Higher Number Of Load Cycles Research Articles

A numerically efficient phase‐field model for fatigue fracture – 1D analysis

AbstractFatigue loads and fatigue failure come along with high numbers of load cycles. This makes the simulation of fatigue fracture computationally very demanding, if each load cycle is simulated comprehensively with its loading and unloading phase. We present a numerically efficient method for the simulation of fatigue fracture, which avoids resolving the loading path and instead requires only one increment per load cycle at most.We combine the phase‐field method for brittle fracture with a classic fatigue concept for lifetime calculation of components, which is applied to the material point here. A local lifetime variable is obtained, which we use to modify the material resistance incrementally in order to consider the progressive weakening of the material. Since we make use of an elasto‐plastic revaluation technique, a linear‐elastic material model is sufficient in the simulations. Furthermore, the phase‐field method allows to describe fatigue crack initiation as well as propagation.In this contribution we describe the method's framework including the mentioned lifetime concept, followed by a proposal for a simulation scheme. Finally, results – for now limited to the one‐dimensional case and constant load amplitudes – are discussed.

Contact resistance between two REBCO tapes: the effects of cyclic loading and surface coating

No-insulation (NI) superconducting REBCO magnets have advantages of self-quench-protection, a very high engineering current density and high mechanical strength, and the potential to reach very high magnetic fields. However, NI REBCO magnets have drawbacks of a long magnet charging time and high field ramp losses. These can be mitigated by controlling the turn-to-turn contact resistivity (Rc). In an effort to control Rc, we consider two approaches. One is coating a REBCO conductor with various resistive thin films, and the other is to use a stainless steel (SS) tape as an interlayer which is also coated with different metallic films. We present experimental results of Rc of an as-received sample under cyclic contact pressure of 2.5–25 MPa up to 30 000 cycles. After an initial increase in Rc for the first 10–20 cycles, Rc decreases to about one tenth of its initial value after a few hundred cycles. A warm-up and cool-down thermal cycle does not significantly change the low Rc resulting from a previously high number of load cycles. We also studied Rc of REBCO tapes that are coated with different resistive layers and interlayers. In order to increase Rc, we experimented with electro- and electroless plating of Ni, Cr, and Ni–P. We also measured Rc with a thin metallic interlayer as a coil co-winding material which included Cu, SS, and SS plated with Ni and Cu. A SS interlayer increases Rc by about three orders of magnitude; while the Cu plated SS interlayer only increases Rc by one order of magnitude. Finally, we treated the as-received REBCO surface by oxidation using an Ebonol® C solution. This controlled oxidation allowed the Rc to be controlled over a wide range.

A numerical study of the return mapping application for the subloading surface model

PurposeMany practical problems in engineering require fast, accurate numerical results. In particular, in cyclic plasticity or fatigue simulations, the high number of loading cycles increases the computation effort and time. The purpose of this study is to show that the return mapping technique in the framework of unconventional plasticity theories is a good compromise between efficiency and accuracy in finite element analyses.Design/methodology/approachThe accuracy of the closest point projection method and the cutting plane method implementations for the subloading surface model are discussed under different loading conditions by analyzing the error as a function of the input step size and the efficiency of the algorithms.FindingsMonotonic tests show that the two different implicit integration schemes have the same accuracy and are in good agreement with the solution obtained using an explicit forward Euler scheme, even for large input steps. However, the closest point projection method seems to describe better the evolution of the similarity centre in the cyclic loading analyses.Practical implicationsThe purpose of this work is to show two alternative implicit integration schemes of the extended subloading surface method for metallic materials. The backward Euler integrations can guarantee a good description of the material behaviour and, at the same time, reduce the computational cost. This aspect is particularly important in the field of low or high cycle fatigue, because of the large number of cycles involved.Originality/valueA detailed description of both the cutting plane and closest point projection methods is offered in this work. In particular, the two integrations schemes are compared in terms of accuracy and computation time for monotonic and cyclic loading tests.

Design of Shaft Respecting the Fatigue Limit for Ultra-High Number of Cycles

Advances in engineering solutions in recent few decades caused that conventional fatigue limit (for steels and cast irons given for number of 107 cycles) is no more sufficient. Construction parts of newly introduced transport vehicles, operating at high velocities and at long distances, reach during their lifetime very high numbers of loading cycles, in order of 109. For this reason, values of fatigue limits for 109 cycles must be considered in design and construction of transport vehicles. In this study, authors present how dramatically will change the design of shaft, when fatigue limit for 109 cycles is considered.

Pedicle screw anchorage of carbon fiber-reinforced PEEK screws under cyclic loading

PurposePedicle screw loosening is a common and significant complication after posterior spinal instrumentation, particularly in osteoporosis. Radiolucent carbon fiber-reinforced polyetheretherketone (CF/PEEK) pedicle screws have been developed recently to overcome drawbacks of conventional metallic screws, such as metal-induced imaging artifacts and interference with postoperative radiotherapy. Beyond radiolucency, CF/PEEK may also be advantageous over standard titanium in terms of pedicle screw loosening due to its unique material properties. However, screw anchorage and loosening of CF/PEEK pedicle screws have not been evaluated yet. The aim of this biomechanical study therefore was to evaluate whether the use of this alternative nonmetallic pedicle screw material affects screw loosening. The hypotheses tested were that (1) nonmetallic CF/PEEK pedicle screws resist an equal or higher number of load cycles until loosening than standard titanium screws and that (2) PMMA cement augmentation further increases the number of load cycles until loosening of CF/PEEK screws.MethodsIn the first part of the study, left and right pedicles of ten cadaveric lumbar vertebrae (BMD 70.8 mg/cm3 ± 14.5) were randomly instrumented with either CF/PEEK or standard titanium pedicle screws. In the second part, left and right pedicles of ten vertebrae (BMD 56.3 mg/cm3 ± 15.8) were randomly instrumented with either PMMA-augmented or nonaugmented CF/PEEK pedicle screws. Each pedicle screw was subjected to cyclic cranio-caudal loading (initial load ranging from − 50 N to + 50 N) with stepwise increasing compressive loads (5 N every 100 cycles) until loosening or a maximum of 10,000 cycles. Angular screw motion (“screw toggling”) within the vertebra was measured with a 3D motion analysis system every 100 cycles and by stress fluoroscopy every 500 cycles.ResultsThe nonmetallic CF/PEEK pedicle screws resisted a similar number of load cycles until loosening as the contralateral standard titanium screws (3701 ± 1228 vs. 3751 ± 1614 load cycles, p = 0.89). PMMA cement augmentation of CF/PEEK pedicle screws furthermore significantly increased the mean number of load cycles until loosening by 1.63-fold (5100 ± 1933 in augmented vs. 3130 ± 2132 in nonaugmented CF/PEEK screws, p = 0.015). In addition, angular screw motion assessed by stress fluoroscopy was significantly smaller in augmented than in nonaugmented CF/PEEK screws before as well as after failure.ConclusionsUsing nonmetallic CF/PEEK instead of standard titanium as pedicle screw material did not affect screw loosening in the chosen test setup, whereas cement augmentation enhanced screw anchorage of CF/PEEK screws. While comparable to titanium screws in terms of screw loosening, radiolucent CF/PEEK pedicle screws offer the significant advantage of not interfering with postoperative imaging and radiotherapy.Graphical abstractThese slides can be retrieved under Electronic Supplementary Material.

Exploring the recovery of fatigue damage in bituminous mixtures: the role of rest periods

Recovery capability of bituminous materials plays a significant role in the development of new technologies for extending the service life of asphalt pavements. This capability originates from various phenomena such as thixotropy, cooling, relaxation of hardening, or healing. However, their real effect on mechanical response is not clear. This article aims to investigate how rest periods (RPs) available between traffic loads can contribute to the damage recovery of bituminous materials. For this purpose, different types and durations of RPs were applied during the laboratory evaluation of fatigue resistance of these materials using the University of Granada Fatigue Asphalt Cracking Test method. The results indicate that the addition of RPs to the loading regime could lead to an extension in the fatigue life of bituminous materials. Additionally, an increase in the RP duration showed a positive impact on the resistance of the materials against cyclic loading. Nonetheless, these benefits are not only related to the recovery of lost properties during RPs, but also a growth in the amount of plastic deformations as a result of the applying RPs could delay the appearance of damages (i.e. cracking). Consequently, the bituminous material can tolerate a higher number of load cycles during fatigue test.

Influence of structure sensitising of the AlSi 316Ti austenitic stainless steel on the ultra-high cycle fatigue properties

Austenitic stainless steels are the wide-spread materials, used mainly in the power industry. In that kind of engineering application, structural parts of rotating elements reach during their lifetime very high numbers of loading cycles, exceeding 107 numbers of cycles. With regard to this fact, the data of ultra-high cycle fatigue properties are needed to be used in the qualified design. Increasing demands on the efficiency cause the increase of the operating temperature, and exposition of these materials to the elevated temperatures can cause some important structural changes, which result in the sensitising of the structure. In this study authors present their own experimental results about fatigue properties of AISI 316Ti austenitic stainless steel after sensitising, in the ultra-high cycle region (Nf = 106 ~ Nf = 3×109 cycles). Fatigue tests were carried out using ultrasonic fatigue testing device with frequency f = 20 kHz at the coefficient of cycle asymmetry R = -1, and temperature T = 20±5°C. In the ultra-high cycle region was observed the continuous decrease of the fatigue properties of the AISI 316Ti, and there was recorded the negative effect of the sensitising on the ultra-high cycle fatigue properties of the AISI 316Ti.

Influence of surface morphology on the very high cycle fatigue behavior of metastable and stable austenitic Cr-Ni steels

The present study investigates conventional and cryogenically turned specimens of metastable austenitic steel AISI 347 and stable austenitic steel AISI 904L in the VHCF regime. The cryogenic turning process includes cooling by CO2 snow and generates a surface layer on the specimens of metastable austenitic steel, which is characterized by a phase transformation from paramagnetic fcc - austenite to ferromagnetic bcc - martensite and grain refinement. The stable austenitic steel retains its purely austenitic structure after cryogenic turning, but also shows grain refinement in the surface layer. The specimens with different surface morphology were cyclically loaded at ambient temperature using an ultrasonic fatigue testing system developed and built at the authors’ institute. The testing machine operates at frequencies of approx. 20 kHz to achieve high numbers of load cycles within a reasonable time. To avoid self heating of the specimen, the tests were performed in pulse-pause mode. All specimens were tested with a load ratio of R = -1. During cyclic loading, the metastable austenitic steel partially transformed from paramagnetic fcc - austenite to ferromagnetic bcc - martensite, while no phase transformation could be detected in the stable austenitic steel.

Timing of PMMA cement application for pedicle screw augmentation affects screw anchorage

IntroductionCement augmentation is an established method to increase the pedicle screw (PS) anchorage in osteoporotic vertebral bodies. The ideal timing for augmentation when a reposition maneuver is necessary is controversial. While augmentation of the PS before reposition maneuver may increase the force applied it on the vertebrae, it bears the risk to impair PS anchorage, whereas augmenting the PS after the maneuver may restore this anchorage and prevent early screw loosening. The purpose of the present study was to evaluate the effect of cement application timing on PS anchorage in the osteoporotic vertebral body.MethodsTen lumbar vertebrae (L1–L5) were used for testing. The left and right pedicles of each vertebra were instrumented with the same PS size and used for pairwise comparison of the two timing points for augmentation. For the reposition maneuver, the left PS was loaded axially under displacement control (2 × ±2 mm, 3 × ±6 mm, 3 × ±10 mm) to simulate a reposition maneuver. Subsequently, both PS were augmented with 2 ml PMMA cement. The same force as measured during the left PS maneuver was applied to the previously augmented right hand side PS [2 × F (±2 mm), 3 × F (±6 mm), 3 × F (±10 mm)]. Both PS were cyclically loaded with initial forces of +50 and −50 N, while the lower force was increased by 5 N every 100 cycles until total failure of the PS. The PS motion was measured with a 3D motion analysis system. After cyclic loading stress, X-rays were taken to identify the PS loosening mechanism.ResultsIn comparison with PS augmented prior to the reposition maneuver, PS augmented after the reposition maneuver showed a significant higher number of load cycles until failure (5930 ± 1899 vs 3830 ± 1706, p = 0.015). The predominant loosening mechanism for PS augmented after the reposition maneuver was PS toggling with the attached cement cloud within the trabecular bone. While PS augmented prior to the reposition, maneuver showed a motion of the screw within the cement cloud.ConclusionThe time of cement application has an effect on PS anchorage in the osteoporotic vertebral body if a reposition maneuver of the instrumented vertebrae is carried out. PS augmented after the reposition maneuver showed a significant higher number of load cycles until screw loosening.

Effects of a large number of cycles on pile shaft resistance analyzed at the grain scale using x-ray tomography

This study presents the results of laboratory-scale cyclic loading tests performed on an instrumented pile in sand using x-ray micro tomography and three-dimensional (3D) image analysis techniques. The macroscopic behaviour of sand-pile interface shows a two–phases evolution during cycles with a non negligible increase of shaft friction in the second phase. A discrete version of Digital Image Correlation (DIC) is employed to analyze quantitatively the mechanisms occurring at the grain scale. Using segmented images this method is able to correlate and follow each grain individually between two configurations. Displacement fields are measured and compared for different amounts of cycles. Grain breakage and density evolution are investigated using grey level measurements. The results provide a better understanding of the phenomena observed at the macroscale for a high number of load cycles.

Behavior of geocell-reinforced granular base under repeated loading

Abstract Major factors that influence the design of paved and unpaved roads are the strength and stiffness of the pavement layers. Among other factors, the strength of pavements depends on the thickness and quality of the aggregates used in the pavement base layer. The strength of pavement increases with increase in the thickness of the base which has a direct implication on construction cost whereas decreasing the thickness of the base makes it weak which results in low load bearing capacity especially for unpaved roads. Due to the lack of availability of aggregates, there is a need to identify alternative methods to minimize the thickness of the base without compromising on the strength of base. The use of geocell, a type of geosynthetics is a potential and reliable solution and studies are conducted in this direction. In the current study the performance of geocell reinforced base layer subjected to repeated loading is investigated. The results of the study are examined in terms of the resilient deformation and permanent deformation for different thicknesses of base layers under different repeated loading. The results showed that geocell reinforces the unbound aggregate thus reducing the deformation along with the reduction in the required thickness of the aggregate layer of unpaved roads. The amount of reduction of permanent deformation is found to be more for reinforced layer with higher thickness. The results also showed that the reinforced layers achieved maximum rut depth reduction compared to unreinforced section. The permanent deformation model modified for the geocell reinforced condition is calibrated using numerical analysis and the model can be used to predict the deformations for higher number of load cycles.

Effects of residual stresses and compressive mean stresses on the fatigue strength of longitudinal fillet-welded gussets

AbstractResults from fatigue testing of small-scale specimens are widely used to study residual stress effects on fatigue of welded structures. It was observed from a literature that welding distortion may cover residual stress effects due to high-bending stresses from clamping. Here, presented are fatigue test results and results from residual stress measurements from welded longitudinal stiffeners in different residual stress conditions. The axial welding distortion was corrected for all samples by straightening reducing effects from clamping and making residual stress effects visible. It was found that the fatigue strength depends strongly on the stabilized residual stresses, especially at high numbers of load cycles. It could be shown that residual stresses at the weld toe either were far below the yield strength or were degraded mainly at the first load cycle but still have major effects. This was investigated at two stress ratios of R = −1 and R = −3.

Very high cycle fatigue of wrought magnesium alloy AZ61

Wrought magnesium alloys show a high strength to weight ratio, which makes them potentially attractive for load bearing components in automotive applications. Structural parts may be subjected to very high numbers of load cycles, and therefore the very high cycle fatigue properties of these materials are of great interest. Fatigue tests in laboratory air were carried out on wrought magnesium alloy AZ61 using ultrasonic fatigue testing equipment. The fatigue strength at 109 cycles is 98 MPa, which is 32% of the tensile strength. Fatigue cracks are predominantly initiated from slip bands at the surface. Near threshold fatigue crack growth was studied in ambient air and vacuum and showed a dominant influence of the environment. The threshold stress intensity amplitude at load ratio R=-1 is Ka,th = 1.1 MPam1/2 in air and 1.9 MPam1/2 in vacuum. Crack propagation as small as 10-12 m/cycle could be documented in vacuum. Lowest growth rates observed in ambient air were about 10-10 m/cycle. In vacuum, a transgranular and ductile crack path is found for all investigated crack growth rates. Corrosive processes induced by ambient air cause a transition from ductile to brittle failure in the near threshold regime.

Modeling and simulation of temperature-dependent cyclic plastic deformation of austenitic stainless steels at the VHCF limit

The exploration of fatigue mechanisms in the VHCF regime is gaining importance since many components have to withstand a very high number of loading cycles due to high frequency or long product life. In this regime, the period of fatigue crack initiation and thus the localization of plastic deformation play an important role. The material that was investigated in this study is the metastable austenitic stainless steel AISI 304 in the initially purely austenitic condition. The experimental investigations during quasi-isothermal fatigue tests revealed that a moderate increase of temperature from room temperature up to 150°C led to a reduced VHCF strength. At both temperatures the 304 grade still undergoes a pronounced localization of plastic deformation in shear bands accompanied by a deformation-induced martensitic phase transformation from the γ-austenite to the α’-martensite during VHCF loading. In the present study, the experimental study is extended by modeling and simulation of the relevant temperature-dependent VHCF deformation mechanisms in order to provide a more profound understanding of the observed cyclic deformation. For this purpose, two-dimensional (2-D) morphologies of microstructures are modeled in the mesoscopic scale by the use of the boundary element method (BEM), and cyclic plastic deformation is considered by certain mechanisms defined in a simulation model. It describes the localization of plastic deformation in shear bands taking into account the formation, plastic sliding deformation and cyclic slip irreversibility of each shear band. The deformation-induced martensitic phase transformation is represented by introducing martensitic nuclei into the modeled microstructure depending on the plastic deformation in shear bands. The influence of temperature is incorporated into the simulation model by the use of empirical data and a kinetic model, each related to the tensile test. The simulated cyclic deformation and phase transformation is compared to experimental observations and allows for assessing the individual influence of deformation mechanisms on the temperature-dependent fatigue behavior. Finally, a temperature-independent ‘limit curve’ for the accumulated irreversible plastic sliding deformation regarding failure in the VHCF regime is proposed.

Push-Off Tests in the Study of Cyclic Behavior of Interfaces between Concretes Cast at Different Times

AbstractThe knowledge about the cyclic behavior and effects of fatigue on interfaces between concrete cast at different times, subjected to shear stress, is still very limited. This aspect is particularly relevant in structures subjected to important cyclic loads, such as railway bridges. In this context, first, a brief state of the art on the monotonic and cyclic behavior of interfaces between concretes cast at different times is presented. Then, the test campaign, which was aimed at obtaining experimental support for modeling the behavior of such interfaces, subjected to a high number of load cycles, is described, and the corresponding results are discussed and analyzed in terms of slip and crack opening, stress variations in the monotonic tests, failure mode, and number of resisting load cycles. It was further observed that the number of resisting load cycles increases as the maximum load level decreases. On the basis of this evidence, an experimental S-N curve was set for this type of interface.

Effect of augmentation techniques on the failure of pedicle screws under cranio-caudal cyclic loading

Augmentation of pedicle screws is recommended in selected indications (for instance: osteoporosis). Generally, there are two techniques for pedicle screw augmentation: inserting the screw in the non cured cement and in situ-augmentation with cannulated fenestrated screws, which can be applied percutaneously. Most of the published studies used an axial pull out test for evaluation of the pedicle screw anchorage. However, the loading and the failure mode of pullout tests do not simulate the cranio-caudal in vivo loading and failure mechanism of pedicle screws. The purpose of the present study was to assess the fixation effects of different augmentation techniques (including percutaneous cement application) and to investigate pedicle screw loosening under physiological cyclic cranio-caudal loading. Each of the two test groups consisted of 15 vertebral bodies (L1-L5, three of each level per group). Mean age was 84.3years (SD 7.8) for group 1 and 77.0years (SD 7.00) for group 2. Mean bone mineral density was 53.3mg/cm3 (SD 14.1) for group 1 and 53.2mg/cm3 (SD 4.3) for group 2. 1.5ml high viscosity PMMA bone cement was used for all augmentation techniques. For test group 1, pedicles on the right side of the vertebrae were instrumented with solid pedicle screws in standard fashion without augmentation and served as control group. Left pedicles were instrumented with cannulated screws (Viper cannulated, DePuy Spine) and augmented. For test group 2 pedicles on the left side of the vertebrae were instrumented with cannulated fenestrated screws and in situ augmented. On the right side solid pedicle screws were augmented with cement first technique. Each screw was subjected to a cranio-caudal cyclic load starting at 20-50N with increasing upper load magnitude of 0.1N per cycle (1Hz) for a maximum of 5000 cycles or until total failure. Stress X-rays were taken after cyclic loading to evaluate screw loosening. Test group 1 showed a significant higher number of load cycles until failure for augmented screws compared to the control (4030 cycles, SD 827.8 vs. 1893.3 cycles, SD 1032.1; p<0.001). Stress X-rays revealed significant less screw toggling for the augmented screws (5.2°, SD 5.4 vs. 16.1°, SD 5.9; p<0.001). Test group 2 showed 3653.3 (SD 934) and 3723.3 (SD 560.6) load cycles until failure for in situ and cement first augmentation. Stress X-rays revealed a screw toggling of 5.1 (SD 1.9) and 6.6 (SD 4.6) degrees for in situ and cement first augmentation techniques (p>0.05). Augmentation of pedicle screws in general significantly increased the number of load cycles and failure load comparing to the nonaugmented control group. For the augmentation technique (cement first, in situ augmented, percutaneously application) no effect could be exhibited on the failure of the pedicle screws. By the cranio-caudal cyclic loading failure of the pedicle screws occurred by screw cut through the superior endplate and the characteristic "windshield-wiper effect", typically observed in clinical practice, could be reproduced.

Static and Dynamic Experimental Stress-Strain Analysis of Axles Loaded by Rotation Bending

Safety assessment of railway axles is based on fatigue strength design. During the design overall lifetime, railway axles undergo a very high number of loading cycles under rotating bending. Due to safety and reliability reasons, fatigue strength design has to be very carefully evaluated. Both theoretical modelling and extensive experimental fatigue testing including full scale tests have to be performed, whereas such tests have to be performed correctly with a high precision. In the paper, problems of dynamic forces during fatigue tests of railway axles are pointed out. Though static calibration can be carried out with a very high precision, dynamic forces can cause a significant redistribution of static stresses along the axle. An example is given and discussed.

Evaluation of Actuator, Sensor, and Fatigue Performance of Piezo-Metal-Composites

Fabrication technologies for multilayer-composites with sensor and actuator functionality were proposed in a previous study. Inside the compounds, macro-fiber-composites (MFCs) are embedded in a layer of epoxy adhesive between two outer aluminum sheets. Forming takes place while the adhesive is still in an uncured state. Relative displacements between the layers and the MFC is possible, which reduces friction loads for the brittle piezoceramic fibers of the MFC. This paper deals with the experimental and numerical characterization of the actuator and sensor functionality dependent on different bending radii and additionally the fatigue behavior of the compounds. The sensor functionality is tested with a shaker, which initiates a defined deflection. The electric response of the integrated MFC is measured and simulated by use of an analytical sensor function model based on strain components. The actuator performance, measured with a distance laser sensor, is modeled with a voltage-temperature-analogy and compared with values from the experiment. The fatigue behavior and the performance reduction of embedded MFC are investigated with cyclic four-point-bending loading. Loads near and above the elastic limit of the sheet metals cause a higher delamination tendency. Specimens that are loaded at a reasonable distance from the elastic limit reach the high cycle fatigue limit of 2.0E + 06 cycles. Thus, the production method for multilayer-composites with embedded piezoceramic fiber modules discussed in this paper is suitable for the manufacturing of structural parts that undergo a high number of load cycles.

Analysis of VHCF damage in a duplex stainless steel using hard X-ray diffraction techniques

Effects of very high cycle fatigue (VHCF) damage were investigated in an austenitic–ferritic duplex stainless steel using the hard X-ray diffraction technique applying a beam diameter in the order of the mean grain size. Diffraction patterns were collected using a large 2D detector as function of the position along the load axis as well as perpendicular to the load axis of hourglass-shaped ultrasonic fatigue specimens. Intensities, angular positions and widths of Bragg reflections from individual grains were studied as a function of load cycles and stress amplitudes. Whereas rocking curves (RC) of ferrite grains behave nearly unaffected by the cyclic load, a splitting of RCs of austenite grains was observed and is taken as an indication for the VHCF damage. The frequency of split RCs of austenite grains increases with the number of load cycles and is found to be a function of the local stress amplitude. The latter one can be modeled by means of the finite element method (FEM). Taken from the 2Θ angles of Bragg peaks the internal compressive lattice strain of ferrite and austenite grains is found to be released for low but increases again for high numbers of load cycles. The evolution of lattice strain and the frequency of split RCs of austenite grains correlate with the appearance of slip bands at the sample surface seen by scanning electron microscopy (SEM) in combination with electron channeling contrast imaging (ECCI) and in the bulk verified by transmission electron microscopy (TEM). Microcrack formation in ferrite grains is assumed originated by the high density of slip bands in austenite grains generated by very high cycle fatigue.

Influence of Process-Related Defects on the Fatigue Behaviour of Welded Aluminium Joints at Very High Cycles

The influence of geometrical notch and weld defects (pores and incomplete fusions) on the fatigue behaviour at very high numbers of loading cycles is shown for welded samples made of the base material EN AW-5083 and EN AW-6082 and the filler material S Al 5183. High frequency fatigue tests with specimens highest stressed cross-sections representing different welding zones show no endurance limit up to 2·109 load cycles. The weakest link of the weld seams are the geometrical notch and (if present) pores and incomplete fusions in the seam interior and at the surface. Considering weld defects in the filler material as fatigue crack initiating notches, a threshold value for the cyclic stress intensity factor (ΔKmin) of 0.9 MPam was found. Using ΔKmin the fatigue life of the samples is discussed on the basis of stress amplitude, the projected area of the defect and its position. X-ray examinations revealed a good correlation between the failure-relevant defect areas and the overall fatigue life of welded samples.