Hydrogen-induced Crack Growth Research Articles

Hydrogen-induced crack growth in a Ni-base superalloy

Hydrogen-induced crack growth in a Ni-base superalloy

Fracture toughness of zirconium hydride and its influence on the crack resistance of zirconium alloys

Measurements of fracture toughness have been made on specimens of δ-zirconium hydride and two-phase mixtures of hydride and zirconium. The tests were carried out over the temperature range of 20 to 300°C on both hydrided zirconium and hydrided Zr-2.5%Nb. The results are discussed in terms of the threshold stress intensity for hydrogen-induced sub-critical crack growth in Zr-2.5%Nb. The effect of hydride on the fracture toughness of zirconium alloys at 20 and 300°C is also discussed. We conclude that the improvement in toughness of hydride-containing zirconium alloys above 150°C is primarily due to the improved ductility of the zirconium phase rather than to improvements in the hydride properties.

Corrosion fatigue of high-strength steel in low-pressure hydrogen gas

AbstractFatigue tests on 835M30 steel in hydrogen gas at atmospheric pressure have confirmed that two distinct forms of corrosion fatigue crack growth can occur. The dominant mechanism of corrosion fatigue is governed by whether the cyclic stress intensity level is above or below the threshold stress intensity value for hydrogen-induced crack growth under static load, and by the stress ratio and cyclic frequency employed during testing. It has been shown that existing mechanistic models are unable to account quantitatively for the observed corrosion fatigue behaviour at stress intensity levels below the static load threshold. However, a model designated the ‘Process Competition Model’ can be used for quantitative predictions of corrosion fatigue behaviour at stress intensity levels above the static load threshold. An extended version of the model is also described in which the effects of cyclic waveform are incorporated. The translation of crack propagation rates to cyclic lifetimes is discussed and the e...

Embrittlement of a 5 Pct Nickel high strength steel by impurities and their effects on hydrogen-induced cracking

Temper embrittlement induced by isothermal aging at 480°C (753 K) in a commercially produced heat of HY 130 steel is shown by Auger electron spectroscopy to be due mainly to intergranular segregation of Si, with additional contributions by P, N, and Sn. Studies of rates of crack growth in gaseous hydrogen at controlled temperature and pressure in specimens aged various amounts showed that, as the impurity concentration increased, the stress intensity for crack growth Kth dropped precipitously, the cracking mode changed from cleavage to intergranular, and the crack growth rate atK > Kth increased. Hydrogen-induced crack growth occurred in a step-wise, rather than continuous, fashion. The impurity-plus-hydrogen effect is discussed in terms of a model adapted from an earlier theory of Oriani.

A film-rupture model of hydrogen-induced, slow crack growth in acicular alpha-beta titanium

A study has been conducted of the terrace-like fracture morphology of gaseous hydrogen-induced crack growth in acicular alpha-beta titanium alloys in terms of specimen configuration, magnitude of applied stress intensity, test temperature, and hydrogen pressure. Although the overall appearance of the terrace structure remained essentially unchanged, a distinguishable variation is found in the size of the individual terrace steps, and step size is found to be inversely dependent upon the rate of hydrogen-induced slow crack growth. Additionally, this inverse relationship is independent of all the variables investigated. These observations are quantitatively discussed in terms of the formation and growth of a thin hydride film along the alpha-beta boundaries and a qualitative model for hydrogen-induced slow crack growth is presented, based on the film-rupture model of stress corrosion cracking.

A short-time diffusion correlation for hydrogen-induced crack growth kinetics

Analysis of hydrogen-stress field interactions have led to kinetic criteria for slow crack growth. Using both elastic and plastic stress fields under opening-mode loading, criteria for stage I, II, III growth are developed in terms of the pressure tensor gradient at the crack tip. It is proposed that stage I (stress-intensity dependent) growth kinetics are predominantly controlled by the elastic stress field while stage II (nearly stress-intensity independent) kinetics are controlled by the plastic stress field. Measurements of slow crack growth in cathodically-charged AISI 4340 steel verify the overall aspects of the correlation. Detailed measurement and analysis of the increase in crack-tip radius with increasing applied stress intensity have led to a proposed decrease in crack growth rate during stage II growth. Some experimental evidence corroborates this later hypothesis and is consistent with long range diffusional flow of hydrogen as the controlling mechanism for crack growth kinetics.

The effect of thickness on hydrogen-induced slow crack growth

The effect of thickness on hydrogen-induced slow crack growth

Gaseous hydrogen-induced cracking of Ti-5Al-2.5Sn

The kinetics of hydrogen-induced cracking have been studied in the Ti-5Al-2.5Sn titanium alloy having a structure of acicular α platelets in a β matrix. It was observed that the relationship between hydrogen-induced crack growth rate and applied stress intensity can be described by three separable regions of behavior. The crack-growth rate at low stress-intensity levels was found to be exponentially dependent on stress intensity but essentially independent of temperature. The crack-growth rate at intermediate stress-intensity levels was found to be independent of stress intensity but dependent on temperature in such a way that crack-growth rate was controlled by a thermally activated mechanism having an activation energy of 5500 cal per mole and varied as the square root of the hydrogen pressure. The crack-growth rate at stress-intensity levels very near the fracture toughness is presumed to be independent of environment. The results are interpreted to suggest that crack growth at high stress intensities is controlled by normal, bulk failure mechanisms such as void coalescence and the like. At intermediate stress-intensity levels the transport of hydrogen to some interaction site along the α-β boundary is the rate-controlling mechanism. The crack-growth behavior at low stress intensities suggests that the hydrogen interacts at this site to produce a strain-induced hydride which, in turn, induces crack growth by restricting plastic flow at the crack tip.