~ Pergamon Acta metall, mater. Vol. 43, No. 8, pp. 2931-2942, 1995 Elsevier ScienceLtd 0956-7151(95)00002-X Copyright © 1995. Acta Metallurgica Inc. Printed in Great Britain. All rights reserved 0956-7151/95 $9.50 + 0.00 ON FATIGUE CRACK PROPAGATION ENHANCEMENT BY A GASEOUS ATMOSPHERE: EXPERIMENTAL AND THEORETICAL ASPECTS G. HENAFF,K. MARCHALand J. PETIT Laboratoire de M6canique et Physique des Mat6riaux, URA CNRS No. 863, ENSMA, Site du Futuroscope, B.P. 109 Chasseneuil-du-Poitou, F-86960 Futuroscope Cedex, France (Received 9 November 1994; in revised form 6 January 1995) Abstract--This paper is concerned with environmentally-assisted stage II fatigue crack propagation and specifically the interaction between the fatigue damage mechanisms and the exposure of the material to an active environment like ambient air. A brief state-of-the-art in the understanding of these well-knit connected processes is first presented. On the basis of original experimental results obtained on a high-strength low-alloy steel exposed to an atmosphere of very low water vapour pressure, a new light is shed on the physical description and on the modelling of environmentally-assisted fatigue damage. The salient feature is that physical adsorption of water vapour on freshly cracked surfaces can by itself enhance the damage process at the crack tip and therefore promote the stage II crack propagation in a wide range of alloys. R6sum&-Ce papier traite de la propagation des fissures de fatigue en stade II assist6e par l'environnement et plus pr6cis6ment des interactions entre l'endommagement par fatigue et l'exposition ~ un environnement actif tel que l'air ambiant. Un rapide 6tat de l'art de la compr6hension de ces processus intimement li6s est tout d'abord dress& Sur la base de nouveaux r6sultats obtenus sous tr~s faible pression de vapeur d'eau sur un acier/l haute r6sistance sous tr~s faible pression de vapeur d'eau, un nouvel 6clairage est port6 sur l'interpr6tation physique comme sur la mod61isation. Le fait marquant est que l'adsorption physique peut elle seule acc616rer le processus d'endommagement en pointe de fissure et par cons6quent la propagation en stade II pour une large vari&6 d'alliages m&alliques. 1. INTRODUCTION Metal fatigue has been known for several decades as one of the major causes for failure of engineering structures. Thanks to intensive resea~'ch in the field several parameters have now been identified to play a significant role in the growth of defects, namely: e the intrinsic parameters like alloying, heat treatment, microstructure and elasto-plastic behaviour; • the mechanical factors like the crack geometry, load amplitude and the stress ratio; • and the physico-chemical parameters including the nature, the composition and the temperature of the environment surrounding the crack tip. In particular, ambient air at room temperature is shown to strongly alter the fatigue crack growth resistance, especially in the slow growth rate regime. An illustration is presented on Fig. 1. However a sound physical explanation of this phenomenon is still elusive. Indeed improved understanding is hampered by the complexity of the different processes involved in environmentally enhanced fatigue crack growth. It requires not only the development of an inter-disci- plinary analysis (fracture and fatigue mechanics, metallurgy, surface chemistry, etc .... ) but also the ability to separate the respective roles of the different parameters. For example, in the threshold region, both environment and crack closure strongly affect the propagation but, in addition, they influence each other. As a matter of fact, up to now, only very few authors have attempted such an approach. This paper first presents a literature overview of the main concepts that attempt to account for the detrimental influence of ambient air. The overview will be primarily concerned with long cracks and stage II propagation at (or near) room temperature. An original experimental work on the influence of very low pressure of active species on the fatigue crack propagation is then presented, and a physical description of the involved processes is proposed. Consequences for the analysis of fatigue crack propagation mechanisms are also discussed. 2. LITERATURE BACKGROUND An extensive review of the deleterious influence of environment on the fatigue resistance of metals has been recently presented [1] and it should be referred to for detailed information. The present overview AM 4 3 / ~ D 2931