The theory of effective elastic properties of cracked solids is used to predict the degradation of mechanical properties of a stainless steel induced by stress corrosion growing cracks. The material is considered as an evolutionary composite with a cracked external volume (damaged elastic behaviour) and a safe internal volume (elastoplastic behaviour). From continuous surface observation of cracks by in situ videomicroscopy, the influence of interactions between multiple cracks is particularly studied. We show that coalescence of cracks can be neglected and that amplification and shielding effects counterbalance each other. This experimental study allows to choose the most suitable method to determine the effective elastic properties. Then, we propose a modelling to represent the overall mechanical behaviour of a solid with many surface cracks. Simulated and experimental slow strain rate curves are compared. (C) 2005 Springer Science + Business Media, Inc.