Microcrack growth and fatigue behavior of a duplex stainless steel

The kinetics of microcrack growth during cycling has been studied in a S32205 duplex stainless steel in the as-received and aged (100 h at 475 °C) conditions. Cylindrical specimens with a shallow notch were subjected to a constant plastic strain range of 0.3% in both thermal conditions. The characteristic features of surface damage and crack growth showed striking differences in microcrack density, nucleation location and propagation rate between the two thermal conditions even though the fatigue lives are comparable. In the as-received material, microcrack density is low and they nucleate mainly at grain and phase boundaries or second-phase particles. In the aged condition, slip markings first appear in the ferritic phase and they are the preferred site for microcrack nucleation. Crack propagation takes place along slip markings in adjacent grains for crack lengths less than 100 μm. A comparison between fatigue life and the relevant parameters of a microcrack growth law was made.     

AN INVESTIGATION OF FATIGUE CRACK GROWTH IN A DUCTILE MATERIAL AT HIGH GROWTH RATES

Abstract— Investigations into tearing-fatigue have been performed using three point bend specimens made of mild steel. A computer controlled testing machine was used which could maintain constant cyclic displacement ranges, or constant cyclic energy input ranges, and hence provide a range of crack tip driving force conditions. Fatigue crack growth rates were measured and compared with the predictions of a model based on the linear summation and non-interaction of fatigue and ductile tearing growth rates. The effects of fatigue crack growth on monotonic crack growth resistance properties were investigated. It is concluded that there is no significant interaction between these two crack propagation mechanisms in this steel. Crack growth rate equations governing propagation in the tearing-fatigue regime are given.     

Fatigue properties of ultrafine grained low carbon steel produced by equal channel angular pressing

Ultrafine grained low carbon (0.15 wt.% C) steel produced by equal channel angular pressing (ECAP) was tested for investigating fatigue properties, including cyclic softening and crack growth rate. Emphasis was placed on investigating the effect of load ratio on the fatigue crack growth rates of ultrafine grained microstructure. The ECAPed steel exhibited cyclic softening. After the first cycle, the tension and compression peak stresses decreased gradually with the number of cycles. Fatigue crack growth resistance and the threshold of ECAPed ultrafine grained steel were lower than that of an as-received coarse grained steel. This was attributed to a less tortuous crack path. The ECAPed steel exhibited slightly higher crack growth rates and a lower ΔK_th with an increase in R ratio. The R ratio effect on growth rates and ΔK_th was basically indistinguishable at a lower load ratio (R>0.3) compared with other alloys, indicating that the contribution of the crack closure vanished. This was explained by the fact that finer grained materials produce a lower opening load P_op due to a relatively less serrated crack path. Consequently, K_min can reach K_op readily with a smaller increment of load ratio. The crack growth rate curve for the ECAPed ultrafine grained steel exhibited a linear extension to the lower growth rate regime than that for the coarse grained as-received steel. This behavior can be explained by a reverse crack tip plastic zone size (r_p) that is always larger than the grain size.     

Assessment of stable tearing on fatigue fracture surfaces

Fatigue fracture surfaces in a range of structural metals sometimes exhibit bands, visible as dull crescent-shaped regions which contrast to the “bright” background fatigue surface. Such bands are believed to be created within a single load cycle under either constant-amplitude or variable-amplitude load conditions. Microscopically, crack advance by tearing has similarity to the final unstable fracture of the component, with the difference that the tearing is stable: it arrests after a certain distance of crack front advance, which is then followed by further fatigue crack growth. Multiple stable tearing bands, separated by regions created by fatigue crack growth, may be visible on a single fracture surface, to the extent that they may even make up the majority of the fracture surface area.Post-failure analysis of fracture surfaces often relies on quantitative fractography to relate various crack-front progression markings to specific loads in the load history, in order to estimate the crack growth history in the component. Matching this data to predicted crack growth, however, is complicated by the fact that stable tearing is not included in fatigue predictive models, and so the presence of significant tearing can greatly complicate the derivation of a crack growth history which can be matched to a crack growth model. The post-failure analysis of fracture surfaces is particularly difficult in cases where the load history record is poor, and that process has been observed to be much more difficult when significant tearing is present. Several empirical models and concepts have been proposed to assist with post-failure analysis of tearing, but as yet we do not have a good understanding of the parameters which influence both the onset and the arrest of the tearing, and the factors which control crack shape change as tearing progresses. As a result, the interpretation of tearing on service fracture surfaces remains difficult.This paper reviews the existing empirical models for tearing analysis, and describes a series of tests which produced tearing in an aircraft aluminium alloy, with the aim of developing a better understanding of these fracture surface features. The analysis of the tearing shapes indicates that one of the empirical models may have broad value as an engineering analysis tool, since it correlates reasonably well with tearing characteristics in these tests. The study reveals that the stress intensity factor is one of the key controlling parameters in tearing onset and arrest, although simple methods for estimation of relevant stress intensity values are problematic because of the key role of crack front curvature. The main conclusions relate to the notable differences between tearing under constant-amplitude and variable-amplitude loading. The constant-amplitude conditions confer some resistance to stable tearing, and this is believed to result from the high loading prior to tear onset; several mechanisms could be involved. This research is part of a program which is developing improved analytical and prognostic models for stable tearing.     

Predictive models for stable tearing crack growth due to overloading in fatigue

Stable tearing is a recurring process in which fatigue crack growth is interspersed by substantial jumps of crack growth, commonly at the central cross‐section of the component while the crack front nearer the surface lags behind. These tearing bands have been observed to start very early during fatigue life and can make up the majority of the fatigue fracture surface. This paper presents the development of predictive models, in which the tongue‐shaped stable tearing band is first idealised as trapezoidal shape, and then two alternative fracture criteria are formulated with the aid of the finite element (FE) method and the Forsyth stable tearing concept. Parametric solutions of the stress intensity factor at the front of the trapezoidal crack front are obtained using the FE method. Comparisons between the model predictions and experimental results indicate that both models produce satisfactory prediction of the stable tearing crack jump length in aluminium alloy coupons of varying cross‐sectional thickness.     

In situ SEM studies on the effects of particulate reinforcement on fatigue crack growth mechanism of aluminium-based metal-matrix composite

The effects of particulate reinforcement on the fatigue behaviour and fatigue mechanisms of two 6061 aluminium-based metal-matrix composites (MMCs) in three different heattreatment conditions were studied in situ with a scanning electron microscope and compared to the unreinforced alloy in the as-received condition. It was observed that the fatigue properties of the MMCs were influenced by the ceramic particles in two ways: firstly the particles increased the fatigue stress intensity threshold mainly by crack-deflection and crack-closure mechanisms, and secondly, the particles raised the fatigue crack growth rates in the Paris region by providing an easy crack path. The effect of ageing was small on the fatigue stress intensity threshold of MMCs, but for the peak-aged MMCs the fatigue crack growth rates in the Paris region were faster. The mechanism of fatigue crack growth was largely associated with the matrix/particle interface and the linkage with subcracks initiated ahead of the main crack at high applied stress intensity factors.     

Fatigue crack growth in welded beams

The fatigue behavior of welded steel beams is evaluated using the fracture mechanics concepts of stable crack growth. A fracture mechanics model for cracks originating from the pores in the web-to-flange fillet weld is developed. Estimates of the stress-intensity factor are made that numerically describe the initial flaw condition. With the final crack size known, a theoretical crack-growth equation was derived from the fatigue test data of the welded beams. The derived relationship compares well with actual crack-growth measurements on a welded beam and available data from crack growth specimens. The regime of crack growth where most of the time is spent growing a fatigue crack in a structural element is shown to correspond to growth rates below 10^?6 in. per cycle. Little experimental crack growth data is available at this level. It is concluded that the fracture mechanics concepts can be used to analyze fatigue behavior and to rationally evaluate the major variables that influence the fatigue life of welded beams.     

Hydrogen embrittlement contributions to fatigue crack growth in a structural steel

The detrimental effects of a hydrogen atmosphere on the fatigue resistance of BS 4360 steel have been assessed by a comparison of crack growth rates in air and hydrogen at a low cycling frequency (0.1Hz), and at a number of temperature (25, 50 and 80 °C). The crack propagation rates in air are almost independent of temperature over this range, but those measured in hydrogen differ by more than an order of magnitude between 25 and 80 °C. The greatest enhancement is seen at 25 °C and at high values of ΔK, the maximum occurring between 40–45 MPa √m at each temperature. There is little hydrogen contribution to crack growth at values of ΔK below 20 MPa √m for R = 0.1.

The enhancement of crack growth rates is reflected by the presence of ‘quasi-cleavage’ facets on the fatigue fracture surfaces of specimens tested in hydrogen. These are most apparent where the greatest increases in growth rate are recorded. The facets show linear markings, which run both parallel and perpendicular to the direction of crack growth. The former are analogous to the ‘river’ lines noted on brittle cleavage facets, and reflect the propagation direction. The latter are more unusual, and indicate that facet formation by hydrogen embrittlement during fatigue is a step-wise process.     

FATIGUE CRACK GROWTH LAWS FOR BRITTLE AND DUCTILE MATERIALS INCLUDING THE EFFECTS OF STATIC MODES AND ELASTIC-PLASTIC DEFORMATION

A fatigue crack growth damage accumulation model is used to derive laws for the fatigue crack growth rates of brittle and ductile materials. The damage accumulated during cyclic loading is assumed to be proportional to the cyclic change in the plastic displacement in the crack tip yielded zone. The static mode contribution to the fatigue damage is assumed to be proportional to some power of the crack tip displacement. The laws are applicable in either the small or large scale yielding regimes provided that the stress ratio remains positive. Static modes are assumed to be controlled by the fracture toughness value in brittle materials, and by the gradient of the crack growth resistance curve in ductile materials. In the analysis of ductile materials it is assumed that the crack growth resistance of the material is not significantly altered by fatigue crack growth.
The growth rate equations are expressed in terms of the near field value of the J -integral, i.e. the value which would be calculated from assuming the material deformed in a non-linear elastic manner during the increasing load part of the fatigue cycle. Examples are given of the predictions of the growth law for ductile materials. It is predicted that after the initiation of stable tearing the crack growth rate, when expressed in terms of the cyclic change in the stress intensity factor, depends on both the structural geometry and the degree of crack tip plastic deformation. In both brittle and ductile materials the fatigue crack growth rate is predicted to accelerate as the failure criteria relevant to static crack instability are approached.     

Effect of pre-charged hydrogen on fatigue crack growth of low alloy steel at 288 °C

Hydrogen effects on mechanical strength and crack growth were studied at high temperatures. The study was motivated by the fact that the environmentally assisted cracking (EAC) of pressure vessel steel SA508 Cl.3 in 288 °C water was suspected to be related to hydrogen embrittlement. Fatigue crack growth rate and tensile tests were performed with hydrogen pre-charged specimens at high temperatures. At 288 °C the fatigue crack growth rate of the hydrogen pre-charged specimen was faster than that of as-received; the fatigue fracture surface of hydrogen pre-charged specimen correspondingly showed EAC like feature. Meanwhile, ductile striation was evident for the case of as-received in both air and argon gas environments. In the dynamic strain aging (DSA) loading condition at 288 °C during tensile tests, the pre-charged hydrogen induced a marked softening (decrease in ultimate tensile strength; UTS) as well as a little ductility loss; this was accompanied by the macrocracks grown from microvoids/microcracks promoted by DSA and hydrogen. These experiments showed that hydrogen embrittlement is an effective mechanism of EAC not only at low temperature but also at the high temperature.     

A methodology for fatigue crack growth and residual strength prediction with applications to aircraft fuselages

The FRANC3D/STAGS software system has been developed to model curvilinear crack growth in aircraft fuselages. Simulations of fatigue crack growth have been reported previously (Potyondy et al. 1995). This paper presents two enhancements to this system. One is the generalization of the representation of cracks that allows the system to represent realistic damaged structures more accurately. With this capability, parameters that may affect the trajectory of a fatigue crack are studied. Results are compared with measurements from a full-scale test. The second enhancement is to model stable tearing for residual strength prediction. A stable tearing simulation along a crack path that captures the material nonlinearities inherent at the crack tip is performed. The CTOA (Crack Tip Opening Angle) is used as a crack growth criterion to characterize the fracture process under conditions of general yielding. Residual strength of cracked structures is predicted.     

EFFECTS OF ENVIRONMENT AND HEAT TREATMENT ON THE TENSILE AND FATIGUE PROPERTIES OF Ti-24 at%A1-11 at%Nb

The α₂+β alloy Ti-24 at%A1-11 at%Nb was evaluated for mechanical properties in the as-received, mill annealed condition (nearly equiaxed) and in the β annealed condition (basketweave). The as-received condition had a moderately strong basal transverse α₂ texture which gave a higher yield strength, higher elastic modulus, and higher fatigue crack growth rate (FCGR) for loading in the transverse direction at room temperature. The β annealed material was stronger and more isotropic in both texture and properties. The fatigue crack growth rate is higher at 732°C in argon than at room temperature. A hydrogen environment results in a FCGR at 732°C comparable to or less than the FCGR in argon at 732°C for the as-received and β annealed conditions respectively. Although the hydrogen in solution at levels as high as several thousand ppm does not embrittle at elevated temperature, embrittlement is found after cooling to room temperature.     

INFLUENCE OF RETAINED AUSTENITE ON FATIGUE CRACK PROPAGATION IN HP 9-4-20 HIGH STRENGTH ALLOY STEEL

Abstract— Industrial multi-pass TIG weldments of HP 9-4-20 high strength alloy steel have been found to contain significant volume fractions (around 10%) of retained austenite which are not readily transformed after stress relieving and subsequent refrigeration procedures. To determine whether the presence of such retained austenite in tempered martensitic structures could be detrimental to fatigue resistance in HP 9-4-20 steel, fatigue crack propagation behavior was examined over six orders of magnitude in growth rate, in commercially heat-treated material (containing less than 3% austenite) and in intercritically heat-treated and tempered material (containing approx. 14% austenite) in an environment of moist, ambient temperature air. Whereas crack propagation rates were unchanged at growth rates exceeding 10⁻⁶ mm/cycle, structures containing 14% austenite showed somewhat superior resistance to near-threshold crack propagation at growth rates less than 10 ⁻⁶ mm/cycle, the threshold for crack growth (Δ K ₀) being over 20% higher than in commercially heat-treated material. The presence of retained austenite further appeared to inhibit the occurrence of intergranular fracture at near-threshold levels. It was concluded that significant proportions of retained austenite are not detrimental to fatigue crack propagation resistance in HP 9-4-20 steel, and may indeed have some beneficial effect at very low, near-threshold growth rates by increasing resistance to environmentally-assisted cracking.     

Fatigue crack growth behavior in weld-repaired high-strength low-alloy steel

Fatigue crack growth properties and Vickers micro-hardness of a weld-repaired high-strength low-alloy steel, known for high strength, low carbon, excellent notch toughness and good weldability and formability, have been studied under the following conditions: as-received high-strength low-alloy, weld-repaired high-strength low-alloy without buffer layer, and weld-repaired high-strength low-alloy with various thickness buffer layers. Those conditions are examined to determine the respective fatigue crack growth behaviors and Vickers hardness distribution, and the effects of different weld-repaired conditions on fatigue characterizations and microscopic features of the fracture and fatigue surface. The extended-compact tension specimen geometry is adopted in this study for all tests. Paris fatigue crack growth curves and the hardness distribution across weld metal, buffer layer and parent metal has been measured together with the relevant scanning electron microscope observations along the fatigue crack growth path, with special attention at and around the interfaces between the weld metal, buffer layer and parent metal. The results show the presence of the BL of a moderate thickness has a significant influence on the fatigue crack growth behavior in the heat-affected zone and around the interface between buffer layer and parent metal. The fatigue resistance of the selected high-strength low-alloy + buffer layer + weld metal tri-metal system is higher than that of the high-strength low-alloy + weld metal bi-metal system.     

Influence of prior cyclic hardening on high temperature deformation and crack growth in type 316L (N) stainless steel

For power generating equipment subjected to cyclic loading at high temperature, crack growth could arise from the combinations of fatigue and creep processes. There is potential for the material to undergo hardening (or more generally changes of material state) as a consequence of cyclic loading. Results of an experimental study to examine the influence of prior cyclic hardening on subsequent creep deformation are presented for type 316L(N) stainless steel at 600°C. Experiments were also carried out to explore creep crack growth at constant load, and crack growth for intermittent cyclic loading. For the as-received material there is substantial primary creep (hardening) at constant load, while for the cyclically hardened material at constant load the creep curves show recovery, and increasing creep rate with increasing time. Specimens subjected to prior cyclic hardening were also used for a series of creep and creep-fatigue crack growth tests. These tests demonstrated that there was accelerated crack growth compared to crack growth in as-received material.     

Fatigue crack growth behavior of X2095 Al–Li alloy

Microstructures and micro-textures of X2095 Al–Li alloy in as-received/superplastic state were characterized by means of SEM/BDS, X-ray diffraction and orientation imaging microscopy (OIM). It was observed that the microstructure of the alloy was typical of a particulate-reinforced composite material, consisting of aluminum matrix and homogeneously distributed T_B(Al₇Cu₄Li) particles with a volume fraction of about 10%. Brass-type texture was the dominant texture component. Both constant amplitude and near-threshold fatigue crack growth rates of the alloy in the L–T and T–L orientations were determined at different stress ratios. Particular attention was paid to the role of the T_B phase in the fatigue crack growth. When a fatigue crack approached a T_B particle, the crack basically meandered to avoid the particle. The T_B particles thus provided a strong resistance to the propagation of fatigue crack by promoting crack deflection and the related crack closure effects. The fatigue crack propagation behavior has been explained by the microstructural features, micro-textures, cracking characteristics and crack closure effects.     

Fatigue crack growth behaviour of Ti-6 Al-4 V metal matrix/continuous SiC and B4C/B fibre composites

The fatigue crack growth (FCG) behaviour of SiC and B₄C/B reinforced Ti-6 Al-4 V metal matrix composites loaded in the transverse direction as a function of modifications of the interface between the fibre and matrix was studied. The interface chemistry, modified by sulphur diffusion during thermal cycling treatment, changed the FCG in air, dry nitrogen and hydrogen environments when compared with the as-received specimens. The FCG rates tend to be higher in a humid environment. The SEM fractrography indicates that the FCG in humid air was by an increased amount of fibre splitting. The FCG in dry nitrogen environment was more often by interface debonding with some fibre splitting and fiber fracture. The FCG rates in dry hydrogen for both as-received and heat-treated specimens were intermediate between the observed rates for dry nitrogen and humid air. During FCG in laboratory air, the sulphur-enriched interface of the specimens thermal cycled in a sulphur environment reacts with the humidity in air to degrade the interface cohesion, resulting in complete separation of the interface from the matrix and the fibre at low strains. This inability of the interface to sustain any strain further increases the FCG rates in the matrix. The results show that the interface does transfer load during fatigue cycling either in an inert environment or if the interface has a minimal amount of impurities.     

Fatigue crack growth rate in welds of WDL61 0D steel

Abstract

In steel welded joint zones microcracks appear very easily, so the study of the fatigue crack growth rate in the welds is very important. To evaluate the fatigue characteristics in welded joints and in the heat affected zone of WDL610D steel, the fatigue crackgrowth rates were obtained through testing ofcompac ttension specimens. Fatigue crack growth rates with probabilities of 97.73% are given. The fatigue crack growth rates of two zones are compared. Also the results are compared with the formula in the 'Safety assessment code of pressure vessels with flaws in service' and the recommendations in the standard BSI PD6463. The results show that fatigue crack growth rates in the WDL610D steel are slower than that in the currently used 16MnR steel within the allowable range.     

The influence of properties of metal and temperature on cyclical crack resistance of power-plant steels

The cyclical crack resistance of power-plant steels is studied. The influence of metal condition (properties) within a specific steel grade on the characteristics of cyclical crack resistance is analyzed. It is shown that, at a room temperature, i.e., in the range of viscous-brittle transition, the cyclical crack resistance of steel decreases with the increase in its tendency to the brittle state. With increasing temperature from room to 300–320°C, the cyclical crack resistance steel decreases, and for state-specific conditions, the degree of fatigue crack growth correlates with the level of yield strength decrease at the same temperature. Taking this into account, the approach to approximation of the kinetic cyclical crack resistance diagram is experimentally proved by a generalized dependence invariant to temperatures in the range 20–300 (320)°C.     

A theory for fatigue crack propagation

A new continuum mechanics model is developed for predicting fatigue crack propagation rates using a fracture mechanics approach. The model demonstrates the critical dependence of fatigue crack growth on the fatigue ductility exponent, the fatigue ductility coefficient, the elastic modulus and the fracture toughness; it is related to the stress intensity range, implying that fatigue crack growth is critically dependent upon the condition at the tip of the crack.Four materials are studied, namely a creep resistant stainless steels, FV535; a $\text{2}\text{1}\text{2}$ per cent nickel-chromium-molybdenum direct hardening steel, 2S96D; a nickel base heat resisting alloy INCO 901; and a ferrous alloy containing titanium carbide in a medium alloy tool steel matrix, known as Ferrotic C.The developed model provides a means of predicting crack propagation rates based on mechanical properties, and the simplified model provides a fundamental basis for a more general form of the Paris relationship.