Fatigue Test Results Research Articles

Experimental and probabilistic study of fatigue crack growth behavior of steel Q345qC under different stress ratios

The stress ratio effects and the scatter of the fatigue test results are the key issues in the design of steel fatigue performance. This paper reports an experimental and probabilistic investigation of the fatigue crack growth (FCG) behavior of steel Q345qC. Firstly, the microstructural characteristics and mechanical properties of steel Q345qC were examined via an optical microscope and tensile tests. Next, the FCG behavior of steel Q345qC was investigated under different stress ratios. The fatigue crack growth rate (FCGR) equations incorporating survival probabilities were proposed based on Paris and Walker laws. Finally, the theoretical FCG life under different survival probabilities in the crack stable growth region is calculated based on the integral formulas of Paris and Walker law and compared with the tested FCG life. The experimental results demonstrate that the larger the stress ratio, the shorter the length of stress intensity factor (SIF) amplitude experienced in the stable growth region and the smaller the SIF at the end of this region. Both probabilistic Paris and Walker equations can be used for FCGR calculations when the survival probability is set as 50%. The FCG life is calculated using the integral formula based on the Walker law with a high safety tolerance and a wide application interval of stress ratio when the survival probability is set as 90%.

Fatigue life determination based on infrared thermographic data for MultiDirectional (MD) CFRP composite laminates

Infrared thermography is a useful tool for the rapid determination of fatigue properties. A previously proposed fatigue life model, which combines the stiffness degradation k(N) with thermographic data ΔTstab, has been successfully applied to UD and ±45° CFRP laminates. In view of more complex damage mechanisms in multidirectional laminates, the model is modified in this work to overcome the conservative predictions. The S-N curves determined by the modified model are compared with the experimental results of traditional fatigue tests on Quasi-Isotropic (QI) and cross-ply CFRP laminates. Very good consistency was observed. In particular, the S-N curve corresponding to fatigue life of Nfl=106 cycles is conservative compared to that from the traditional fatigue test results, but the maximum safety factors are no more than 1.15 and 1.08 for QI and cross-ply laminates, respectively. Both S-N curves corresponding to Nfl=107 cycles for two tested laminates lie within the 95% confidence interval of traditional experimental results. Therefore, based on infrared thermographic data, the modified model proposed in this study is more general. It allows to determine the S-N curves of QI and cross-ply CFRP laminates just in about ten hours of testing machine time instead of one month and a half using traditional fatigue tests, even though more experimental data (other types of MD stacking sequences, composite materials, under other fatigue modes, etc.) are necessary to demonstrate the applicability of modified model to all types of MD laminates.

Fatigue life assessment of GFRP composite laminate under axial and transverse fatigue loading

The strong cyclic nature of the loads that the structures are designed to support can lead to fatigue, which is one of the primary failure modes that can occur in the structures. Throughout their service lives, composite materials experience many kinds of fatigue loads. In this experiment, we look at how glass fiber-reinforced laminates with different stacking sequences react to axial and transverse loading. Hand lay-up method followed by the press molding technique was adapted for the fabrication of laminates. Experimental studies were carried out on universal testing machine to characterize GFRP laminate under both static and dynamic loading conditions. Tensile and flexural fatigue tests were performed according to ASTM D-3479 and ASTM D790-17 standards respectively on symmetric [(0/90) / (±45)2/ (0/90)] s and asymmetric glass fiber reinforced laminates (0/90) / (±45)2/ (0/90) // (±45) / (0/90)2/ (±45). Fatigue tests were conducted at stress ratio(R) = 0.1 and frequency = 3 Hz for tensile fatigue and 1 Hz for transverse fatigue at 90%, 70%, and 50% stress levels of their respective ultimate strength. Results were presented in terms of stress–strain, wohler curve, and hysteresis loops. Results of tensile and flexure fatigue tests showed that symmetric laminates have a better fatigue life.

Application of Statistical Criteria for Checking the Homogeneity of Dispersions of Results of Fatigue Tests of Heat-Resistant Alloys Produced by Selective Laser Melting

Application of Statistical Criteria for Checking the Homogeneity of Dispersions of Results of Fatigue Tests of Heat-Resistant Alloys Produced by Selective Laser Melting

Effect of mastic specifications on the fatigue and low-temperature performance of hot mix asphalt

ABSTRACT This study aimed to evaluate the effect of mastic specifications on the intermediate and low temperatures performance of hot mix asphalt. In this research study, four different asphalt mixtures were fabricated with two types of aggregate and bitumen at three different mastic densities (defined as F/B ratios). At intermediate temperature, Four-Point Bending (4PB) beam fatigue tests were carried out at 8 Hz frequency and the strain levels of 460, 750, and 1100 microstrain. Also, the low-temperature performance of mixtures was evaluated by using Semi-Circular Bending (SCB) tests at −6°C, −18°C and −12°C, −24°C for PG 64–16 and PG 58–22 fabricated mixtures, respectively. Fatigue test results showed that the fatigue life of asphalt mixtures was more influenced by the mastic density at the lower strain level. Moreover, a minimum mastic density was necessary for improving fatigue performance. The SCB test results revealed that increasing the mastic density makes higher values in the mixture’s fracture energy, fracture toughness, cracking resistance index (CRI), and tensile strength. Also, the low-temperature performance of asphalt mixtures containing aggregate with a high abrasion value (AAV) was more dependent on the mastic density.

Experimental observation and numerical investigation of fatigue crack propagation in a multi-tubular heat exchanger specimen

Novel high-cycle fatigue test results are presented for a multi-tubular heat exchanger specimen. The unique test specimen is developed with multiple tubes. Cracks were sequentially generated at the welded regions. These cracks were enlarged under tensile cyclic load. Fatigue crack propagation (CP) was produced with complicated-shaped crack geometries. Stress analysis and CP simulation are performed to analyze the experimental observations. The complicated CP phenomenon is successfully reproduced through numerical simulations. Fracture patterns and CP behaviors are discussed. Finally, the fatigue CP characteristics of the multi-tubular heat exchanger specimen are studied from the fracture mechanics viewpoint.

Creep–fatigue mechanical characteristics of salt rocks under triaxial loading: An experimental study

Salt rock is a sedimentary rock that is widely distributed in the crust. Since salt rock has a low permeability, rheological behavior, and self-healing characteristics, it has been extensively exploited for subsurface energy storage and waste disposal. The construction of compressed air energy storage (CAES) facilities using salt caverns for solution mining is a practical solution to achieve carbon peaking and carbon neutrality goals worldwide. The stability of the salt cavern surrounding rock is critical to the function of a CAES plant. Considering the triaxial stress condition of the rock surrounding such underground salt caverns, a number of triaxial creep–fatigue mechanics tests were performed on salt rock using an MTS815 mechanical testing device. The creep–fatigue damage to the salt rock was examined under various confining pressures and stress levels. The main research results and conclusions are as follows. First, the triaxial creep–fatigue stress–strain curves show a similar tendency to classical triaxial fatigue results: the strength and ultimate deformation of salt rocks increase as the confining pressure increases. Second, in contrast to typical fatigue test results, the residual strain before the high-stress plateaus is larger than that after the plateaus during a creep–fatigue test. However, as the confining pressure increases, this disparity decreases. Third, fatigue loading increases creep deformation, and this effect weakens when the confining pressure increases. Fourth, the residual strain differential before and after the high-stress plateau increases together with the stress level, and the contribution of creep strain increases correspondingly. Last, as the confining pressure increases, the salt rock behavior transitions from a brittle response to a fully ductile response. A high confining pressure prevents relative sliding between the salt rock particles. Thus, the effect of confining pressure on the creep–fatigue mechanical behavior of the salt rock reaches a threshold.

Interfacial fatigue debonding retardation in wrapped composite joints: Experimental and numerical study

Debonding crack propagation at the composite-to-steel interface has been found to be an important failure mechanism for wrapped composite joints under static and fatigue loads. Friction at the interface behind the crack tip may deviate fatigue debonding of the joints from the linear-fracture-mechanics behaviour. This paper presents static and fatigue tests of axial wrapped composite joints. 3D DIC and optical fiber system is employed to monitor displacements and crack propagation. A finite element model is established and validated against static and fatigue test results, where friction is considered at the cracked interface. Through FE modelling, it is proved that the friction at the interface significantly reduce the strain energy release rate (SERR) at the crack tip, leading to retardations of crack growth and stiffness degradation. Parametric study is conducted finally to investigate the influence of friction coefficient, failure modes as well as Paris relationship parameters on the predicted fatigue behaviour of wrapped composite joints.

Effect of Anodizing on Very High Cycle Fatigue Properties of Aluminum Alloy A2014-T6 in Laboratory Air and 3 % NaCl Solution

Abstract To examine the effect of oxide layer thickness on fatigue behavior, rotating bending fatigue tests were performed on the aluminum alloy A2014-T6 with oxide layers of 3 different thicknesses in laboratory air and 3 % sodium chloride (NaCl) solution over a wide cycle-life region (104–109 cycles). Tests were also conducted on untreated specimens for comparison. Based on the results of fatigue tests in laboratory air, deterioration of the fatigue strength was observed for anodized specimens having an oxide layer in comparison with untreated specimens. The fatigue strength of anodized specimens decreased with increasing oxide layer thickness. On the other hand, fatigue test results in 3 % NaCl solution showed that anodizing improved fatigue life, which increased with increasing oxide layer thickness. In addition, the thickness of the oxide layer was found to be important from the viewpoint of fracture mechanics.

Influence of distortion ratio on distortion-induced fatigue behavior of steel girder bridges

The physical fatigue test and a new proposed research methodology called the digital fatigue test are used to investigate distortion-induced fatigue performance at web gap regions in steel girder bridges. Digital fatigue test based on the extended finite element method can combine factors such as residual welding stress, initial defects, and fatigue action, to simulate the propagation of mixed-mode cracks. Physical and digital fatigue test results show fatigue strength decreases significantly with increasing distortion ratio, which is the ratio of the minimum and maximum distortion at web gaps. When distortion ratio and nominal stress ratio increased from 0.2 to 0.5, the fatigue strength of 60 mm horizontal gusset plate–vertical​ stiffener web gap and 40 mm vertical stiffener web gap decreases by 43% and 50%, respectively.

Anisotropic fatigue performance of directed energy deposited Ti-6Al-4V: Effects of build orientation

Directed energy deposition (DED), an important branch of additive manufacturing technology, has unique processes that attribute to the fatigue performance anisotropy associated with the build orientation of DED materials. However, the mechanism of fatigue performance anisotropy still remains unclear due to multiple factors of the defect and microstructure, which has limited further application of DED materials. This paper investigates the effects of build orientation on the anisotropic fatigue performance of DED Ti-6Al-4V treated with stress relief annealing, specifically the impact of defects and microstructures on fatigue performance anisotropy. The defects in the fully dense DED Ti-6Al-4V are non-directional porosity defects with smooth morphology and high sphericity; the microstructures are mainly characterized by prior-β columnar grains, α colonies and basketweave structures. To represent fatigue performance anisotropy of the material, standard round bar specimens were sampled from three build orientations, i.e., X, Y and Z directions, where Z-direction represents the build direction. These specimens were subsequently subjected to constant amplitude high cycle fatigue tests at the same loading condition and room temperature. The fatigue test results showed that the logarithmic mean fatigue lives of the X- and Y-direction specimens were close to each other, but significantly longer than those of the Z-direction specimens. Fractography and EBSD tests were conducted to explain the causes of fatigue performance anisotropy, suggesting that microstructures are more likely to explain the anisotropy caused by the build orientation than the porosity defects, with crystallographic orientation and α colony being two critical factors.

Finite element modelling of the fatigue damage in an explosive welded Al-dual-phase steel

Abstract In this study, production of dual phase steel-Al 1100 aluminium alloy laminated composite by explosive welding and investigation the microstructural and mechanical properties especially fatigue life of the composite and its components were aimed. Finite element analysis (FEA) was used to simulate the bending fatigue tests of the Al 1100-DP steel (LITEC 1050) bimetal. A semi-wave at the explosive interface with intermetallic was obtained as a microstructural result. Hardness was increased near the interface due to deformation and intermetallic formation at the explosive weld interface. The impact test results showed that the samples’ impact toughness was enriched by fabricating steel-Al laminated composite. The results also showed that the Al 1100-DP steel (LITEC 1050) bimetal composite properties were appropriate to the composite theory and satisfied strength-ductility relations accomplished. Additionally, the bending fatigue test and FEA results were compatible. Moreover, fatigue cracks were observed at the explosive weld interface between Litec DP steel and intermetallic but not at the Al-intermetallic interface.

Parametric study on fatigue behavior of steel friction connections in shear

A parametric study on the fatigue behavior of steel friction connections was conducted in this paper. Fatigue failure mechanism of the connections and two periods of fatigue life were discussed in detail. The procedure to predict the fatigue life of steel friction connections was proposed using the finite element method, and was verified by the fatigue test results. A parametric study was then performed based on the prediction procedure. Considering the connection type, bolt related parameters, and plate related parameters, a total of ten cases were included in the parametric study. The results show that the fatigue strength of single lap connections is much lower than that of double lap connections, which is caused by the local bending of bolted plates and contact status change on the interface. Fatigue behavior of the single lap connections is greatly affected by the number of bolts, steel grade, and hole-making process, while the fatigue behavior of the double lap connections is greatly affected by the bolt preload, slip factor, main plate thickness, steel grade, and hole-making process.

Integrating SMA and CFRP for fatigue strengthening of edge-cracked steel plates

Inclined fatigue cracks of U-ribs and diaphragms in orthotropic steel-box girders are challenging to be repaired using prestressing techniques. Integrating shape memory alloys (SMAs) and carbon fibre reinforced polymers (CFRPs) can achieve a hybrid self-prestressing strengthening approach for such fatigue cracks. To examine the strengthening effect of this approach on inclined cracks, 39 specimens were prepared in four groups and tested under static and fatigue tensioning in this study. Both static and fatigue test results showed that SMA/CFRP composite strengthening effectively retard steel cracking. Compared to control specimens, SMA/CFRP composite strengthening enhanced the notch tip yield load by 128% and 111% for specimens with a notch inclination of 0° and 30°, and the corresponding fatigue life by 7.62 and 8.69 times, respectively. Besides, the strengthening effect of combining SMA and CFRP (SMA/CFRP composite) on cracked steel plates was greater than that of either SMA or CFRP alone, particularly for the steel plates with an inclined crack. Finally, the calculation of mixed-mode I/II effective stress intensity factors confirmed the superior strengthening effect of SMA/CFRP composite on steel plates with an inclined crack. The findings of this study can provide a reference for the reinforcement of orthotropic steel-box girders.

Assessment of nylon versus polyester ropes for mooring of floating wind turbines

With the development of floating wind turbines comes the need for mooring systems suitable for shallow water depths. Compliance becomes challenging for mooring systems composed of chain and wire rope. Polyester ropes can improve compliance, but the even lower stiffness of nylon may further reduce mooring line loads. However, fatigue lifetime of nylon ropes has been limiting. This paper presents results of stiffness and fatigue tests on long lay polyester and nylon ropes and evaluates the performance of two mooring systems for a floating wind turbine. The fibre ropes’ complex stiffness behaviour is modelled by the Syrope model, with mooring analyses based on fully coupled time-domain simulations. Nylon ropes reduce the dynamic loads in the mooring system, particularly wave frequency loads. This improves the fatigue lifetime of all components, allowing a reduction of the chain cross-section area by ∼40% compared to the polyester-based mooring system. The long lay nylon ropes show excellent fatigue properties. With the reduced loading, the lifetime is longer for the nylon ropes than the polyester ropes. Ultimate loads are also reduced when using nylon, allowing reduction of the breaking load by ∼30%. This gives a thinner and lighter rope than the equivalent polyester rope.

Life prediction method based on damage mechanism for titanium alloy TC4 under multiaxial thermo-mechanical fatigue loading

In this paper, a life prediction method based on damage mechanism for titanium alloy TC4 under multiaxial thermo-mechanical fatigue loading is proposed. Temperature and time dependent influence factors are proposed and introduced into the strain amplitude-life curve method to consider the temperature and time dependence of damage, respectively. The increase in fatigue and oxidation damages caused by tensile mean stress and non-proportional additional hardening is expressed by the mean stress in Macaulay brackets and by using a multiaxial damage parameter, respectively. A fast cracking influence factor is proposed to consider the influence of fast cracking of materials caused by the combined action of high temperature, tensile stress and shear stress on damage behavior. The material constants are identified by test data under uniaxial and proportional loadings, and are further used for prediction under non-proportional loading. The failure life results of uniaxial isothermal fatigue tests with and without dwell time, uniaxial and multiaxial thermo-mechanical fatigue tests are used to verify the proposed method. The value of the scatter band of life prediction result is less than ± 2, as well as the maximum and mean relative errors between predicted and experimental strain amplitudes are 18.49% and 6.54%, respectively.

Vibration fatigue behavior and life prediction of directionally solidified superalloy based on the phase transformation theory

Directionally solidified nickel base superalloy DZ125L is the main material that has been widely used to manufacture aviation gas turbine blades and hot-end structural parts. Aeroengine is under vibration load for a long time in the service process, which makes vibration fatigue become one of the main failure modes of aero-engine structures. In this study, vibration fatigue behavior and excitation frequency response curves of DZ125L alloy under different stress levels were investigated through vibration fatigue tests. The fatigue crack initiation and growth mechanism of DZ125L alloy under vibration load is studied based on the results of the vibration fatigue test and finite element simulation. A vibration fatigue life prediction model based on the phase transformation theory is developed. Each parameter has clear physical significance in the model. The comparison between the model prediction results and the test results shows that the theoretical prediction results have reasonable accuracy.

Gigacycle Fatigue Properties of Wrought Aluminum Alloys

Fatigue data sheets of the National Institute for Materials Science disclose the results of gigacycle fatigue tests conducted on wrought aluminum alloys. The materials tested were hot-rolled plates and extruded round bars of A5083P-O (two heats), A7075-T6 (three heats), and A6061-T6 (three heats). The fatigue testing methods were rotating-bending at 100 Hz, ultrasonic at 20 kHz, and uniaxial at 100 Hz. The rotating-bending fatigue tests were conducted for 3 years to reach 1010 cycles. The ultrasonic and uniaxial fatigue tests were also conducted at high stress ratios. This study analyzed the fatigue life data, clarifying frequency effects, presence or absence of fatigue limits, and stress ratio effects. The three types of fatigue tests revealed equivalent results, meaning that frequency effects and stress gradient effects were negligible in the wrought aluminum alloys. Frequency effects were visible in several cast aluminum alloys owing to the influence of humidity, but the wrought aluminum alloys were observed to be insensitive to humidity. Many specimens were fractured at >107 cycles, whereas the degradation of fatigue strength after 107 cycles was very small in A5083P-O but large in A7075-T6 and A6061-T6. A5083P-O revealed fatigue limits at around 107 cycles. The heat treatment effects determined the presence or absence of fatigue limits because the equivalent tensile strengths between A6061-T6 and A5083P-O excluded the possibility of tensile strength effects. The stress ratio effects on gigacycle fatigue strengths of the wrought aluminum alloys are reasonable and can be estimated in conventional ways.

Rational relationship between the fatigue curves of asphalt mixes obtained from tension/compression and 4-point bending tests

The fatigue properties of asphalt mixes are usually established according to a series of laboratory cyclic loading tests at fixed temperature (θ) and frequency (f). Unfortunately, the results are different when considering different types of tests. This paper proposes a rational method to link the 4-point bending (4PB) and tension/compression (TC) fatigue test results. First a theoretical analysis of the 4PB fatigue test is presented. This analysis allows to introduce a link between the fatigue curves (modulus or damage versus number of cycles) of this type of test with the results from uniform TC tests. The 4PB fatigue curve is superimposed with the TC fatigue curve if the reference strain amplitude is correctly chosen. This amplitude must not be the maximum strain at the boundary fibre of the beam (ε 0max), as usually considered, but a lower proposed value (ε 0h0). Then, the previous approach is extended to the fatigue failure Wöhler’s law, which is a line in the Log–Log plot of the number of cycles at failure versus the loading amplitude (stress or strain). An experimental campaign on a Canadian asphalt concrete (HMAC) validates the developped approach. It is an important output from this research as fatigue life from one type of test can be obtained from the other type of test in the case of strain (or displacement) control tests.

An investigation on fatigue crack propagation behavior of welded connections in UHPC reinforced OSDs using 3D crack simulation

Fatigue performance of orthotropic steel decks (OSDs) is one of the concerns that should be carefully considered, since OSD is reported as one of the most structures experiencing fatigue defects. In this study, the fatigue crack propagation behavior of welded connections in OSDs was investigated using 3D crack simulation based on the linear elastic fracture mechanics (LEFM). Fatigue testing results of rib-to-deck and rib-to-floorbeam welded specimens were considered to validate the applicability and efficiency of the crack propagation analysis. Stress intensity factors (SIFs) of the test specimens were computed using the M−integral approach, where Mode I (opening mode) and mixed-mode (i.e., combining Mode I, II, and III) of failure were considered. Additionally, the effect of the applied stress range amplitude, initial crack size, and UHPC layer thickness on the crack propagation behavior of welded joints under consideration were explored. The results indicated that the crack simulation analysis is acceptable and applicable for the evaluation of fatigue crack propagation of welded connections in OSDs. The computed SIFs demonstrated that the governing propagation mode of rib-to-deck and rib-to-floorbeam welded joints is Mode I (opening mode) and the effect of shear and tearing is limited. For both details (i.e., RD and RF welded joints) the decrease of the stress range amplitude and the increase of the UHPC layer thickness considerably slowed the crack propagation, while the increase of the initial crack size significantly accelerated the crack propagation. Comparison of the test and numerical results with the SN curves showed that FAT100 curve of the IIW specification is recommended to predict the fatigue strength of RD and RF welded joints in OSDs reinforced by UHPC layer.