International Journal of Fracture 47: R3-R10, 1991. R3 © 1991 Kluwer Academic Publishers. Printed in the Netherlands. A FATIGUE CRACK CLOSURE MODEL AND MEASUREMENT TECHNIQUE Majid Mirzaei and James W. Provan Mechanical Engineering Department McGill University, Montreal, Quebec, Canada H3A 2K6 This note describes the present status of a simple closure measurement technique, based upon a new analytic interpretation, and is initially applied to aluminum and titanium alloys. The proposeed procedure implements only one experimentally determined parameter for the assessment of the closure load-dis- placement characteristics. The procedure outlined also removes some of the current ambiguities related to the specimen dimensions and geometry. Practical fatigue crack growth prediction models consider the stress intensity factor range AK = K - Kr~ as a field parameter that correlates crack growth rate data. Furthermore, Elber [1], discovered that fatigue cracks remain closed during a significant portion of each loading cycle. The occurrence of crack closure, which can be due to a variety of mechanisms [2,3], significantly affects the crack driving force and plays a crucial role in fatigue crack growth or arrest. The quantitative knowledge of the crack opening stress level, S , is required to define . . . . ~ . AKfe = K_, - K which is an appropriate field parameter for correlating crack op. . . . . growth rates under both constant-amphtude and varlable-amphtude loading conditions. However, pronounced ambiguities exist concerning the interpretation of the nature and distribution of the residual stress patterns on the crack wake and behind the crack tip [4]. While a large number of techniques have been proposed for closure measurement [5], the reported results are not consistent and there is no standard technique which can be agreed upon. Among the different closure measurement techniques the CMOD compliance method appears to be the most popular. The objectives of the investigation partially reported in this note may be summarized as: i) the introduction of a new analytic approach to the closure phenomenon which accounts for specimen size and geometry; ii) the introduction of a modified CMOD-compliance closure load measurement technique based upon the proposed model; and iii) the verification of the new technique by comparing the results with those obtained using the available techniques Based on the general interpretation depicted in Fig. 1, the development of crack closure in a C(T) specimen is considered to be the manifestation of the interaction between the bulk of the specimen and the residual material which builds up within the crack wake as it penetrates through the specimen*. The closure model introduced in this note conjectures the existence of a hypothetical *The residual material represents each or a combination of plastic stretch, oxide layer, and asperity interactions. Int Journ of Fracture 47 (1991)