Low Fatigue Loading Research Articles

Determination of fatigue life under low frequency fatigue loading based on temperature evolution

The wind turbine blades are key components of wind turbines. During the working process, the blades are subjected to fatigue loading, resulting in the failure of wind turbine blades. The surface temperature of blade materials will change when subjected to fatigue loading and the temperature rise rate of blade surface under fatigue loading can be used as an index to predict fatigue life. However, the frequency of fatigue loading on the wind turbine blades is low, the non-adiabatic response under low frequency fatigue loading would lead to temperature dissipation. In this paper, fatigue life prediction under low frequency fatigue loading was studied. Based on the law of energy conservation and the laws of heat conduction, heat convection, heat radiation, the temperature evolution of materials under low frequency fatigue loading was theoretically analyzed. Epoxy resin was selected as testing material and fatigue tests were conducted on epoxy resin specimens. During the fatigue test, the infrared thermal imager was used to monitor the samples surface temperature in real-time. The results show that the predicted life considering temperature dissipation effect is closer to the real life. Meanwhile, the general application of life prediction model is further verified by metal material. This work can be used for predicting the fatigue life of wind turbine blades under low frequency.

Experimental Investigation into the Mechanical and Piezoresistive Sensing Properties of Recycled Carbon-Fiber-Reinforced Polymer Composites for Self-Sensing Applications.

This study investigates the mechanical and piezoresistive sensing properties of recycled carbon-fiber-reinforced polymer composites (rCFRPs) for self-sensing applications, which were prepared from recycled carbon fibers (rCFs) with fiber lengths of 6, 12, 18, and 24 mm using a vacuum infusion method. Mechanical properties of the rCFRPs were examined using uniaxial tensile tests, while sensing characteristics were examined by monitoring the in situ electrical resistance under cyclic and low fatigue loads. Longer fibers (24 mm) showed the superior tensile strength (92.6 MPa) and modulus (8.4 GPa), with improvements of 962.1% and 1061.1%, respectively. Shorter fibers (6 mm) demonstrated enhanced sensing capabilities with the highest sensitivity under low fatigue testing (1000 cycles at 10 MPa), showing an average maximum electrical resistance change rate of 0.7315% and a gauge factor of 4.5876. All the composites displayed a stable electrical response under cyclic and low fatigue loadings. These results provide insights into optimizing rCF incorporation, balancing structural integrity with self-sensing capabilities and contributing to the development of sustainable multifunctional materials.

Experimental investigation on the effect of forced assembly on fatigue behavior of single-lap, countersunk composite bolted joints

Forced assembly is a common countermeasure for eliminating the interface gap during composite airframe assembly, which modifies the bolt-hole bearing behavior and brings failure risks. In this study, an experimental investigation on the effect of forced assembly on the fatigue behavior of single-lap, countersunk composite bolted joints is presented. The hole elongation and dynamic stiffness are obtained based on ASTM D6873. The S-N curves are fitted according to ASTM E739. The fatigue damage is observed through microscopy, and the residual strength is acquired by static tensile tests. The results indicate that forced assembly significantly shortens the fatigue life of joints. After forced assembly, the hole elongation evolves faster and reaches the fatigue failure threshold earlier. In addition, the initial dynamic stiffness is reduced, and the loss of stiffness is accelerated. Apart from the designed hole bearing failure, another failure mode, i.e., bolt head cracking, is observed under low fatigue load level (70% of 2% offset bearing static strength), leading to faster degradation of fatigue property. Under equal hole elongation (4% of the hole diameter), the joints with forced assembly display severe hole bearing damage, and visible shearing cracks are observed, lowering the residual strength.

Fractional-order numerical modeling to study chloride ion transport in concrete with fly ash or slag additions.

To better study the chloride ion migration in concrete with fly ash or ground granulated blast furnace slag under low fatigue load, a Caputo time fractional-order chloride diffusion model is developed in this paper. The model, grounded in Fick's second law with a fractional-order derivative, employs an implicit numerical method for discretization, resulting in a fractional-order numerical scheme. The stability and convergence of the scheme are rigorously proven within the paper. The model's unknown parameters are estimated using genetic algorithm with a grid method. To validate the model's effectiveness, its numerical solution is juxtaposed with experimental results from chloride erosion studies. Furthermore, the fitting efficacy of the Caputo time fractional-order numerical scheme is compared with that of the classical Fick's second law numerical scheme and analytical solution. The research findings demonstrate that the fractional-order numerical scheme can more accurately simulate the chloride concentration in concrete containing fly ash or slag. Additionally, the model shows promise in predicting the service life of fly ash or slag concrete.

A comparative study into the fracture toughness properties of duplex stainless steels

Beside their high mechanical strength, duplex stainless steels are a suitable choice in highly corrosive environments. These types of steels are used in steel bridges more and more frequently exposed to low temperatures and fatigue loads. However, for low temperature applications, it must be guaranteed that brittle fracture is avoided since duplex stainless steel shows a toughness-temperature relationship similar to that of carbon steel. For this reason, in the frame of the German national FOSTA research project “P 1390”, comprehensive investigations have been started into the material selection of duplex stainless steel to avoid brittle fracture considering the fracture mechanic based background of EN 1993-1-10. For this purpose, Charpy-V impact tests and fracture toughness tests have been systematically carried out for various duplex stainless steels in order to create the basis for the development of toughness requirements for new duplex classes. The validity of the already existing Master Curve concept and the applicability of the transition temperature correlation for duplex stainless steels based on experimental fracture toughness and Charpy-V impact tests have been investigated. The aim of this contribution is to present first results of these investigations.

Fatigue failure mechanism and estimation of aluminum alloy self-piercing riveting at different load levels

In order to investigate the mechanical properties of self-piercing riveting (SPR) joints in low ductility aluminum alloys, the study utilized three different types of aluminum alloy sheets: 7075, 6061, and 5754. These sheets were joined together using self-piercing riveting technology. The research compared the geometric parameters, microhardness, quasi-static mechanical properties, and fatigue properties formed with the joints of the three aluminum alloys. Additionally, the fatigue life of the self-piercing riveting joint in the 7075 aluminum alloy was analyzed using the Paris law and its corollary. The findings revealed that effective connections were established in all three aluminum alloys, although the expansion of the rivet was comparatively lower in the 7075 alloy. Plastic deformation resulted in an increase in microhardness of joined sheets. The hardness of the joined sheets followed the order: 7075 > 5754 > 6061. Furthermore, the 7075 alloy exhibited superior static mechanical and fatigue properties due to its inherent material. However, despite its lower material strength, the 5754 alloy outperformed the 6061 alloy at low fatigue load levels as a result of work hardening. The failed joints exhibited black oxide, and fretting damage under fatigue loading exacerbated the fatigue failure of the joint, serving as the primary cause. In the case of the 7075 alloy, three types of failure occurred due to fretting damage, which was influenced by the unique nature of the lower die. Fretting between the lower sheet and the rivet leg primarily caused lower sheet fracture and rivet failure, while fretting wear between the rivet head and the upper sheet or between the two sheets mainly led to upper sheet fracture failure. The fatigue life estimation method employed in the study, based on the Paris law, demonstrated good agreement with the experimental results.

Effect of rivet arrangement on fatigue performance of electromagnetic riveted joint with [formula omitted]10 mm diameter rivet

Electromagnetic riveting (EMR) can effectively form large-diameter rivets and produce high-quality joints. In this paper, the effect of rivet arrangement on fatigue performance of electromagnetic riveted joint was investigated. Quasi-static shear tests, fatigue tests, numerical simulation and microstructure observation were carried out. The results showed that the rivet arrangement had little effect on the shear performance and had a great impact on the fatigue performance of the EMR joint. Specifically, the difference between the average maximum shear load of the V-joints (60.49 kN) and the H-joints (60.41 kN) was small. The fatigue life of the H-joints was higher than that of the V-joints at high fatigue load levels (Fmax>30.225 kN), while the result was opposite at low fatigue load levels (Fmax<30.225 kN). Furthermore, the fatigue fracture analysis showed that there were two typical failure modes for the joints: (I) the rivet fracture, and (II) the sheet fractured on the manufactured head side. The V-joints mainly displayed the coexistence of failure mode I and mode II, while the H-joints displayed failure mode II at all fatigue load levels. At high fatigue load levels, the significant stress concentration in both rivets resulted in the rivet fracture of the V-joints within a relatively short time. Due to multiple extension directions and a smaller extension zone, the fatigue life of H-joints was lower than that of the V-joints at low fatigue load levels. Therefore, the fatigue performance of the component could be improved by choosing a different rivet arrangement.

A modified model for thermoelastic stress analysis under low frequency fatigue loading

Thermoelastic stress analysis (TSA) can be used to determine the stress field of structures or materials under cyclic loadings through temperature under adiabatic conditions. So high-frequency cyclic loadings are considered as a fundamental condition. However, loading frequencies in many practical applications do not meet the adiabatic conditions. In order to improve the accuracy of TSA under low-frequency fatigue loadings, the non-adiabatic response in TSA was derived by combining the classical thermoelastic equation and heat conduction equation. This response was used to create a modified model that accounted for heat transfer and temperature distribution under low-frequency loading conditions. Otherwise, fatigue test and finite element method were used to evaluate the correctness of the modified model using epoxy resin material. The results showed that the temperature change obtained from the modified model were in good agreement with the experiment results. The modified model can effectively evaluate the structural stress under low-frequency fatigue loadings.

Discussion on the spatial-temporal inhomogeneity characteristic of horizontal-axis wind turbine's wake and improvement of four typical wake models

Previous studies on wake of horizontal-axis wind turbines (HAWTs) mainly focus on the spatial distribution but ignore the time-varying characteristics of the wake profile which highly affect the operation strategy of the downstream HAWTs to ensure the high-efficiency and low-fatigue load. This paper proposed a mathematical method for studying the spatial-temporal inhomogeneity of wake profiles. Considering the anisotropy of wake expansion, the turbulence intensity was modified to avoid repeated experimental calculation of empirical parameters such as wake expansion coefficient. In addition, the super-Gaussian function was used to modify the near wake model, and the modified near wake model was combined with four typical far wake models to obtain four temporal wake models that can express the time-varying characteristics of the wake. The accuracy of the improved four temporal wake models were verified by wind tunnel test, and the relative errors were analyzed. Results show that the prediction errors of the modified two-dimensional temporal wake model are basically within 4%, which is smaller than the other three wake models. The method presented in this paper can effectively describe the spatial distribution and time-varying characteristics of wake expansion, and can provide a reference for optimizing the control strategy of HAWTs.

Mechanical Properties of B1500HS/AA5052 Joints by Self-Piercing Riveting

Self-piercing riveting (SPR) is a suitable technology to join various materials and has attracted more attention in the automotive industry. In this work, the effects of forming parameters on the forming qualities and mechanical properties of B1500HS steel/AA5052 aluminum alloy SPR joints were analyzed. The results show that the sheet stack sequence has little influence on the peak tensile load and rigidity of SPR joints. When the steel sheet is placed on the aluminum sheet, the failure displacement, energy absorption, and ductility factor are, respectively, 2.77, 2.13, and 2.28 times larger than those of the joints with the aluminum sheet placed on the steel sheet. The SPR joints with steel sheets placed on aluminum sheets have better mechanical stability. Meanwhile, when the steel sheet is placed on the aluminum sheet, the fatigue life of the joint can be increased by about 98.4%, 88.3%, and 118.1%, respectively, under high, medium, and low fatigue loads. A joint with opposite riveting direction has the optimal fatigue performance and the fatigue life is 1.64 and 2.14 times those of the other two-rivet joints. Generally, the fatigue fractures of aluminum alloy sheets in SPR joints occurred in fatigue tests. The fatigue fracture of a joint with a steel sheet stacked on an aluminum sheet extends uni-directionally to the edge of the sheet from the riveting point, while a symmetric fatigue crack of aluminum occurs for joints with the opposite sequence. The distribution of fatigue cracks is related to fatigue load, and fatigue cracks mainly originate in the fretting wear area of the contact interface between the rivet leg, upper sheet, and lower sheet.

Multifunctional Hybrid Fiber Composites for Energy Transfer in Future Electric Vehicles.

Reducing the weight of electric conductors is an important task in the design of future electric air and ground vehicles. Fully electric aircraft, where high electric energies have to be distributed over significant distances, are a prime example. Multifunctional composite materials with both adequate structural and electrical properties are a promising approach to substituting conventional monofunctional components and achieving considerable mass reductions. In this paper, a hybrid multifunctional glass-fiber-reinforced composite containing quasi-endless aluminum fibers with a diameter of is proposed for electric energy transfer. In addition to characterizing the material’s behavior under static and fatigue loads, combined electrical-mechanical tests are conducted to prove the material’s capability of carrying electric current. Light microscopy, thermal imaging and potentiometry-based resistance characterization are used to investigate the damage behavior. It is found that a volume fraction of about work-hardened aluminum fibers does not affect the static fiber-parallel material properties significantly. Under transverse loading, however, the tensile strength is found to decrease by 17% due to the weak bonding of the aluminum fibers. The fiber-parallel fatigue strength of the multifunctional laminate containing work-hardened aluminum fibers is comparable to that of the reference material. In contrast, the integration of soft-annealed aluminum fibers decreases the tensile strength (−10%) and fatigue life (−21%). Concerning the electrical properties, electrical resistance is nearly unchanged until specimen rupture under quasi-static tensile loads, whereas under cyclic loading, it increases up to 60% within the last third of the fatigue life. Furthermore, the material’s capability of carrying currents up to 0.32 A/mm2 (current density of 4.5 A/mm2 in the aluminum phase) is proven. Under combined electrical-mechanical loads, a notable reduction in the fatigue life (−20%) is found at low fatigue loads, which is attributed to ohmic specimen heating. To the best knowledge of the authors, this is the first study on the electrical and mechanical material properties and damage behavior of glass-fiber-reinforced composites containing aluminum fibers tested under combined electrical-mechanical loads.

Fatigue performance of CFRP reinforced pretensioned prestressed beams

This study presents an experimental investigation on the fatigue performance of CFRP reinforced pretensioned prestressed beams. A total of 10 specimens were designed and tested under monotonic or fatigue conditions. The investigated variables included the prestressing level, fatigue load range, and the absence or presence of novel additional aluminum alloy ribs (ARs) anchorage. Specimens using a high prestressing level or subjected to a low fatigue load range would have a reduced deflection and crack width development compared to those in their counterparts. In addition, the fatigue loading was found to cause cumulative bond damage of pretensioned FRP bar to concrete and finally led to an uncontrollable slippage failure of the beam. Such bond issue was demonstrated to have been avoided by using the ARs anchorage at beam ends. The applicability of three theoretical models proposed for steel reinforced beams to predicting the deflection of CFRP reinforced pretensioned prestressed beams under fatigue loading were evaluated, and CEB-FIP model was found to be the most suitable one to describe the deflection development of the tested specimens.

Model-based design of a wave-feedforward control strategy in floating wind turbines

Abstract. Floating wind turbines rely on feedback-only control strategies to mitigate the negative effects of wave excitation. Improved power generation and lower fatigue loads can be achieved by including information about incoming waves in the turbine controller. In this paper, a wave-feedforward control strategy is developed and implemented in a 10 MW floating wind turbine. A linear model of the floating wind turbine is established and utilized to understand how wave excitation affects rotor speed and so power, as well as to show that collective pitch is suitable for reducing the effects of wave excitation. A feedforward controller is designed based on the inversion of the linear model, and a gain-scheduling algorithm is proposed to adapt the feedforward action as wind speed changes. The performance of the novel wave-feedforward controller is examined first by means of linear analysis and then with non-linear time-domain simulations in FAST. This paper proves that including some information about incoming waves in the turbine controller can play a crucial role in improving power quality and the turbine fatigue life. In particular, the proposed wave-feedforward control strategy achieves this goal complementing the industry-standard feedback pitch controller. Together with the wave-feedforward control strategy, this paper provides some insights about the response of floating wind turbines to collective-pitch control and waves, which could be useful in future control-design studies.

Effect of pneumatic shot peening on the high and low cycle combined fatigue life of K403 turbine blades

AbstractThe effect of pneumatic shot peening (PSP) on the fatigue properties of K403 turbine blades has been investigated under the high and low cycle combined fatigue (CCF) load with two types of turbine blades: untreated blades and PSP treated blades. It is found that there is a threshold vibration stress for the blades in this study. The PSP has a positive effect on the CCF life of blades mainly due to the compressive residual stress resulting in the reduction of the number of crack sources and propagation rate, when vibration stresses are below the threshold vibration stress. However, the PSP treatment has no effect or a negative effect when vibration stresses are above the threshold value. The compressive residual stress is released along with the microstructure changes of K403 alloy. Meanwhile, the microstructure changes, reflected in the precipitation of the lamellar MC carbides and σ phases, can accelerate the processes of crack initiation and propagation.

A Comparative Study of Local Heat Treatment for Enhancing Overall Mechanical Properties of Clinched Joints

Clinching technology is widely used for joining lightweight materials. It is well known that the fatigue life of clinched joints is outstanding, but its static strength is inferior to other conventional joints such as resistance spot welding joints. In this study, a post-processing method was proposed to enhance the overall mechanical performances of clinched joints in material of steel plate cold commercial steel. Based on the failure mechanism analysis, we performed quenching, a local heat treatment process, on the joining zone to improve the static strength of clinched joints. The static strength and fatigue life of clinched joints before and after quenching were compared via experimental study. The experimental results showed that the tensile–shearing strength of clinched joint increased by 60.65% and the fatigue life was extended when the joint was under high fatigue load levels after local heat treatment. Besides, metallographic test and fracture analysis were conducted to analyze the mechanism of the change of mechanical behaviors. Lath martensite was observed in the cross section of clinched joint after local heat treatment, which is the main reason for the tensile–shearing strength increase as martensite has higher strength than ferrite and pearlite. The fracture analysis showed that the fatigue fracture mode became transcrystalline rupture after local heat treatment when the clinched joint was under low fatigue load levels, which lead to the fatigue behavior of clinched joints weakened. Nonetheless, the overall mechanical properties of clinched joints were improved.

Behaviour of steel beams retrofitted with anchored carbon-fibre-reinforced polymer strips

Steel bridge beams can be damaged due to increased traffic loads and environmental impacts. An experimental study on the use of bonded and mechanically anchored carbon-fibre-reinforced polymer (CFRP) strips was undertaken to assess the retrofitting of such steel beams. The number of mechanical anchors used in the ends of the bonded strips was varied from zero to eight. The steel beam samples were tested under four-point loading. Loading was applied as an increasingly high static load and a low repetitive fatigue load. The load–displacement behaviour under the effects of static and fatigue loading and the strain distributions along the strips were measured and interpreted. The results showed that retrofitting cracked steel beams with CFRP strips is an effective method. In addition, retrofitting with anchored strips increased the performance of cracked beams under the effects of both static and fatigue loading.

Semi-submersible wind turbine hull shape design for a favorable system response behavior

Floating offshore wind turbines are a novel technology, which has reached, with the first wind farm in operation, an advanced state of development. The question of how floating wind systems can be optimized to operate smoothly in harsh wind and wave conditions is the subject of the present work. An integrated optimization was conducted, where the hull shape of a semi-submersible, as well as the wind turbine controller were varied with the goal of finding a cost-efficient design, which does not respond to wind and wave excitations, resulting in small structural fatigue and extreme loads.The optimum design was found to have a remarkably low tower-base fatigue load response and small rotor fore-aft amplitudes. Further investigations showed that the reason for the good dynamic behavior is a particularly favorable response to first-order wave loads: The floating wind turbine rotates in pitch-direction about a point close to the rotor hub and the rotor fore-aft motion is almost unaffected by the wave excitation. As a result, the power production and the blade loads are not influenced by the waves. A comparable effect was so far known for Tension Leg Platforms but not for semi-submersible wind turbines. The methodology builds on a low-order simulation model, coupled to a parametric panel code model, a detailed viscous drag model and an individually tuned blade pitch controller. The results are confirmed by the higher-fidelity model FAST. A new indicator to express the optimal behavior through a single design criterion has been developed.

Adaptive cycle jump and limits of degradation in micromechanical fatigue simulations of fibre-reinforced plastics

This research investigates the influence of numerical parameters of micromechanical fatigue damage models on the obtained progressive damage behaviour of fibre-reinforced plastics at transverse tensile fatigue loads. The simulated damage behaviour is evaluated using experimentally observed crack patters published in the literature. The investigated numerical model parameters are (1) whether or not the model considers static failure within a simulated load cycle, (2) the degree of material property degradation after sudden failure and (3) the size of the cycle jump. The results reveal a significant influence of the degree of material degradation and of the cycle jump on the simulated matrix crack formation at both higher and lower fatigue loads. Static failure within a simulated load cycle primarily affects the damage behaviour at higher fatigue loads. The paper gives recommendations of the parameter choice for plausible progressive fatigue damage simulation results. Regarding the cycle jump, an adaptive algorithm is proposed and implemented. This approach leads to plausible fatigue damage results paired with a significant reduction of computation time comparing to a cycle-by-cycle analysis.

Supraspinatus Tendons Have Different Mechanical Properties Across Sex.

Sex differences in the mechanical properties of different musculoskeletal tissues and their impact on tendon function and disease are becoming increasingly recognized. Tendon mechanical properties are influenced by the presence or absence of sex hormones and these effects appear to be tendon- or ligament-specific. The objective of this study was to determine how sex and hormone differences in rats affect supraspinatus tendon and muscle properties. We hypothesized that male supraspinatus tendons would have increased cross-sectional area but no differences in tendon material properties or muscle composition when compared to supraspinatus tendons from female or ovariectomized (OVX) female rats. Uninjured supraspinatus tendons and muscles from male, female, and OVX female rats were collected and mechanical and histological properties were determined. Our analysis demonstrated decreased dynamic modulus and increased hysteresis and cross-sectional area in male tendons. We found that male tendons exhibited decreased dynamic modulus (during low strain frequency sweep and high strain fatigue loading), increased hysteresis, and increased cross-sectional area compared to female and OVX female tendons. Despite robust mechanical differences, tendon cell density and shape, and muscle composition remained unchanged between groups. Interestingly, these differences were unique compared to previously reported sex differences in rat Achilles tendons, which further supports the concept that the effect of sex on tendon varies anatomically. These differences may partially provide a mechanistic explanation for the increased rate of acute supraspinatus tendon ruptures seen in young males.

Experimental and computational studies on honeycomb sandwich structures under static and fatigue bending load

Sandwich structures with glass fiber face sheets and aluminum honeycomb core are investigated computationally and experimentally. A three point bending load arrangement is conducted to examine the static and fatigue performance of honeycomb sandwich panel. Under static loading, the load and displacement response is indicated in five phases. The decrease in fatigue life with load level was observed in approximately linear manner. The visual and Scanning Electron Microscopic (SEM) analysis were carried out to analyze the failure modes. For static and high amplitude fatigue load, the failure initiates due to face yielding, while for low fatigue load failure initiates as a result of delamination at core and skin interface. However, in all cases principle failure mode is indentation. The honeycomb sandwich structure was also modeled with commercially available finite element packages ANSYS and the fatigue analyses were carried out to determine the life of specimens under load-displacement response. The experimental results were in good agreement with the Finite Element Analysis (FEA) results in both static and fatigue loads, and fracture modes prediction.