High Cycle Regime Research Articles

Elucidating the Effect of Additive Friction Stir Deposition on the Resulting Microstructure and Mechanical Properties of Magnesium Alloy WE43

In this work, the effect of processing parameters on the resulting microstructure and mechanical properties of magnesium alloy WE43 processed via Additive Friction Stir Deposition (AFSD), a nascent solid-state additive manufacturing (AM) process, is investigated. In particular, a parameterization study was carried out, using multiple four-layer deposits, to identify a suitable process window for a structural 68-layers bulk WE43 deposition. The parametric study identified an acceptable set of parameters with minimal surface defects and excellent consolidation for the fabrication of a bulk WE43 deposition. Microstructural, tensile, and fatigue life characterization was conducted on the bulk WE43 deposition and compared to commercially available wrought material to elucidate the process-structure-property-performance (PSPP) relationship of the AFSD process. This study shows that the bulk WE43 deposit exhibited a refined homogenous microstructure and a texture shift relative to the wrought material. However, a reduction in hardness and tensile behavior was observed in the as-deposited WE43 compared to the wrought control. Additionally, fatigue specimens extracted from the bulk deposition exhibited a decrease in life in the low-cycle regime but performed comparably to the wrought plate in the high-cycle regime. The outcomes of this study illustrate the potential of the AFSD process in additively manufactured structural load-bearing components made with magnesium alloy WE43 in the as-built condition.

Effects of Specimen Size and Welded Joints on the Very High Cycle Fatigue Properties of Compressor Blade Steel KMN-I

The effects of specimen size and welded joints on the very high cycle fatigue properties of compressor blade steel KMN-I were studied by ultrasonic fatigue testing. It was found that the S-N curve of large specimens had a slow decline above 107 cycles, and fatigue failure still occurred in the very high cycle regime (>107 cycles), while the very high cycle fatigue characteristics of welded specimens was less obvious, and the fatigue limit was observed. Metallographic observation and SEM analysis were carried out on the fracture of the specimens. The results showed that surface fractures were mostly observed in the large specimens, and only a small number of cracks initiated from non-metallic inclusions above 107 cycles. The cracks of welded specimens initiated from the surface below 107 cycles and initiated from the internal matrix above 107 cycles. In addition, the formation mechanism of GBF (granular bright facet) was analyzed by the “dispersive decohesion of spherical carbide” theory, and the fatigue strength and fatigue life were predicted, which was consistent with the experimental results.

Short- to long-term deformation behavior, failure, and service life of amorphous polymers under cyclic torsional and multiaxial loadings

When investigating materials to be utilized, attention inevitably focuses on their resistance over the service life. Despite the popularity of amorphous polymers, ranging from their applications in structural components to their ability to increase the toughness of biocomposites, the investigation of their short- to long-term resistance has been considerably limited to date. Here, an improved testing equipment and model are proposed to describe the resistance of amorphous polymers under cyclic loadings. Two failure mechanisms are considered for the low- to high-cycle regimes: plastically induced and fatigue, and a history dependent fatigue damage model and a plastic evolution law with a relaxed shear resistance are proposed. Against state-of-the-art models, the proposed model is able to simulate the experimentally observed ultralow- to high-cycle failure and service life under torsional and multiaxial loads. The experimental and model results are similar, suggesting that the model is a capable tool for simulating costly and time-consuming tests. Interestingly, the predicted progress of material failure with plastic deformation was found to resemble the observed development of accumulated void volume. The failure (void volume) development strongly influenced the onset and growth of tertiary cyclic creep and thus, the entire service life. The amorphous structure also appeared to effectively resist failure under torsion.

Fatigue Improvement of Additive Manufactured Ti–TiB Material through Shot Peening

In this work, fatigue improvement through shot peening of an additive manufactured Ti–TiB block produced through Plasma Transferred Arc Solid Free-Form Fabrication (PTA-SFFF) was investigated. The microstructure and composition were explored through analytical microscopy techniques such as scanning and transmission electron microscopy (SEM, TEM) and electron backscatter diffraction (EBSD). To investigate the isotropic behavior within the additive manufactured Ti–TiB blocks, tensile tests were conducted in longitudinal, diagonal, and lateral directions. A consistent tensile behavior was observed for all the directions, highlighting a nearly isotropic behavior within samples. Shot peening was introduced as a postmanufacturing treatment to enhance the mechanical properties of AM specimens. Shot peening led to a localized increase in hardness at the near-surface where stress-induced twins are noted within the affected microstructure. The RBF-200 HT rotating-beam fatigue machine was utilized to conduct fatigue testing on untreated and shot-peened samples, starting at approximately 1/2 the ultimate tensile strength of the bulk material and testing within low- (<105 cycles) to high-cycle (>105 cycles) regimes. Shot-peened samples experienced significant improvement in fatigue life, increasing the fitted endurance limit from 247.8 MPa for the untreated samples to 318.3 MPa, leading to an increase in fatigue resistance of approximately 28%.

Consideration of salt-corrosion fatigue for lightweight design and proof of aluminium safety components in vehicle applications

Most of the available knowledge regarding the influence of corrosion on fatigue strength exists for constant amplitude loading resulting in Woehler-lines. However, the fatigue behaviour under spectrum loading, resulting in Gassner-lines, and simultaneously repeated corrosion is an area that is seldom investigated. The present paper presents the current state of the art regarding salt-corrosion fatigue behaviour, especially under spectrum loading, for forged, cast and welded aluminium vehicle safety components of the material families 5000, 6000, 42000 and 43000. The results obtained allow the estimation of fatigue-lines in air as well as under corrosion conditions.While wet-dry cycles or continuous irrigation with 5 %NaCl reduce the fatigue strength under constant amplitude loading by about 50% in the high-cycle regime, under spectrum loading the decrease in fatigue strength is, however, about 20%. The lower strength reduction under combined spectrum loading and corrosion enables experimental durability proofs in air without corrosive media by either increasing the amplitudes by about 15% or by extending the length of the applied spectrum by a factor of 2 to 3 to compensate for the fatigue life reducing effect of salt-corrosion. The favourable behaviour under spectrum loading opens lightweight design options despite the existence of corrosion conditions.

Fatigue Behavior of Linear Friction Welded Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo-0.1Si Dissimilar Welds.

The use of joints fabricated from dissimilar titanium alloys allows the design of structures with local properties tailored to different service requirements. To develop welded structures for aerospace applications, particularly under critical loading, an understanding of the fatigue behavior is crucial, but remains limited, especially for solid-state technologies such as linear friction welding (LFW). This paper presents the fatigue behavior of dissimilar titanium alloys, Ti–6Al–4V (Ti64) and Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242), joined by LFW with the aim of characterizing the stress versus number of cycles to failure (S-N) curves in both the low- and high-cycle fatigue regimes. Prior to fatigue testing, metallurgical characterization of the dissimilar alloy welds indicated softening in the heat-affected zone due to the retention of metastable β, and the typical practice of stress relief annealing (SRA) for alleviating the residual stresses was effective also in transforming the metastable β to equilibrated levels of α + β phases and recovering the hardness. Thus, the dissimilar alloy joints were fatigue-tested in the SRA (750 °C for 2 h) condition and their low- and high-cycle fatigue behaviors were compared to those of the Ti64 and Ti6242 base metals (BMs). The low-cycle fatigue (LCF) behavior of the dissimilar Ti6242–Ti64 linear friction welds was characterized by relatively high maximum stress values (~ 900 to 1100 MPa) and, in the high-cycle fatigue (HCF) regime, the fatigue limit of 450 MPa at 107 cycles was just slightly higher than that of the Ti6242 BM (434 MPa) and the Ti64 BM (445 MPa). Fatigue failure of the dissimilar titanium alloy welds in the low-cycle and high-cycle regimes occurred, respectively, on the Ti64 and Ti6242 sides, roughly 3 ± 1 mm away from the weld center, and the transitioning was reasoned based on the microstructural characteristics of the BMs.

In vitro corrosion-fatigue behavior of biodegradable Mg/HA composite in simulated body fluid

Magnesium and its composites as biodegradable materials offer especial capabilities to be used as bio-absorbable implants. However, their poor corrosion and fatigue properties in the physiological environment can restrict their applications. In this study, corrosion-fatigue tests have been performed on the extruded magnesium and magnesium/hydroxyapatite (Mg/HA) composites in a high cycle regime. To produce the composites, pure magnesium was reinforced by 2.5 wt.% and 5 wt.% of hydroxyapatite submicron particles using an electromagnetic-mechanical stirring method and hot extrusion process. The experimental density measurement exhibits that the porosity increases in the extruded samples with more hydroxyapatite particles. To investigate the corrosion and corrosion-fatigue behavior of the specimens, simulated body fluid (SBF) was used during in vitro tests. The results of the potentiodynamic polarization corrosion test show that the composite with 2.5 wt.% of hydroxyapatite (Mg/2.5 wt.% HA) and the pure magnesium specimen exhibit the highest and the lowest corrosion resistance, respectively. Regarding the elemental mapping analysis of the corroded samples, this behavior could be due to the formation of strong phosphorus-calcium based layers on the composite surface. The results obtained from the mechanical tests indicate that Mg/5 wt.% HA offered the highest tensile and compressive yield strengths, as well as the most promising high cycle fatigue behavior. During the corrosion-fatigue test, the simultaneous effects of fatigue and corrosion damages led to a similar corrosion-fatigue behavior in both composites. The fracture surfaces of the corrosion-fatigue samples suggest that the cracks are initiated in the corroded regions of the samples surface, which reduces the crack initiation step and subsequently decreases the fatigue life. In comparison with the pure magnesium, both composites exhibit more promising corrosion and corrosion-fatigue behaviors with a significant fatigue life improvement in the physiological environment.

Is 2D numerical modelling of welding process able to capture the residual notch stress intensity factor values?

Because of its local nature, the influence of residual stresses on fatigue life of welded joints in the high cycle regime can be quantified by the residual notch stress intensity factor (R-NSIF). The standard methodology used for its assessment requires a very fine mesh and this is the reason why 2D numerical models were used first in literature. The calculation of R-NSIF values via 3D models is very recent. It was found possible thanks to rapid numerical techiques such as the peak stress method. This approach was proven to work well but R-NSIF values coming from 3D models were not consistent with those obtained through 2D models. This work is aimed at solving that apparent discrepancy. 2D and 3D welding process numerical simulations applied to a carbon steel were carried out taking into account effects of metallurgical transformations and by varying the main geometrical parameters of the plates. It was found that 2D models are able to capture the R-NSIF value that is observed in the stationary residual stress distribution of the corresponding 3D models.

Effect of forging process on high cycle and very high cycle fatigue properties of TC4 titanium alloy under three‐point bending

AbstractThe ultrasonic fatigue tests under two three‐point bending loading modes were carried out on TC4 specimens with the equiaxed, bimodal, and lamellar microstructure in the range of 105 to 109 cycles. The S‐N curves with different shapes were obtained, and it was found that the crack initiation in very high cycle regime changed from surface to subsurface. The crack initiation mechanism with different microstructures was revealed by fracture analysis. The axial tension influence mechanism is that it changes the axial stress distribution on the specimen cross section and makes the crack origin migrate to the interior.

Proposal of a Probabilistic Model on Rotating Bending Fatigue Property of a Bearing Steel in a Very High Cycle Regime

In S-N diagrams for high strength steels, the duplex S-N curves for surface-initiated failure and interior inclusion-initiated failure were usually confirmed in the very high cycle regime. This trend is more distinct in the loading type of rotating bending, due to the stress distribution across the section. In the case of interior failure mode, the fish-eye is usually observed on the fracture surface and an inclusion is also observed at the center of the fish-eye. In the present work, the authors attempted to construct a probabilistic model on the statistical fatigue property in the interior failure mode, based on the distribution characteristics of the location and the size of the interior inclusion at the crack initiation site. Thus, the P-S-N characteristics of the bearing steel (SUJ2) in the very high cycle regime were successfully explained.

An Overview of Fatigue Testing Systems for Metals under Uniaxial and Multiaxial Random Loadings

This paper presents an overview of fatigue testing systems in high-cycle regime for metals subjected to uniaxial and multiaxial random loadings. The different testing systems are critically discussed, highlighting advantages and possible limitations. By identifying relevant features, the testing systems are classified in terms of type of machine (servo-hydraulic or shaker tables), specimen geometry and applied constraints, number of load or acceleration inputs needed to perform the test, type of loading acting on the specimen and resulting state of stress. Specimens with plate, cylindrical and more elaborated geometry are also considered as a further classification criterion. This review also discusses the relationship between the applied input and the resulting local state of stress in the specimen. Since a general criterion to classify fatigue testing systems for random loadings seems not to exist, the present review—by emphasizing analogies and differences among various layouts—may provide the reader with a guideline to classify future equipment.

Probabilistic S-N curves for CFRP retrofitted steel details

Experimental fatigue data of non-cracked open-hole steel specimens retrofitted with bonded carbon fiber reinforced polymer (CFRP) are collected from the literature and assessed in this paper. The specimens are representative of steel plates in tensile riveted or bolted connections. The CFRP substantially extends the fatigue life of specimens at stress range Δσ<220MPa where the fatigue data points converge towards a fatigue limit. These are promising results since most of the ancient metallic riveted assemblies are subjected to stress levels below 150 MPa. The fatigue life extension ratio decreases when the stress level increases due to the fatigue degradation of the bonding joint and/or the CFRP. The fatigue data of specimens retrofitted with CFRP follow a nonlinear trend and code-based linear S-N curves with a slope m = 3 do not capture this behavior. The commonly used methods that assume the fatigue life extension ratio as constant are not adequate to predict the full fatigue life of CFRP retrofitted specimens. To assess the fatigue behavior of the CFRP retrofitted specimens, probabilistic methods are adopted in this study using: the Normal distribution; the two-parameter Weibull; the Gumbel; and the three-parameter Weibull model developed by Castillo & Fernández-Canteli (CFC). Regardless of the assumed distribution function, linear probabilistic S-N curves (P-S-N) are not adequate to model the complex behavior of retrofitted specimens with CFRP. The nonlinear CFC model fitted fairly well the different trends of the fatigue data including the fatigue life at high cycle regime and captured the transition between medium–high cycle range. P-S-N curves derived using the CFC model are proposed for CFRP retrofitted specimens.

High cycle fatigue behaviors of API X65 pipeline steel welded joints in air and H2S solution environment

To investigate the mutual effect of hydrogen, microstructures and stress concentration on the fatigue failure, fatigue behaviors of X65 steel welded joints in both air and saturated H2S solution were investigated at high cycle regime. The experimental result demonstrates that due to lower dislocation density observed by electron backscattered diffraction (EBSD), the fine grain heat affected zone (FGHAZ) is prone to induce cyclic strain localization and further lead to fatigue crack propagating along the FGHAZ in air. Furthermore, the quasi-cleavage with brittle-like fatigue striations and secondary crack on the fracture surface in saturated H2S solution is attributed to hydrogen embrittlement. Moreover, compared with base metal (BM) and FGHAZ, the weld metal (WM) and coarse grain heat affected zone (CGHAZ) are composed of bainite and martensite/austenite (M/A) phase, and more sensitive to hydrogen. Therefore, the fatigue crack is prone to grow along the interface between WM and CGHAZ under the normal applied stress.

Extremely Slow Fatigue Crack Propagation of High Strength steel in Very High Cycle Regime

Extremely Slow Fatigue Crack Propagation of High Strength steel in Very High Cycle Regime

Relationship between non-inclusion induced crack initiation and microstructure on fatigue behavior of bainite/martensite steel in high cycle fatigue/very high cycle (HCF/VHCF) regime

High cycle and very high cycle fatigue (HCF/VHCF) in a bainite/martensite (B/M) multiphase steel with varying inclusion size and microstructural features was studied using ultrasonic axial cycling test. The fatigue crack initiation was predominantly induced by inclusions in specimens with large inclusion size, whereas fatigue crack initiated from the sub-surface microstructure in specimens with coarse microstructure. The fatigue life from granular bright facet (GBF) to fish-eye and from fish-eye to the critical crack size was calculated to obtain an estimate of the contribution to fatigue life by GBF, for the two modes of crack initiation within the HCF/VHCF regime. The results demonstrated that the majority of fatigue life was consumed by the crack initiation process along with the formation of GBF irrespective of whether the crack initiated from inclusions or from sub-surface microstructure. In the case of crack initiation from sub-surface microstructure, the ratio of fatigue crack initiation life to total fatigue life (Ni/Nf) had a wide scatter, which is attributed to varying B/M hierarchical structure in individual prior austenite grains.

Role of mean stress on fatigue behavior of a 316L austenitic stainless steel in LWR and air environments

Mean stress effects on fatigue life of a stainless steel in air and boiling water reactor (BWR) environments were evaluated by load-controlled tests. In stress-life, non-zero mean stresses increase life in low cycle fatigue regime in both environments and in high cycle regime in air. In high cycle fatigue regime in BWR environment, higher fatigue life was observed with negative mean stress while a decrease of the fatigue limit was found for positive one. In strain-life, the mean stress effects are insignificant. All data are well correlated with a Smith-Watson-Topper parameter showing no interaction between mean stress and BWR environment.

Experimental investigation and multiscale simulation on the bending fatigue of 2D SiCf/SiC composites

In order to investigate the bending fatigue of two-dimensional(2D) plain woven SiCf/SiC composites upto relatively high-cycle regime, experimental set-up and testing procedure were detailedly designed. The in-situ natural frequency variation and in-situ damage images evolution during the experiment were obtained. Afterwards, multiscale fatigue simulation framework was developed on the basis of General Method of Cells(GMC) and cycle jump technique. The simulated damage evolution is in good agreement with the observed images. In the end, the comparison between predicted and experimental fatigue life demonstrates a scatter band of ±2.36, proving that the developed framework possesses good capability for life prediction.

Effect of microstructure and cycling frequency on the torsional fatigue properties of 17-4PH stainless steel

Torsional fatigue tests in the high and very high cycle regime were performed with martensitic stainless steel 17-4PH. Tests were conducted with a servo-hydraulic machine and an ultrasonic equipment at cyclic frequencies of 15–35 Hz and 19 kHz, respectively. Fatigue lifetimes were significantly longer for smooth specimens tested at 19 kHz. Fatigue limits were not affected by testing frequency. This behaviour was explained by the presence of δ ferrite grains, in which crack initiation at conventional frequency is accelerated, but their size is too small to significantly decrease the fatigue limit. In the presence of detrimental defects – i.e. small drilled holes that were introduced into test specimens –, no frequency effect could be observed. The geometry of non-propagating shear cracks emanating from δ ferrite grains and in the martensitic matrix was measured by stepwise polishing of a run-out specimen. Fracture-mechanics considerations suggest that the threshold condition for the propagation of Mode-I cracks determines the fatigue limit under torsional loading.

Fatigue crack growth in micro specimens as a tool to measure crack–microstructure interactions

AbstractThe process of short fatigue crack growth plays a significant role for the lifetime of materials in the high and very high cycle regimes. Fatigue crack growth is strongly influenced by interactions with microstructural obstacles, such as grain boundaries or phase boundaries, requiring a better understanding of these interactions to enhance the lifetime in these load regimes and improve lifetime calculations. Although it is possible to obtain crack growth rates from fatigue cracks in the lower micrometre range, further information like the exact position and type of the obstacle are mostly unavailable during the experiment. To overcome this issue, we propose a testing methodology of fatigue crack growth in micro specimens, which allows for an exact positioning of the crack relative to the obstacles and for monitoring the crack behaviour in a scanning electron microscope. The capabilities of this method are demonstrated for the interaction of fatigue cracks with grain boundaries.

Effect of shot peening on very high cycle fatigue of 2024-T351 aluminium alloy

To investigate the influence of shot peening (SP) on very high cycle fatigue (VHCF) performance of 2024-T351, the specimens with three surface conditions were performed under ultrasonic fatigue tests: mechanicallypolished without peening (NP), ceramic shot peening (SP1), steel and glass mixed shot peening (SP2). The roughness, microhardness, residual stress, fractography measurement and scanning electron microscopy (SEM) were applied before fatigue test to characterize the effective layer induced by the peening treatment. For the failed specimens, the fracture surface were analysed using SEM to study the mechanisms of fatigue crack propagation. In addition, the fatigue life curve in ultra-high cycle region continuously decreased in the three series of specimens. However, the experimental results revealed that fatigue strength improvement resulting from shot peening treatment was negligible in very high cycle regime. Furthermore, the stress intensity factor for the surface crack initiation (SCI) and interior crack initiation (ICI) was discussed based on quantitative analysis on the fracture surface. The average values of ΔKfish-eye for NP, SP1 and SP2 specimens are about 2.22, 1.48 and 1.61 MPa · m1/2, respectively.