PSI - Issue 2_B

Chretien Gaëlle et al. / Procedia Structural Integrity 2 (2016) 950–957

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Gaëlle Chretien et al. / Structural Integrity Procedia 00 (2016) 000–000

δ. This displacement is measured by a strain gage slicked on the back face of the specimen for ambient tests and by a capacitive sensor placed on the front face thanks to the assembly presented on figure 5 for tests at 400°C.

Fig. 3. Procedure of successive threshold evaluations at increasing short crack length illustrated with two thresholds realized on specimen 1.

Fig. 4. Schematic plot of the complaisance method to measure the crack closure.

The specimen compliance χ = ∂P/∂δ is extracted from the linear part of the P – δ diagram. The lower end of the linear part of this P – δ diagram corresponds to the opening load P op . The identification of P op can be optimized by the differential Kikukawa compliance diagram P – δ’ with: χ P δ 'δ - � ( 1) The value of Pop is more easily determined. A numerical acquisition for P and δ is performed through a computer interface. The data processing is performed as detailed by Kim by calculating the third derivative of the differential displacement δ’. The derivatives are calculated using a linear least squares fit and crack opening corresponds to the point where the minimum of the third derivative is reached as shown on figures 4 and 5. This method is specially adapted for short crack closure measurements as shown on figure 5.

Fig. 5. Illustration of the use of Kim’s method to determine crack opening of a short crack of 230 µm.