numero25

Yu. G. Matvienko et alii, Frattura ed Integrità Strutturale, 25 (2013) 20-26; DOI: 10.3221/IGF-ESIS.25.04 21 M AIN PRINCIPALS AND RELATIONS odified version of the crack compliance method resides in recording interference fringe patterns, which correspond to a difference between two in-plane displacement component fields. Each field is referred to a crack of close but different length. The first exposure is made for a crack of initial length a n-1 (see Fig. 1). Then initial crack length is increased by small increment Δa n so that new total crack length becomes equal to a n = a n-1 +Δa n and the second exposure is performed. Required interference fringe patterns are visualized by numerical subtraction of two images recorded for two cracks [11]. Two interferograms, which are obtained by this way for thin plate with through edge crack of mode I, are shown in Fig. 2. Positive direction of x -axis in Fig. 1 and Fig. 2 coincides with a direction of the crack propagation. Figure 1 : Polar co-ordinate system related to a crack tip and the notation adopted. (a) (b) Figure 2 : Specimen #3V. Interference fringe pattern obtained in terms of in-plane displacement component u (a) and v (b) . Initial crack length a 4 = 7.18 mm with increment Δa 5 = 1.81 mm Procedure of deriving required fracture mechanics parameters from interference fringe patterns is based on Williams’ formulation [12]. In-plane displacement field near a crack tip is expressed as an infinite series for each in-plane displacement component. When x -direction coincides with the crack line these series for mode I condition have the following form: M