High Cycle Fatigue Regime Research Articles

Fracture Toughness and Fatigue Crack Growth Characteristics of UFG Microalloyed and IF Steels Processed by Critical Phase Control Multiaxial Forging

The aim of the current study is to design multiaxial forging (MAF) schedules in order to achieve submicron-grained (<1μm) structure in a microalloyed (MA) steel as well as an interstitial-free (IF) steel, which could impart a good combination of yield strength and tensile ductility. At the same time, an effort has been made to evaluate the fracture toughness characteristics by conducting 3-point bend tests and computing the KQ, Kee and J-integral values of ultrafine grained (UFG) samples and correlating them with the microstructure, besides evaluating the other mechanical properties. Fatigue strength in the high cycle fatigue (HCF) regime were also investigated and fracture mechanisms analyzed and comparison established between differently processed samples. The microstructural analysis was performed using transmission electron microscopy (TEM) and Electron backscatter diffraction (EBSD) and results corroborated with the mechanical properties. Superior combinations of yield strength (YS), ductility (% El.), fracture toughness (Kee) and high cycle fatigue strength (σf) were obtained under certain conditions, i.e., i) MA steel: intercritical (α+γ) phase regime (~Ar1) controlled and 15-cycle multiaxially forged (MAFed) (YS=1027MPa, %El.=8.3%, σf=355MPa and Kee=90MPa√m), and ii) IF steel: ferritic region (<Ar1) controlled 18-cycle MAFed (YS=881MPa, %El.=11.2%, σf=255MPa and Kee=97MPa√m). In the case of MA steel, an enhancement of the fatigue and fracture toughness properties can be ascertained following the formation of uniformly distributed nanosized fragmented cementite (Fe3C) particles (~35nm size) present in the submicron sized (average ~280nm size) ferritic microstructure. In contrast, in the case of IF steel, this is ascribed to the development of submicron sized ferrite grains (average ~320nm) along with a high density of dislocation substructures. These fine dislocation cells/substructures along with the nanosized Fe3C particles could effectively block the initiation and propagation of cracks and thereby enhance the fatigue endurance and fracture toughness of the steel. Superior fracture toughness along with high mechanical properties in submicron-grained condition render the two steels highly useful for high-strength structural applications.

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.

Sensitivity analysis of stress, pre-corrosion, nano-particles and heat treatment on fatigue lifetime of aluminum alloy

This article presents the sensitivity analysis of the stress level, the pre-corrosion phenomenon, the addition of nano-particles and applying the heat treatment on the high-cycle bending fatigue lifetime of the aluminum-silicon alloy. For this objective, gravity and stir-casting processes were done for aluminum alloy and nano-clay-composite specimens and then, standard samples were machined from initial casted cylinders. Furthermore, rotary fatigue tests were performed under cyclic bending loadings, through the high-cycle fatigue regime. Some samples were pre-corroded on in the sulfuric acid for 200 hours. Based on the sensitivity analysis on experimental data by the Minitab software, the obtained results indicated that the stress level was the effective parameter on the fatigue lifetime. The meaningful regression model was calculated and calibrated on the logarithmic scale of the fatigue lifetime. Then, the second sensitive parameter was demonstrated as the pre-corrosion, which caused a significant degradation of fatigue properties in the material. The last-ranked factor was related to nano-particles for the beneficial effect on the improvement of the high-cycle fatigue lifetime.

Effect of transformed β phase on fish-eye ductile crack initiation of a Ti-6Al-4V alloy in very high cycle fatigue regime

Very high cycle fatigue (VHCF) properties and damage mechanisms were investigated for a Ti-6Al-4V alloy with needle-like β phase. It was revealed that the fatigue damage is dominated by interior crack initiation with a fish-eye pattern. The center of the fish-eye area is covered by peaks, dimples, microvoids and grooves, exhibiting micro-ductile characteristics instead of the commonly reported brittle ones. It was found that the formation of this particular appearance resulted from the nucleation and coalescence of microvoids and grooves essentially occurring in the transformed β phase (βt). The βt phase played a dominant role in the crack initiation process of the Ti-6Al-4V in the VHCF regime.

Initiation and Propagation Processes of Internal Fatigue Cracks in β Titanium Alloy Based on Fractographic Analysis

Uniaxial fatigue tests were conducted for a β titanium alloy Ti-22V-4Al up to a very high cycle fatigue (VHCF) regime. The initiation and propagation processes of the internal fatigue cracks were investigated using 3D fractographic analysis. Multiple facets were observed at the crack initiation site. Three facet initiation models were proposed based on the surface appearances and the 3D facet bonding patterns of the multiple facets, and the major facet was determined to be the true crack initiation site. Using the size of the major facet, a Tanaka–Akiniwa model, which can determine the material constants for the Paris law using only conventional fatigue tests, was applied to reveal the propagation process of the internal cracks. A reverse fatigue life prediction was also conducted to evaluate the accuracy of the material constants obtained using the Tanaka–Akiniwa model. When the facet initiation models were applied, the predictions showed less deviation and better agreement than when the facet initiation process was not considered. The findings of this study indicate that the formation of multiple facets in β titanium alloys is sequential rather than simultaneous.

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.

Mechanical behaviors of electron beam welded titanium alloy up to very high cycle fatigue under different process conditions

The mechanical behavior of electron beam welded joints for titanium alloy has been investigated in static and cyclic loading up to very high cycle fatigue. It can be concluded that a higher welding voltage parameter and optimal heat treatment condition in combination will produce higher static and cyclic properties of titanium alloy welded joints. The metastable martensite α′ structures in the β grains of the fusion and heat-affected zones are wholly decomposed into small lamellar α phases, forming α + β phases through the post-weld heat treatment (PWHT). Hence, the welded joints by PWHT could transform the “concave” shape microhardness distribution into “convex” shape distribution. The fatigue cracks are initiated from the internal pores of the material in very high cycle fatigue regime. The higher voltage parameter applied to the welding process could reduce the density of pores in the material, which is in favour of the fatigue life of the welded joints. • A higher voltage and optimal heat treatment in combination produce higher mechanical properties of welded joints. • Microstructure and microhardness of welded joints were analyzed before and after heat treatment. • The higher voltage reduced the pores density in the welded joints, which improved fatigue life.

Analysis of Fatigue Indicator Parameters for Ti-6Al-4V microstructures using extreme value statistics in the HCF regime

Fatigue Indicator Parameters (FIPs) have been introduced to serve as surrogate measures of driving force to form and grow fatigue cracks. With Ti-6Al-4V, we consider that transgranular cracks may form along slip bands due to irreversible cyclic plasticity or disruption of phase boundaries impinged by dislocation pile-up under mean stress. FIPs that capture these mechanisms are examined over several microstructure and loading conditions in the High Cycle Fatigue (HCF) regime. The peaks-over-threshold approach determines extreme FIP values and rank-orders fatigue behavior of microstructures. It is shown that fatigue crack formation is dominated by alpha-phase grains in the HCF regime.

The Effect of Baking Heat Treatment on the Fatigue Strength and Life of Shot Peened 4340M Landing Gear Steel.

In this study, the effect of baking heat treatment on fatigue strength and fatigue life was evaluated by performing baking heat treatment after shot peening treatment on 4340M steel for landing gear. An ultrasonic fatigue test was performed to obtain the S–N curve, and the fatigue strength and fatigue life were compared. The micro hardness of shot peening showed a maximum at a hardened depth of about 50 μm and was almost uniform when it arrived at the hardened depth of about 400 μm. The overall average tensile strength after the baking heat treatment was lowered by about 80–111 MPa, but the yield strength was improved by about 206–262 MPa. The five cases of specimens showed similar fatigue strength and fatigue life in high cycle fatigue (HCF) regime. However, the fatigue limit of the baking heat treated specimens showed an increasing tendency rather than that of shot peening specimens when the fatigue life was extended to the very high cycle fatigue (VHCF) regime. The effect of baking heat treatment was identified from improved fatigue limit when baking heat was used to treat the specimen treated by shot peening containing inclusions. The optimum temperature range for the better baking heat treatment effect could be constrained not to exceed maximum 246 °C.

Fourier transform fatigue analysis of the stress in tension/tension of HDPE and PA6

AbstractMechanical fatigue under strain controlled tension/tension (T/T) of rectangular un‐notched and notched specimens of high density polyethylene and polyamide 6 was performed. Under large amplitude oscillatory elongation, the stress response is nonlinear asymmetric due to a different stress response during loading and unloading and was analyzed via discrete Fourier transform. Dynamic strain (ε0) sweep tests revealed odd (I3/1 α ε02) and even (I2/1 α ε01) harmonics in the stress, well described by the Neo‐Hooke's law. Before crack onset for the Nc specimens, the storage and loss moduli and I2/1 are constant, but I3/1 increases with fatigue. The first derivative of I3/1 (dI3/1/dN) and the cumulative nonlinearity Qf parameter (integral of I3/1/ε02) were found to be strong criteria predicting fatigue. To break the unnotched specimens in the low and high cycle fatigue regime, large strain amplitudes were applied, resulting for both materials in initial plastic deformation followed by buckling when a critical compression force is exceeded. This results in a different time evolution of the linear and nonlinear parameters, especially as a minimum in the I2/1 curve. Finally, a model is proposed to describe the nonlinear viscoelastic specimen response under T/T and the effect of buckling.

Properties of the Fine Granular Area and Postulated Models for Its Formation during Very High Cycle Fatigue—A Review

Understanding the mechanisms leading to very high cycle fatigue is necessary to make predictions about the behavior under various conditions and to ensure safe design over the whole lifetime of high-performance components. It is further vital for the development of possible measures to increase the very high cycle fatigue strength. This review therefore intends to give an overview of the properties of the fine granular area that have been observed so far. Furthermore, the existing models to describe the early crack initiation and crack growth within the very high cycle fatigue regime are outlined and the models are evaluated on the basis of the identified fine granular area properties. The aim is to provide an overview of the models that can already be considered refuted and to specify the respective open questions regarding the other individual models.

Effect of Turning on the Surface Integrity and Fatigue Life of a TC11 Alloy in Very High Cycle Fatigue Regime

In this work, the effect of a turning process on fatigue performance of a Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11) titanium alloy is studied in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes. For this purpose, the surface characteristics including surface morphology, surface roughness and residual stress were investigated. Moreover, axial fatigue tests were conducted with an ultrasonic fatigue testing system working at a frequency of 20 kHz. The results show that the turning process deteriorated the fatigue properties in both HCF and VHCF regimes. The fatigue strength at 1 × 108 cycles of turned samples is approximately 6% lower than that of electropolished ones. Fracture surface observations indicate that turning marks play a crucial role in the fatigue damage process, especially in the crack initiation stage. It was observed that the crack of all the turned samples originated from turning marks. In addition, the compressive residual stress induced by the turning process played a more effective role in resisting crack propagation in the VHCF regime than in the HCF regime.

Strain-based fatigue reliability assessment of welded joints in ship structures

A strain-based fatigue reliability assessment of welded ship structural joints is presented. To obtain a complete description of the fatigue stress acting on a butt-welded joint in a ship, a probabilistic distribution for the low cycle fatigue stress range is developed considering the uncertainty in the static and dynamic stresses. A model considering the effect of a secondary notch embedded into the primary notch is developed considering the low and high cycle fatigue regimes. The misalignment, secondary notch, residual stress and fatigue limit effects are incorporated into the notch strain approach on which the fatigue reliability assessment is based in the present study. The efficient Advanced Mean Value method is used to derive the distribution of the fatigue crack initiation life where a detectable crack is formed. The influence of different modelling options is analysed, and the importance of accounting for the misalignment, residual stress and especially the proposed secondary notch effect is discussed. The sensitivity of the reliability estimates to the variables’ uncertainty is investigated based on the First Order Reliability Method, and the dominant ones are identified.

Evaluation of multiaxial high-cycle fatigue criteria under proportional loading for S355 steel

Multiaxial stresses are usually present in engineering structures and are often associated to multiaxial fatigue failures. However, multiaxial fatigue is an open topic, full of questions and different points of view. Therefore, an experimental campaign of uniaxial and multiaxial fatigue tests under proportional loading was conducted aiming at evaluating the multiaxial fatigue behaviour of S355 structural steel in the high-cycle fatigue regime. Five different multiaxial models were used and evaluated, namely the Sines, Findley, McDiarmid, Dang Van and Susmel-MWCM. Each of them was applied to experimental data and the mean fatigue curves obtained from it were evaluated and compared. The coefficients present in each model definition were studied and determined through different methods. The Dang Van’s multiscale approach and Susmel model showed great accuracy in the description of the fatigue behaviour of the S355 steel, providing the best correlation of the uniaxial and multiaxial experimental data.

Bending Fatigue Behavior of 316L Stainless Steel up to Very High Cycle Fatigue Regime.

Effect of microstructure on the crack initiation and early propagation mechanism in the very high cycle fatigue (VHCF) regime was studied in 316L stainless steel (316L SS) by atomic force microscope (AFM) and electron back scattered diffraction (EBSD). The results show that small fatigue cracks initiate from the slip band near the grain boundaries (GBs) or the twin boundaries (TBs). Early crack propagation along or cross the slip band is strongly influenced by the local microstructure such as grain size, orientation, and boundary. Besides, the gathered slip bands (SBs) are presented side by side with the damage grains of the run-out specimen. Finally, it is found that dislocations can either pass through the TBs, or be arrested at the TBs.

Integral treatment of butt joints for the fatigue life assessment in the low cycle fatigue regime

The framework for a fatigue assessment of welded joints under service loading conditions of crane structures from the low cycle to the high cycle fatigue regime includes the consideration of elastic-plastic material behavior, variable amplitude loading, and acceptable calculation times. Therefore, an integral treatment of butt joints has been developed for fatigue life estimation. The butt weld is considered in its entirety, so that it can be described by its cyclic behavior. The evaluation of the cyclic stress-strain behavior and tri-linear strain-life curves of butt joints for different high-strength, fine-grained structural steels, derived by strain-controlled fatigue tests, is the basis for this description. This procedure is not limited to conventionally applied gas metal arc welding only, but also the fatigue assessment of laser beam welding is possible, for example. Cyclic transient effects have been analyzed and a distinctive cyclic softening is described by linearization of Ramberg-Osgood parameters, depending on the damage content of each cycle derived from constant amplitude, strain-controlled tests. On the basis of the cyclic behavior in combination with memory and Masing behavior, a simulation of the stress-strain paths of investigated butt welds, under constant and variable amplitude loading, has been performed. Damage parameters are used to accumulate the damage cycle by cycle in order to derive the fatigue lifetime. Finally, calculated fatigue lives were compared with experimentally determined lives, showing the impact of this procedure.

Influence of processing parameters of selective laser melting on high‐cycle and very‐high‐cycle fatigue behaviour of Ti‐6Al‐4V

AbstractOrthogonal experiment design together with the analysis of variance was used to examine the processing parameters (laser power, scan speed, layer thickness and hatch spacing) of selective laser melting (SLM) for superior properties of SLM parts, in which nine groups of specimens of Ti‐6Al‐4V were fabricated. The results clarify that the influence sequence of individual parameter on the porosity is laser power > hatch spacing > layer thickness > scan speed. Ultrasonic fatigue tests (20 kHz) were conducted for the SLMed specimens in high‐cycle fatigue (HCF) and very‐high‐cycle fatigue (VHCF) regimes. The S‐N data show that the fatigue strength is greatly affected by the porosity: the group with the smallest porosity percentage having the highest fatigue strength in HCF and VHCF regimes. Then, the tests on the validation group were performed to verify the optimal combination of SLM processing parameters. Moreover, the observations by scanning electron microscopy revealed that fatigue cracks initiate at lack‐of‐fusion defects in the cases of surface and internal crack initiation.

Evolution of the residual stresses of types I, II, and III of duplex stainless steel during cyclic loading in high and very high cycle fatigue regimes

Residual stresses (RSs) in the very high cycle fatigue regime have to be evaluated, as in some cases they are comparable with the applied stress, affecting the fatigue behaviour. In this work, the evolution of RSs in terms of types I, II, and III was investigated for a duplex stainless steel (DSS) during cyclic loading in high and very high cycle fatigue regimes. The results showed that the type I RS can increase the fatigue strength and prolong the fatigue life remarkably while the types II and III RSs varying in micro-scale dominate the micro-crack initiation mechanism of the DSS.

Comparison of the damage evolution in glass fiber-reinforced polyurethane and epoxy in the HCF and VHCF regimes investigated by intermittent in situ X-ray computed tomography

The aim of this work is the comparative characterization of the fatigue and damage behaviors of GFR-polyurethane and GFR-epoxy with application-relevant quasi-isotropic layer setup in the high cycle and very-high cycle fatigue regimes. Therefore, a high-frequency test method based on a resonant testing system (1 kHz) has been further developed and assessed with special consideration of self-heating. In intermittent test procedures, the damage state has been explored by in situ X-ray computed tomography analysis after certain numbers of cycles. It was shown that the overall damage state in the VHCF regime is reduced by a factor of three compared to the HCF regime and accompanied by delayed initiation and propagation of delamination. The latter was proven to be the main reason for a decreased inclination of the S/N-curve in the VHCF regime by 50-60%.

Calculation of characteristic size and fatigue life of structural members with blunt notches

In order to evaluate notch effects and size effects on fatigue performances, smooth specimens without stress gradient and a group of notched specimens particularly designed are investigated in this paper. From the perspective of probability, the statistical size effect of the latter is worth studying for bodies weakened by notches. A unified model of combination of the improved theory of critical distance (TCD), along with the concept of highly-stressed-volume (CHSV) is established to identify the initiation crack size, evaluate the fatigue strength, and determine the fatigue lifetime of notched members, and even similar large-scale components in high cycle fatigue regime. The predicted results are compared with the experimental data sets taken from literatures. Fortunately, we find that, the novel model not only inherits the generality of existing methods, such as TCD, but also provides necessary support for predictions of characteristic length and fatigue lifetime of components containing any blunt-notch geometries.