CORROSION FATIGUE OF HSLA STEELS IN SALT SPRAY

The corrosion fatigue propagation behaviour of high strength low alloy Cr-Ni and Si-Mnsteels has been investigated in 3.5 %NaCl solution spryly at the frequencies of 0.1 and 5.5 Hz.It was shown that the fatigue crack initiation resistances of the two steels are significantly re-duced in salt spray;the fatigue crack growth rates of steels increase with the decrease of fre-quency and are much higher in salt spray than in air within low ΔK range.A critical stress in-tensity factor was observed for each steel and the crack growth stoppage will occur if the ΔKvalues are lower than it.It was found that the active dissolution makes great contribution tothe fatigue crack growth within low ΔK range.     

Effect of HIP Treatment on Fatigue Crack Growth Behavior of Ti–6Al–4V Alloy Fabricated by Electron Beam Melting

The effect of hot isostatic pressing treatment on the fatigue crack growth behavior of Ti–6Al–4V alloy fabricated by electron beam melting was investigated.The results indicate that the fatigue crack growth rate of the HIPed samples is higher than that of the as-fabricated one under certain stress intensity factor(DK\18 MPa m~(1/2)).With further increase in DK,the fatigue crack growth rates of the studied two samples become similar.The variation of a lamella thickness and the pore defects distribution have an effect on the fatigue crack growth rates in the studied samples,and the latter plays the dominant role.     

ON FATIGUE CRACK PATH DEVIATION AT ELEVATED TEMPERATURE IN ELECTRON BEAM REPAIRED WELDMENTS OF TURBINE DISK

Fatigue crack growth behaviors in electron beam weldments of a nickel-base superalloy are. studied. The objective of this paper is to discuss effects of the inhomogeneity of mechanical performance on fatigue crack growth (FCG) rate and crack path deviation (CPD). The base metal served in a turbine disk of aerospace engine was selected to fabricate bead-on-plate weldments by using electron beam welding. Some wedge-type opening loading specimens, notched in three different zone of weld metal, HAZ and base metal, were employed and performed fatigue crack growth tests at 650℃. The results show that the fatigue crack growth of electron beam welded joints is instable due to the influence of mechanical heterogeneities. Owing to the crack deviation at the weld metal and heat-affected-zone (HAZ), the effective growth driving force at the tip of fatigue crack was reduced with the reduction of the effective stress intensity factor (SIF) which finally causes fatigue crack rate decrease. Fatigue crack was strongly affected by size and the symmetrical characteristics of the plastic zone at the crack tip, which means that the integrity of the welded structure containing the fatigue crack mainly depended on the toughness of the low strenqth zone.     

EFFECT OF CRACK CLOSURE ON SLOW PROPAGATION IN DUAL-PHASE STEEL

The effect of ferrite content in ferrite-martensite dual-phase steel on the initiation and prop-agation of fatigue crack and the plastic deformation at crack tip has been studied.In the rangeof ferrite content from 24.2 to 41.5％,the optimum seems to be 33.8％,of which the crack ini-tiation will be prolonged,the threshold value increased,the propagation rate decreased and theclosure stress intensity factor increased.As the propagation force is described by effectivestress intensity factor,three steels with various ferrite contents will show the same propagationbehaviour on da/dN vs △ K_(eff)curve.It is shown that the closure effect increases with thedecrease in △K at the fatigue crack tip.When △K equals to △K_(th),the closure effect reachesa maximum value of0.7 in a dual-phase steel with 33.8％ferrite.     

Effects of precipitates on fatigue crack growth rate of AA 7055 aluminum alloy

The effects of precipitates on the fatigue crack growth rate of AA 7055 Al alloy subjected to different ageing treatments were investigated using transmission electron microscope and fatigue crack growth testing.The results show that the T77 treated samples exhibit the lowest crack growth rate,while the crack growth rate of over-aged samples is the highest.In terms of the model based on the reversibility of dislocation motion within the plastic zone close to the crack tip,the improved crack growth resistance is attributed to many precipitates that are coherent with Al matrix in the under-aged and T77 treated samples.When the precipitate is coherent with the Al matrix,the larger the precipitate is,the slower the fatigue crack grows.The effects of grain boundary precipitates and precipitate free zone on the fatigue crack growth resistance are less significant than those of precipitates within grains of the alloy.     

MIXED MODE SCC OF 18-8 STAINLESS STEEL IN BOILING 42% MgCl_2 SOLUTION

A series of mixed mode tests were carried out on 18-8 stainless steel in boiling42% MgCl_2 solution.The results show that for any K_Ⅱ/K_Ⅰ ratio,the SCC direction coin-cides well with the crack tip maximum normal stress plane,while the SCC resistance of thematerial reduces as the ratio of K_Ⅱ/K_Ⅰ increases.The experimental results were discussed inthe light of anode dissolving mechanism and the effect of mixed mode loading on crack tipstress and strain.It is concluded that for fracture analysis if mixed mode cracks were simplytaken into account as mode Ⅰ cracks,and only mode Ⅰ testing results as mode Ⅰ fracture criter-ion were employed,it may not be safe.     

(Ⅰ+Ⅱ)MIXED MODE HYDROGEN INDUCED CRACKING

Hydrogen induced cracking(HIC)of 0.3% C,1% Cr,1% Mn,1% Si high strength steel hasbeen studied under simple mode Ⅰ,mode Ⅱ and(Ⅰ+Ⅱ)mixed mode loading conditions.Af-ter being hydrogen-charged in IN H_2SO_4 solution,the material behaved hydrogenembrittlement in all the cases studied.The threshold K_(ⅡH)/K_(ⅡX) of HIC under mode Ⅱ load-ing was 0.27,which was nearly the same as that K(ⅠH)/K_(ⅠX)=0.29 under mode Ⅰ loading.While the thresholds of-HIC under(Ⅰ+Ⅱ)mixed mode loading were 0.36,0.41 and 0.37 cor-responding to the K_Ⅱ/K_Ⅰ ratio of 0.27,0.4 and O.81.The results show that simple mode Ⅰor mode Ⅱ loading is more susceptible to hydrogen embrittlement than(Ⅰ+Ⅱ)mixed mode.For explaining the experimental results,the effects of triaxial stress as well as plasticdeformation ahead of crack tip has been discussed.     

Fatigue crack growth of titanium alloy joints by electron beam welding

Electron beam welding（EBW） has been widely used in the manufacture of titanium alloy welded blisk for aircraft engines. Based on fatigue crack growth tests on titanium alloy electron beam welding（EBW） joints, mechanism of fracture was investigated under scanning electron microscope（SEM）. The results show that fatigue crack growth rate increases as the experimental load increases under the same stress ratio and stress intensity factor range. At the beginning of crack growth, the extension mechanism of fatigue crack is the typical mechanism of cleavage fracture. In the steady extention stage, crack extends along the weld seam firstly.Then, crack growth direction changes to extend along the base metal. The extension mechanism of fatigue crack in the weld seam is the main mechanism of cleavage fracture and the extension mechanism of fatigue crack in the base metal is the main extension mechanism of fatigue band. In the instantaneous fracture stage, the extension mechanism of fatigue crack is the typical dimple-type static fracture mechanism.Crack growth was simulated by conventional finite element method and extended finite element method.     

EFFECT OF NON-PROPORTIONAL OVERLOADING ON FATIGUE CRACK GROWTH

The effect of proportional and non-proportional overloading on mode l fatigue crack growthhave been studied,and the influences of crack tip plastic zone,crack tip blunting as well ascrack closure were discussed.Proportional(model I)overloading may cause more seriouscrack growth retardation than non-proportional(mixed mode)overloading.Therefore,forestimating the fatigue life of engineering structures to simplify a real overload which may of-ten be non-proportional as a proportional one is not always safe.     

FATIGUE CRACK PROPAGATION IN Al-Li ALLOY 8090

Fatigue crack growth rates of Al-Li alloy 8090 in air were found to be strongly dependent up-on the aging conditions.The naturally aged and underaged specimens showed the highest re-sistance to the fatigue crack propagation.The fatigue crack growth resistance of the overagedspecimen is the lowest and that of the peakaged specimen in between.As compared to air,3.5% NaCl solution does not change the effect of aging conditions on the fatigue crackgrowth,but causes a decrease of the resistance to the fatigue crack growth under the same ag-ing condition.Both short cracks from notch and physically short cracks showed much higherrates of fatigue crack propagation in comparison with long cracks under the same aging condi-tion and stress intensity level.The growth behavior of the short crack depends on its type.Thegrowth rate of short crack from notch decreases first to a minimum and then increases with in-creasing △K.However,the physically short crack grows at a progressively increasing rate.The effects of aging conditions are explained in terms of the slip planarity of dislocations andthe cyclic slip reversibility.The observed short crack behavior is considered to be dependent onthe crack closure and the local plasticity near the crack tip.     

MICROFRACTOGRAPHY OF NEAR-THRESHOLD FATIGUE CRACK PROPAGATION IN DUAL-PHASE STEELS

SEM microfractography of near-threshold fatigue crack propagation were carried out in thedual-phase steels of 3 martensite morphologies and 6 volume fractions of martensite (V_m).All of them are featured by cyclic cleavage characteristics in near-threshold region,i.e.,main-ly controlled by mode Ⅱ stress.In the higher ΔK regions,the fracture surfaces are character-ized by mixed modes including cyclic cleavage facets,two types of secondary cracks andstriations,etc..The roughness-induced crack closure of fracture surface is attributedprimarily to extreme high fatigue crack growth threshold values.     

SLOW PROPAGATION OF FATIGUE CRACK NEAR THRESHOLD

The effect of pearlite interlamellar spacing(d)from 0.31 to 0.77 μm of T8 steel on fatiguecrack propagation near threshold has been investigated at different stress ratios(R).Theclosure stress intensity factor(K_(cl)),the threshold value(ΔK_(th))and crack tip opening dis-placement range(ΔCOD)were measured.The results show that pearlite interlamellar spac-ing in this range has no effect on K_(cl),but has effect on the threshold value in low stress ratio.The threshold value is higher for materials with larger interlamellar spacing.No matter whatthe stress ratio is,during the propagation near threshold,the propagation rate will be thesame,provieded the crack tip opening displacement range is the same.Metallurgical examinations were also carried out to show that the crack propagationpath is sensitive to mierostructure.The crack always propagates along either boundary ofpearlite colony or ferrite lamellar in pearlite.     

TEM OBSERVATION OF LOCAL DEFORMATION BAND AT CRACK TIP

The thin foil specimen of a ferrite-austenite duplex stainless steel was tensiled under transmis-sion electron microscope(TEM).It was found that both in ferrite and austenite the localdeformation band at crack tip was formed near to the crack propagating direction.Its forma-tion was related with the crack tip Schmid factor,dislocation shielding,latent hardening andhardening coefficient.When the crack tip emitted dislocations to a slip system by the action ofpure mode Ⅱ stress resolute,and the decreasing rate of hardening coefficient was suitable,lo-cal deformation band was easy to form.     

Creep crack growth in a Cr-Mo-V type steel： experimental observation and prediction

In this study, the creep crack growth (CCG) properties and fracture mechanism of a Cr-Mo-V steel at 566 C in compact tension (CT) specimens were investigated, and the CCG rate was predicted by using the NSW model. The results show that the CCG rate measured by CT specimens is much lower than that predicted by the NSW model under plane-strain state. This means that the NSW model prediction for the CCG rate of the steel is over-conservative. In addition, the CCG rate da/dt versus C measured by the experiments shows the piecewise linear relation on log-log scale instead of a single linear relation predicted by the NSW model. The main reasons for these results are that the actual creep fracture mechanism of the steel and the actual creep crack tip stress field in the CT specimens have not been fully captured in the NSW model. The experimental observation shows that the creep crack propagates in a discontinuous way (step by step) at meso-scale, and the cracks at micro-scale are usually formed by the growth and coalescence of voids on grain boundaries. The NSW model based on the creep ductility exhaustion approach may not correctly describe this creep fracture process. In addition, the opening stress and triaxial stress ahead of crack tips calculated by three-dimensional finite element method is lower than those predicted by the HRR stress field which is used in the NSW model under plane-strain state. The use of the high HRR stress field will cause high CCG rates. The change in the creep fracture mechanism at micro-scale in different ranges of C may cause the piecewise linear relation between the da/dt and C . Therefore, it is necessary to study the actual CCG mechanism in a wide range of C and the actual creep crack tip stress field to establish accurate CCG prediction models.     

Numerical Simulation and Experimental Verification of CMOD in SENT Specimen: Application on FCGR of Welded Tool Steel

Single-edged notched tension (SENT) specimen is used to study the fatigue crack growth rate (FCGR) behavior of AISI 50100 steel using MTS 810. Calibration tests are run to get plots of crack mouth opening displacement (CMOD) vs. load and CMOD vs. crack length-to-width ratio with the known crack lengths. Numerical simulation is also done to try to establish a relation between crack length and CMOD. FCGR of welded and un-welded specimens are plotted against stress intensity factor range to show the effect of welding on fatigue crack growth rate of AISI 50100 steel. The experimentally obtained CMOD values are compared with values obtained by numerical simulation using ABAQUS/StandardTM software package. Results show that numerical values are in good agreement with experimental data for small crack lengths and lower values of applied load.     

Higher order asymptotic fields for mode Ⅰ crack in functionally gradient material

Higher order stress fields for a mode Ⅰ crack perpendicular to the direction of property variation in a functionally gradient material(FGM), which has an exponential variation of elastic modulus along the gradient direction， were obtained through an asymptotic analysis. The Poisson‘s ratio of the FGMs was assumed to be constant throughout the analysis. The first five terms in the asymptotic expansions of crack tip stress fields were derived to bring out the influence of nonhomogeneity on the structure of the stress field explicitly. The analysis reveals that only the higher order terms in the expansion are influenced by the material nonhomogeneity. Moreover, it can be seen from expressions of higher order stress fields that at least three terms must be considered in the case of FGMs in order to explicitly account for the nonhomogeneity effects on the structure of crack tip stress fields.     

EFFECT OF SULPHURIZING-OXIDIZING ENVIRONMENT ON CREEP CRACK GROWTH OF Ni-BASE SUPERALLOY GH30

An investigation was made on the creep crack growth behaviour under static load at 1023 Kfor Ni-base superalloy GH30 exposed to air and air+10% SO_2.The results showed that inthe region of low stress intensity factor,the creep crack growth rate is higher in air+10%SO_2 than in air only,while in the high region,it is reverse.The fractograph of specimens hasbeen analyzed,and the mechanism of creep crack growth together with the influences of sul-phur and oxygen has been discussed as well.     

Mechanism of high-temperature oxidation effects in fatigue crack propagation and fracture mode for FGH97 superalloy

The high-temperature fatigue crack growth behaviors in powder metallurgy(P/M)Ni-based superalloy FGH97 for turbine disk application were investigated at different temperatures(650,700 and 800 ℃)in air using a combination a servohydraulic test system,fractographic and microanalytical investigations.It is found that there is a temperature-sensitive region in which the fatigue life of FGH97 alloy decreases sharply.To further evaluate the crack propagation mode and oxidation effects,interruption experiments were conducted at 700 and 300 ℃,respectively.The results indicate that the reduction of the fatigue lifetime for FGH97 takes place when the fracture mechanism transforms from a predominantly transgranular mode to an intergranular one as the temperature increases.Although the microstructures and mechanical properties may vary with the temperature,they are not the dominating factors contributing to the temperature sensitivity of fatigue property for FGH97.It is the oxidation that governs the fatigue crack growth behaviors in air at elevated temperature.The enhanced thermal activity of oxygen and certain active metal elements result in accelerated oxidation reaction.The brittle oxide intrusions formed at the crack tip and grain boundaries of crack frontier lead to grain boundary weakness,which is responsible for the transformation of crack growth mode and degradation of the fatigue property of FGH97 alloy.     

CREEP CRACK PROPAGATION IN CONVENTIONAL CAST AND DIRECTIONALLY SOLIDIFIED SUPERALLOY RENE 80

The creep crack propagation in superalloy René80 of two different microstructures,i.e.,equiaxed grain structure by conventional casting and columnar grain by directionalsolidification,was investigated under static load at 1123 K.The creep crack growth rate,da/dt,seems to be correlated with the stress intensity factor,K.The creep crack growth ratein the directionally solidified alloy is lower than that in the conventional cast alloy,owing tothe elimination of transverse grain boundaries.The effect of microstructure on creep crackpropagation has also been discussed.     

The Relationship Between Oxidation and Thermal Fatigue of Martensitic Hot-Work Die Steels

Thermal fatigue behaviors of two forged hot-work die steels subjected to cyclic heating(650 °C)–water quenching were investigated. A martensitic hot-work die steel containing 10% Cr(HHD), showing superior oxidation resistance and thermal fatigue resistance to the commercial martensitic hot-work die steel(Uddeholm DIEVARò), was developed. The maximal crack length in HHD was 35% shorter than that in DIEVAR after 2000 thermal cycles, and the hot yield strength at 650 °C of HHD was 14% lower than that of DIEVAR prior to thermal fatigue testing, which is 30% higher after 1500 cycles. It is found that cracks initiated and propagated along the oxide layers in the grain boundaries, suggesting that the oxidation-induced thermal fatigue cracks can significantly reduce the mechanical performance and service life for the hotwork die steel. High-temperature oxidation behavior is crucial for thermal fatigue crack formation, while high-temperature yield strength and ductility play a less important role.