High-cycle fatigue and fracture behaviours of SLM AlSi10Mg alloy

The high-cycle fatigue and fracture behaviours of the selective laser melting(SLM)AlSi10Mg alloy were investigated.Flat specimens were designed directly in the shape required for the fatigue tests under pulsating loading in tension(R=0,R is the dynamic factor).The fatigue−life(S−N)curves were modelled with a conditional Weibull’s probability density function,where the real-valued genetic algorithm(GA)and the differential ant-stigmergy algorithm(DASA)were applied to estimating the needed Weibull’s parameters.The fractography of the fatigue specimens showed that the fatigue cracks initiated around the surface defects produced by SLM and then propagated in an unstable manner.However,the presence of large SLM defects mainly influenced the crack initiation period and did not have a strong influence on the crack propagation.The obtained experimental results present a basis for further investigation of the fatigue behaviour of advanced materials and structures(e.g.cellular metamaterials)fabricated by additive manufacturing(AM).Especially,in the case of two-dimensional cellular structures,the cross-section of cellular struts is usually rectangular which corresponds to the specimen shape considered in this work.

High-cycle Fatigue Fracture Behavior of Ultrahigh Strength Steels

The fatigue fracture behavior of four ultrahigh strength steels with different melting processes and therefore different inclusion sizes were studied by using a rotating bar two-point bending fatigue machine in the high-cycle regime up to 107 cycles of loading. The fracture surfaces were observed by field emission scanning electron microscopy （FESEM）. It was found that the size of inclusion has significant effect on the fatigue behavior. For AtSI 4340 steel in which the inclusion size is smaller than 5.5 μm, all the fatigue cracks except one did not initiated from inclusion but from specimen surface and conventional S-N curve exists. For 65Si2MnWE and Aermet 100 steels in which the average inclusion sizes are 12.2 and 14.9 μm, respectively, fatigue cracks initiated from inclusions at lower stress amplitudes and stepwise S-N curves were observed. The S-N curve displays a continuous decline and fatigue failures originated from large oxide inclusion for 60Si2CrVA steel in which the average inclusion size is 44.4 pro. In the case of internal inclusion-induced fractures at cycles beyond about 1×10^6 for 65Si2MnWE and 60Si2CrVA steels, inclusion was always found inside the fish-eye and a granular bright facet （GBF） was observed in the vicinity around the inclusion. The GBF sizes increase with increasing the number of cycles to failure Nf in the long-life regime. The values of stress intensity factor range at crack initiation site for the GBF are almost constant with Nf, and are almost equal to that for the surface inclusion and the internal inclusion at cycles lower than about 1×10^6. Neither fish-eye nor GBF was observed for Aermet 100 steel in the present study.

Effect of heat treatment on high-cycle fatigue behavior of Mg-Zn-Y-Zr alloy

High-cycle fatigue (HCF) behavior of as-forged-T5 Mg-Zn-Y-Zr wrought alloy with stress-ratio R=-1 at ambient environment was presented. The relationship between the maximum stress and the number of cycles to failure was constructed. The results show that the fatigue strength at 107 cycles of the as-forged alloy in T5 state is higher than that of the alloy in T4 state. However, in T6 state, the fatigue strength at 107 cycles is higher than those of the alloys in both T5 and T4 states.

High-cycle fatigue behavior of nickel-base single crystal superalloy

High-cycle rotating bending fatigue behavior of SRR99 nickel-base single crystal alloy at 700 and 900℃ was investigated. The fatigue strengths for 107 cycles are 350 and 335MPa at 700 and 900℃, respectively. The total fatigue life becomes shorter when the temperature increases regardless of the loading stress and frequency. With the number of cycles decreasing, the difference in fatigue strength at the two temperatures becomes smaller. Typical fatigue rupture process including crack initiation site, crack propagation region and final rupture region exhibits at 700℃. The fracture surface is basically characterized by cleavage rupture at 900℃.

Fatigue cracking criterion of high-strength steels induced by inclusions under high-cycle fatigue

Fatigue properties of high-strength steels become more and more sensitive to inclusions with enhancing the ultimate tensile strength (UTS) because the inclusions often cause a relatively low fatigue strength and a large scatter of fatigue lives. In this work, four S–N curves and more than 200 fatigue fracture morphologies were comprehensively investigated with a special focus on the size and type of inclusions at the fatigue cracking origin in GCr15 steel with a wide strength range by different heat treatments after high-cycle fatigue (HCF). It is found that the percentage of fatigue failure induced by the inclusion including Al2 O3 and TiN gradually increases with increasing the UTS, while the percentage of failure at sample surfaces decreases conversely and the fatigue strength first increases and then decreases. Besides, it is interestingly noted that the inclusion sizes at the cracking origin for TiN are smaller than that for Al2 O3 because the stress concentration factor for TiN is larger than that for Al2 O3 based on the finite element simulation. For the first time, a new fatigue cracking criterion including the isometric inclusion size line in the strength-toughness coordinate system with specific physical meaning was established to reveal the relationship among the UTS, fracture toughness, and the critical inclusion size considering different types of inclusions based on the fracture mechanics. And the critical inclusion size of Al2 O3 is about 1.33 times of TiN. The fatigue cracking criterion could be used to judge whether fatigue fracture occurred at inclusions or not and provides a theoretical basis for controlling the scale of different inclusion types for high-strength steels. Our work may offer a new perspective on the critical inclusion size in terms of the inclusion types, which is of scientific interest and has great merit to industrial metallurgical control for anti-fatigue design.

Effect of laser shock peening on combined low- and high-cycle fatigue life of casting and forging turbine blades

Laser shock peening （LSP） is a novel effective surface treatment method to improve the fatigue performance of turbine blades. To study the effect of LSP on combined low- and high-cycle fatigue （CCF） life of turbine blades, the CCF tests were conducted at elevated temperatures on two types of full-scale turbine blades, which were made of K403 by casting and GH4133B by forging. Probabilistic analysis was conducted to find out the effect of LSP on fatigue life of those two kinds of blades. The results indicated that LSP extended the CCF life of both casting blades and forging blades obviously, and the effect of LSP on casting blades was more evident; besides, a threshold vibration stress existed for both casting blades and forging blades, and the CCF life tended to be extended by LSP only when the vibration stress was below the threshold vibra- tion stress. Further study of fractography was also conducted, indicating that due to the presence of compressive residual stress and refined grains induced by LSP, the crack initiation sources in LSP blades were obviously less, and the life of LSP blades was also longer; since the compressive residual stress was released by plastic deformation, LSP had no effect or adverse effect on CCF life of blade when the vibration stress of blade was above the threshold vibration stress.

Comparative study of tensile and high-cycle fatigue properties of extruded AZ91 and AZ91-0.3Ca-0.2Y alloys

Mg-Al-Zn-Ca-Y alloys with excellent ignition and corrosion resistances—termed SEN alloys(where the letters"S,""E,"and"N"stand for stainless,environmentally friendly,and non-flammable,respectively)—have been developed recently.In this study,the microstructure,tensile properties,and high-cycle fatigue properties of an extruded Mg-9.0Al-0.8Zn-0.1Mn-0.3Ca-0.2Y(SEN9)alloy are investigated and compared with those of a commercial Mg-9.0Al-0.8Zn-0.1Mn(AZ91)alloy extruded under the same conditions.Both the extruded SEN9 alloy and the extruded AZ91 alloy have a fully recrystallized structure comprising equiaxed grains,but the former has a smaller average grain size owing to the promoted dynamic recrystallization during extrusion.The extruded AZ91 alloy contains coarse Mg_(17)Al_(12) discontinuous precipitate(DP)bands parallel to the extrusion direction,which are formed during its cool down after extrusion.In contrast,the extruded SEN9 alloy contains relatively fine undissolved Al_(2)Ca,Al_(8)Mn_(4)Y,and Al_(2)Y second-phase particles,which are formed during the solidification stage of the casting process.The tensile strength of the extruded SEN9 alloy,which has finer grains and more abundant particles,is slightly higher than that of the extruded AZ91 alloy.However,the difference in their strengths is relatively small because the stronger solid-solution hardening and precipitation hardening effects in the extruded AZ91alloy offset the stronger grain-boundary hardening and dispersion hardening effects in the extruded SEN9alloy to some extent.The tensile elongation of the extruded AZ91 alloy is significantly lower than that of the extruded SEN9 alloy because the large cracks formed in the DP bands in the former cause its premature fracture.Although the extruded SEN9 alloy has higher tensile properties than the extruded AZ91alloy,the high-cycle fatigue life and fatigue strength of the former are shorter and lower,respectively,than those of the latter.The DP bands in the extruded AZ91 alloy do not act as fatigue crack initiation sites,and therefore,fatigue cracks initiate on the specimen surface at all stress amplitude levels.In contrast,in most of the fatigue-fractured specimens of the extruded SEN9 alloy,fatigue cracks initiate on the undissolved Al_(2)Ca and Al_(2)Y particles present on the surface or subsurface of the specimens because of the high local stress concentration on the particles during cyclic loading.This particle-initiated fatigue fracture eventually decreases the high-cycle fatigue resistance of the extruded SEN9 alloy.

Microstructural evolution and mechanical properties of duplex-phase Ti6242 alloy treated by laser shock peening

The effects of laser shock peening(LSP)on the microstructural evolution and mechanical properties of the Ti6242 alloy,including the residual stress,surface roughness,Vickers microhardness,tensile mechanical response,and high-cycle fatigue properties,were studied.The results showed that the LSP induced residual compressive stresses on the surface and near surface of the material.The maximum surface residual compressive stress was−661 MPa,and the compressive-stress-affected depth was greater than 1000μm.The roughness and Vickers micro-hardness increased with the number of shocks,and the maximum hardness-affected depth was about 700μm after three LSP treatments.LSP enhanced the fraction of low-angle grain boundaries,changed the grain preferred orientations,and notably increased the pole density ofαphase on the near surface from 2.41 to 3.46.The surface hardness values of the LSP samples increased with the increase of the number of shocks due to work hardening,while the LSP had a limited effect on the tensile properties.The high-cycle fatigue life of the LSP-treated sample was significantly enhanced by more than 20%compared with that of the untreated sample,which was caused by the suppression of the initiation and propagation of fatigue cracks.

Modeling of ratcheting accumulation of secondary deformation due to stress-controlled high-cyclic loading in granular soils

An objective of this work is to develop a validated computational model that can be used to estimate ratcheting accumulation behavior of granular soils due to high-cyclic loading. An accumulation model was proposed to describe only the envelope of the maximum plastic deformations generated during the cyclic loading process, which can calculate the accumulated deformation by means of relatively large load cycle increments. The concept of volumetric hardening was incorporated into the model and a so-called overstress formulation was employed to describe the evolution of the accumulated volumetric deformation as a state parameter. The model accounted for ratcheting shakedown and accumulation such as a pseudo-yield surface(a shakedown surface) associated with loading inside the current virgin yield surface which was implemented into the well-known modified Cam-clay model. Finally, the model was calibrated using data from the stress-controlled drained cyclic triaxial tests on homogeneous fine grained sands. It is seen that the model can successfully represent important features of the ratcheting accumulation of both volumetric and deviatoric deformation caused by repeated drained loading over a large number of cycles.

Temperature evolution and fatigue life evaluation of AZ31B magnesium alloy based on infrared thermography

The surface temperature of extruded AZ31B alloy plate was measured by infrared thermograph in air during tension and high-cycle fatigue tests. The mechanism of heat production was discussed and the value of critical fatigue damage temperature was calculated according to the P—ΔT curve. Results show that the variation trend of temperature is different between tension and fatigue tests. The temperature evolution in tension test consists of four stages： linear decrease, reverse linear increase, abrupt increase, and final drop. The initial decrease of temperature is caused by thermal elastic effect, which is corresponding to the elastic deformation in tension progress. When cyclic loading is above the fatigue limit, the temperature evolution mainly undergoes five stages： initial increase, steep reduction, steady state, abrupt increase, and final drop. The peak temperature in fatigue test is caused by strain hardening that can be used to evaluate the fatigue life of magnesium alloy. The critical temperature variation that causes the fatigue failure is 3.63 K. When ΔT≤3.63 K, the material is safe under cyclic loading. When ΔT3.63 K, the fatigue life is determined by cycle index and peak temperature.

High Cycle Fatigue Properties of Die-Cast Magnesium Alloy AZ91D with Addition of Different Concentrations of Cerium

The effect of addition of different concentrations of Ce on high-cycle fatigue behavior of die-cast magnesium alloy AZ91D was investigated. Mechanical fatigue tests were conducted at the stress ratio of R = 0.1, and fatigue strength was evaluated using up-and-down loading method. The results show that the grain size of AZ91D alloy is remarkably refined, and the amount of porosity decreases and evenly distributes with the addition of Ce. The fatigue strength of AZ91D alloy at room temperature increases from 96.7 up to 116.3 MPa （ 1% Ce） and 105.5 MPa （2 % Ce）, respectively, at the number of cycles to failure, Nf = 1 × 10^7. The fatigue crack of AZ91D alloy initiates at porosities and inclusions, and propagates along grain boundaries. The fatigue striations on fractured surface appear with Ce addition. The fatigue fracture surface of test specimens shows mixed-fracture characteristics of quasi-cleavage and dimple.

Experimental investigation on fatigue of blade specimen subjected to resonance and effect of a damping hard coating treatment

Failures due to high-cycle fatigue have led to a high cost in aerospace engineering over the past few decades.In this paper,the experimental results of the fatigue behavior of compressor blade specimen subjected to resonance and the effects of a damping hard coating on relieving the fatigue progress are presented.The crack initiation and propagation processes were observed under resonance of the first bending mode by using the resonant frequencies as the indicator.Significant nonlinear features were observed in the spectrum of the blade with a fatigue crack.The finite element model considering the breathing crack was established with nonlinear contact based on the crack localization and size,which was obtained by ultrasonic phased array technology.The simulation results of the vibration behavior of the cracked blade were obtained and consistent with the experimental results.A NiCrAlY coating was deposited on the blade,and increases in the fatigue life were observed under the same condition.The results of this paper can help to better understand the fatigue of a compressor blade subjected to resonance and provide a preference for the application of a damping hard coating on compressor blades.

Fracture of two three-dimensional parallel internal cracks in brittle solid under ultrasonic fracturing

Similar to hydraulic fracturing(HF), the coalescence and fracture of cracks are induced within a rock under the action of an ultrasonic field, known as ultrasonic fracturing(UF). Investigating UF is important in both hard rock drilling and oil and gas recovery. A three-dimensional internal laser-engraved crack(3D-ILC) method was introduced to prefabricate two parallel internal cracks within the samples without any damage to the surface. The samples were subjected to UF. The mechanism of UF was elucidated by analyzing the characteristics of fracture surfaces. The crack propagation path under different ultrasonic parameters was obtained by numerical simulation based on the Paris fatigue model and compared to the experimental results of UF. The results show that the 3D-ILC method is a powerful tool for UF research.Under the action of an ultrasonic field, the fracture surface shows the characteristics of beach marks and contains powder locally, indicating that the UF mechanism includes high-cycle fatigue fracture, shear and friction, and temperature load. The two internal cracks become close under UF. The numerical result obtained by the Paris fatigue model also shows the attraction of the two cracks, consistent with the test results. The 3D-ILC method provides a new tool for the experimental study of UF. Compared to the conventional numerical methods based on the analysis of stress-strain and plastic zone, numerical simulation can be a good alternative method to obtain the crack path under UF.

A two-parameter model to predict fatigue life of high-strength steels in a very high cycle fatigue regime

In this paper, ultrasonic （20 kHz） fatigue tests were performed on specimens of a high-strength steel in very high cycle fatigue （VHCF） regime. Experimental results showed that for most tested specimens failed in a VHCF regime, a fatigue crack originated from the interior of specimen with a fish-eye pattern, which contained a fine granular area （FGA） centered by an inclusion as the crack origin. Then, a two-parameter model is proposed to predict the fatigue life of high-strength steels with fish-eye mode failure in a VHCF regime, which takes into account the inclusion size and the FGA size. The model was verified by the data of present experiments and those in the literature. Furthermore, an analytic formula was obtained for estimating the equivalent crack growth rate within the FGA. The results also indicated that the stress intensity factor range at the front of the FGA varies within a small range, which is irrespective of stress amplitude and fatigue life.

Multivariate relationships between microstructure evolution and strengthening mechanisms in laser powder bed fusion of Al-Mn-Sc alloy:towards improved fatigue performance

The effects of direct aging treatment(at 300℃ for 5 hours)on selective laser melted(SLMed)Al-4.5Mn-1.5Mg-0.9Sc-0.2Zr alloy were investigated in this work,with the microstructure,fatigue behaviors,and fracture characteristics examined to determine the primary cause of fatigue crack source.The results revealed that the microstructure of the investigated alloy comprised fine equiaxed and columnar grains.Upon aging treatment,a significant number of nano-scaled Al3(Sc,Zr)precipitates were dispersed within the grains,leading to a substantial increase in strengths.The yield strength improved from 431 MPa to 568 MPa,representing an increase of more than 32%,while the fatigue strength improved from 180 MPa to 220 MPa after aging treatment.Nevertheless,the fracture toughness decreased significantly from 25.1 MPa·√m to 12.3 MPa·√m.The results of the fatigue fracture characteristics indicate that the Mn-rich phase and the formation of defects such as pores and poor powder fusion are the sources of fatigue cracking.Although direct aging treatment can significantly increase the yield strength,decrease the rate of fatigue crack propagation,and thus improve the fatigue performance,it deteriorates the fracture toughness,and thus shortens the fatigue life of the alloy as well.

Fatigue Life Prediction of Gray Cast Iron for Cylinder Head Based on Microstructure and Machine Learning

Conventional fatigue tests on complex components are difficult to sample,time-consuming and expensive.To avoid such problems,several popular machine learning(ML)algorithms were used and compared to predict fatigue life of gray cast iron(GCI)with the complex microstructures.The feature analysis shows that the fatigue life of GCI is mainly influenced by the external environment such as the stress amplitude,and the internal microstructure parameters such as the percentage of graphite,graphite length,stress concentration factor at the graphite tip,matrix microhardness and Brinell hardness.For simplicity,collected datasets with some of the above features were used to train ML models including back-propagation neural network(BPNN),random forest(RF)and eXtreme gradient boosting(XGBoost).The comparison results suggest that the three models could predict the fatigue lives of GCI,while the implemented RF algorithm is the best performing model.Moreover,the S–N curves fitted by the Basquin relation in the predicted data have a mean relative error of 15%compared to the measured data.The results have demonstrated the advantages of ML,which provides a generic way to predict the fatigue life of GCI for reducing time and cost.

Fatigue behavior of CoCrFeMnNi high-entropy alloy under fully reversed cyclic deformation

Bulk ultrafine-grained(UFG) CoCrFeMnNi high-entropy alloy(HEA) with fully recrystallized microstructure was processed by cold rolling and annealing treatment. The high-cycle fatigue behaviors of the UFG HEA and a coarse-grained(CG) counterpart were investigated under fully reversed cyclic deformation.The fatigue strength of the UFG HEA can be significantly enhanced by refining the grain size. However,no grain coarsening was observed in the UFG HEA during fatigue tests. Mechanisms for the superior mechanical properties of the UFG HEA were explored.

Review on fatigue life prediction models of welded joint

Fatigue assessment of welded joint is still far from being completely solved now,since many influencing factors coexist and some important ones should be considered in the developed life prediction models reasonably.Thus,such influencing factors of welded joint fatigue are firstly summarized in this work;and then,the existing life prediction models are reviewed from two aspects,i.e.,uniaxial and multiaxial ones;finally,significant conclusions of existing experimental and theoretical researches and some suggestions on improving the fatigue assessment of welded joints,especially for the low-cycle fatigue with the occurrence of ratchetting,are provided.

Corrected stress field intensity approach based on averaging superior limit of intrinsic damage dissipation work

Corrected stress field intensity obtained by averaging the superior limit of intrinsic damage dissipation work in critical domain, which considers thoroughly thermodynamic consistency within irreversible thermodynamic framework, was proposed for predictions of high-cycle fatigue endurance limits. Simultaneously, the effects of mean stress, additional hardening behavior related to non-proportional loading paths and stress gradients on multiaxial high-cycle fatigue are taken into account in the proposed approach. The approach is an extension of the general stress field intensity. For a better comparison, existing multiaxial high-cycle fatigue criteria were employed to predict the endurance limits of different metallic materials subjected to different multiaxial loading paths, and it is shown that present proposal performs better from statistical value of error indexes, which make the proposed approach of corrected stress field intensity and its associated concepts provide a new conception to predict endurance limits of multiaxial high-cycle fatigue with high accuracy.

Effect of cold deformation on corrosion fatigue behavior of nickel-free high nitrogen austenitic stainless steel for coronary stent application

Due to the excellent mechanical properties, good corrosion resistance, high biocompatibility and nickel- free character, the high nitrogen nickel-free austenitic stainless steel （HNASS） becomes an ideally alternative material for coronary stents. Stent implantation works in harsh blood environment after a balloon dilatation, i.e., the material is used in a corrosive environment with a permanent deforma- tion. The present study attempts to investigate effects of pre-straining on high-cycle fatigue behavior and corrosion fatigue behavior of HNASS in Hank＇s solution and the relevant mechanism for coronary stents application. It is found that higher pre-straining on HNASS results in higher strength and maintains almost same corrosion resistance. Fatigue limit of 0% HNASS is 550 MPa, while corrosion fatigue limit is 475 MPa. And improvement in fatigue limit of 20% and 35% pre-strained HNASS is in comparison with the 0% HNASS, while corrosion would undermine the fatigue behavior of HNASS. In a suitable range, the pre- straining had a beneficial effect on corrosion fatigue strength of HNASS, such as nearly 300 MPa improved with 20% cold deformation. This result provides a good reference for predicting the life of HNASS stent and as well its design.