Load-controlled Fatigue Tests Research Articles

Fatigue crack initiation in cold spray coated AZ31B-H24 with AA7075 powder

The effect of cold spray coating parameters on the fatigue life and cracking mechanism of AZ31B-H24 coated with AA7075 powder is investigated. Two sets of coated samples are fabricated based on the selection of different coating parameters. An in-situ control of heat transfer is performed to obtain different residual stress states and microstructure at the aluminum/magnesium interface. Subsequently, the samples are tested under load-controlled fatigue tests at different load amplitudes. Fatigue lives are obtained and the cracking behavior of the two samples is studied and compared with that of uncoated baseline samples. It is revealed that the samples with compressive residual stress at the coating/substrate interface have significantly longer lives (approximately 85% improvement at the same stress) compared with that of the baseline samples. In contrast, samples with tensile residual stress at the interface have similar or slightly improved lives (approximately 24%) compared with that of the baseline samples. The cracking mechanisms of these two samples are considerably different. In the case of compressive samples, cracks initiate at the coating surface and propagate through the splat boundaries of the cold spray coating to the substrate. Conversely, in the case of tensile samples, delamination and cracking initiate at the interface and subsequently propagate to the substrate and through the splats in the coating. The different lives and cracking mechanisms obtained are attributed to the differences in the initial state of stress, details of the microstructure of the nano-size interface layer, and the morphology of the substrate grains adjacent to the interface.

A Novel, Open-Source, Low-Cost Bioreactor for Load-Controlled Cyclic Loading of Tendon Explants.

A major risk factor for tendinopathy is tendon overuse (i.e., fatigue loading). Fatigue loading of tendon damages the extracellular matrix and induces tissue degeneration. However, the specific mechanisms linking tendon fatigue damage with tissue degeneration are unclear. While explant models of tendon fatigue loading have been used to address this knowledge gap, they predominantly employ bioreactors that apply cyclic displacements/strains rather than loads/stresses, which are more physiologically relevant. This is because of the technical complexity and cost of building a load-controlled bioreactor, which requires multiple motors, load cells, and computationally intensive feedback loops. Here, we present a novel, low-cost, load-controlled bioreactor that applies cyclic loading to multiple tendon explants by offloading weights from a single motorized stage. Using an optional load cell, we validated that the bioreactor can effectively provide load-controlled fatigue testing of mouse and rat tendon explants while maintaining tissue viability. Furthermore, all the design files, bill of materials, and operating software are available "open source"1 so that anyone can easily manufacture and use the bioreactor for their own research. Therefore, this novel load-controlled bioreactor will enable researchers to study the mechanisms driving fatigue-induced tendon degeneration in a more physiologically relevant and cost-effective manner.

Fatigue Life and Reliability of Steel Castings through Integrated Simulations and Experiments

The quality and performance of steel castings is always a concern due to porosities formed during solidification of the melt. Nowadays, computational tools are playing a pivotal role in analyzing such defects, followed by their minimization through mold design optimization. Even if the castings are produced with defects in a permissible range, it is important to examine their service life and performance with those defects in a virtual domain using simulation software. This paper aims to develop a methodology with a similar idea of simulation-based optimization of mold design and predictions of life and reliability of components manufactured with minimized casting defects, especially porosities. The cast parts are standard fatigue specimens which are produced through an optimized multi-cavity mold. X-ray imaging is done to determine the soundness of cast parts. Experimental work includes load-controlled fatigue testing under fully reversed condition. The fatigue life of specimens is also simulated and compared with the experimental results. The classical strength-stress model is used to determine the reliability of cast parts through which a safe-load induced stress of steel castings is determined. Finally, probability distributions are fit to the reliability results to develop the reliability models. It is found that porosities can be minimized significantly in the mold design phase using casting simulations. Nevertheless, some porosities are bound to exist, which must be included in realistic estimation of fatigue life and reliability of cast parts.

Effect of specimen configuration and notch root angle on fatigue behavior of novel dissimilar resistance spot welds of AA5754 to HSLA steel

Facing stringent requirements for fuel economy and regulations for greenhouse gas emissions, structural lightweighting using multi-material solutions has become commonplace in the automotive industry. When joining dissimilar materials such as aluminum to steel by resistance spot welding (RSW), a thin layer of brittle intermetallic compound forms at the aluminum-steel interface and dominates mechanical behavior of the joint. In this contribution, RSW of 1.1 mm thick AA5754 sheet to 2.0 mm thick high-strength low-alloy (HSLA) steel sheet was performed using multi-ring domed (MRD) electrodes and multiple solidification weld schedules to achieve acceptable static joint strength. Load-controlled fatigue tests were conducted and the results show that the fatigue life is longer in the AA5754 to HSLA steel spot welds than that of the 1.1 mm thick AA5754 joined to itself (aluminum-aluminum). Structural stress analysis revealed that all fatigue life data points from both the lap shear and coach peel configurations fall onto a single master curve indicating that the weld nugget diameter is the controlling parameter for fatigue life. Finite element simulation considering material inhomogeneity in the weld further confirms that a large notch root angle at the weld nugget is beneficial to yield longer fatigue life as less maximum principal strain occurs in the aluminum sheet in the AA5754-HSLA steel spot welds.

Investigation of the Effect of Dwell Period in Load Controlled Fatigue Tests of Inconel718 Superalloy

The mechanical property of Inconel718 is examined in terms of fatigue behavior. The main concern of this work is to determine the material's fatigue properties under pure bending fatigue conditions with additional interruptions and thermal effects. Test conditions will include room temperature, 450°C, and dwell times to simulate the material's operating environment in the engine application. Additional to fatigue testing hardness measurements and nano indentation testing were performed at ambient temperature. Fractographic and microstructural examinations of the tested specimens are performed with an optical microscope and SEM (scanning electron microscope) to understand the material's reaction to different deformation factors.

Influence of compressed hydrogen on the fatigue behavior of a ferritic stainless steel

Metallic materials, e.g. used for fuel cell components, have to show a high corrosion fatigue resistance against gaseous hydrogen in context of hydrogen susceptibility. This work shows the influence of gaseous hydrogen on the fatigue behavior of a ferritic stainless steel in the low cycle (LCF) and the high cycle fatigue regime. Therefore, strain-controlled and load-controlled fatigue tests are conducted in gaseous hydrogen under pressure and as a reference in ambient air.The evaluation of the test results focuses on the determination of the stress-strain behavior and the cyclic softening and hardening behavior. Based on this evaluation the investigated ferritic stainless steel shows a strong reduction in lifetime at high strain amplitudes under gaseous hydrogen compared to the reference investigations in air. The evaluation of the stress-strain behavior in form of the cyclic stress-strain curve and the cyclic deformation curves showed no significant difference, instead. However, a lifetime reducing effect of the hydrogen can be observed.On the other hand, it can be shown by the help of the load-controlled fatigue tests that the damaging influence of the hydrogen decreases with the reduction of the load level. If the local stress or strain falls below a level where only elastic material behavior can be assumed, the determined fatigue strength under air and hydrogen coincide. In this case, the generation of dislocation is drastically reduced, lowering the damaging influence of on the investigated material.

Load Controlled Fatigue Behaviour of Microplasma Arc Welded Thin Titanium Grade 5 (6Al-4V) Sheets.

The current study investigates the load controlled fatigue properties of the microplasma arc welded thin titanium Grade 5 (6Al-4V) sheets. In order to explore the effect of weld geometry on the fatigue, two different welded joints were used in the experimental studies. Load controlled fatigue test results were evaluated to present an outlook on the behaviour of microplasma welded titanium alloy Grade 5 sheets under cyclic loading. Even though the previously published monotonic tests showed successful use of microplasma arc welding to join thin titanium Grade 5 sheets with mechanical properties comparable to the base metal, fatigue life of the welded joints was lower than the lives of samples without welds. In particular, the fatigue performance of overlap joints was very poor. This was presumed to be due to the changed material properties of the heat affected zone which was formed by the excess heat of the welding process as fractures often occurred at such locations. Based on experimental findings and fractographic observations, a clear adverse effect of welding process in material behaviour was discovered. Despite the concentrated heat of microplasma arc welding, post-weld heat treatment of the weld area is recommended to improve the mechanical behaviour of the welded joints.

Use of a Micromagnetic Nondestructive Test in the Evaluation of the α’-Martensitic Transformation Generated in the Mechanical Fatigue Process of the AISI 304L Stainless Steel

The detection of the strain-induced martensitic transformation with the magnetic Barkhausen noise (MBN) was investigated in the high-cycle fatigue (HCF) of the AISI 304L stainless steel at different points of the surface of an hourglass-type specimen. Load-controlled fatigue tests with loads below and above the fatigue limit were performed. Each fatigue test was interrupted at certain numbers of cycles, prior to fracture, and MBN measurements were performed without applied loading. Comparing with the initial condition of the steel under study, no large changes in the MBN measurements were detected in the specimens subjected to loads near the fatigue limit, i.e., no $\alpha '$ -martensitic transformation was detected. In the specimen subjected to the greatest load, it was possible to detect an uncommon distribution of the MBN signals, which are correlated with the strain-induced $\alpha '$ -martensite and were verified with optical microscopy and X-ray diffraction (XRD). The results show that the MBN is a sensitive technique to detect the strain-induced $\alpha '$ -martensite in HCF tests, which can be used as an indirect parameter to show the fatigue damage of materials.

Improvement of Fatigue Properties of AZ31B Extruded Magnesium Alloy through Forging

Axial monotonic and load-controlled fatigue tests were performed to investigate the influence of forging at various temperatures and different deformation rates, on both the microstructural and mechanical behaviour of extruded AZ31B magnesium alloy. The obtained microstructural analysis showed that the extruded AZ31B magnesium alloy possesses a bimodal grain structure with strong basal texture. In contrast, once forged, the material showed refined grains and a modified texture. A monotonic yield and ultimate tensile strength of about 223 MPa and 278 MPa were observed for the forged samples showing an increase of 18%, from the as-extruded material. The optimum forging condition was determined to be the coldest of the investigated temperatures, based on the improvement in both monotonic and cyclic properties vs. the as-extruded material. The fractographic analysis of the failure surfaces showed that ductile type fractures occurred in both as-extruded and forged samples. However, more dimples and plastic deformation were identified in the fracture surfaces of the forged specimens. A significant improvement of fatigue life was also observed for all of the forged samples, in particular those forged at 400°C and 39 mm/min. Forging improved the fatigue life via a combination of grain refinement and texture modification resulting in improved strength and ductility.

Analysis of the Influence of Surface Integrity of Cemented Carbides Machined by Sinking EDM on Flexural Fatigue

Abstract Bulk forming is a manufacturing technology characterized by high tool loads. In order to reduce form deviations and increase surface qualities this manufacturing technology aims for even higher process forces, which result in larger stresses during the forming process. Consequently, the tool life of steel tools decreases drastically. A material that offers an alternative are cemented carbides due to their ability to withstand enormous compressive stresses and their resistance to wear. However, for the same reason, cemented carbides are hard to machine with conventional cutting technologies. A cost-efficient alternative is electrical discharge machining (EDM), because of its independence from mechanical material properties. Due to the brittle character of cemented carbides, one of the challenges in EDM, the reduction of the influence on the surface integrity, becomes a paramount concern. Thermal stresses caused by machining can initiate cracks that may lead to a failure of the tool after a few load cycles. The risk of crack initiation can be reduced with an appropriate machining strategy. In order to correlate the influence of surface integrity with flexural strength, load-controlled fatigue tests are conducted with two different types of cemented carbides that have each been machined with three different EDM-strategies. Along with the fatigue evaluation, the cross sections of the specimens are examined regarding their surface integrity. With these results it is possible to evaluate the impact of surface integrity of EDMed cemented carbides on the flexural fatigue.

Fatigue life prediction and reliability assessment of ductile iron castings using optimized mold design

Metal castings are often questioned for their service life and reliability due to a lot of uncertainties associated with the process. It is important to virtually analyze these aspects of a cast product before it actually put into service. This paper presents a new approach to predict the fatigue life and reliability of ductile iron castings through coupled simulations and experiments. In this work, fatigue specimens are considered as sample cast products which are produced using an optimized mold design for minimum porosity. Radiographic examination is done to observe the quality of cast specimens. Experimental load-controlled fatigue testing of selected specimens is done and compared with simulated fatigue lives in fe-safe (Dassault Systems UK Limited 2015). Reliability of cast specimens under fatigue is computed using stress-strength model by considering the variability in both stress and strength. A safe load-induced stress is estimated to obtain infinite life of ductile iron parts under consideration. Furthermore, the reliability models are developed by fitting probability distributions to reliability estimates and determining the distribution parameters. It is concluded that the mold design optimization using casting simulations can lead to nearly defect free castings which can serve for the similar fatigue lives as their sound counterparts. Moreover, the probability of survival of the cast components for an infinite life can be computed using the classical stress-strength reliability model based on which safe loading conditions can be determined for almost any engineering application.

Monotonic and cyclic anisotropies of an extruded Mg–Al–Ca–Mn alloy plate: Experiments and crystal plasticity studies

Mechanical anisotropies of an extruded Mg–Al–Ca–Mn alloy plate were investigated under both monotonic and cyclic conditions through a combination of experiments and crystal plasticity finite element simulations. To this end, monotonic tensile, compression as well as fully-reversed strain-controlled and load-controlled fatigue tests were conducted on specimens aligned either in the extrusion or transverse direction. Plastic activities of the deformation mechanisms for the different loading conditions were quantitatively analyzed based on the results of numerical simulations. The asymmetrical mechanical response between tension and compression along the two loading directions was found to be associated with the lack of axi-symmetry of the relatively strong basal texture. In addition, the peculiar and lower tensile flow stress in the transverse direction was explained by the activation of deformation twinning in grains with their c-axis aligned along the loading direction. Under low-cycle fatigue conditions, a lower asymmetry of stress–strain hysteresis curves was observed in the transverse direction due to the occurrence of twinning/detwinning in both the tensile and compressive portions. High-cycle fatigue experiments revealed lower fatigue lives in the transverse direction than in the extrusion direction but similar fatigue limits. Crystallographic analyses of multiple secondary cracks revealed crack initiation from basal slip, twin bands, grain boundaries and non-metallic inclusions. The evaluation of slip-induced and twin-induced fatigue criteria from the results of fatigue simulations of explicitly modeled polycrystals suggested that the most detrimental crack initiation mechanism was crack initiation from basal slip as it predicted a significantly lower initiation life in the transverse direction than in the extrusion direction, as observed experimentally.

Role of retained austenite in hydrogen trapping and hydrogen-assisted fatigue fracture of high-strength steels

The Interaction of hydrogen with retained austenite under fatigue loading of dual-phase and complex-phase high-strength steels with strength of about 1200 MPa was studied. A load-controlled fatigue test was performed in the air with the maximum applied tensile strength of 900 MPa, which is just below the offset yield point of the studied steels. The trapping of hydrogen accumulated into the studied steels under the fatigue loading was studied by thermal desorption spectroscopy. Measurements of hydrogen trapping evolution and microstructure changes during fatigue testing reveal a complicated hydrogen trapping behavior driven by hydrogen interaction with deformation defects and retained austenite. Hydrogen concentration increases in the studied steels during the fatigue testing in the air without preceding hydrogen charging. The fracture surfaces were studied by scanning electron microscopy evidencing the relationship between the hydrogen concentration increase related to retained austenite and initiation of the intergranular fatigue fracture. The role of retained austenite in hydrogen-assisted fatigue cracking is discussed and a possible mechanism of the hydrogen-assisted fatigue crack initiation in the high-strength steels is proposed.

Fatigue life curve – A continuous Wöhler curve from LCF to VHCF

Abstract In order to consider cyclic material behavior in the course of the design process of cyclically loaded components and safety relevant parts, the importance of local strain-based fatigue design approaches has been growing continually. For the damage impact of load-time histories on components such as chassis parts, standard service loads with amplitudes settled in the high cycle fatigue and very high cycle fatigue regimes as well as overloads and misuse with load amplitudes from the low cycle fatigue regime, have to be considered, in order to perform a reliable fatigue life estimation. Therefore, a continuous fatigue life curve from the LCF to the VHCF regime, which covers all relevant damage mechanisms, is required. Hence, a fatigue life curve from low cycle fatigue (LCF) across to high cycle fatigue (HCF), and culminating at very high cycle fatigue (VHCF), derived from a combined method using strain- and load-controlled fatigue tests, will be discussed. This continuous fatigue life curve for aluminum wrought alloys, based on the evolution of the behavior of elastic-plastic material as well as the results of high frequency testing up to VHCF, will be presented.

Microstructure and fatigue behavior of novel Multi-Ring Domed resistance spot welds for thin X626-T4 aluminum sheets

Resistance spot welding of aluminum alloys is increasingly used in the automotive industry for vehicle lightweighting and better fuel efficiency. In this contribution, General Motor’s aluminum spot welding process including Multi-Ring Domed (MRD) electrodes and Conditioning, Shaping, and Sizing (CSS) weld schedule were used for welding of 0.8-mm thick X626-T4 aluminum alloy sheet. The resultant spot welds had an equiaxed grain structure adjacent to the weld nugget periphery. Within the weld nugget, fine columnar grains were found at the nugget perimeter, while welding defects were concentrated in the centre of the nugget. Using modified shear specimens, it was determined that the weld nugget is the weakest location while the heat affected zone is enhanced by precipitation aging due to a combination of welding and paint bake thermal cycles. We then derived a new equation for calculating minimum weld nugget diameters to ensure that interfacial fracture would not occur during tensile and fatigue testing. Load controlled fatigue test results showed that using structural stress analysis, fatigue life curves for both lap-shear and coach-peel configurations fell onto a single master curve indicating that the weld nugget size is the controlling parameter for fatigue life. While fracture mode varied with load amplitude from full button pullout at high amplitude to eyebrow-like fracture with kinked crack paths at low amplitude, no interfacial fracture occurred during fatigue testing due to the large size of the weld nugget even though the weld nugget is the weakest location.

Fatigue behaviour of corrosion pits in X65 steel pipelines

Corrosion pits are a form of geometrical discontinuity that lead to stress and strain concentration in engineering components, resulting in crack initiation under service loading conditions and ultimately fracture and failure. Initiation and propagation of cracks in offshore pipelines can lead to loss of containment and environmental and commercial impacts. In order to prevent such failures, tools to predict the structural integrity of pipelines need to be improved. This work investigates the fatigue behaviour of corrosion pits in API-5L X65 grade steel pipeline utilising numerical and analytical methods. Firstly, load-controlled fatigue tests were carried out on smooth X65 steel samples to establish S–N data. Secondly, local stress–strain behaviour at corrosion pits and its effect on fatigue crack initiation were investigated using elastic-plastic finite element analysis of samples containing a single corrosion pit under cyclic loading. Analysis of stabilised stress–strain hysteresis loops at corrosion pits showed that the local stress ratio at the pit changes from 0.1 to −0.4 while the applied stress amplitude increases with the same stress ratio of 0.1. Analytical methods were also used to predict the local maximum stress and strain at the pit, which showed a similar local stress ratio to the finite element analysis result but lower stress and strain ranges. Finally, fatigue crack initiation life was predicted using the combination of finite element stress and strain analysis and the Smith–Watson–Topper strain–life approach. An advantage of this method for life estimation is that this approach considers the local stress and strains at corrosion pits rather than applied stress.

Fatigue behavior of 316L austenitic stainless steel in air and LWR environment with and without mean stress

The fatigue life design curves in nuclear codes are generally derived from uniaxial straincontrolled fatigue test results. Evidently, the test conditions are very different from the actual components loading context, which involves much more complex thermo-mechanical loading including mean stress, static load holding time and variation in water chemistry, etc. In this work, the mean stress and environmental effects on fatigue life of 316L austenitic stainless steel in air and light water reactor (LWR) environment were studied using hollow fatigue specimens and testing under load-controlled condition. Both positive (+50 MPa) and negative (-20 MPa) mean stresses showed beneficial effect on fatigue life in LWR environment and in air. This is tentatively attributed to mean stress enhanced cyclic hardening, which leads to smaller strain response at the same loading force. -20 MPa mean stress was found to increase fatigue limit, whereas the effect of +50 MPa mean stress on fatigue limit is still unclear. The preliminary results illustrate that the environmental reduction of fatigue life is amplified in load-controlled fatigue tests with tensile mean stress.

The effect of corrosive environment on fatigue life and on mean stress sensitivity factor

An experimental investigation of the effect of corrosive environment on the fatigue life of circular cross section specimens made from structural low carbon steel S355 is presented in this paper. Uniaxial load controlled fatigue tests in the regime of 105 to 107cycles were performed in two different environments: at room temperature in air (without corrosion) and in-situ fresh water solution environment. Various approaches to evaluate mean stress effect on stress-life behaviour in inert environment are reviewed and used to estimate the mean stress sensitivity factors both for air and corrosive environment. Mean stress sensitivity factor is calculated based on experimental results and predicted using the different approaches. Moreover, a comparison between Haigh Diagram predicted by these models and experimental results is proposed and the Walker approach is found to be the one that best fits experimental data.

A study on fatigue behavior of MIG-welded Al-Mg alloy with different filler-wire materials under mean stress

Cylindrical specimens of ϕ5 mm were cut from the weld bead of 50 mm thick MIG-welded Al-Mg alloy (Al-5083) plates with two different filler materials: Al-5183 (commonly used) and Al-5.8%Mg (special hardened new alloy). Load-controlled fatigue tests were done at different stress-ratios (R=-1,-0.5,0.1,0.5) using sinusoidal wave at 30 Hz and the performance of three popular finite-life empirical models: Gerber’s parabola, Smith-Watson-Topper’s model and Walker’s model, was studied using the experimental data. Fatigue lives and endurance limits of both materials did not differ significantly, but they got reduced when the R-ratio was increased and the endurance limit followed the Gerber’s parabola. A non-monotonic trend in the slope of S-N curves was observed: first increases and then decreases. Fracture surface observations revealed majority of surface-initiated failures at lower R-ratios (-1,-0.5), while for higher R-ratios (0.1,0.5), defect-induced failures were predominant. This shift in crack initiation site was attributed to the local cyclic plasticity due to stress-concentration and demonstrated using elastic-plastic finite element (FE) simulations. The detrimental effect of mean-stress on fatigue lives was attributed to combined effects of reduction in crack-nucleation time: due to stress concentration at defects and the crack closure effects. The three fatigue life models predicted monotonic decrease in slopes of S-N curves and thus did not give best results. A new model, based on Walker’s model, was proposed which was able to capture the observed non-monotonic trend of the slopes of S-N curves and predicted fatigue lives better than these models.

Strain accumulation during microstructurally small fatigue crack propagation in bcc Fe-Cr ferritic stainless steel

Strain accumulation was studied by digital image correlation technique (DIC) during microstructurally small fatigue crack propagation in polycrystalline 18%Cr ferritic stainless steel. Load-controlled fatigue testing was performed with R-ratio of 0.1 and frequency 10 Hz. The maximum applied stress was well below the yield stress of the studied material. The effect of the observed strain field on crack growth rate variation is discussed. Fracture surfaces were studied by scanning electron microscopy (SEM) evidencing the connection between the mechanism of the fatigue crack growth, accumulated strain and crack growth rate. Detailed study of fracture surface morphology was carried out by atomic force microscopy (AFM). Results indicate two processes of material damage accumulation and failure during cyclic loading: 1) local shear strain zones form successively ahead of the crack tip, and 2) fatigue crack growth occurs by both single- and multiple-slip mechanisms. The place and intensity of shear strain localization zones vary during the crack growth that is related closely to the local variation of crack growth rate.