare events. The predicted fatigue crack patterns are self-updated and self-corrected by the evolving crack patterns. The end-to-end approach is validated by representative examples with fatigue cracks in plates, which showcase the digital-twin scenario in real-time structural health monitoring and fatigue life prediction for maintenance management decision-making.

From Zhentong Gao Method to Generalized Zhentong Gao Method

It is found that the data of structural fatigue life are mostly in accordance with Weibull distribution rather than Gaussian distribution, but the biggest obstacle affecting the application of Weibull distribution is that the estimation of its three- parameters will encounter difficulties in mathematical derivation and complicated calculation. Based on the estimation of correlation coefficient, the author has proposed Zhentong Gao method (Z.T. Gao method) which can avoid these difficulties. Further research found that the idea of Z.T. Gao method can be used to directly calculate the maximum value of the likelihood function of three-parameter Weibull distribution and estimate the three parameters simultaneously, which is called Generalized Zhentong Gao method. At the same time, it is also found that Nonlinear Programming method and Boltzmann Annealing method in machine learning can estimate the three parameters of Weibull distribution by directly calculating the maximum of likelihood function, so they can be regarded as Generalized Z. T. Gao methods in a sense. The results obtained by these methods are compared and some enlightening conclusions are obtained.

Construction of Fishing Vessel Molding with A Case Study of Ship Objects for The Java South Sea Region

In the process of building fiberglass boats (FRP), mold or molding is needed to make the hull and superstructure. Molding can be made of several kinds of main constituent materials including steel, wood, aluminium, HDPE and others. The object of the case study was carried out by collecting data from existing ships at the South Coast of the Java Sea. Molding is the main means in the fabrication process for fiberglass boats. To find out how long the mold or molding can be used, a service life analysis is needed. In terms of current technology, the analysis method uses structural fatigue analysis or fatigue analysis. In the calculations, the design is carried out earlier by conducting modelling simulations. In the fatigue analysis of the molding structure, material data is required to be applied to the molding. This will provide input data at the preprocessing stage in the finite element method. Furthermore, if the lifetime results are known at the age of the mold, then it can be continued with making the fishing boat molding product. So that later it can provide an illustration for shipbuilders that the time or age of the mold can be used for the time that has been designed.

Fatigue Life Assessment in the Typical Structure of Large Container Vessels Based on Fracture Mechanics

Welding defects are known to cause crack propagation and reduce structural fatigue performance. Based on the Paris theory of fracture mechanics, research is conducted on evaluation methods for analyzing fatigue crack propagation by adopting random loads with long-term distribution that follows the Weibull distribution for the stress ranges of fatigue loads. This approach is combined with the corrective stress intensity factor (SIF) equation and the method for calculating the reference stress and failure criterion. A large container ship is selected for a simulation, and fatigue crack propagation analysis is conducted on typical critical locations. A detailed comparison of the forecasted fatigue life is carried out between fracture mechanics theory and the S-N curve. The results indicate that the fatigue life values obtained using the two methods are of the same magnitude. In general, for the welded structure, the fatigue life value obtained via the fracture mechanics method is shorter than that obtained via the S-N curve method, while, for the free edge of the structure and the unwelded structure, the predicted fatigue life value is closer than that predicted via the S-N curve method. Moreover, the influence of initial crack defects on the fatigue life is investigated, and the results show that the depth of the initial crack will greatly affect the fatigue life of the target ship in typical locations, but the influence of the shape ratio on the fatigue life is limited. Therefore, in the actual ship construction process, controlling the initial crack depth of components is effective for limiting crack propagation and improving fatigue life. The above conclusions and suggestions can serve as a reference for the structural design and fatigue life evaluation of large container ships.

Effect of shot peening forming and shot peening strengthening post-treatment on the fatigue behavior of bolt-connected 2024HDT alloy

To improve surface integrity and suppress fretting fatigue of bolt-connected 2024HDT alloy plate-hole structure, the effect of shot peening forming (SPF) and shot peening strengthening (SPS) post-treatment on the surface integrity and fatigue behavior of 2024HDT alloy was investigated. The results showed that the SPF treatment increased the fretting fatigue resistance of the 2024HDT alloy plate-hole structure, and the failure mode changed from fretting fatigue to plain fatigue. The SPS post-treatment with three different intensities investigated here all improved the plain fatigue resistance of the SPF sample, whose failure mode also manifested as plain fatigue. This was due to the SPS treatment that flattened the surface of the SPF sample and introduced a compressive residual stress (CRS) field with a larger value and deeper distribution, while increasing the dislocation density and hardness of the SPF sample, which suppressed the initiation and propagation of fatigue cracks. SPS with a medium intensity produced an excellent surface CRS field with the lowest roughness and slightest surface damage, resulting in an improved plain fatigue resistance of 7.92%.

Laser powder bed fused 304L steel shot-peened with various ceramic shot sizes: Surface/subsurface characteristics, tensile behavior, and fatigue behavior

Shot peening is an efficient post-processing technique for eliminating surface manufacturing defects, inducing compressive residual stress, and improving the mechanical properties of laser powder bed fused (L-PBFed) metallic materials. With a series of microstructural characterizations, this work comprehensively evaluated the influence of ceramic shot size on the surface characteristics, subsurface characteristics, and mechanical properties, especially fatigue performance, of L-PBF manufactured 304L austenitic stainless steel (SS). The mechanical testing process was in situ observed using an infrared thermal camera, and the tensile-fractured and fatigue-fractured microstructures were systematically examined, aiming to explore the life improvement mechanisms caused by different ceramic shot sizes. The results revealed that when compared to small-sized AZB 100 shots, the large-sized AZB 300 shots could result in higher surface roughness, larger thickness reduction, more apparent surface gradient structure, higher strength, and longer fatigue lifetimes. The strength improvement could be associated with the size reduction of L-PBF induced subsurface defects, the grain refinement at the surface layer, and the α'-martensitic transformation during tensile deformation. The life improvement could be primarily attributed to the subsurface defect strengthening, which resulted in the fatigue crack initiation transition from subsurface defects to surface defects. During the fatigue process, the energy dissipation originating from the core materials dominated the thermal equilibrium temperature, whereas the density of micro-defects (crystalline defects) at the surface layer dominated the crack tip temperature. These findings provide a basic understanding of how to advance the shot peening process for L-PBFed materials.

Research on the Residual Stress Field of a Compression Bushing-Lug Plate in Cold Expansion Strengthening

The accurate acquisition of the residual stress field is the key to clarifying the cold expansion strengthening mechanism of compression bushings, optimizing the extrusion process parameters, and improving the structural fatigue life. In the actual cold expansion strengthening process, the resultant distribution of residual stresses is influenced by the relative extrusion amount, mandrel structure, material properties, and extrusion speed. In this article, the distribution patterns of residual stress after cold extrusion are investigated through a combination of finite element simulation and experimental measurements using a micro-region stress tester. To examine the redistribution law of the stress field of cold expansion reinforcement under external load, the compression bushing-lug-plate-reinforced structure is loaded and unloaded. The results show that large circumferential residual compressive stresses are distributed in the hole wall of the compression bushing after cold expansion. Radial residual stresses are also compressive stresses, although the values are small. In addition, the reinforced structure after cold extrusion presents a large difference in the stress redistribution rules compared with cold extrusion after the load is applied and removed.

Simultaneous energy utilization and vibration suppression study of a rolling-structured triboelectric nanogenerator for the vortex-induced vibration of a cylinder

Vortex-induced vibration (VIV) is a prevalent phenomenon observed in marine submersible buoys subjected to ocean currents, significantly compromising the structural fatigue life and sensor stability. Meanwhile, the provision of in-situ energy supply and long-term sustainability pose significant challenges for marine submersible buoys. To tackle these issues, this paper presents a novel approach that entails the simultaneous utilization of VIV energy and suppression of VIV through the implementation of a rolling-structured, freestanding triboelectric-layer-based nano-generator (RF-TENG). An experimental investigation has been conducted to explore the power generation mechanism of RF-TENG when subjected to VIV excitation, as well as the vibration suppression effect of RF-TENG on VIV. The following conclusions have been drawn: (1) The maximum power generation is observed within the filling rate range of 50%–60%, where the conversion of kinetic energy to electrical energy resulting from particle friction prevails. Correspondingly, the maximum vibration suppression rate is achieved within the filling rate range of 60%–90%, primarily attributed to energy dissipation due to particle collisions. Notably, at approximately 60%–70% filling rate, RF-TENG exhibits a relatively efficient and balanced performance in terms of both energy utilization and vibration suppression. (2) Particle motion irregularity is a prominent factor that greatly influences the power conversion efficiency, which is manifested as the presence of the third harmonic component in the output voltage and current. The power conversion efficiency of RF-TENG attains its peak value at a filling rate of 50%. (3) Within a certain range, an increase in particle diameter contributes to enhancing the robustness and VIV suppression efficiency of RF-TENG. However, it comes at the cost of reduced power generation capacity. In this study, under the excitation of VIV experienced by a cylinder with an effective amplitude of approximately 1.56 cm and a frequency of approximately 1.62 Hz, RF-TENG achieves a power generation density of 80 mW/m3 and a VIV amplitude suppression rate of 24%. These results suggest that RF-TENG is an effective method for synchronously collecting energy and suppressing VIV in marine structures.