Corrosion fatigue short crack growth behaviour in a high strength steel

The influence of an aggressive environment (0.6 M, aerated NaCl solution) on short fatigue crack initiation and growth behaviour has been studied. The study involved three major test series, namely: air fatigue, corrosion fatigue, and intermittent air fatigue/corrosion fatigue. The above tests carried out under fully reversed torsional loading conditions at a frequency of 5 Hz, showed that it was the non-metallic inclusions which took part in crack initiation resulting from debonding at metal matrix/inclusion interface and pitting of inclusions in both air and corrosove environments, respectively. Short fatigue crack growth results in these two environments obtained by using plastic replication technique, indicated a large effect of microstructure i.e. prior austenite grain boundaries. The stage/stages at which the environmental contribution was dominant has been discussed by considering the results achieved from intermittent tests. However, the mechanisms involved in corrosion fatigue short crack growth have also been described in the light of results obtained from futher investigations carried out by conducting corrosion fatigue tests under applied cathodic potential conditions and tests on hydrogen pre-charged specimens under air fatigue and uniaxial tension conditions.     

Crack closure and fatigue crack growth near threshold of a metastable austenitic stainless steel

In this paper R-ratio effects on fatigue crack growth near threshold region of a metastable austenitic stainless steel (MASS) in two different conditions, i.e. annealed and cold rolled, is investigated. The authors present two approaches to correlate FCGR data for R = 0.1, 0.3, 0.5, 0.7 and K_max = 23 MPa√m using a two-parameters approach (ΔK, K_max and α in Kujawski’s model) and crack closure model (using Elber’s K_op and in Donald’s ACRn2 approaches). The K_op and ACRn2 were experimentally measured on a single edge tension specimens. The K_op measurements were performed using a modified method and based on ASTM standards. While the two driving force approaches correlate data well in the Paris region, they fail to correlate them in the threshold region. However, this correlation can be improved in the threshold region when a different α value from the Paris region is used. The authors indicated that two different mechanisms operate; one in the Paris region and another in the near threshold. Hence, they proposed to combine the two-parameter and crack closure approaches where ΔK is replaced by ΔK_eff (estimated by a new method proposed in this paper), which is shown to correlate the FCGR data for different stress ratios for annealed steel. The correlation for cold rolled condition shows improvement with the new approach but is not as good as for the annealed one. The author further suggests to modify K_max in the two-parameter approach.     

Effect of tensile overloads on fatigue crack growth of high strength steel wires

Fatigue of the tensile armor wires is the main failure mode of flexible risers. Techniques to increase the life of these components are required to improve the processes safety on oil exploration. This work evaluates the crack growth retardation of high strength steel wires used in flexible pipelines. Fracture toughness tests were performed to establish the level of stress intensity factor wherein the wires present significant plastic deformation at the crack tip. The effect of tensile overload on fatigue behavior was assessed by fatigue crack growth testing under constant ΔK control and different overload ratios with two different load ratios. The outcomes show that the application of controlled overloads provides crack retardation and increases the fatigue life of the wires more than 31%. This behavior is also evident at stress ratio of 0.5, in spite of the crack closure effect being minimized by increasing the applied mean stress.     

A numerical study on the crack growth behavior of a low and a high strength steel

Geometry and size effects on the crack growth resistance curves in CT-specimens of a low and high strength material are numerically investigated. The fracture process is controlled by experimentally determined critical values of the crack tip opening displacement (CTOD_i for crack growth initiation and the crack tip opening angle (CTOA_C) for stable crack growth. The results show that for the low and the high strength material the geometry and size effects are different. This is mainly due to the fact that the load vs. load line displacement records are basically different for the two materials which results from the large difference in the scaled fracture toughness parameters, CTOD_i/_O and CTOA_C/_O, of the two materials.Presented at the Far East Fracture Group (FEFG) International Symposium on Fracture and Strength of Solids, 4–7 July 1994 in Xi'an, China.     

Analysis of high temperature fatigue crack growth behavior

High temperature fatigue crack growth has been examined in the light of the new concepts developed by the authors. We observe that the high temperature crack growth behavior can be explained using the two intrinsic parameters ΔK and K_max, without invoking crack closure concepts. The two-parameter requirement implies that two driving forces are required simultaneously to cause fatigue cracks to grow. This results in two thresholds that must be exceeded to initiate the growth. Of the two, the cyclic threshold part is related to the cyclic plasticity, while the static threshold is related to the breaking of the crack tip bonds. It is experimentally observed that the latter is relatively more sensitive to temperature, crack tip environment and slip mode. With increasing test temperature, the cycle-dependent damage process becomes more time-dependent, with the effect that crack growth is dominated by K_max. Thus, in all such fracture processes, whether it is an overload fracture or subcritical crack growth involving stress corrosion, sustained load, creep, fatigue or combinations thereof, K_max (or an equivalent non-linear parameter such as J_max) remains as one essential driving force contributing to the final material separation. Under fatigue conditions, cyclic amplitude ΔK (or an equivalent non-linear parameter like ΔJ) becomes the second necessary driving force needed to induce the characteristic cyclic damage for crack growth. Cyclic damage then reduces the role of K_max required for crack growth at the expense of ΔK.     

Short-crack growth in corrosion fatigue for a high strength steel

Fatigue crack growth tests of HY130 steel were conducted under either constant-load-range or constant-stress-intensity-range conditions, by using four-point-bend and compact-tension specimens with a crack of length 0.4–41 mm. The tests were conducted in 3.5% NaCl solution under either the freely corroding condition or with the specimen coupled to a sacrificial zinc anode.Crack growth rates for the zinc-coupled case were higher than those for the freely corroding case, for either the long crack or short crack specimen. The growth rates of short cracks were faster than those of long cracks for the same environment. The maximum amount of crack growth acceleration due to crack size effect was about a factor of two for these alloy-environment combinations. With increasing crack length, the rate for short cracks converges to that for long cracks. Since there was no crack-size effect in air, the additional enhancement in the rate of crack growth for short cracks must be chemical in nature.The upper limit of chemically short cracks (as delineated by the point of convergence of rates) was shorter for the freely corroding case than for the zinc-coupled case, and was longer for the higher stress intensity range, for both the constant-load-range and the constant-stress-intensity-range tests. The observed effect of crack size on crack growth rates is discussed in terms of the hydrogen embrittlement mechanisms.     

微观组织对贝氏体钢疲劳裂纹扩展行为的影响

为了研究组织对疲劳裂纹扩展行为的影响,对3种不同贝氏体组织钢进行了疲劳裂纹扩展实验,并采用SEM和EBSD等方法对裂纹进行了分析.结果表明,板条贝氏体组织在近门槛区和稳定扩展区阻碍裂纹扩展的能力最强,具有最小的裂纹扩展速率.板条贝氏体组织中的大角度晶界使裂纹更容易发生偏折,导致断口表面粗糙度增加,裂纹扩展受到较强的粗糙度诱导裂纹闭合效应的作用.随着ΔK的增大,塑性诱导裂纹闭合效应取代粗糙度诱导裂纹闭合效应开始占据主导作用,是板条贝氏体组织中裂纹扩展速率对ΔK的变化较敏感的原因.     

The effect of random grain distributions on fatigue crack initiation in a notched coarse grained superalloy specimen

Coarse grained superalloys are of large interest in high temperature applications, and can be found in e.g. gas turbine components, where great care must be given with respect to high temperature fatigue. Due to the large grain size, the material behaviour at e.g. sharp notches cannot be considered homogeneous. As a consequence, the fatigue behaviour is likely to expose a large variation. In order to numerically investigate this variation, a Monte Carlo analysis has been carried out by 100 FE-simulations of notched specimens, where placements and orientations of the grains were randomised. Furthermore, each grain was modelled as a unique single-crystal, displaying both anisotropic elastic and plastic behaviour and tension/compression asymmetry. The effect of randomness was investigated by the obtained dispersion in fatigue crack initiation life. It was concluded that the fatigue life behaviour of coarse grained nickel-base superalloys may show a considerable variation, which cannot be captured by one single deterministic analysis based on data for a homogenised material. Furthermore, the dispersion is of such a magnitude that it needs to be taken into account in industrial applications where highly stressed coarse grained materials are used.     

Influence of stress ratio on the threshold level for fatigue crack propagation in high strength steels

Centrally cracked specimens of JIS SM58Q and HT80 steels were fatigued. The fatigue crack growth rates, da/dn, and the stress intensity threshold levels, ΔK_th were measured over the range of stress ratio, R, from ?1 to 0.8 by the use of an automatic method of continuously decreasing stress intensity factor with crack extension. The measured ΔK_th was well represented as |ΔK_th/2|_R=(1?R)γ|ΔK_th/2|_R=0; and the propagation rate, as da/dn = A(1?R)^γm[(ΔK/2)^m ? {(1 ? R)^γ|ΔK_th/2|R=0}^m] for ?1≦R≦0.33 or da/dn = A(1 ? 0.33)^?γm [(ΔK/2)^m {(1 ? R)^γ |ΔK_th/2|R=0}^m] for 0.33 < ≦ 0.8.     

Effects of nitriding on fatigue strength of quenched and tempered steel: role of interstitial nitrogen

Abstract

This paper describes an investigation into the effects of nitriding, denitriding, and renitriding, on the rotating bending fatigue strength of notched specimens of the quenched and tempered steels 24CrMo13 and 42CrMo4. The effects of the different treatments were characterised by microstructural investigation, hardness measurements, determination of the nitrogen and carbon contents in the diffusion layer, and determination of surface macrostresses. Nitriding induced a doubling of the fatigue limit. Denitriding only partly removed the effect of the nitriding treatment, because the strength increase caused by precipitation of alloying element nitrides remained unchanged, and only the effect of interstitially solved nitrogen disappeared. For nitrided and renitrided specimens an approximately linear relationship was found between the surface hardness and the fatigue strength. Denitriding caused a larger decrease in the fatigue strength than could be explained by the loss of surface hardness. The change in macrostresses from compressive to tensile also played a part. The role of interstitially dissolved nitrogen in determining the fatigue strength was found to be larger than assumed thus far.     

Micro-defect effect on gigacycle fatigue S-N property and very slow crack growth of high strength low alloy steel

Abstract

Transformation induced plasticity (TRIP) assisted steels contain a small quantity of carbon enriched retained austenite, which transforms into martensite during the course of plastic deformation. Transformation of this kind can be induced by both stress and plastic strain. The detailed mechanism by which the martensite is induced is different for these two scenarios. An attempt is made here to discover the relative importance of these mechanisms and it is found that stress affected transformation can explain much of the variation in retained austenite content as a function of plastic strain.     

Vacuum crack growth behavior of austenitic stainless steel under fatigue loading

The fatigue crack growth behavior of an austenitic stainless steel in a vacuum environment has been studied. Fatigue crack growth tests were performed with the compact tension specimens in laboratory air and vacuum, and the environmental effect on the crack growth behavior was examined. The crack growth rate data were expressed in terms of the J-integral range during fatigue loading, while an elastic–plastic finite element analysis was employed to calculate the J-integral range. The fractographic examinations were also carried out to assess the crack growth mechanisms and correlate with fatigue characteristics. The results show an accelerated fatigue crack growth in air compared to that in vacuum and this environmental effect depends on the load ratio.     

Modeling of fatigue crack growth threshold development for a microstructurally small crack

A method for predicting the fatigue crack growth threshold using finite element analysis is investigated. The proposed method consists of monitoring the plastic strain hysteresis energy dissipation in the crack tip plastic zone, with the threshold being defined in terms of a critical value of this dissipated energy. Two-dimensional plane-strain elastic-plastic finite element analyses are conducted to model fatigue crack growth in a middle-crack tension M(T) specimen. A single-crystal constitutive relationship is employed to simulate the anisotropic plastic deformation near the tip of a microstructurally small crack without grain boundary interactions. Variable amplitude loading with a continual load reduction is used to generate the load history associated with fatigue crack growth threshold measurement. Load reductions with both constant load ratio R and constant maximum stress intensity K_max are simulated. In comparison with a fixed K_max load reduction, a fixed R load reduction is predicted to generate a 35% to 110% larger fatigue crack growth threshold value.     

Proposed fatigue damage measurement parameter for shot-peened carbon steel based on fatigue crack growth behavior

A new technique for evaluating fatigue-damage accumulation in shot-peened (SP) carbon steel based on variations in residual stress is proposed. Using findings from previous studies, a fatigue damage parameter for a material treated with SP based on the change in induced compressive residual stress (CRS) is examined. A plastic replica method with the focused ion beam (FIB) technique is used to assess the relationship between the residual stress state and the fatigue crack growth (FCG) behavior of SP specimens over the fatigue lifespan. It is found that the residual stress relaxation phenomenon can be used as an effective parameter for determining the fatigue damage growth, provided the residual stress relaxation rate of each mechanical load and the critical threshold relaxation boundary of each material is known.