High Cycle Fatigue Tests Research Articles

Effect of layer architecture on the mechanical behavior of accumulative roll bonded interstitial free steel/aluminum composites

Multi-layered interstitial free (IF) steel/aluminum (Al) composites were fabricated by the accumulative roll bonding (ARB) method. Two types of IF steel/Al6061 dissimilar layered metal composites (LMC) with varied stacking of aluminum layers were processed to examine the effect of the layer architecture. Microhardness and uniaxial tensile experiments were applied to analyze the surface and bulk monotonic mechanical properties. Besides, the cyclic mechanical response of the processed materials was investigated via high cycle fatigue (HCF) tests with positive mean stress. Microstructure and mechanical characteristics of composites with various layer architectures were analyzed up to five ARB passes. It is revealed that the monotonic and cyclic performances of all LMCs are significantly enhanced as compared to the base alloy with an aluminum layered structure. Moreover, composites with aluminum as the outer layer exhibited the highest fatigue life, due to crack branching at the interface region during propagation from the softer to the harder layer. Fracture morphology analysis of composites demonstrated that in addition to the significant impact of surface cracks on the outer layers, propagation of cracks initiating from the interface layers led to failure under cyclic loading.

Experimental Evidence of Specimen-Size Effects on EN-AW6082 Aluminum Alloy in VHCF Regime

The present paper investigates the influence of the specimen size of EN-AW6082 wrought aluminium alloy subjected to very high cycle fatigue (VHCF) tests. The hourglass specimens were tested under fully reversed loading condition, up to 109 cycles, by means of the ultrasonic fatigue testing machine developed by Italsigma® (Italy). Three specimens groups were considered, with a diameter in the middle cross-section ranging from 3 mm up to 12 mm. The stress field in the specimens was determined numerically and by strain gauge measurements in correspondence of the cross-section surface. The dispersion of experimental results has been accounted for, and data are reported in P-S-N diagrams. The decrease in fatigue resistance with increasing specimen size is evident. Theoretical explanation for the observed specimen-size effect is provided, based on Fractal Geometry concepts, allowing to obtain scale independent P-S*-N curves. The fatigue life expectation in the VHCF regime of the EN-AW6082 aluminium alloy full-scale components is rather overestimated if it is assessed only from standard small specimens of 3 mm in diameter. Experimental tests carried out on larger specimens, and a proper extrapolation, are required to assure safe structural design.

Microstructure and fatigue damage mechanism of 6082-T6 aluminium alloy welded joint

In this paper, 6mm thick 6082-T6 high-strength aluminium alloy is taken as the research object, and CLOOS ROMAT-3500 industrial robot MIG welder is used for welding. High-cycle fatigue test is carried out on welded joint and 6082-T6 base metal. Microstructure, microhardness, tensile properties and fatigue properties are studied, and fatigue fracture is observed and analyzed. The test results show that: equiaxed crystals exist in the center of the weld. Broken equiaxed crystals are found in the weld due to stirring in the molten pool, a large number of pore and columnar crystal layers are found at the junction of the two beads. A small amount of pore and cellular dendrite are found in the fusion zone. A softening zone with a minimum hardness of 65.5HV exists in the heat affected zone(HAZ). The conditional fatigue limits for base metal and weld specimens are 77.26MPa and 48.69MPa at the set target cycle of 107 respectively. In fatigue fracture, there are a lot of cleavage fracture characteristics and ductile fracture characteristics. The crack initiation is quasi-cleavage fracture characteristic. In the past studies, I have studied the fatigue life of joints, but as the research progresses, I believe that the fatigue behavior and fatigue damage mechanism plays an important role in fatigue life. In this paper, Fatigue fracture process is analyzed by correlation calculation of stress intensity factor. Two crack initiation modes are proposed based on the welding process. The first is fatigue crack initiation caused by stress concentration at the grain boundary where hydrogen elements converged, the second is fatigue crack initiation caused by the gap between equiaxed and columnar crystals inside and outside the central pore of the weld.

An accelerated corrosion-fatigue testing methodology for offshore wind applications

Offshore wind turbines are subjected to cyclic loading conditions during their operational lifespan which is typically between 20 and 25 years. An important issue in fatigue design and integrity assessment of offshore wind turbine foundations is the examination of the long-term fatigue and corrosion-fatigue behaviour of steel structures in the high cycle region. High cycle fatigue tests, particularly at low frequencies in a seawater environment, are time-consuming and costly. Therefore, there is an essential need to perform accelerated tests to predict the long-term behaviour of the structures under realistic operational loading conditions. In this work, the existing fatigue acceleration mechanisms have been reviewed and a novel methodology has been proposed for accelerated testing and analysis of fatigue data in different environments (i.e. air, salt-spray and seawater) at higher temperatures. Two distinct equations have been developed and proposed for the calibration and prediction of S-N fatigue life and crack growth behaviour of steels in different environments. The proposed methodology has been validated through comparison with the existing data in the literature and predictions have been made at operational temperatures using high temperature data. The proposed approach is relatively simple to calibrate for a material of interest and enables accelerating S-N fatigue and crack growth testing of the examined materials by a factor of two and three, respectively. The proposed methodology and the obtained results have been discussed in terms of the need for accelerated testing for fatigue design and integrity assessment of offshore wind monopiles, especially those which are close to the end of initial design life and need a comprehensive engineering analysis for life extension or decommissioning.

Shape Optimization of Rectangular Plates for Vibration-Based Fatigue Testing

Abstract In vibration-based high cycle fatigue testing, a base-excited plate is driven at a high frequency resonant mode until failure. In one vibration-based method involving a cantilevered square plate, a mode often referred to as the “two-stripe” mode is sometimes used because it exists at high frequencies and produces large uniaxial bending stresses along the free edge that are suitable for fatigue testing. The purpose of this work is to precisely investigate how the dimensions of a more generally rectangular plate influence performance when driven at the two-stripe mode. Included are the results of many thousands of modal analysis simulations. From these simulations, general trends with respect to resonant frequencies, frequency isolation, and stress fields in the plate are examined. Results of select geometries were then experimentally validated using a 1000 lb shaker. It is generally shown that, compared with square plates, rectangular plates with 1.37 length-to-width ratio exhibit more favorable stress distributions and frequency isolation. Recommendations are also given for how to quickly select preferable plate dimensions when planning a test based around the operating frequencies of the test setup.

The effect of manufacturing defects on the high-cycle fatigue of electron-beam-welded Ti-6Al-4V titanium alloy: experimental and numerical analysis

PurposeElectron-beam welding has been widely used in industry to join different titanium alloys (Ti-6Al-4V) components. During welding production defects, such as porosity, lack of penetration or thinning are often observed. High-cycle fatigue (HCF) tests have been performed on welded specimens to understand the effect of weld defects on fatigue capabilities. The fatigue life of different types of “defective” welds has been compared against a non-welded reference specimen.Design/methodology/approachThe results of the experimental campaign have been correlated with finite elements models.FindingsIt is concluded the geometry produced by the weld process, e.g. toe radius and under-bead shape, and the related stress raisers play a relevant role on fatigue capabilities of welds. This conclusion is valid only for a Ti-6Al-4V T-joint weld and only for flaw initiation. Knock down in materials properties has not been considered.Originality/valueThere is a lack of HCF fatigue data for welds of this geometry and material in the open literature. The paper is of relevance for industrial application and practical interest, although a lot more validation tests are required to draw a final conclusion.

Effect of Platform Temperature and Post-Processing Heat Treatment on the Fatigue Life of Additively Manufactured AlSi7Mg Alloy

The effect of platform temperature combined with a T5 heat treatment on the fatigue life of additively manufactured aluminum alloy AlSi7Mg was characterized and understood. High-cycle fatigue tests were carried out on samples built with four platform temperatures (35 °C, 60 °C, 80 °C and 200 °C) and post-processing heat treatment strategies (F and T5). Microstructural and fractographic observations combined with microhardness measurements were performed. A log-normal statistical distribution regressed with 90% B-basis probabilities of survival revealed that specimens produced on a platform maintained at 80 °C and post-processed with a T5 heat treatment presented the highest fatigue life among the conditions tested. Precipitation of silicon within the aluminum cells during the T5 heat treatment is the proposed explanation for the improved fatigue life of the T5 samples. In the as-built condition, specimens produced at 200 °C were found to be less resistant to fatigue than the specimens built at lower temperatures. The coarser microstructure and lowest microhardness resulting from high-temperature manufacturing explain this reduced fatigue strength. All fatigue cracks initiated from manufacturing discontinuities. This led to a fatigue life prediction model based upon linear elastic fracture mechanics. The model was fitted to the experimental results of the F and T5 samples separately. With the exception of the 35 °C—T5 specimens, the predicted fatigue lives agree with the experimental results and literature.

A new numerical approach to estimate stress intensity factors under very high cycle fatigue testing

Ultrasonic very high cycle fatigue testing finds many advantages in fatigue studies especially in very low fatigue crack growth rate investigations. In this application, the determination of the stress intensity factor has a key role and presenting a general approach to calculate stress intensity factor in reasonable time and ultimate accuracy is necessary. The present research proposes a new numerical approach (hybrid method) based on reduce order modeling (dynamic substructuring), multi-harmonic balance method, and M-integral to evaluate stress intensity factors under very high cycle fatigue testing in the most accurate and fast way possible. The verification of the hybrid method was done through a benchmark and implicit solver implemented in ABAQUS commercial software as the reference solution. Investigations proved that the presented hybrid method here could determine stress intensity factor in ultrasonic regime with the ultimate accuracy and save computational time at least 50%.

Fatigue and fracture behavior of ultrafine-grained near β titanium alloy produced by radial shear rolling and subsequent aging

The fatigue and fracture behavior of an ultrafine-grained near β Ti-5Al-5V-5Mo-1Cr-1Fe alloy produced by radial shear rolling and subsequent aging was investigated by very high cycle fatigue (VHCF) testing with the stress ratio R = −1. It was shown that the formed structure contributes to a high fatigue strength of the alloy in tests with a stress amplitude of 850 MPa and the number of cycles to failure 2.78 · 107. Fractographic analysis revealed an internal mode of fatigue crack development with ΔK values higher than 27 MPa m1/2. The fatigue crack initiation region was an area about 25 μm in diameter showing the presence of up to 10 μm long microcracks and micron-sized microvoids. The fatigue microcracks initiated at the interfaces between the primary α (α") phase particles and the transformed β matrix. A fatigue fracture model of the ultrafine-grained titanium alloy under VHCF loading was proposed which agrees well with the obtained experimental data.

The Effect of Polyurea Coating on Fatigue Life of Mild Steel

The main objective of this study is to investigate experimentally the effect of polyurea coating on fatigue strength of grade-A marine steel (Mild steel). Some preliminary specimens are prepared by different cutting and surface finishing methods, using two types of primer. Based on roughness measurement of surface and pull-off test results, the milling method for cutting and grinding machine method for surface preparation with Rayston as a primer are used. Mechanical properties of steel and coating are determined by applying tensile tests on five specimens. The results of tensile tests show that polyurea coating is low-strength, high-ductile material with good energy absorption capability. Then thirty specimens are fabricated and high-cycle axial fatigue tests with a stress ratio of 0.05 and maximum stress as a fraction of yield stress are employed. The fatigue strength of coated specimens can be as more than 128% of uncoated specimens in high-cycle fatigue. This is due to the high absorption of energy by coating when the applied stress is in the elastic range of steel. In low cycle fatigue tests, the results of coated specimens are almost the same as for uncoated specimens. It was concluded that the type of primer has a very significant effect on the adhesion and fatigue strength of coated specimens.

Experimental and crystal plasticity modelling study on the crack initiation in micro-texture regions of Ti-6Al-4V during high cycle fatigue tests

Micro-textured regions (MTRs) are the presence of large regions of α grains aggregates with strong local texture, which are detrimental to the fatigue performances in titanium alloys. This work proposes an EBSD map-image based crystal plasticity model to study the fatigue crack initiation in MTRs. Crystal plasticity simulations reveal that the crack initiation may be attributed to the significantly high plastic strain accumulated on prismatic a slip systems leading to significantly high dislocation pile-ups, and high stress concentrations on the cluster of hard grains leading to the stress redistribution and cleavage fracture of the specimen.

Fatigue behaviors of 2205 duplex stainless steel with gradient nanostructured surface layer

Fatigue behaviors of 2205 duplex stainless steel (initially composed of austenite and ferrite phase) with a gradient nanostructured (GNS) surface layer induced by the ultrasonic rolling process (USRP), are investigated in both strain-controlled high-cycle fatigue (HCF) and low-cycle fatigue (LCF) tests. Results showed that the fatigue life is improved in HCF (~5 times longer under 0.3% strain) but decreased in LCF (~53% under 0.8% strain). This is different from the simultaneous enhancement of fatigue behaviors in both HCF and LCF in other materials with GNS. The effect of martensite phase transformation (MPT), residual compressive stress as well as the GNS layers have been investigated to clarify the mechanisms of the fatigue behavior of the USRP samples. Besides the suppression of the surface crack initiation and propagation by residual compressive stress and GNS layers, the MPT during the USRP and cyclic loading processes greatly affects the fatigue behaviors. For HCF tests, not only the hard-brittle martensite phase induced by the USRP process improved the material strength but also the MPT process during cyclic loading absorbed the strain energy released from the crack growth, which increased the HCF life. However, for LCF tests, the hard-brittle martensite phase decreased the ductility of the GNS layer and accelerated the crack growth rate under higher strain amplitude, resulting in the decrement of LCF life. This work presents the first investigation on fatigue behaviors of duplex stainless steel with GNS structure.

Stress concentrations and fatigue behavior of bird-beak SHS overlap K-joints subjected to brace in-plane force

This paper focuses on the stress concentration factors (SCFs) and fatigue behavior of bird-beak square hollow section overlap K-joints under brace in-plane force. Four bird-beak overlap K-joint specimens were statically tested to determine the SCFs and to predict the critical cracking locations. Subsequently, high-cycle fatigue tests were performed to investigate the fatigue behavior including crack initiation and propagation, fatigue life and rigidity degradation. The results demonstrate that the crack always initiated at the locations with the greatest SCFs, specifically, at the overlap region and at the brace crown area for square and diamond bird-beak joints, respectively. The experimental fatigue lives were compared with the predictions by the International Institute of Welding (IIW) Srhs-N design curves. The comparison shows that the IIW design curve for conventional joints can conservatively estimate the fatigue life of bird-beak overlap K-joints.

A test method for manufacturing effect assessment in HCF

Surface conditions produced by manufacturing processes are known to significantly affect the fatigue strength of metallic materials. However, fatigue tests on planar specimens usually present the onset of fatigue crack in a specimen edge, where the superposition of different machining techniques and subsequent machining operations can hide the effects of the studied process. This study deals with the design of a fatigue test able to reliably assess the effects on the high cycle fatigue strength of any machining process used to produce planar surfaces. It was devised a test configuration where the specimen geometry and the loading conditions are aimed to locate the crack nucleation in the middle of the surface machined by the investigated process. This condition is achieved by employing a notched specimen and a three-point bending test configuration, where the position and geometry of the notches are designed to relieve the specimen’s edges. Nevertheless, in the critical region, the stress state is quite uniaxial and uniform in the longitudinal direction. The specimen is manufactured by using the same parameters and processes adopted for full-scale components, thus producing a surface with the same properties as the real component. The designed test configuration was validated in a high cycle fatigue test aimed to compare the effects of broaching and wire electrical discharge machining on the Wöhler curve (S-N curve). The specimen geometry was tailored for a broaching process so that it has sufficient stiffness both in the directions parallel and perpendicular to the broaching direction. Fatigue tests were carried out on a resonant machine, that allowed to detect the crack nucleation by monitoring the test frequency.

Probability-based service safety life prediction approach of raw and treated turbine blades regarding combined cycle fatigue

To avoid the use of specific conversion coefficients with high expense and unacceptable prediction accuracy, a probability-based prediction method is proposed by considering probabilistic feature parameters, to predict the service safety life (SSL) of aeroengine turbine blades. The direct correlation between laboratory remaining life (LRL) and SSL was firstly established by considering probabilistic feature parameters. By conducting Combined high and low Cycle Fatigue (CCF) tests of turbine blades, the effectiveness of the developed method was validated based on the failure event. The proposed method was further verified by predicting the SSL of treated blades with certain operation time. In respect of the studies, it is illustrated that (1) the SSL of turbine blade can be reasonably reflected by the LRL in respect of probabilistic feature parameters; (2) the prediction errors of the raw and treated blades are 2.2% and 12.7%, respectively, indicating that the developed probability-based prediction method has acceptable prediction precision and is an effective method in the SSL prediction of aeroengine turbine blades; (3) the developed method needs less samples than the specific conversion coefficients method, indicating that the SSL prediction of turbine blade needs fewer time and costs. The efforts of this study provide a promising approach for the SSL prediction of turbine blades, offer a useful guidance for the service life management of aeroengine turbine blades to reduce the cost of expense and time and enhance the safety of aeroengine operation.

Prediction of combined cycle fatigue life of TC11 alloy based on modified nonlinear cumulative damage model

The nonlinear cumulative damage model is modified to have high prediction accuracy when the high-low cycle stress frequency ratio m is large (m ≥ 500). The low cycle fatigue (LCF) tests, high cycle fatigue (HCF) tests and combined high and low cycle fatigue (CCF) tests of TC11 titanium alloy were carried out, and the influencing factors of CCF life were analysed. The CCF life declines with the decrease of the ratio of high-low cycle stress frequency m. Both linear and nonlinear cumulative damage models are used to predict the CCF life. The CCF life prediction error of the linear cumulative damage model is great and the predictions tend to be overestimated, which is dangerous for engineering application. The accuracy is relatively high when the high-low cycle stress frequency ratio m ≤ 500. The accuracy of nonlinear cumulative damage model is higher than that of linear model when the high-low cycle stress frequency ratio m ≥ 500. Based on the relationship between high cycle average stress σmajor and material yield limit σp,0.2, a correction term is added to the nonlinear cumulative damage model and verified, which made the modified model more accurate when m ≥ 500.

Defect-correlated fatigue resistance of additively manufactured Al-Mg4.5Mn alloy with in situ micro-rolling

Hybrid in situ rolled wire + arc additive manufacturing (HRAM) is a potential 3D printing method that combines traditional wire and arc additive manufacturing (WAAM) with in situ micro-rolling. Here, the manufacturing defects and mechanical and fatigue strength were explored for an HRAM- and WAAM-processed Al-Mg4.5 Mn alloy. The results show that advanced HRAM can result in significant grain refinement and enhance the material failure resistance compared with standard WAAM. High-resolution X-ray tomography was adopted to identify the defect density, morphology, distribution, and size, which showed that the defect population and size produced by HRAM were considerably lower than those produced by WAAM. Based on high-cycle fatigue tests, fatigue failure was observed entirely from metallurgical defects. Finite element simulations of the defect images based on computed tomography scans showed that lack-of-fusion defects are more dangerous than gas porosity in terms of increasing the local stress. Finally, the defect-controlled fatigue lifetime was predicted by combining the peaks-over-threshold method with a modified NASGRO model.

Dislocation Source and Pile-up in a Twinning-induced Plasticity Steel at High-Cycle Fatigue

Dislocation behaviour of a twinning-induced plasticity (TWIP) steel subjected to high-cycle fatigue tests is investigated in the present study. Grain boundaries are the important sources of dislocation generation during fatigue tests, contributing to the increase in dislocation density. Continuous emission of dislocations from grain boundaries is observed in many grains. Inclusions can sustain large dislocation pile-ups at the inclusion interfaces, leading to a high stress concentration and therefore acting as potential sites of microcrack nucleation. In contrast, annealing twin boundaries are relatively weak boundaries for dislocation pile-ups. When the number of dislocations in a pile-up is large, dislocations can crossover twin boundaries and glide inside the annealing twins. The stress concentration at the twin boundary is relatively low so that twin boundaries could not act as the sites for microcrack initiation.

Distributed Fiber Optic Sensor-Based Strain Monitoring of a Riveted Bridge Joint Under Fatigue Loading

Riveted steel bridges, which were built in the early 20th century, require their regular structural integrity assessment to avoid any catastrophic failure. This article presents continuous strain monitoring of a single riveted lap joint, which is a representative critical element of riveted steel bridges through an optical frequency domain reflectometry (OFDR)-based distributed fiber optic sensor (DFOS). The aim of this study was to instrument a DFOS on a single riveted lap joint for monitoring the surface and critical strains experienced by the rivet joint under two fatigue loading conditions and also to compare the strain transfer between the two commonly used adhesives for bonding the DFOS. Initially, through finite element analysis (FEA), a location for installing the DFOS was identified, and also a strategy was developed for monitoring the critical location of the joint during fatigue loading. Subsequently, the DFOS was instrumented on the riveted joint at the identified location in two segments, where similar strain levels were expected with the aid of two types of adhesives: cyanoacrylate and epoxy. The strains on the rivet joint were monitored under high cycle fatigue (HCF) for up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times {10}^{6}$ </tex-math></inline-formula> loading cycles with constant stress amplitude and followed by low cycle fatigue (LCF) loading with increasing stress amplitude until the failure of the specimen. The results showed that the DFOS could continuously sense the cyclic peak strain of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 223\,\,\mu \varepsilon $ </tex-math></inline-formula> under HCF conditions and a peak maximum strain of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$- 1244\,\,\mu \varepsilon $ </tex-math></inline-formula> under LCF conditions. Furthermore, the internal critical strain on the rivet joint during loading could be monitored with the application of the developed damage monitoring strategy and DFOS strain data. Finally, the DFOS segment bonded using cyanoacrylate measured marginally high strains than epoxy adhesive during the HCF test.

Improvement of experimental device for high cycle fatigue tests of copper alloy strips and characterizing fatigue properties

Improvement of experimental device for high cycle fatigue tests of copper alloy strips and characterizing fatigue properties