PSI - Issue 66

Hendrik Baarssen et al. / Procedia Structural Integrity 66 (2024) 305–312 Author name / Structural Integrity Procedia 00 (2025) 000–000

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The crack length is estimated based on CMOD compliance as indicated in ASTM 647-00. The relation between the crack size and the CMOD compliance is: =1.0010 − 4.6695 +18.460 2 − 236.82 3 +1214.9 4 − 2143.6 5 with = [ 1 ] 1 2 +1 (2) with is the crack length, is the effective width, is the thickness, is the modulus of elasticity and is the CMOD compliance. Equation 2 is valid for crack sizes of 0.20 ≤ ⁄ ≤ 0.975 .

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Figure 2. a) Drawing of the specimen b) Experimental setup

In addition, the crack length is also measured using a Kyowa KV-25b crack gauge. A speckle pattern is applied to the same surface as the crack gauge. A stereoscopic DIC setup consisting of two 12MPx cameras captures the images, which are processed in Instra4D to determine the displacement field. The y-deformation with the rigid body motion removed is post-processed using an algorithm in MATLAB R2023b to determine the crack path and fatigue crack growth, the algorithm is visualized in Figure 3. The first step of the algorithm is to find the plane of the point cloud and re-orient it to align with the XY-plane by a roto-translation, resulting in the matrix of displacement A ij for each image. By considering each vector A j , the displacements are extracted along virtual line gauges with a pitch depending on the cell size. The Chow coefficients are determined as discussed in Section 2. An exponential distribution is fitted over the Chow coefficients obtained along every line gauge in the first 10 images, which are taken before crack initiation. In this study, we present the sensitivity of the threshold by considering a probability of occurrence corresponding to + , +2 , +3 , and +4 . To avoid problems with early outliers in the noise, the threshold has to be exceeded for all successive images in order to A j to be considered cracked. The crack length is determined by the difference between the location of the notch and the location of the furthest cracked path. Finally, the crack length is matched with the number of cycles recorded at the time of the capturing the image to determine the fatigue crack growth. The method is validated by comparing the crack length obtained with the proposed methodology, the CMOD compliance method, and the crack gauge measurement.