Enhanced Fatigue Crack Growth Research Articles

Fatigue of 1018 steel in hydrogen

Tests are performed at different hydrogen pressures and mechanical load parameter values (applied load, stress concentration factor, applied load frequency) to study enhanced fatigue crack growth in SAE 1018 steel. Oxygen was introduced as a hydrogen-oxygen gas mixture environment in order to compare the role of oxygen in crack inhibition with that previously reported for both static and cyclic loading of higher strength steels. In a number of experiments, a thin film of palladium was applied to the specimen surface to facilitate the study of steel-hydrogen interaction in the absence of any surface impedance to hydrogen entry. The kinetics of the diffusing atoms are found to be influenced by the stress field around the notch tip and the applied frequency. An increment in environmental effect occurred when applied load or stress concentration factor was increased.

Use of the overpressure test to indicate fitness for purpose of a ferritic pressure vessel

A study is presented of the efficacy of customary proof testing practices. The fracture mechanics methods used in the study are first shown to estimate approximately the immunity observed after proof testing experimental vessels. The overstress required to assure a useful number of operating cycles is calculated for some typical cases, by considering growth in service of the maximum cracklike defect which could survive the pressure test, allowing for tearing. A wide range of possible fracture toughness, effects of geometry and operating temperature are included. The significance of fatigue crack growth enhancement during approach to fracture is estimated. The calculations and a discussion of more general issues confirm that the overpressure test is a valuable demonstration of pressure vessel integrity.

Fatigue crack growth surface reactions for titanium alloys exposed to water vapor

Parallel fracture mechanics and surface chemistry studies have been carried out to develop understanding of environmentally assisted fatigue crack growth in titanium alloys. Room temperature crack growth response in water vapor was determined for annealed Ti-5wt.%Al2.5wt.%Sn alloy and for Ti-6wt.%Al-4wt.%V alloys in the solution-treated and solutions-treated plus overaged conditions as a function of water vapor pressure from 0.266 to 665 Pa at a frequency of 5 Hz and a load ratio of 0.1.Reference data were obtained in vacuum. The kinetics of reactions of water vapor and oxygen with fresh surfacesof Ti-5wt.%Al-2.5wt.%Sn alloy were measured at room temperature. The results showed that the reactions with water vapor and with oxygen are both rapid, with an initial sticking coefficient of 1.0 for water vapor and of 0.54–0.84 for oxygen. The reaction with water vapor produced a monolayer of oxide (TiO), whereas two to three layers of TiO resulted from the reaction with oxygen. Enhancement of fatigue crack growth by water vapor is believed to result from embrittlement by hydrogen that is produced by the reaction of water vapor with the fresh crack surfaces. The fatigue crack growth response was shown to conform to a model for transport-controlled crack growth.

Corrosion Fatigue Crack Growth in Reactor Pressure Vessel Steels in PWR Primary Water

Experimental results for corrosion fatigue crack growth in several different ferritic pressure vessel steels including A533-B and A508-III exposed to simulated PWR primary water at 288°C are described. Since relatively little influence of environment was detected in low-frequency tests on these materials, in contrast with data reported from another laboratory, specimens have been exchanged and tested in each laboratory. These results are described, and possible reasons for the observed differences when each tested the same material are discussed. The environmental influence on corrosion fatigue crack growth is analyzed on the basis of a model of a sequence of protective oxide rupture/repassivation events during fatigue cycling. The repassivation kinetics and the controlling electrochemical factors are judged to be of crucial significance in determining the extent of environmental enhancement of fatigue crack growth.

Elevated temperature fatigue crack growth in incoloy alloy 800 in sulfidizing environments

Rates of fatigue crack growth in INCOLOY alloy 800 were measured in He-SO2 and He-H2S environments at a frequency of 0.1 Hertz over a temperature range of 316 to 650 °C. Increasing concentrations of sulfur and increasing temperatures increased the growth rates at higher △K values. H2S environments were more aggressive than SO2-containing gases, probably because of the inhibiting effects of oxygen when SO2 reacted with the alloy. At lower crack growth rates, sulfide particle formation at the crack tips caused cracks to decelerate and stop due to crack tip blunting and inhibition of crack closure. Sulfide particle formation also promoted the formation of periodic crack opening jumps. Under static loading, cracking could be produced only by a combination of prior cyclic deformation and the presence of a sulfidizing environment. The results suggest that sulfidizing environments enhanced fatigue crack growth through enrichment of sulfur ahead of the crack tip, producing a time-dependent crack growth process.

Threshold for fatigue macrocrack propagation in some aluminum alloys

Measurement of the threshold for fatigue macrocrack propagation, ΔKo, in a number of aluminum alloys has shown an increase with grain size and decrease with increase in strength as with steels. The results are not primarily due to environmental enhancement of fatigue crack growth because an even larger variation in ΔKo with microstructural change is noted at 77 K than at 300 K. In particular, ΔKo of high purity 2124-T4 increases much more on cooling from 300 to 77 K than does ΔKo of 2024-T4. It is suggested that ΔKo is determined by the stress necessary to operate a dislocation source near the crack tip. A Frank-Read type source is proposed for 2024-T4 with constituent particles acting as pinning points while double cross-slip, a thermally activated process, is proposed for the source in high purity 2124-T4.

Fatigue crack growth rates of irradiated pressure vessel steels in simulated nuclear coolant environment

Fatigue crack growth rates, in a simulated pressurized water reactor primary loop environment, are presented for A508-2 and A533B steels in both the irradiated and unirradiated conditions. The initial results, for specific experimental conditions, constant load amplitude waveforms, and load ratio = 0.2, indicate that irradiation does not enhance fatigue crack growth rates more than the enhancement due to the environment alone. This article outlines the experimental techniques involved in this testing, and describes the results and their possible significance with respect to the existing safety analysis codes.

Effect of hydrogen on flaw growth in Ti-6A1-2Sn-4Zr-2Mo-0.1Si

The influence of hydrogen on fatigue-crack-growth kinetics of Ti-6A1-2Sn-4Zr-2Mo-0.1Si (Ti-6242S) was studied, using a fracture mechanics approach, as a function of temperature, stress-intensity level, and cyclic load frequency. External gaseous hydrogen at pressures < 400 Pa does not increase the fatigue-crack-growth rate of Ti-6242S. Internal hydrogen at a level of 530 parts per million by weight (wppm), however, increases the fatigue-crack-growth rate significantly, with the effect being more pronounced at intermediate and high stress-intensity levels, low cyclic-load frequencies, and low temperatures. At temperatures above 400 K, internal hydrogen has no significant effect on fatigue-crack-growth rates. The results suggest that enhancement of fatigue-crack growth by internal hydrogen in the Ti-6242S occurs through a stress-assisted hydride-precipitation mechanism.

Fracture mechanics and surface chemistry studies of fatigue crack growth in an aluminum alloy

Fracture mechanics and surface chemistry studies were carried out to develop further understanding of the influence of water vapor on fatigue crack growth in aluminum alloys. The room temperature fatigue crack growth response was determined for 2219-T851 aluminum alloy exposed to water vapor at pressures from 1 to 30 Pa over a range of stress intensity factors (K). Data were also obtained in vacuum (at < 0.50 μPa), and dehumidified argon. The test results showed that, at a frequency of 5 Hz, the rate of crack growth is essentially unaffected by water vapor until a threshold pressure is reached. Above this threshold, the rates increased, reaching a maximum within one order of magnitude increase in vapor pressure. This maximum crack growth rate is equal to that obtained in air (40 to 60 pct relative humidity), distilled water and 3.5 pct NaCl solution on the same material. Parallel studies of the reactions of water vapor with fresh alloy surfaces (produced either byin situ impact fracture or by ion etching) were made by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The extent of surface reaction was monitored by changes in the oxygen AES and XPS signals. Correlation between the fatigue crack growth response and the surface reaction kinetics has been made, and is consistent with a transport-limited model for crack growth. The results also suggest that enhancement of fatigue crack growth by water vapor in the aluminum alloys occurs through a “hydrogen embrittle ment” mechanism.

Some Mechanistic Observations on the Crack Growth Characteristics of Pressure Vessel and Piping Steels in PWR Environment

Abstract The fatigue crack growth behavior of A533B and A508 pressure vessel steel and Types 304 and 316 steels used in reactor coolant piping has been studied in a pressurized water reactor environment at 288 C (550 F). The influence of stress ratio (Pmin/Pmax), frequency, ramp times, specimen orientation, and material microstructures were included in the study. While none of the materials showed evidence of static crack growth in the environment, the ferritic steels did show an enhanced fatigue crack growth rate at test frequencies of 5 cpm and lower. Based on fractographic examinations, the enhanced growth rate is not the result of environmentally induced intergranular or cleavage modes of crack propagation, instead, striation spacing measurements were found to agree with the macroscopic crack growth rate. This indicates that a time dependent environmental interaction exists which introduces a frequency dependent enhancement of the mechanically developed striations. Crack growth experiments using hold ...

Development of fatigue life criteria for experimental fusion reactor first-wall structures

An approach to the rational design of fusion reactor first-wall structures against fatigue crack growth is proposed. The approach is motivated by microstructural observations of fatigue crack growth enhancement in unirradiated materials due to volumetric damage ahead of a propagating crack. Examples are cited that illustrate the effect of mean stress on void nucleation and coalescence, which represent the dominant form of volumetric damage at low temperature, and of grain boundary sliding and creep cavitation, which are the dominant volumetric damage mechanisms at high temperature. The analogy is then drawn between these forms of fatigue crack growth enhancement and those promoted by irradiation exposure in the fusion reactor environment, such as helium embrittlement and atomic displacement. An enhanced strain range is suggested as a macroscopic measure of the reduction in fatigue life due to the higher fatigue crack growth rates. The enhanced strain range permits a separation of volumetric and cyclic effects, and assists in the assignment of rational design factors to each effect. A series of experiments are outlined which should provide the numerical values of the parameters for the enhanced strain range.

Environmentally Enhanced Fatigue Crack Growth of a High-strength Steel Sensitive to Stress Corrosion Cracking

Low-cycle fatigue tests and sustained load tests of a high-strength steel (SNCM8) sensitive to stress corrosion cracking (SCC) have been carried out under various hydrogen contents. The threshold stress intensity facto for cyclic SCC under cyclic stress KFSCC is smaller than the threshold value for static SCC under sustained load KISCC, KFSCC being smaller with an increasing hydrogen content. In a steel tempered from a low temperature no hydrogen embrittlement is caused until diffused hydrogen is concentrated into a plastic zone at crack tips under small hydrogen content, so that the influence of stress waveforms becomes identical with that in a usual corrosion fatigue. Under high hydrogen content cyclic SCC plays a predominant role and a holding period during one cycle most enhances the crack growth. In a steel tempered from a high temperature the corrosion fatigue crack growth characteristics are the same as those in a usual low-strength tough material.

Fatigue Crack Growth in 2-1/4-Cr-1Mo Steel Exposed to Hydrogen Containing Gases

Steels used in coal gasification vessels and piping (externals) can be exposed to mixtures of hydrogen, water vapor (steam), hydrogen sulfide, methane, carbon monoxide, carbon dioxide, and other gases at temperatures and pressures up to 600°K and 10 MPa. Such mixtures, under certain operating conditions, can either enhance or inhibit crack growth in these steels. As a part of a program to identify thermodynamic conditions for this enhancement or inhibition, fatigue crack growth experiments have been carried out on a 2-1/4Cr-1 Mo (ASTM A542, Class 2) steel in hydrogen, water vapor, and hydrogen sulfide at low pressures (below 133 kPa). The results indicate considerable enhancement of fatigue crack growth by some of these environments and also indicate that the apparent immunity of this material to stress corrosion cracking does not imply the same immunity to corrosion fatigue. The results will be discussed in terms of the influences of temperature, gas pressure and loading variables, and will be interpreted in terms of chemical reaction kinetics.

Temperature Influence on Corrosion Fatigue Behavior of 5Ni-Cr-Mo-V Steel

Abstract An investigation of the below KIscc corrosion fatigue crack propagation behavior of 5Ni-Cr-Mo-V steel in a 3½ percent NaCl environmental solution is conducted with particular emphasis on the influence of frequency and temperature. The behavior is described using a generalized equation which incorporates the influence of temperature, frequency, and stress intensity range. The generalized equation 1dadN∣C=dadN∣F{H(ΔK,f,T)} relates the corrosion fatigue crack growth rate, dadN∣C, to the baseline cycle-dependent fatigue crack growth rate, dadN∣F. The function H is an environmental enhancement multiple. The severity of the temperature influence on environmentally enhanced fatigue crack growth rates of steels in aqueous environments appears to be related to the inherent resistance of the material to environmental cracking.