High Cycle Fatigue Crack Initiation Research Articles

Effect of microstructure and local stress on small crack initiation and propagation paths in coarse‐grained FeCrAl alloys

AbstractThis study investigated the high cycle fatigue crack initiation and propagation behaviors in coarse‐grained FeCrAl alloys, with a focus on the relationship between the path of small cracks and the local microstructure. The results show that the small cracks initiate along the {100} slip plane due to the synthetical influence of local microstructure and shear stress. The initiation zone forms inclined areas with layered fine‐grain areas caused by cyclic shear stress. Quantitative analysis of the stress intensity factor (SIF) indicates that in the realm of mixed‐mode loading, the SIF of Mode I takes the lead in determining the intensity of stress and strain fields at the crack tip. As the crack propagates further, the SIF steadily increases. Once it reaches a specific threshold, the propagation mode within a coarse grain shifts from Mode II to Mode I. Moreover, fatigue life increases with a tendency for decreased SIF during crack deflection.

Fatigue life prediction of Ti60 titanium alloy under very high cycle loading at different temperatures

The very high cycle fatigue crack initiation mechanism of Ti60 titanium alloy at different temperatures was studied. The results showed that the initiation of cracks occurred on oversized facets in the subsurface. Based on this crack initiation mechanism, a full-life prediction model based on crack propagation at room temperature was first proposed, and then a very high cycle fatigue life prediction model considering temperature was proposed based on the degradation law of tensile strength at different temperatures. The experimental and predicted life results of Ti60 specimen at room temperature with 300 °C and 500 °C showed that the proposed method can effectively predict the very high cycle fatigue life, and the error is within a factor of 3.

Influence of notch root radius on high cycle fatigue properties and fatigue crack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure

Notch high cycle fatigue (HCF) properties and microcrack initiation behavior of Ti-55531 alloy with a multilevel lamellar microstructure under various notch radii were systematically investigated. Results indicate that the reduction of notch root radius significantly promotes fatigue microcrack initiation, and then dramatically reduces the HCF life and strength of the alloy. Cyclic deformation of the alloy is mainly controlled by the slipping and deformation twinning in α plates. The primary fatigue crack initiation micro-mechanism is α/β interface cracking induced by slipping and twinning at all notch HCF specimens. Moreover, the volume fraction of twinning would increase with decreasing of the notch root radius. Interestingly, when the notch root radius is the smallest (R = 0.14 mm) and the stress concentration factor is the largest (Kt = 4), in addition to slipping and twinning, basal stacking faults promoting the cracking of α/β interface could be another crucial HCF microcrack initiation mechanism of the alloy. Furthermore, with decreasing of notch root radius and increasing of stress concentration factor, the size of cycle plastic deformation zone at notch root gradually reduces to the size of α colony and even the α plate. Therefore, the order of influential degree of three different levels microstructure on the crack initiation mechanism of notched HCF can be arranged as α plate > α colony > β GB.

Quantitative estimation of contribution of pore size and the distance between nearest neighbour pores on high cycle fatigue (HCF) crack initiation in Ti-6Al-4V MIM-sintered materials

The fatigue damages due to pores of MIM-sintered Ti64 were investigated using the synchrotron X-ray CT imaging. The effect of pores on the fatigue strength limits was analysed independently from that of grain size by introducing the fatigue limit ratio RF. The pores near to the maximum-stress area were identified by elastic FEM. The maximum stress intensity factor KI-max at the edge of the identified pores showed a negative correlation with RF. Moreover, the initial cleavage in the crack was influenced by the mutual interference of both stress fields by the proximity of the identified pore and its adjacent pores.

Solute segregation improves the high-cycle fatigue resistance of nanocrystalline Pt-Au

Fatigue-induced grain growth has been shown to drive high-cycle fatigue crack initiation in nanocrystalline metals. As a potential strategy to suppress such grain growth, particular solute additions are known to stabilize grain boundaries through both thermodynamic and kinetic mechanisms. In previous work, Au additions in nanocrystalline Pt were confirmed to stabilize grain boundaries against thermally-driven grain boundary migration, however, the corresponding effect on fatigue resistance was unknown. The present work compared the high-cycle fatigue performance of stabilized Pt-10at%Au to reference pure Pt with a similar initial grain size, using a combination of ex-situ and in-situ scanning electron microscope fatigue tests. Ex-situ tests revealed that the Pt-10Au exhibits substantial improvement in overall fatigue life over Pt, including a 75% elevation of the fatigue endurance limit. The improvement was attributed to enhanced crack initiation resistance, since the Pt-10Au exhibited diminished resistance to crack propagation compared to Pt. Electron backscatter diffraction, transmission kikuchi diffraction, and transmission electron microscopy showed that the pure Pt exhibited extensive fatigue-induced abnormal grain growth, with the largest grains growing to 10 times the size of the initial grain size, whereas the Pt-10Au exhibited inhibited grain growth, with the largest grains growing by approximately a factor of two. This study provides clear evidence that thermodynamic strategies used to impart thermal stability can also contribute to improved high-cycle fatigue resistance via suppression of grain-growth induced crack initiation.

Micro-mechanism associated with very high cycle fatigue crack initiation of advanced DQ&P processed steel

In the present study, very high cycle fatigue (VHCF) characteristics and properties of a direct quenched and partitioned (DQ&P) medium carbon (0.4 wt.% C) steel having ultrahigh-strength (UTS > 2 GPa) was investigated using an ultrasonic-fatigue testing technique (∼19 kHz frequency) up to ∼1010 cycles at the load ratio (R) of −1 (fully reversed tension-compression). The occurrence of crack initiation in the VHCF regime was observed at interior non-metallic inclusions. The microstructural changes following VHCF failure underneath the fracture surfaces near the crack initiation sites were further investigated through transmission electron microscopy. Formation of a clear nanograined subsurface layer adjacent to the crack initiating site at the interior inclusion was noticeably evident. The variation of average grain size (60 − 140 nm) and thickness (300 − 1200 nm) of nanograined layers at different number of cycles to failure were measured in detail. Apparently, localized plastic deformation caused the fragmentation of martensitic laths and hence, the formation of aforesaid ultrafine-grained layers in the microstructures.

­Solute Segregation Improves the High-Cycle Fatigue Resistance of Nanocrystalline Pt-Au

Fatigue-induced grain growth has been shown to drive high-cycle fatigue crack initiation in nanocrystalline metals. As a potential strategy to suppress such grain growth, particular solute additions are known to stabilize grain boundaries through both thermodynamic and kinetic mechanisms. In previous work, Au additions in nanocrystalline Pt were confirmed to stabilize grain boundaries against thermally-driven grain boundary migration, however the corresponding effect on fatigue resistance was unknown. The present work compared the high-cycle fatigue performance of stabilized Pt-10at%Au to reference pure Pt with a similar initial grain size, using a combination of ex-situ and in-situ scanning electron microscope fatigue tests. Ex-situ tests revealed that the Pt-10Au exhibits substantial improvement in overall fatigue life over Pt, including a 75% elevation of the fatigue endurance limit. The improvement was attributed to enhanced crack initiation resistance, since the Pt-10Au exhibited diminished resistance to crack propagation compared to Pt. Electron backscatter diffraction, transmission kikuchi diffraction, and transmission electron microscopy showed that the pure Pt exhibited extensive fatigue-induced abnormal grain growth, with the largest grains growing to 10 times the size of the initial grain size, whereas the Pt-10Au exhibited inhibited grain growth, with the largest grains growing by approximately a factor of two. This study provides clear evidence that thermodynamic strategies used to impart thermal stability can also contribute to improved high-cycle fatigue resistance via suppression of grain-growth induced crack initiation.

Slip-driven and weld pore assisted fatigue crack nucleation in electron beam welded TC17 titanium alloy joint

High cycle and very high cycle fatigue (VHCF) crack initiation mechanism of TC17 alloy joint with heterogeneous microstructures in FZ has been studied at the stress ratio of 0.1. Intrinsic deficiencies of slip deformation in β grains with few martensites and cracking coarse grain boundary between prior β grains with martensite colonies account for the high cycle fatigue (HCF) of joints. The average diameter of intergranular welding pores regarding VHCF crack nucleation is measured to be ∼36 μm. The driving force governing the crack nucleation from intrinsic deficiencies in HCF to porosity defects in VHCF is ∼1.66 MPa·m1/2.

Review of the Mechanical Properties of High-Strength Alloys at Cryogenic Temperatures

To advance the understanding of metallic materials for structural components at cryogenic temperatures, a review of previous studies on the mechanical properties of austenitic stainless steels, titanium alloys, aluminum alloys, and copper alloys is presented herein. In particular, the evaluation of strength and toughness balance intends to clarify key differences between these alloys to facilitate better alloy design and potentially bolster further alloy development. Several key points on cryogenic mechanical tests are also introduced. Temperature control, testing tools, and test conditions should be properly calibrated to avoid experimental errors. Furthermore, substantial progress for subsurface crack initiation in high-cycle fatigue at cryogenic temperatures is presented, in which the mechanism of subsurface crack generation is highlighted as a key design consideration in microstructure of high-strength alloys.

Understanding main factors controlling high cycle fatigue crack initiation and propagation of high strength maraging stainless steels with Ti addition

Abstract The microstructure and high cycle fatigue (HCF) property of maraging stainless steels with Ti addition have been systematically studied. The two steels showed similar hardening responses in the aging process, which should be attributed to their identical microstructure characters, typical lath martensite immerged with dispersive precipitates Ni3Ti and R phase. It is proposed that the HCF crack initiation mechanism depends on the value of normalized applied stress (NAS). At high NAS level (slip band control region), the yield strength dominated by nano-size precipitates is the main factor controlling the HCF lifetime. At low NAS level (inclusion control region), the distance between TiN inclusion and the specimen surface is the main factor controlling the HCF lifetime. TiN inclusion could sharply increase the fatigue crack growth rate. It is found that TiN inclusion acted as the nucleation site of microcrack at front of main crack tip, thus deteriorated the crack propagation resistance of the maraging stainless steel.

Modelling the influence of the microstructure on the high cycle fatigue crack initiation in short fibre reinforced thermoplastics

In this work, a new criterion to predict fatigue crack initiation in plain Short Fibre Reinforced Thermoplastics is proposed. Several experimental observations in the literature showed that the initiation of a macro crack results from the accumulation of damage in the matrix at the micro-scale. Accordingly, in the proposed criterion the life to crack initiation is predicted through an effective stress acting in the most critical regions in the matrix (close to the fibre ends and lateral surfaces). More precisely, the effective stress is evaluated within a control volume defined as a certain percentage of the matrix volume subjected to the highest stresses. A multi-scale (macro-meso-micro) procedure is adopted for estimating the local stress fields in the polymer. The criterion was proved to successfully predict the influence of the material microstructure (fibre content and orientation distribution) and load orientation.

Planar slip-driven fatigue crack initiation and propagation in an equiatomic CrMnFeCoNi high-entropy alloy

High-cycle fatigue crack initiation and propagation in an equiatomic CrMnFeCoNi high-entropy alloy were investigated using smooth specimens. The microstructural deformation characteristics, i.e., planar dislocation slip, significantly affected the fatigue crack initiation and small fatigue crack propagation. The deformation localization associated with dislocation planarity led to multiple crack initiation on the slip planes. The crack propagation mechanism comprised crack formation on slip planes around the main crack tip and subsequent coalescence. The fatigue crack propagation mechanism shifted to Mode I type as the crack length increased.

Relationship between microstructures, facet morphologies at the high-cycle fatigue (HCF) crack initiation site, and HCF strength in Ti-6242S

The effect of microstructures on the high-cycle fatigue (HCF) properties of Ti-6242S (Ti-6Al-2Sn-4Zr-2Mo-0.1Si) was investigated. Five different microstructural conditions were generated through combinations of hot-deformation and heat treatments, and each was fatigue-tested using smooth round bar specimens at an R ratio of 0.1. The HCF behavior was found to be highly dependent on microstructures. The bimodal microstructure showed the highest HCF strength in the whole cycle range with 750 MPa at 1.0E+07 cycles. The fatigue ratio S of the HCF strength at 1.0E+07 cycles to tensile strength showed its highest value of 0.65 for the acicular α-microstructure whereas it showed its lowest value of 0.49 for the full lamellar microstructure. The flat facets were observed at the fatigue crack initiation sites irrespective of the microstructures, and were always located in the subsurface regions. Concurrent observation of the fatigue initiation facet and the underlying microstructure was made using the precision sectioning method. Based on these experimental results, the main factor contributing to the improvement of HCF strength was analyzed, along with the correlation between facet morphologies and fatigue life-controlling microstructural units, the fatigue initiation mechanisms and the dependence of HCF strength on the facet size.

The behavior of crack initiation and early growth in high-cycle and very-high-cycle fatigue regimes for a titanium alloy

The behavior of crack initiation and early growth in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes for a TC4 titanium alloy with equiaxed microstructure was investigated. Fatigue tests were conducted via ultrasonic axial cycling (20 kHz) superimposed by an amount of tensile load. The effect of tensile mean stress on fatigue endurance was plotted with a Haigh diagram formulated by Goodman, Gerber and the present formula. Fractography was observed by scanning electron microscopy, and three failure types of crack initiation were classified: surface-without-RA (rough area), surface-with-RA and interior-with-RA. Profile samples from the crack initiation region were prepared and examined by transmission electron microscopy with selected area electron diffraction. The observations show that a nanograin layer prevails underneath the fracture surface in the RA region only for the VHCF case with a stress ratio R = −1. The nanograin formation mechanism was explained by the numerous cyclic pressing (NCP) model.

The role of local plasticity during very high cycle fatigue crack initiation in high-strength steels

Cracks leading to failure in very high cycle fatigue (VHCF) of high-strength steels often initiate at subsurface inclusions with stress intensity factors (SIF) below the classical threshold SIF Kth for long cracks, especially for negative stress ratios. A characteristic fine granular area (FGA) can then be observed at the fracture surface in the vicinity of those crack-initiating inclusions. According to various researchers, the FGA formation might be responsible for the late initiation of a propagable long crack. Microstructural investigations reveal strong local grain refinement along the fracture surface inside the FGA. The origin of the FGA and its influence on the crack initiation in VHCF is discussed controversially in the literature. The grain refinement might be the result of local plasticity around inclusions in VHCF prior to crack initiation or propagation and thereby causal for the late failure. If this is true, there should be a correlation between the FGA dimension and the local plastic zone size. This hypothesis is investigated in detail in this study. For this purpose, the spatial geometry of FGA is systematically analyzed for different fatigued specimens of the high-strength steel 100Cr6. The results are compared to plastic zone size models and the influence of local plasticity on VHCF is discussed. Based on the results, the VHCF failure mechanism proposed by Grad et al. (2012) [1] is extended and improved.

High cycle fatigue strength assessment methodology considering punching effects

Since a decrease of the fatigue strength may result from punching operations, this study proposes a methodology for designing punched parts against high cycle fatigue crack initiation. To reach this goal, high cycle fatigue tests are performed on different specimens configurations with either punched or polished edges. Due to punching effects, the fatigue strength of punched specimens is significantly decreased. Fracture surfaces observations reveal that crack initiation occurs always on a punch defect. Additional investigations are combined to characterize how the edges are altered by the punching operations. High tensile residual stress levels along the loading direction are quantified using X-Ray diffraction techniques. Furthermore, micro-hardness measurements and X-Ray diffraction results reveals a strong hardness gradient due to punching operation. For a better understanding of crack initiation mechanisms, the edge geometries have been scanned with 3D optical microscopy, allowing us to identify the most critical defect (and its real geometry) by comparing the edges before and after fatigue failure. Finally, FEA are performed on identified defects. A non-local high cycle multiaxial fatigue strength criterion has been used as post-processing of FEA to take into account the effect on the HCF strength of defects and the strong stress-strain gradients around them.

Correlation between micro-mechanical property and very high cycle fatigue (VHCF) crack initiation in friction stir welds of 7050 aluminum alloy

The S–N curves at very high cycle fatigue (VHCF) regime for the base material and the friction stir welding (FSW) joint of 7050 aluminum alloy present multi-stage shapes with an inflection point at about 108 cycles in the study. In the regime between 1×106 and 5×107, the fatigue crack initiates at the thermo-mechanically affected zone (TMAZ) on the advancing side; during the life between 5×107 and 1×109 cycles, most of failure originates from specimen surface without preferential site on both the advancing side and the retreating site of TMAZ. The correlation between micro-mechanical properties and fatigue crack initiation location and the effects of cyclic loading level on the failure site transition are investigated. It is believed that inhomogeneous microstructure of FSW joint leads to the selective strain localization at relatively high stress level and the load-dependence of the irreversible cyclic slip formation, as validated in the full-field strain measurement by digital image correlation (DIC) technique. At final, considering fatigue crack initiation site and its corresponding hardness, fatigue strength in the VHCF range is accurately estimated by the modified Murakami’s model.

Influence of mean stress and notches on the very high cycle fatigue behaviour and crack initiation of a low‐pressure steam turbine steel

AbstractThe aim of the present work is to investigate the influence of different stress concentration factors (1.09, 1.31, and 2.42) and several R‐values on the fatigue behaviour of martensitic steel X10CrNiMoV12‐2‐2 up to 2 · 109 load cycles at ambient temperature. The tests were performed using an ultrasonic fatigue testing system developed at the authors' institute. The S‐N curves of specimens with the lowest investigated stress concentration factor of 1.09 show for both R = −1 and R = 0.5 a flat slope with transition from surface to volume crack initiation at about 1–2 · 107 load cycles. The maximum number of load cycles where fracture occurs increases with increasing R‐value. The √area approach by Murakami describes lifetime behaviour within about 4 decades of lifetime and a wide range of load ratios. In case of internal crack initiation at R = −1, the fracture surface contains a fine granular area (FGA) around the inclusion within the typical “fish‐eye” fracture. With increasing notch factor, the fatigue strength up to 2 · 109 load cycles decreases and the slope of the S‐N curve becomes steeper in the low cycle fatigue and high cycle fatigue regime, especially for αk = 2.42. In this case, damage at internal defects is suppressed and fatigue cracks are generally initiated at the notch root.

Multiaxial fatigue strength assessment of electroslag remelted 50CrMo4 steel crankshafts

This work investigates the fatigue strength of multiaxially-loaded gas engine crankshafts incorporating electroslag remelted 50CrMo4 steel. Uniaxial fatigue tests with small-scale round specimens before and after performing the ESR process are conducted. The results reveal that the high-cycle fatigue strength at the run-out region significantly decreases by 21% if no ESR-treatment is performed. Multiaxial fatigue tests under proportional normal and shear stress loading demonstrate that the elliptical model by Gough and Pollard leads to a minor underestimation of 5% compared to the test results. An application of several state-of-the-art equivalent stress hypotheses utilizing the critical plane approach is presented and the estimated results are compared to the experiments in terms of high-cycle fatigue resistance and crack initiation angle.

Very High Cycle Fatigue Crack Initiation Mechanism in Nugget Zone of AA 7075 Friction Stir Welded Joint

Very high cycle fatigue behavior of nugget zone in AA 7075 friction stir welded joint was experimentally investigated using ultrasonic fatigue testing system (20 kHz) to clarify the crack initiation mechanism. It was found that the fatigue strength of nugget zone decreased continuously even beyond 107 cycles with no traditional fatigue limits. Fatigue cracks initiated from the welding defects located at the bottom side of the friction stir weld. Moreover, a special semicircular zone could be characterized around the crack initiation site, of which the stress intensity factor approximately equaled the threshold of fatigue crack propagation rate. Finally, a simplified model was proposed to estimate the fatigue life by correlating the welding defect size and applied stress. The predicted results are in good agreement with the experimental results.