Effect of long-period stacking ordered structure on very high cycle fatigue properties of Mg-Gd-Y-Zn-Zr alloys

Magnesium alloys with a long-period stacking ordered(LPSO)structure usually possess excellent static strength,but their fatigue behaviors are poorly understood.This work presents the effect of the LPSO structure on the crack behaviors of Mg alloys in a very high cycle fatigue(VHCF)regime.The LPSO lamellas lead to a facet-like cracking process along the basal planes at the crack initiation site and strongly prohibit the early crack propagation by deflecting the growth direction.The stress intensity factor at the periphery of the faceted area is much higher than the conventional LPSO-free Mg alloys,contributing higher fatigue crack propagation threshold of LPSO-containing Mg alloys.Microstructure observation at the facets reveals a layer of ultrafine grains at the fracture surface due to the cyclic contact of the crack surface,which supports the numerous cyclic pressing model describing the VHCF crack initiation behavior.

Effect of main inclusions on crack initiation in bearing steel in the very high cycle fatigue regime

This work aims to investigate the effect of main inclusions on crack initiation in bearing steel in the very high cycle fatigue（VHCF） regime. The size and type of inclusions in the steel were quantitatively analyzed, and VHCF tests were performed. Some fatigue cracks were found to be initiated in the gaps between inclusions（Al2 O3, Mg O-Al2 O3） and the matrix, while other cracks originated from the interior of inclusions（Ti N, Mn S）. To explain the related mechanism, the tessellated stresses between inclusions and the matrix were calculated and compared with the yield stress of the matrix. Results revealed that the inclusions could be classified into two types under VHCF; of these two, only one type could be regarded as holes. Findings in this research provide a better understanding of how inclusions affect the high cycle fatigue properties of bearing steel.

Very high cycle fatigue behavior of bridge steel welded joint

Very high cycle fatigue（VHCF） behaviors of bridge steel（Q345） welded joints were investigated using an ultrasonic fatigue test system at room temperature with a stress ratio R = -1. The results show that the fatigue strength of welded joints is dropped by an average of 60% comparing to the base metal and the fatigue failure still occurred beyond 10~7 cycles.The fatigue fracture of welded joints in the low cycle regime generally occurred at the solder while at the heat-affected zone（HAZ） in the very high cycle regime.The fatigue fracture surface was analyzed with scanning electron microscopy（SEM）,showing welding defects such as pore,micro-crack and inclusion were the main factors on decreasing the fatigue properties of welded joints.The effect of welding defects on the fatigue behaviors of welded joints was discussed in terms of experimental results and finite element simulations.

Correlation of crack growth rate and stress ratio for fatigue damage containing very high cycle fatigue regime

A model is proposed to correlate the crack growth rate and stress ratio containing very high cycle fatigue regime.The model is verified by the experimental data in literature.Then a formula is derived for the effect of mean stress on fatigue strength,and it is used to estimate the fatigue strength of a bearing steel in very high cycle fatigue regime at different stress ratios.The estimated results are also compared with those by Goodman formula.

Effect of low cycle fatigue pre-damage on very high cycle fatigue

Carbon-manganese steel is often applied in components of pipes in nuclear plant. Ultrasonic fatigue tests following low cycle fatigue （LCF） cycles damaged are used to study the strength of very high cycle fatigure （VHCF）. The comparison of test results of simple VHCF and cumulative fatigue （LCF plus VHCF） shows that LCF load influences the following VHCF strength. Continuum damage mechanics model is extended to VHCF region.

Very high cycle fatigue for GCr15 steel with smooth and hole-defect specimens

Very high cycle fatigue（VHCF） properties of a low temperature tempering bearing steel GCr15 with smooth and hole-defect specimens are studied by employing a rotary bending test machine with frequency of 52.5 Hz.Both smooth and hole-defect specimens break in VHCF regime with some difference in fatigue crack initiation.For smooth specimens,a fine granular area（FGA） is observed near the grain boundary in the fracture surface of the specimens broken after 10~7 cycles. But no FGA is observed in the hole-defect specimens broken in VHCF regime,and the VHCF crack does not initiate from the small hole at the surface as it does at low or high cycle fatigue regime. Internal stress is employed to explain the VHCF behavior of these two types of specimens.At last,an advanced dislocation model based on Tanaka and Mura model is proposed to illustrate the internal stress process and to predict fatigue crack initiation life with FGA observed in the fracture region.

Improving fatigue strength of bainite/martensite dual-phase steels in very high cycle fatigue regime by refining microstructures

Very high cycle fatigue behaviors of two bainite/martensite dual-phase steels were investigated.One of the steels was cyclic rapid heat treated and its microstructures were refined. Fatigue strength of the steel is 225 MPa higher than that without refining.Observation of fracture surfaces show that the fatigue cracks initiate at bainites for non-refined steel and at non-metallic inclusions for the refined steel.The size of inclusions is much smaller than that of bainites which results in the improvement of fatigue strength.

Comparison of the very high cycle fatigue behaviors of INCONEL 718 with different loading frequencies

In order to clarify the differences of very high cycle fatigue(VHCF) behavior of nickel based superalloy IN718 with different loading frequencies,stress-controlled fatigue tests were carried out by using ultrasonic testing method(20 KHz) and rotary bending testing method(52.5 Hz),both at room temperatures,to establish stress versus cycles to failure(S-N) relationships.Results disclosed that cycles to failure at a given stress level increased with an increase of the applied frequency,i.e.,the higher frequency produced an upper shift of the S-N curves.Fractographic analysis suggested that crack initiation and propagation behaviors had large differences:cracks in low-frequency tests preferentially initiated from multiple sources on the specimen surface,while in high-frequency tests,cracks mostly originated from a unique source of subsurface inclusions.Subsequently,frequency-involved modeling was proposed,based on the damage accumulation theory,which could well illustrate qualitatively those comparisons due to different loading frequencies.

Influence of Hydrogen on GBF in Very High Cycle Fatigue of High Strength Steel

The relationship of hydrogen, GBF （granular bright facet） and very high cycle fatigue properties of high strength spring steels 60Si2CrV with three different hydrogen contents were studied using hydrogen thermal desorp- tion analysis and ultrasonic fatigue test. The results showed that the influence of hydrogen on the relationship between fatigue life and the ratio of GBF to inclusion size was obvious, and the expression between fatigue life and with different hydrogen contents can also be obtained. In addition, based on the research of hydrogen diffusion and GBF, it was explained why the GBF cannot form below 106 cycles. At last, the estimated critical fa tigue life of GBF formation can be expressed accurately.

Effect of stress ratio on very high cycle fatigue properties of Ti-10V-2Fe-3Al alloy with duplex microstructure

In fatigue critical applications, Ti-10V-2Fe-3Al alloy components are expected to endure cyclic loading with cycles above 109. To assess their operating safety, S-N relations of Ti-10V-2Fe-3Al alloy in very high cycle fatigue （VHCF） regime are of concern and have been investigated in this work. Fatigue behavior including S-N curves and crack initiation mechanisms is reported. Two transitions of fatigue crack initi- ation mechanism, from internal crack initiation to surface crack initiation and from αp cleavage to αS/β decohesion, occur when the stress ratio （R） and stress level are reduced. Fatigue limits exist at Nr = 6×10^7 cycles for all stress ratios except for 0.5. In the VHCF regime two kinds of internal crack initiation mechanisms exist, i.e., coalescence of cluster of αp facets and αS/β decohesion. Their mutual competition depends on the stress ratio and can be interpreted in terms of different stress character required for promotion on different internal crack initiation mechanism. Small crack propagation is discussed to be life controlling process under the stress ratio range from -0.5 to 0.1 during VHCF regime while under the stress ratio 0.5 VHCF, life almost refers to the life required for crack initiation.2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.

Current understanding of ultra-high cycle fatigue

The fatigue life of numerous aerospace,locomotive,automotive and biomedical structures may go beyond 10~8 cycles.Determination of long life fatigue behavior becomes extremely important for better understanding and design of the components and structures.Initially,before the invention of ultrasonic fatigue testing,most of the engineering materials were supposed to exhibit fatigue life up to 10~7 cycles or less.This paper reviews current understanding of some fundamental aspects on the development of accelerated fatigue testing method and its application in ultra-high cycle fatigue,crack initiation and growth mechanisms of internal fracture,S-N diagram,fatigue limit and life prediction, etc.

Fatigue life assessment of a high strength steel 300 M in the gigacycle regime

The fatigue behavior of a high strength steel 300 M in the gigacycle regime was investigated. Fully reversed tension - compression fatigue tests at ambient temperature were performed using an ultrasonic fatigue system operating at 20 kHz.The staircase test method was employed to obtain accurate values of the mean fatigue strength corresponding to fixed numbers of cycles up to 10~9.These results were compared to the curve which is estimated by the data tested in the mid-long life regime on conventional servo hydraulic test machine at 20 Hz.Results indicate that the fatigue strength determined from ultrasonic fatigue testing is lightly higher than conventional testing in the range of 10~6-10~7 cycles.It is obvious that nucleations of fractures tend to occur below the surface, if fractures happen after more than 10~7 cycles.All the fractured specimens fails from internal SiO_2 inclusions or smaller carbides and carbide clusters.

GIGACYCLE FATIGUE BEHAVIOR OF CAST ALUMINUM IN TENSION AND TORSION LOADING

An improved understanding of fatigue behavior of a cast aluminum alloy（2-AS5U3G-Y35）in very high cycle regime is developed through the ultrasonic fatigue test in axial and torsion loading.The new developed torsion fatigue system is presented.The effects of loading condition and frequency on the very high cycle fatigue（VHCF）are investigated.The cyclic loading in axial and torsion at 35 Hz and 20 kHz with stress ratio R=-1 is used respectively to demonstrate the effect of loading condition.S-N curves show that the fatigue failure occurs in the range of 105—1010 cycles in axial or torsion loading and the asymptote of S-N curve is inclined,but no fatigue limit exists under the torsion and axial loading condition.The fatigue fracture surface shows that the fatigue crack initiates from the specimen surface subjected to the cyclic torsion loading.It is different from the fatigue fracture characteristic in axial loading in which fatigue crack initiates from subsurface defect in very high cycle regime.The fatigue initiation is on the maximum shear plane,the overall crack orientation is on a typical spiral 45° to the fracture plane and it is the maximum principle stress plane.The clear shear strip in the torsion fatigue fracture surface shows that the torsion fracture is the shear fracture.

Fatigue Crack Initiation Potential from Defects in terms of Local Stress Analysis

The competition of surface and subsurface crack initiation induced failure is critical to understand very high cycle fatigue（VHCF） behavior, which necessitates the elucidation of the underlying mechanisms for the transition of crack initiation from surface to interior defects. Crack initiation potential in materials containing defects is investigated numerically by focusing on defect types, size, shape, location, and residual stress influences. Results show that the crack initiation potency is higher in case of serious property mismatching between matrix and defects, and higher strength materials are more sensitive to soft inclusions（elastic modulus lower than the matrix）. The stress localization around inclusions are correlated to interior crack initiation mechanisms in the VHCF regime such as inclusion-matrix debonding at soft inclusions and inclusion-cracking for hard inclusions（elastic modulus higher than the matrix）. It is easier to emanate cracks from the subsurface pores with the depth 0.7 times as large as their diameter. There exists an inclusion size independent region for crack incubation, outside which crack initiation will transfer from the subsurface soft inclusion to the interior larger one. As for elliptical inclusions, reducing the short-axis length can decrease the crack nucleation potential and promote the interior crack formation, whereas the long-axis length controls the site of peak stress concentration. The compressive residual stress at surface is helpful to shift crack initiation from surface to interior inclusions. Some relaxation of residual stress can not change the inherent crack initiation from interior inclusions in the VHCF regime. The work reveals the crack initiation potential and the transition among various defects under the influences of both intrinsic and extrinsic factors in the VHCF regime, and is helpful to understand the failure mechanism of materials containing defects under long-term cyclic loadings.

Mechanism of subsurface microstructural fatigue crack initiation during high and very-high cycle fatigue of advanced bainitic steels

Advanced bainitic steels with the multiphase structure of bainitic ferrite,retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue(HCF/VHCF)regimes.The subsurface microstructural fatigue crack initiation,referred to as“non-inclusion induced crack initiation,NIICI”,is a leading mode of failure of bainitic steels within the HCF/VHCF regimes.In this regard,there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF.To address this aspect,we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure.This was accomplished by uniquely combining electron microscopy,three-dimensional confocal microscopy,focused ion beam,and transmission Kikuchi diffraction.Interestingly,the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels,including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI,which resulted in the different distribution of fine grains in the crack initiation area.The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries.The formation of fine grains assisted the advancement of small cracks.Another important aspect discussed is the role of retained austenite(RA)during cyclic loading,on crack initiation and propagation in terms of the morphology,distribution and stability of RA,which determined the development of localized cyclic plastic deformation in multiphase structure.

Numerical investigation of the influence of shear band localization on the resonant behavior in the VHCF regime

The monitored resonant behavior of fatigue specimens of metastable austenitic stainless steel （AISI304） is correlated with its damage accumulation in the very high cycle fatigue （VHCF） regime. The resonant behavior is stud- ied experimentally and shows a distinct transient characteristic. Microscopic ex- aminations indicate that during VHCF a localized plastic deformation in shear bands arises on the specimen surface. of damage accumulation in shear bands Hence, this work focuses on the effect on the resonant behavior of AISI304 in the VHCF regime. A microstructural simulation model is proposed that takes into account specific mechanisms in shear bands proven by experimental results. The simulation model is solved numerically using the two-dimensional bound- ary element method and the resonant behavior is characterized by evaluating the force-displacement hysteresis loop. Simulation of shear bands agrees well with microscopic examinations and plastic deformation in shear bands influences the transient characteristic of the resonant behavior.

Ultrasonic Fatigue Damage Behavior of 304L Austenitic Stainless Steel Based on Micro-plasticity and Heat Dissipation

Very high cycle fatigue behavior （107 --109 cycles） of 304L austenitic stainless steel was studied with ultra- sonic fatigue testing system （20 kHz）. The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation. It was found that micro-plasticity （slip markings） could be observed on the specimen surface even at very low stress amplitudes. The persistent slip mark- ings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure. By detailed investigation using a scanning electron microscope and an infrared camera, slip markings appeared at the large grains where the fatigue crack initiation site was located. The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack. Finally, the cou- pling relationship among the fatigue crack propagation, appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior.