Near-threshold Fatigue Crack Growth Behavior Research Articles

Influence of microstructure on near-threshold fatigue crack growth behavior of 18Ni maraging steel weldment

Influence of microstructure on near-threshold fatigue crack growth behavior of 18Ni maraging steel weldment

Near-threshold fatigue crack growth behavior of the weld metal and heat-affected zone in an ex-service CrMoV steel joint

Threshold value (ΔKth) and fatigue crack growth behavior at different temperatures and stress ratios were investigated in weld metal (WM) and heat-affected zone (HAZ) of an ex-service CrMoV steel joint. Compared with the ΔKth at 25 ℃, the ΔKth at 500 ℃ and 540 ℃ were still high, because significant oxide-induced crack closure (OICC) occurred in WM and HAZ during high-temperature tests. The high-temperature ΔKth of HAZ were higher than those of WM, due to their more severe oxidation and more significant OICC. This was owing to the lower Si and Cr content and smaller grain size in HAZ.

Near-threshold fatigue crack growth behavior of 10% Cr martensitic steel welded joint with 9% Cr weld metal in high temperature air

The near-threshold fatigue crack growth (FCG) behavior under different stress ratios (R) of a 9%–12% Cr steel welded joint was studied in 538 °C air. The most significant oxide-induced crack closure (OICC) occurred in weld metal (WM) due to the lowest Cr content, resulting in the highest threshold value (ΔKth) of WM. The base metal (BM) with coarser grains and denser lath structures had more obvious roughness-induced crack closure (RICC) and more severe crack deflection, which led to a higher ΔKth of BM than heat affected zone (HAZ). The ΔKth of WM, HAZ and BM decreased as R increased.

An Abrupt Transition to an Intergranular Failure Mode in the Near-Threshold Fatigue Crack Growth Regime in Ni-Based Superalloys

Cyclic near-threshold fatigue crack growth (FCG) behavior of two disk superalloys was evaluated and was shown to exhibit an unexpected sudden failure mode transition from a mostly transgranular failure mode at higher stress intensity factor ranges to an almost completely intergranular failure mode in the threshold regime. The change in failure modes was associated with a crossover of FCG resistance curves in which the conditions that produced higher FCG rates in the Paris regime resulted in lower FCG rates and increased ∆Kth values in the threshold region. High-resolution scanning and transmission electron microscopy were used to carefully characterize the crack tips at these near-threshold conditions. Formation of stable Al-oxide followed by Cr-oxide and Ti-oxides was found to occur at the crack tip prior to formation of unstable oxides. To contrast with the threshold failure mode regime, a quantitative assessment of the role that the intergranular failure mode has on cyclic FCG behavior in the Paris regime was also performed. It was demonstrated that even a very limited intergranular failure content dominates the FCG response under mixed mode failure conditions.

Nonuniqueness of the fatigue threshold

The fatigue crack growth prediction near the threshold regime remains one of the most challenging fields in fatigue research. In previous studies, the threshold effects have been incorporated into fatigue crack growth models upon modification of the Paris Law in an empirical fashion. In this work, without a priori assumptions and empirical constants, we derived the threshold levels with a combination of molecular dynamics and continuum calculations. We illustrate that the threshold value is non-unique and depends on the state of the microstructure surrounding the crack. Also, in reality, we envisage very low fatigue crack growth rates near the ‘threshold’ and not a cut-off value, which is consistent with the experimental trends. In the model, the microstructure is characterized by the grain boundary types, the grain size, and the initial dislocation density. We derive friction stresses for the forward and the reverse motion of the crack tip dislocations interacting with the grain boundaries which allow determination of the irreversible crack tip displacements. We illustrate the benefits of sigma-3 grain boundaries, finer grain sizes and the shielding dislocations at the crack tip on improving the near-threshold fatigue crack growth behavior.

Effect of long-term aging on near-threshold fatigue crack growth of Ni–Cr–Mo–V steel welds

Effect of thermal aging at 350°C for 3000h on near-threshold fatigue crack growth (FCG) behavior of a Ni–Cr–Mo–V steel welded joint was investigated at various load ratios, R. FCG resistance of base metal after long-term aging was decreased and found to be dominated by microstructure influence; whilst FCG was controlled by R in weld metal due to microstructure homogeneity. Degradation of FCG threshold was assessed using a quantitative parameter. A microscopic interpretation of FCG threshold showed the predominant role of characteristic microstructure size. A model for fatigue threshold prediction involving the long-term aging effect was developed, and able to evaluate the stability of near-threshold FCG resistance of the welds in case of long-term service exposure.

Influence of precipitates on low-cycle fatigue and crack growth behavior in an ultrafine-grained aluminum alloy

The strain-controlled fatigue and near-threshold fatigue crack growth behavior of an ultrafine-grained (UFG), age-hardening aluminum alloy after severe plastic deformation by equal-channel angular pressing (ECAP) are discussed. The main question addressed is how different precipitate morphologies affect low-cycle fatigue (LCF) and fatigue crack growth. An AlMg0.5Si0.4 alloy is subjected to two and eight passes of ECAP to obtain different degrees of grain refinement and fragmentation of the initially semi-coherent precipitates. Furthermore, a thermally recovered condition with newly formed, small coherent precipitates, which is obtained by aging after two ECAP passes, is considered. Strain-controlled fatigue tests and ΔK-controlled crack growth measurements are conducted and microstructural evolution during cycling and fracture surfaces are carefully analyzed using scanning and transmission electron microscopy. Most importantly, the results of this study show that newly formed, coherent precipitates in the thermally recovered condition directly contribute to a more planar slip behavior, to slip localization and to early failure during LCF loading. It is clearly demonstrated that precipitate morphology also affects fatigue crack propagation, and that this is closely related to slip reversibility, even in the UFG regime.

Effect of sigma phase on near-threshold fatigue crack growth behavior and ultrasonic evaluation in AISI 316L stainless steel

In this work, we examined the influence of microstructural changes, such as an intermetallic sigma (σ) phase, on the fatigue behavior of high-temperature aged AISI 316L stainless steel. Nondestructive ultrasonic test and fatigue crack growth tests were performed to determine the threshold stress intensity factor of these artificially aged specimens. Ultrasonic test results characterizing the microstructural changes were compared with those of the fatigue tests to propose an empirical formula capable of predicting the threshold stress intensity factor by a nondestructive method. We observed a strong correlation between the increase in the volume fraction of the σ phase and the decrease of ΔKth. Ultrasonic velocity increased in response to the coarsening behavior of the σ phase in the vicinity of the grain boundaries.

Microstructure vs. Near-threshold Fatigue Crack Growth Behavior of an Heat-treated Ductile Iron

Perferritic isothermal ductile iron (IDI®) is an intermediate grade between the low-strength grades of austempered ductile iron (ADI) and pearlitic ductile iron (DI) recently developed by Zanardi Fonderie Italy. IDI is produced by heat-treating an unalloyed nodular cast iron. The specific matrix microstructure is called “Perferritic” and consists predominantly of ferrite and pearlite. Compared to the pearlitic grades of nodular ductile iron, IDI combines similar strength with higher toughness as a result of the isothermal heat treatment. In this contribution the fatigue crack growth resistance and Kath of IDI are investigated and correlated to mechanical properties and microstructural features. The threshold Ka was determined using the load shedding technique as per ASTM Standard E-647 using CT specimens extracted from a cast block. Tensile specimens were extracted from the broken CT halves and used to determine the static mechanical properties. A metallographic investigation was carried out to correlate structural features and mechanical properties.DOI: http://dx.doi.org/10.5755/j01.ms.18.1.1336

OS0505 ニッケル基超合金材の下限界近傍における大気中および真空中高温疲労き裂進展挙動の荷重波形依存性とその破面(OS5-1き裂進展,OS-5 材料の疲労挙動と損傷評価1)

Loading waveform dependence of near-threshold fatigue crack growth behavior of Ni-based superalloy at elevated temperatures was examined in the air and in a vacuum using constant Kn. and ΔK decreasing test method. It was found that the crack growth behavior at 850℃ and 900℃ on both environments was dependent on loading waveform. A difference was observed in appearance of fracture surfaces in correspondence to each C-P and P-C waveform at 850℃ in the air.

Near- threshold Fatigue Crack Growth Behavior and Crack Closure of Natural Gas Pipeline Steels

The near-threshold fatigue crack growth behavior and crack closure of three pipeline steels, - X65, X70 and X80 steels, - were investigated in air at a load ratio (R) of 0.1. With increase in the grade of pipeline steel, the average grain size of ferrite decreased and yield and tensile strength increased. In the near-threshold regime, the X80 steel had the slowest fatigue crack growth rate while the X65 steel had the fastest fatigue crack growth rate. The fatigue crack growth rate of the X70 steel was in-between those of the X65 and X80 steels in the near-threshold regime. For the X65 and X70 steels, it had been shown that the crack closure levels (Kc/Kmax) were 0.1 and independent of stress intensity factor range (AK). In contrast, the crack closure level of the X80 steel was 0.54 at the fatigue threshold and decreased with increasing AK. From analysis of fracture surface roughness, it was found that the factors characterizing roughness-induced crack closure for three pipeline steels in air were average fracture surface roughness, Ra, and the effective range of the crack tip opening displacement, (ACTOD)eff. It is concluded that the crack closure phenomenon of the X80 steel in the low ΔK region is roughness-induced.

1103 真空中および大気中におけるニッケル基超合金の下限界近傍高温疲労き裂進展挙動の繰り返し速度依存性(OS11-1 材料の疲労挙動と損傷評価-き裂進展-)

Loading frequency dependence of near-threshold fatigue crack growth behavior at elevated temperature, was examined in the air and in a vacuum in Ni-based Superalloy. It was found that crack growth behavior at 800℃, 850℃, and 900℃ in the air and 900°C in a vacuum was dependent on loading frequency, while crack growth behavior at 800°C and 850℃ in a vacuum was independent of loading frequency.

Effect of microstructure on appearance of near-threshold fatigue fracture in Cr–Mo–V steel

Near-threshold fatigue crack growth behavior in 25Cr2NiMo1V steel with different microstructures was investigated by utilizing the load-shedding technique at ambient temperature. Crack surface morphology was observed by SEM with special emphases on the incidence of intergranular fracture and the influence on crack growth rates. Results show that the maximum intergranularity occurs at the ΔK corresponding to the cyclic plastic zone size being equivalent to the prior austenitic grain size. Two types of crack growth mode were observed in the near-threshold regime, i.e., the crystallographic mode of crack growth and the striation mode of crack advance. The incidence of faceted fracture was mainly rationalized by comparing the cyclic plastic zone size with the grain size. It is concluded that, in the crystallographic mode, lower crack growth rates in samples with higher heat treatment temperatures are caused by a greater degree of roughness-induced crack closure (RICC), faceted fracture induced crack closure (FFICC), and oxide-induced crack closure (OICC). The faceted fracture shows negligible influence on crack growth rates when cracks grow in a striation controlled mode.

Loading Frequency Dependence of Near-Threshold Fatigue Crack Growth Behavior in SUS304 Stainless Steel at Elevated Temperatures in a Vacuum

The loading frequency dependence of fatigue crack growth in the near-threshold region in SUS304 stainless steel was investigated using constant Kmax and ΔK decreasing test method at 350°C, 550°C and 650°C in a vacuum below 2.7×10-4Pa. Loading frequency was changed from 30 Hz to 3Hz and vice versa, and crack growth rates at f=30 Hz and 3 Hz were compared. It was found that crack growth behavior in the region below the conventional threshold value was dependent on the loading frequency. Furthermore crack growth rates per unit hour at f=30 Hz and 3 Hz were compared in order to examine the effect of oxides on the crack surfaces on crack growth behavior. It was suggested that the amount of crack growth was rather time-dependent than cycle-dependent in the region below the conventional threshold in a vacuum.

Fatigue and fracture behaviour of structural Al-alloys up to very long life regimes

Very long life fatigue and near-threshold fatigue crack growth behaviour of 7075 and 6061 Al-alloys in T6 condition were investigated using accelerated fatigue testing at 19.5 kHz on a piezoelectric machine. Significant interfacial voids were observed in the two Al-alloys during crack initiation and early fatigue crack growth process. Fatigue failure in Al-alloys could be ascribed to: the formation of a number of fatigue voids, their growth and coalescence, and the subsequent formation and propagation of macroscopic cracks. Moreover, fatigue crack growth rates of small cracks in these alloys were found to be greater than those of large cracks for the same stress intensity factor range.

1113 Ni基超合金の下限界近傍における高温疲労き裂伝ぱ挙動とその破面の定量評価の試み(GS-3,4 材料の疲労)

1113 Ni基超合金の下限界近傍における高温疲労き裂伝ぱ挙動とその破面の定量評価の試み(GS-3,4 材料の疲労)

Closure mechanisms in Al–Si–Mg cast alloys and long-crack to small-crack corrections

Crack growth behavior of small and long cracks has been a subject of much research; the differences observed in the growth response were largely attributed to crack closure. In this study, cast Al–Si–Mg alloys of various residual stress levels and Si content/morphology were investigated, and the dominant closure mechanisms were identified. The near-threshold fatigue crack growth behavior of the alloys is controlled by two types of closure: residual stress-induced (due to residual stresses introduced during heat treatment) and microstructure/roughness-induced (due to variations in Si content/morphology). These closure mechanisms were individually discussed and closure corrective techniques were applied to long crack growth results. The near-threshold closure corrected long crack growth data compare favorably with small crack growth data.

Interactions of plasticity and oxide crack closure mechanisms near the fatigue crack growth threshold

An advanced analytical model developed earlier has been utilized to predict interactions between plasticity and oxide crack closure mechanisms that influence near-threshold fatigue crack growth behavior. Near-threshold closure is complex because multiple closure mechanisms are likely, including plasticity and oxide-induced crack closure. A series of experiments has been performed to validate analytical results. Analytical and experimental results suggest that interactions between plasticity and oxide result in high closure levels near the fatigue crack growth threshold. At sufficiently low ΔK, the combined effects of plasticity and oxide completely closes fatigue cracks resulting in high fatigue crack growth threshold values.

OS11W0034 Microstructural effects on fatigue crack growth behavior near threshold in a Ni-base superalloy

Microstructural effects on near-threshold fatigue crack growth behavior in a Ni-base superalloy have been studied. TEM and SEM observations have been performed on corresponding microstructures near crack tip and fracture surfaces. The fatigue crack growth resistance increases with increasing grain size, i.e. the threshold value increases and the fatigue crack growth rate (FCGR) decreases; Correspondingly the fracture surface changes from "fatigue striations" into remarkable "facets". However, increasing γ' particle size at aging-peak condition leads to decrease of the fatigue crack growth resistance, i.e., the threshold value decreases and the FCGR increases; In this case the fracture surface shows "fatigue striations" feature. The high resistance to fatigue crack growth is closely related to the planar slip mode of dislocations in the plastic zone near crack tip, which gives rise to lower accumulation of damage and enhances crack closure resulted from facet-feature cracking. The optimum microstructure for best fatigue crack resistance is suggested as coarse grains with fine γ' particles just before aging-peak is reached.

Near-threshold fatigue crack growth behavior of 2195 aluminum-lithium-alloy—prediction of crack propagation direction and influence of stress ratio

Tensile properties and fatigue crack propagation behavior of a 2195-T8 Al-Li alloy were investigated at different stress ratios, with particular emphasis on their dependence on specimen orientation. Specimens with orientations of 0, 15, 30, 45, and 90 deg to the rolling direction were tested. The alloy contained a strong brass-type texture and a profuse distribution of platelike precipitates of T 1 (Al2CuLi) phase on {111} matrix planes. Both tensile strength and fatigue thresholds were found to be strongly dependent on the specimen orientation, with the lowest values observed along the direction at 45 deg to the rolling direction. The effect of stress ratio on fatigue threshold could generally be explained by a modified crack closure concept. The growth of fatigue crack in this alloy was found to exhibit a significant crystallographic cracking and especially macroscopic crack deflection. The specimens oriented in the L-T + 45 deg had the smallest deflection angle, while the specimens in the L-T and T-L orientations exhibited a large deflection angle. The dependence of the fatigue threshold on the specimen orientation could be rationalized by considering an equivalent fatigue threshold calculated from both mode I and mode II values due to the crack deflection. A four-step approach on the basis of Schmid’s law combined with specific crystallographic textures is proposed to predict the fatigue crack deflection angle. Good agreement between the theoretical prediction and experimental results was observed.