MATERIALS CHARACTERIZATION 43:21–26 (1999) © Elsevier Science Inc., 1999. All rights reserved. 1044-5803/99/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S1044-5803(98)00055-2 21 Boundary Identification Criteria and Geometry Analysis of Al 7050 Stretch Zone from Elevation Profiles Luis Rogerio de O. Hein*, Jean-Jacques Ammann † , and Ana Maria M. Nazar † *Departamento de Materiais e Tecnologia, Faculdade de Engenharia de Guaratinguetá, Universidade Estadual Paulista, Guaratinguetá, P.O. Box 205, Sao Paolo, Brazil 12500-000; and † Departamento de Engenharia de Materiais, Faculdade de Engenharia Mecânica, Universidade Estadual de Campinas, Campinas, P.O. Box 6122, Sao Paolo, Brazil 13083-970 This article describes the development of a method for analysis of the shape of the stretch zone surface based on parallax measurement theory and using digital image processing techniques. Accurate criteria for the definition of the boundaries of the stretch zone are es- tablished from profiles of fracture surfaces obtained from crack tip opening displacement tests on Al-7050 alloy samples. The elevation profiles behavior analysis is based on stretch zone width and height parameters. It is concluded that the geometry of the stretch zone profiles under plane strain conditions can be described by a semi-parabolic relationship. © Elsevier Science Inc., 1999. All rights reserved. INTRODUCTION A stretch (or stretched) zone is formed un- der a critical state provided by stable crack propagation in the fracture process [1]. As a fractographic description of blunting be- fore unstable crack propagation, the stretch zone can be related to the local fracture toughness [2]. This region, located between the stable crack propagation zone and the rupture zone, can be recognized by the presence of a stretching mechanism. How- ever, it is very difficult to define the refer- ences for its limits and, especially, to obtain representative measurements of its geomet- rical parameters [3]. A recent round robin of the European Group on Fracture (EGF) on stretch zone width determination found difficulties nor- malizing practical criteria for these bound- ary definitions [4]. One solution to the problem suggests that a complete analysis of the region profiles is needed to address the topographic characteristics. Thus the goals in the present study were to try to de- fine the representative criteria for the stretch zone boundaries and to describe the profile geometry of this crack blunting re- gion. To this end, surface elevation mea- surements were determined by an image processing technique based on the parallax method which provides high spatial reso- lution for profiles. EXPERIMENTAL PROCEDURE Crack-tip opening displacement (CTOD) specimens, both transversely- and longitu- dinally-oriented, were cut from the same Al 7050 T73651 plate in conformance with ASTM standard E 1290-93 [5]. The CTOD tests were applied to 25mm thick speci- mens using a MTS 810 servo-hydraulic sys- tem. Stereo pairs from the crack surfaces were obtained with a JEOL JXA-840A scan- ning electron microscope. The positions for the stretch zone observations were selected