Acceleration Of Fatigue Crack Growth Rate Research Articles

Enhanced hydrogen embrittlement of low-carbon steel to natural gas/hydrogen mixtures

Fatigue crack growth rate (FCGR) tests were used to investigate the fatigue property of low-carbon steel in actual natural gas/hydrogen mixtures and carbon dioxide/hydrogen/nitrogen mixtures. It is found for the first time that the acceleration of FCGRs in the mixtures are much faster than that in hydrogen or natural gas. Natural gas/hydrogen mixtures changed the fracture mode from ductile fatigue striation to brittle cleavage fracture , and restricted the deformation activities around the crack. Synergy effect of carbon dioxide and hydrogen on the material is proposed to be the major reason for the abnormal performance of steel in the mixtures.

Effect of electrical discharge machining on corrosion and corrosion fatigue behavior of aluminum alloys

Electrical discharge machining (EDM) is often used in the manufacturing of fatigue test coupons because it allows for the production of complex shapes and sharp starting notches. However, the effect that the machining process has on the corrosion susceptibility due to the surface alterations is unknown. This work focuses on the effect the machining method has on the corrosion and corrosion fatigue behavior of aluminum alloys. The study is aimed at understanding the influence of EDM processes on environmentally assisted cracking evaluations, namely crack growth rate. Conventional milling and electrical discharge machining were the machining process examined in this study; the machining parameters used were best practices for laboratory fatigue sample production. The following aluminum alloys (AA) 2024-T351, 5083-H116, 6061-T6 and 7075-T651 were evaluated in this study. EDM was performed using a 0.152 mm (0.006 in.) hard brass wire. Surface quality evaluations, microstructural analysis, electrochemical tests and corrosion fatigue testing in sodium chloride (NaCl) solution were completed. The study found corrosion rates are affected by the EDM machining in all aluminum alloys. In final testing, crack growth rate tests were completed in sodium chloride solution with starter notches that were traditionally cut and EDM machined for 7xxx and 2xxx series alloys. The testing showed accelerated fatigue crack growth rates for the samples with EDM notches as compared to cut notches.

Assessing Gaseous Hydrogen Assisted Fatigue Crack Growth Susceptibility of Pipeline Steel Weld Fusion Zones and Heat Affected Zones

Abstract The objective of this work was twofold: (1) measure reliable fatigue crack growth relationships for X65 steel and its girth weld in high-pressure hydrogen gas to enable structural integrity assessments of hydrogen pipelines, and (2) evaluate the hydrogen accelerated fatigue crack growth susceptibility of the weld fusion zone and heat-affected zone relative to the base metal. Fatigue crack growth relationships (da/dN versus ΔK) were measured for girth welded X65 pipeline steel in high pressure hydrogen gas (21 MPa) and in air. Hydrogen assisted fatigue crack growth was observed for the base metal (BM), fusion zone (FZ), and heat-affected zone (HAZ), and was manifested through crack growth rates reaching nearly an order of magnitude acceleration over rates in air. At higher ΔK values, crack growth rates of BM, FZ, and HAZ were coincident; however, at lower ΔK, the fatigue crack growth relationships exhibited some divergence with the fusion zone having the highest crack growth rates. These relative fatigue crack growth rates in the BM, FZ, and HAZ were provisional, however, since both crack closure and residual stress contributed to the crack-tip driving force in specimens extracted from the HAZ. Despite the relatively high applied R-ratio (R = 0.5), crack closure was detected in the heat affected zone tests, in contrast to the absence of crack closure in the base metal tests. Crack closure corrections were performed using the adjusted compliance ratio method and the effect of residual stress on Kmax was determined by the crack-compliance method. Crack-tip driving forces that account for closure and residual stress effects were quantified as a weighted function of ΔK and Kmax (i.e., Knorm), and the resulting da/dN versus Knorm relationships showed that the HAZ exhibited higher hydrogen accelerated fatigue crack growth rates than the BM at lower Knorm values.

Hydrogen accelerated fatigue crack growth of friction stir welded X52 steel pipe

Friction stir welded steel pipelines were tested in high pressure hydrogen gas to examine the effects of hydrogen accelerated fatigue crack growth. Fatigue crack growth rate (da/dN) vs. stress-intensity factor range (ΔK) relationships were measured for an X52 friction stir welded pipe tested in 21 MPa hydrogen gas at a frequency of 1 Hz and R = 0.5. Tests were performed on three regions: base metal (BM), center of friction stir weld (FSW), and 15 mm off-center of the weld. For all three material regions, tests in hydrogen exhibited accelerated fatigue crack growth rates that exceeded an order of magnitude compared to companion tests in air. Among tests in hydrogen, fatigue crack growth rates were modestly higher in the FSW than the BM and 15 mm off-center tests. Select regions of the fracture surfaces associated with specified ΔK levels were examined which revealed intergranular fracture in the BM and 15 mm off-center specimens but an absence of intergranular features in the FSW specimens. The X52 friction stir weld and base metal tested in hydrogen exhibited fatigue crack growth rate relationships that are comparable to those for conventional arc welded steel pipeline of similar strength found in the literature.

Application of crack tip plasticity based fatigue model on high temperature alloy HAYNES® 282®

Confined crack tip plasticity model is employed to predict time independent fatigue crack growth rate (FCGR) behavior of HAYNES® 282® alloy at temperatures 1200F and 1400F. Crack growth tests were done in lab air, vacuum and steam environments at load ratios R=Kmin/Kmax ranging from 0.05 to 0.5. Calibrated model predicts average cyclic crack growth rate behavior of the material reasonably well. Predictions do not capture the accelerated fatigue crack growth rates observed in the data at low load levels. Such effects are believed to be caused by environmentally driven factors, which are not expected to be predicted by plasticity based models.

Effect of frequency on fatigue crack growth behavior of magnesium alloy AZ61 under immersed 3.5 mass% NaCl environment

Fatigue crack growth test of AZ61 magnesium alloy was carried out under immersed NaCl environment at frequencies of 15, 5 and 0.5Hz under a stress ratio of 0.1. In order to investigate the effect of frequency on fatigue crack growth behavior in detail, additional tests at frequencies ranged from 15 to 0.01Hz were conducted under a constant ΔK of 3.25MPam1/2. Effect of frequency was clearly observed in low ΔK region, where fatigue crack growth rate decreased with decreasing frequency. Crack closure would be a dominant factor for the frequency effect observed under immersed NaCl environment at frequencies ranged from 15 to 0.5Hz. However, fatigue crack growth rates at frequencies lower than 0.05Hz were higher than those at frequencies higher than 0.5Hz. The accelerated fatigue crack growth rates at frequencies lower than 0.05Hz would be attributed to the corrosion attack at the crack tip.

Fatigue Crack Growth Threshold of Austenitic Stainless Steels in Simulated PWR Primary Water

Many studies have revealed that fatigue crack growth (FCG) rate of austenitic stainless steels is accelerated in light water reactor environment compared to that in air at room temperature. Major driving factors in the acceleration of FCG rate are stress ratio, temperature and stress rise time. Based on this knowledge, FCG curves have been developed considering these factors as parameters. Hewever, there are few data of FCG threshold ΔKth in light water reactor environment. Hence it is necessary to clarify FCG rate under near-threshold condition for more accurate evaluation of fatigue crack growth behavior under cyclic stress with relatively low ΔK. In the present study, therefore, ΔKth was determined for austenitic stainless steels in simulated PWR primary water, and FCG behavior under near-threshold condition was revealed by collecting fatigue crack propagation data. The results are summarized as follows : No propagation of fatigue crack was found in high temperature water, and there was a definite ΔKth. Average ΔKeff, th was 4.3MPa·m0.5 at 325°C, 3.3MPa·m0.5 at 100°C, and there was no considerable reduction compared to currently known ΔKeff, th in air. Thus, it was revealed tha ambient conditions had minimal effect, on ΔKeff, th, ΔKth increases with increasing temperature and decreasing frequency. As a result of fracture surface observation, oxide-induced-crack-closure was considered to be a cause of the dependency described above. In addition, it was suggested that changes in material properties also had influence on ΔKth, since ΔKeff, th itself increased at elevated temperature.

3511 SUS310S鋼の疲労き裂伝ぱ速度に及ぼす水素雰囲気の影響およびその周波数依存性(S20-3 水素ガス雰囲気での疲労特性,S20 材料強度特性に及ぼす水素の影響)

In order to investigate whether hydrogen enhance the fatigue crack growth (FCG) rate of stable austenitic stainless steels or not, FCG tests of a type 310 steel were performed in a hydrogen gas environment and in a nitrogen gas environment. The FCG rate tested at 5.0Hz in hydrogen is higher than that in nitrogen. It seems that the acceleration of FCG rate in hydrogen does not necessarily need the formation of strain-induced α' martensite, though it increases the degree of the acceleration. The cause of the acceleration of type 310 seems to be hydrogen enhanced concentration of slip at a crack tip. The degree of the acceleration became smaller in a frequency of 0.1Hz. It seems that the generation of heat at the crack tip concern. Further investigation is necessary.

The effect of potential upon the high-temperature fatigue crack growth response of low-alloy steels, part 1: crack growth results

Corrosion-fatigue crack propagation experiments were conducted on several low-alloy steels in elevated temperature aqueous environments and experimental parameters included temperature, sulfer content of the steel, applied potential and dissolved hydrogen (and in one case, dissolved oxygen) concentration in the water. Specimen potentials were controlled potentiostatically and the observation (or non-observation) of accelerated fatigue crack growth rates was a complex function of the above parameters.The interaction of electrochemical potential and sulfides in the crack enclave are discussed in Part 2, as well as the postulated explanation for the complex behaviour.

Influence of precipitate morphology on the high temperature fatigue properties of SRR99

Fatigue crack growth (FCG) tests have been conducted in air at 650°C and 850°C on 〈001〉 oriented single crystals of SRR99 having the γ′ particles in the form of: (A) 0.3 μm cuboids; (B) 0.2 μm ogdoadical cuboids; and (C) a coarse, rafted γ′ structure. In general, reducing the frequency and increasing the temperature enhances crack-tip shielding at low Δ Ks due to increasing oxide induced crack closure. In material A at 650°C the crack path changes from one of γ′ precipitate cutting on {001} to propagation within the matrix as Δ K increases. Enhanced crack branching at 850°C improves the Paris regime behaviour compared with that seen at 650°C. In material B at 650°C, greater cross slip at lower frequency reduces slip reversibility, thus enhancing the fatigue crack growth rate (FCGR). At 850°C crack tip blunting and meandering, associated with γ′ cutting, improves the high Δ K FCG response and on a strength/modulus normalized basis is comparable with that seen for material A. Material C shows a similar FCG resistance to A at 650°C, but there is an acceleration in FCGR at 850°C, which can be accounted for in terms of the lower proof stress and modulus of this microstructure.

Role of oxides in fatigue crack propagation

AbstractOxides and oxidation play a very important role in fatigue crack propagation for many materials, not only at elevated temperatures, but also at ambient temperature. The simplest approach to modelling environmental effects on crack growth is to attempt to add a purely mechanical crack growth increment to an environmental component: the oxidation rate multiplied by the cycle time. Limited success has been achieved with models of this type, both for high temperature fatigue in air and in high pressure water environments. One significant result has been to emphasise that at temperatures up to about half the melting point of the material, it is environmental effects (and thus, frequently, oxidation) rather than creep which give rise to accelerated fatigue crack growth rates. However, the problem with these simple models is that there is no account taken of the interaction between the environmental processes taking place. There are many types of interaction between the mechanical and the chemical proces...

Various electrochemical measurements in a simulated corrosion fatigue crack: Charles, E.A., Congleton, J. and Parkins, R.N. Corrosion Sept. 1988, 44, (9), 599–605

Corrosion-fatigue crack propagation experiments were conducted on several low-alloy steels in elevated temperature aqueous environments and experimental parameters included temperature, sulfer content of the steel, applied potential and dissolved hydrogen (and in one case, dissolved oxygen) concentration in the water. Specimen potentials were controlled potentiostatically and the observation (or non-observation) of accelerated fatigue crack growth rates was a complex function of the above parameters.The interaction of electrochemical potential and sulfides in the crack enclave are discussed in Part 2, as well as the postulated explanation for the complex behaviour.

Roles of precipitates on corrosion fatigue crack growth of high-strength steels in corrosive environments

The effect of microstructures on resistance to corrosion fatigue cracking and fracture surface morphology for age-hardened steels were investigated in a 3.5% NaCl aqueous solution under a cathodic potential of −0.85 V (Ag/AgCl). The free corrosion was about −0.63 V (Ag/AgCl). The resistance to corrosion fatigue cracking of materials containing coherent precipitates in the matrix (underaged conditions) was less than that of materials containing incoherent precipitates (reheated conditions) at equal strength levels. Accelerated fatigue crack growth rates of the underaged material in the aqueous solution were followed by cracking along prior-austenite grain boundaries, due to hydrogen embrittlement, while the overaged material did not show accelerated fatigue crack growth rates and had fracture surfaces similar to those in air. The difference in the fracture surfaces of both materials in air and in the aqueous solution was considered to depend on the ease of diffusion of hydrogen to the prior-austenite grain boundaries. It is concluded that incoherent precipitates in the matrix made hydrogen accumulation at prior-austenite grain boundaries much slower than for coherent precipitates.

Frequency and wave-form effects on the fatigue crack growth behavior of alloy 718 at 298 K and 823 K

The fatigue crack growth rate (FCGR) of Alloy 718 was measured on CT type specimens at 298 and 823 K. At 823 K, the influence of frequency was studied in the range between 5 – 10-3 Hz and 20 Hz, using a sinusoidal wave form signal. A substantial increase in FCGR occurred, particularly at low stress intensity levels, as the temperature was increased from 298 to 823 K and as the frequency was decreased at 823 K. At elevated temperature, the effect of cyclic stress wave form was equally investigated, using triangular and square wave form signals producing the same frequency of 5.10-2 Hz. The triangular load led to higher FCGR than the square wave form. In addition the hold time of 10 s both at the maximum and the minimum load associated with the square load had no significant effect on the FCGR. Electron microscopy was used to observe the substructures that developed ahead of fatigue cracks. These observations showed that in certain circumstances plastic deformation proceeded by the propagation of planar bands which were identified as twins. At room temperature, twinning was found to be abundant only in the threshold regime. At 823 K, twinning was observed in the domain of higher FCGR, particularly at low frequencies. Fractography was carried out to study the micromechanisms of crack propagation. At 823 K. intergranular cracking occurred as the frequency was decreased. The comparison between the substructures formed in low cycle fatigue and those associated with the plastic zones of propagating cracks is made. The importance of planar deformation and twinning on intergranular cracking and on the acceleration of FCGR when the loading rate is decreased at 823 K, is discussed.