Near-threshold Fatigue Crack Growth Behavior Research Articles

Near-Threshold Fatigue Crack Growth Behavior of SUS304 Steel at High Temperatures Using Interferometric Strain/Displacement Gage. 1st Report, Crack Closure Behavior.

Interferometric strain-displacement gauge(I.S.D.G.) testing system was developed for the measurement of the crack opening displacement at high temperatures. Helium-Neon gas laser and center-cracked specimens of type SUS304 austenitic stainless steel were used. Fatigue crack growth tests were conducted at 450, 550, 650 and 700°C at stress ratios of R=-0.1 and 0.5. Using I.S.D.G.for the measurement of crack closure modified using platinum plate and optical filter, crack opening displacement was found to be measured with good accuracy at all temperatures tested. Results showed that crack closure was observed at R=-0.1, while no crack closure was observed at R=0.5 even at 650°C. From the results of the measurement of fatigue crack closure in the near-threshold regime at all temperatures investigated, it was found that rapid increase and decrease in crack growth rate da/dN were observed when crack opening ratio U showed rapid increase and decrease, respectively.

Fatigue crack growth behavior in niobium-hydrogen alloys

Near-threshold fatigue crack growth behavior has been investigated in niobium-hydrogen alloys. Compact tension specimens (CTS) with three hydrogen conditions are used: hydrogen-free, hydrogen in solid solution, and hydride alloy. The specimens are fatigued at a temperature of 296 K and load ratios of 0.05, 0.4, and 0.75. The results at load ratios of 0.05 and 0.4 show that the threshold stress intensity range (ΔK th ) decreases as hydrogen is added to niobium. It reaches a minimum at the critical hydrogen concentration (C cr ), where maximum embrittlement occurs. The critical hydrogen concentration is approximately equal to the solubility limit of hydrogen in niobium. As the hydrogen concentration exceeds C cr , ΔK th increases slowly as more hydrogen is added to the specimen. At load ratio 0.75, ΔK th decreases continuously as the hydrogen concentration is increased. The results provide evidence that two mechanisms are responsible for fatigue crack growth behavior in niobium-hydrogen alloys. First, embrittlement is retarded by hydride transformation-induced and plasticity-induced crack closures. Second, embrittlement is enhanced by the presence of hydrogen and hydride.

Crack Closure Effects on Fatigue Crack Growth Thresholds and Remaining Life in an HSLA Steel

The effects of crack closure on the near-threshold corrosion fatigue crack growth behavior of Mil S-24645 HSLA steel and its weld metal have been investigated in air, ASTM seawater at the free corrosion potential, and ASTM seawater at −0.8V and −1.0V (SCE) using frequencies of 10, 2, and 0.2 Hz, and a stress ratio, R = 0.1. Remaining life, in the presence and absence of crack closure, has been estimatedas a function of applied stress range for a structure containing a 3-mm-deep surface semi-elliptical flaw.

Near-Threshold Fatigue Crack Growth Behavior of SUS304 Steel at High Temperatures Using Interferometric Strain/Displacement Gage. 1st Report. Crack Closure Behavior.

Fatigue crack growth tests using center-cracked specimens [M (T) specimens] of SUS304 steel were conducted to examine the behavior of fatigue crack growth in the near-threshold regime at 450, 550, 650, and 700°C. Using the interferometric strain/displacement gage (I.S.D.G.), the crack closure behavior, which was thought to be significant due to oxide film accumulated on crack surfaces, was examined. It was shown that the crack closure level was measured with good accuracy even at high temperatures by I.S.D.G. modified using a platinum plate and a filter. It was found from the behavior observed at 650°C that crack closure did not occur at R=0.5, and that changes in the behavior of crack growth rate, da/dN, corresponded to those in the behavior of crack opening ratio, U, at each test temperature.

Effects of dendrite cell size and particle distribution on the near-threshold fatigue crack growth behaviour of cast AlSiCp composites

Fatigue crack growth tests and crack closure measurements were performed for A356 cast aluminium alloys reinforced by 10 or 20% SiC particles and their matrix A356 alloys with systematically controlled dendrite cell size and particle distribution. The cell size dependence of the fatigue crack growth behaviour in the composite was found to be quite similar to that of the matrix alloy. This suggests that the cell size rather than the particle-crack tip interaction is the most important factor to control the fatigue crack growth of the composites. Near threshold fatigue crack growth properties were improved in the composites with coarser cell size and inhomogeneous particle distribution due to the enhanced roughness induced crack closure effect. These results were compared to those of powder-metallurgically-processed materials.

A model for sliding mode crack closure part I: Theory for pure mode II loading

A physical model has been developed to quantify the sliding mode crack closure (SMCC) experienced in cyclic mode II loading conditions. Idealized crystallographic surface geometry and Coulomb friction behaviour were assumed in the model. The results reveal that faceted crack surfaces will generate a local wedging mode I component while frictional attenuation tends to reduce the effective mode II stress intensity factor range. It is found that asperity angle and friction coefficient are two fundamental factors influencing near-threshold fatigue crack growth behaviour during the entire fatigue cycle. The local stress-strain field is significantly altered due to the wedging mode I component and frictional attenuation of the nominal mode II component. It is concluded that ideal mode II loading may never be achieved in practice for polycrystalline materials due to the inevitable existence of local mixed mode loading conditions.

Influence of dynamic strain aging on the near-threshold fatigue crack growth behavior of a new single crystal nickel-based superalloy

In recent years, the design requirements of advanced gas turbine engines have led to the development of directionally solidified single crystal nickel-based superalloys. Among these alloys, CMSX-4 is a new second-generation single crystal nickel-based superalloy containing rhenium. This alloy has been developed from the first-generation CMSX alloys. It has superior hot corrosion resistance and higher creep-rupture strength than first-generation single crystal nickel-based superalloys and is an attractive candidate for complex components that require continuous exposure to very high temperatures. Due to its potential applications in turbine blades, rotors and nuclear reactors, etc., the fatigue crack growth rate (FCGR) and fatigue threshold (FT) data of this material are extremely important for safe life prediction as well as failure safe design at elevated temperatures. Preliminary studies by these authors have indicated that this material is probably undergoing dynamic strain aging (DSA) in the temperature range of 260 to 800 C. However, the effect of DSA on FCGR and FT is not well known or established for single crystal nickel-based superalloys. The objective of this investigation was to determine the effect of DSA on FCGR and FT of CMSX-4 at the temperature range of 650 and 800 C.

Influence of stress ratio and loading condition on near-threshold fatigue crack growth behaviour: Chen, D., Weiss, B., Stickler, R. and Wang, Z. J Mater Sci Technol (China) (Feb 1993) 9 (1), 47–52

Influence of stress ratio and loading condition on near-threshold fatigue crack growth behaviour: Chen, D., Weiss, B., Stickler, R. and Wang, Z. J Mater Sci Technol (China) (Feb 1993) 9 (1), 47–52

The influence of crack closure on the positive load ratio ( [formula omitted]) dependence of fatigue threshold stress intensity factor range ( [formula omitted]) of AISI 316 stainless steel in air at room temperature

Near-threshold fatigue crack growth (FCG) behaviour of 316 SS at positive R- ratios in air at room temperature is influenced by crack closure. Empirical modifications of McEvily-Groeger and Zhao Xiaopeng equations are proposed to incorporate crack closure effects; the modified equations are found to provide excellent correlation for the R- dependence of the present ΔK th data.

Near-Threshold Fatigue Crack Growth Behavior of Mil S-24645 HSLA Steel and Its Weldments

The near-threshold corrosion fatigue crack propagation behavior of Mil S-24645 HSLA steel, its weld metal and heat-affected zone have been studied in simulated marine environments. Tests were conducted in air and ASTM seawater, at both the free corrosion potential and −1.0V (SCE), using a frequency of 0.2 Hz and a stress ratio, R = 0.1. The results were compared with previous data for Mil S-24645 and ASTM A710 steel tested at frequencies of 10 and 2 Hz. The influence of frequency, microstructure, and cathodic protection on crack closure contributions in the near-threshold regime will be discussed.

Near-threshold fatigue crack growth behaviour of a ferritic stainless steel at elevated temperatures

Near-threshold fatigue crack propagation (FCP) behaviour has been studied in an 18%CrNb stabilized ferritic stainless steel at temperatures ranging from room temperature to 700 °C. At a stress ratio of 0.1 increasing the test temperature from room temperature to 500 °C resulted in an increase of the growth rates in the midrange growth regime and a sharply defined threshold at a δK level higher than the room temperature threshold, giving rise to a crossover type of behaviour. At temperatures higher than 500 °C increased crack tip plasticity predominates and the fatigue crack growth rates decrease smoothly with a decreasing value of ΔK to thresholds lower than the room temperature value. Crack closure measurements suggest that asperity-induced closure dominates at room temperature but transitions to plasticity-induced closure dominate at 500 °C. A constant- K max, increasing R-ratio (CKIR) test procedure was utilized at room temperature and at 500 °C in an attempt to identify near-threshold FCP data in the absence of crack closure. However, the type of crossover behaviour identified with constant R-ratio tests at room temperature and 500 °C was also observed in the CKIR tests. This is attributed to a change in the closure mechanism from a roughness-induced one to one involving crack tip plasticity.

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)* was investigated. The investigation also examined the influence of heat treatment on resultant microstructures and the near-threshold fatigue crack growth behavior. In addition, the influence of load ratios(R), material strength, and grain size on fatigue threshold was studied. Compact tension specimens prepared from Incoloy 825 with transverse-longitudinal (TL) orientation in the as-received, as well as two different heat treated conditions, were used. The heat treatment studies revealed a peak hardness condition after solution treatment at 1200 °C for 1/2 hr, followed by aging at 600 °C for 434 hr. Among all the heat treated conditions, the fatigue threshold was the highest and the near-threshold crack growth rate was lowest in this peak aged condition. Fatigue threshold values were observed to decrease with an increase in load ratio, whereas an increased grain diameter resulted in a higher fatigue threshold. An earlier mathematical model was found applicable to characterize the relationship between load ratio and fatigue threshold. Preferential etching of grain boundary suggests formation of a thin film of carbide precipitation along the grain boundary region in the aged specimens. This carbide precipitation facilitated intergranular crack growth in these samples, resulting in higher roughness-induced crack closure. The highest fatigue threshold in the peak aged condition can be attributed to this large roughness-induced crack closure process.

Ｈａｓｔｅｌｌｏｙ‐Ｘ分布微小き裂材の高温疲労き裂成長下限界

Near-threshold fatigue crack growth behavior for distributed cracks in Hastelloy-X has been examined with reference to the effects of crack size and environment on the threshold stress intensity ΔK.The threshold stress intensity value for a large crack in air was lower than in vacuum. This result was same for a small crack in air and vacuum experiments. But the value for a small crack was about 50% lower than for a large crack in vacuum. In air, the small crack threshold was lower than the large one. These results were discussed in terms of fatigue crack closure. Observation of crystallographic fracture surface in vacuum suggested the roughness induced crack closure mechanisms.The distributed-small-cracks specimen was obtained by low cycle fatigue loading tests. The threshold stress intensity for the distibuted-fatigue-crack growth could be estimated by the maximum crack size of the distributed cracks. The coalescence of small cracks did not affect the near-threshold fatigue crack growth behavior.

The effect of stress ration on the near-threshold fatigue crack growth behaviour of Ti8Al1Mo1V at elevated temperature: Salivar, G.C., Heine, J.E. and Haake, F.K. Eng. Fract. Mech. 1989 32, (5), 807–817

The effect of stress ration on the near-threshold fatigue crack growth behaviour of Ti8Al1Mo1V at elevated temperature: Salivar, G.C., Heine, J.E. and Haake, F.K. Eng. Fract. Mech. 1989 32, (5), 807–817

The effect of stress ratio on the near-threshold fatigue crack growth behavior of Ti-8A1-1Mo-1V at elevated temperature

An investigation of the effects of stress ratio and temperature on the near-threshold fatigue crack growth characteristics of Ti-8Al-1Mo-1V was performed. Computer-controlled decreasing stress intensity range tests using compact tension specimens were conducted for various stress ratios at temperatures of 76°F (24°C) and 500°F (260°C) in laboratory air. Crack opening load was determined throughout the tests based on automated compliance measurements and was used to adjust the fatigue crack growth rate data to effective stress intensity range ( ΔK eff ). The effects of stress ratio at a constant temperature were able to be explained by the concept of crack closure and ΔK eff . However, crack closure was not able to account for the effects of temperature at a fixed stress ratio of 0.1, as higher near-threshold crack growth rates were observed at 500°F (260°C) when the data were plotted as a function of ΔK eff . This difference in crack growth rate is believed to be attributable to significant crack front branching and secondary cracking as a result of the increased oxidation at 500°F (260°C). A unique value of fatigue crack growth rate threshold, ΔK th was observed for all of the test conditions when crack closure was accounted for in the data analysis.

The near-threshold high [formula omitted] fatigue crack growth characteristics of SA508 cl III reactor pressure vessel steel

This paper describes the effect of frequency and environment on the near-threshold fatigue crack growth behaviour of SA508 cl III reactor pressure vessel (RPV) steel. The study has shown that in the near-threshold regime microstructure and environment markedly affect fatigue crack growth behaviour. In an aqueous environment, fatigue crack growth behaviour became even more sensitive to microstructure, and the fatigue crack growth rate increased by a factor of four in the case of the 3 Hz test, while that for the 0·3 Hz test was increased by a factor of approximately sixteen. This environmental enhancement manifested itself in the form of intergranular failure. For the 0·3 Hz test the percentage intergranular failure decreased from 18% to <1% with an increase in ΔK level. The transition from microstructure-sensitive to microstructure-insensitive occurs when the cyclic plastic zone size is of the order of the prior austenite grain size.

Near-threshold fatigue crack growth properties for SB42 and SUS304 steels at elevated temperatures.

Near-threshold fatigue crack growth behavior for SB 42 carbon steel and SUS 304 stainless steel was investigated at elevated temperatures of 200 and 400°C at a frequency of 50 Hz. A new test procedure was atempted in order to avoid crack closure: ΔK was decreased under increasing minimum load. A unique ΔKth-value of about 2.8 MPa·m1/2 was obtained independent of material and temperature by the closure-free test procedure. The crack growth rate, however, was found to be accelerated in the ΔK region higher than 4 MPa·m1/2 for both materials at elevated temperatures. The fractographic features influenced by microstructure and environment were discussed in terms of the mechanism of crack growth.

クリープ条件下の疲労き裂伝ぱ下限界挙動とき裂開閉口

The fatigue crack growth behavior of SUS 304 stainless steel at 650°C was investigated under low stress ratios R on the basis of the non-linear fracture mechanics. Emphasis was placed upon the near-threshold fatigue crack growth behavior and crack closure under creep conditions. It was found that in the very low da/dN region and the high da/dN region the relation between the cycle-dependent crack growth rate da/dN and stress intensity factor range ΔK obtained for -1≤R<0.2 deviated from the same correlation obtained under 0.5≤R. The values of fatigue threshold ΔKth for low R were much greater than those obtained for high R. Such a strong R dependence of the crack growth behavior found in the low R region was eliminated by the use of J-integral range ΔJf, eff, in which the effect of crack closure as well as that of material non-linearity were taken into account. The near-threshold crack growth behavior was found to be very sensitive to the loading history including frequency. This was rationalized in terms of the effect of oxide-induced crack closure enhanced by fretting as well as that of time-dependent creep deformation.

Near-threshold fatigue crack growth properties for SB 42 and SUS 304 steels at elevated temperature.

Near-threshold fatigue crack growth behavior for SB 42 carbon steel and SUS 304 stainless steel was investigated at elevated temperatures of 200 and 400 °C at a frequency of 50 Hz. A new test procedure was attempted in order to avoid crack closure : ΔK was decreased under increasing minimum load while maintaining maximum load. A unique ΔKth-value of about 2.8 MPa·m1/2 was obtained independently of material and temperature by the closure-free test. The crack growth rate, however, was found to be accelerated in the ΔK region higher than 4 MPa·m1/2 for both materials at elevated temperatures. The fractrographic features influenced by microstructure and environment were discussed in terms of the mechanism of crack growth.

Threshold and closure of fatigue crack propagation for low alloy steels, stainless steels, nickel alloy and aluminium alloys.

The near-threshold fatigue crack growth behavior is investigated for carbon steel (S25C), low alloy steel (SM50B), stainless steels (SUS430, SUS304 and SUH310), nickel alloy (Hastelloy X) and aluminium alloys (A2017 and A5083) in laboratory and dry air conditions. At higher load ratios, such as 0.8, where there is no closure, ΔKth-values are proportional to Young's modulus, E. At lower load ratios, such as 0.1, however, ΔKth/E-values are dependent on material and atmosphere, due to crack closure. Oxide-induced closure appears on carbon and low alloy steels in laboratory air, while oxide-thickening within the crack is prevented by passive film on stainless steels, nickel alloy and aluminium alloys. Dry air reduces oxide formation on the fracture surface. Roughness-induced closure is stronger in nickel and aluminium alloys than in steels. Fracture surfaces are more crystallographic in the former alloys. Phase-transformation-induced closure results from martensitic transformation inherent to austenitic stainless steels. This transformation, however, opens the crack near the threshold rather than closes it.