Int J Fatigue 14 No 4 (1992) pp 211-218 Environmental influence on the near-threshold fatigue crack propagation behaviour of a high- strength steel G. Hrnaff, J. Petit and B. Bouchet The near-threshold fatigue crack propagation behaviour of a high-strength low-alloy steel has been investigated in ambient air and in vacuum so as to determine the role of the environment precisely. The analysis of the results is conducted by taking crack closure effects into account. It is concluded that fatigue crack growth rates measured in ambient air depend upon three processes: intrinsic fatigue crack propagation as observed in vacuum, adsorption of water vapour molecules on freshly created rupture surfaces, which enhances crack propagation, and a subsequent step of hydrogen-assisted cracking. The appearance of intergranular ruptures and oxide layers on rupture surfaces in ambient air is also discussed. Key words: environmental influence; adsorption; hydrogen-assisted cracking Many engineering structures contain initial defects that may grow during service. Knowledge of the fatigue crack growth behaviour of the materials used in such structures is essential to prevent these defects from reaching a critical size. As these structures may sometimes endure a high number of loading cycles during their operating lives, special attention is generally paid to the regime where the fatigue crack growth rate is slow. Several parameters have been identified as influencing the threshold value of the cyclic stress intensity factor AKth, below which no crack growth can be detected, as well as growth rates in the near-threshold regime. More particularly, it has been shown that the presence of water vapour or hydrogen in the test environment may induce higher crack growth rates than those that pertain in an inert atmosphere. This crack propagation enhancement in an aggressive environ- ment must be analysed by comparing the measured crack growth rate with the rate obtained in an inert environment such as a high vacuum, for the same value of the effective cyclic stress intensity factor AKecf (ie by separating out crack closure effectsl). Wei and Simmons have proposed a sequential step process to describe the influence of aggressive species on fatigue crack propagation in aluminium alloys and in steels. 2"3 According to these authors the observed crack growth enhancement should be attributed solely to bulk hydrogen embrittlement. However Bouchet4 was first to suggest that the adsorption of water vapour molecules on crack surfaces could by itself promote fatigue crack propagation by lowering the surface energy ,/(the Rehbinder effectS). On the basis of experiments conducted in a high vacuum, purified nitrogen that contains traces of water vapour and ambient air, Petit et al 6-s have distinguished two different mechanisms of environmentally assisted fatigue crack propagation. (a) At crack growth rates higher than a critical value (da/dN)c. which depends upon different parameters such as partial pressure of the water vapour, load ratio R and test frequency, the crack growth rate is controlled by plasticity. This regime can be described by a modified Weertman relationship7,9 da AK~. -A - - (1) dN vL~U where w represents the shear modulus, ~0 a strength parameter and U the energy necessary to create a unit surface of crack by fatigue. The value of U can be modified by water vapour adsorption. On the basic assumption that U is proportional to ~, Petit 7 has described environmentally assisted fatigue crack growth by introducing the rate of coverage O of freshly created surfaces by adsorbed molecules, as defined by Langmuir, l° in the following form: and: ~o=~ +0 ~ -~ v e,s v (2) 0 - ~/~-'-~/vl (3) ~.s~ _.y~l with the suffixes meaning: e, environmental; v, vacuum; e,s, saturated environment. (b) At crack growth rates lower than (da/dN)cr a hydrogen-assisted crack growth mechanism can occur and lead to highly accelerated crack growth rates and lower values of AKth when compared with propagation in a high vacuum. In the present study the near-threshold fatigue crack propagation behaviour of a high-strength low-alloy steel has been investigated in ambient air and in a high vacuum in order to obtain more insight into the different processes involved in environmentally assisted fatigue crack growth. 0142-1123/92/040211-08 © 1992 Butterworth-Heinemann Ltd Int J Fatigue July 1992 211