PSI - Issue 75

Martin Edgren et al. / Procedia Structural Integrity 75 (2025) 555–563 Martin Edgren et Al. / Structural Integrity Procedia (2025)

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2.4 Strain monitoring

There are several non-destructive testing methods available to indicate surface breaking cracks, which of some (magnetic testing and penetrant testing) was used in the study by Edgren et al [6]. However, these techniques are not suitable for crack depths characterisation. For crack depth characterisation, a volumetric method like ultrasonic testing (UT) can be used. Baby et al [14] and Edgren et al [15] showed that UT time-of-flight diffraction (TOFD) can be used for accurate sizing of surface breaking cracks. However, as concluded by Jie et al [16] the effectiveness of ultrasonic time-of-flight diffraction (TOFD) in detecting and quantifying near-surface defects is limited as a result of the presence of a dead zone. Furthermore, the accuracy of the estimated crack is dependent of the thickness of test specimen [17]. In this study the cracks are too shallow, and the test specimen have an unfavourable geometry for the use of TOFD. Moreover, the scanning system utilized in this study was designed for notch evaluation and, therefore, is not configured to perform crack depth characterization. As a result, it cannot be employed for crack depth estimation.

Figure 3 a) Crack depth 1.2 mm at 20% strain range drop, b) crack depth 1.5 mm at 30% strain range drop.

The use of strain monitoring has been implanted in several studies [18], [19], [20] to monitor changes in the stiffness related to crack growth and crack depth. Strain monitoring is particularly advantageous in scenarios involving fatigue crack growth monitoring, where continuous measurement can provide real-time data on crack propagation without requiring operational interruptions. In this study strain monitoring has been implemented and a stop criterion based on a strain range drop value has been adopted by aid of FEA and qualitative tests. Monitoring was conducted using strain gauges with a gauge length of 5 mm and width of 1 mm, temperature-compensated for steel (11.7 × 10 6 /°C). Data acquisition and control were performed using a commercial data acquisition unit in combination with a relay control box. Figure 3 shows crack depths at two different strain range drop values for the in-plane gusset plate test specimens. The strain range drop was monitored without the initial mild notch to get a qualitative number on the relation between strain range drop and crack depth (strain gauge placement differed to analysed position in this study). It is observed that most of the fatigue life for the specimens used for qualitative tests is spent in the crack initiation phase, as projected in the study by Y. Ono et al [21]. Strain gauge positions were measured and used as inputs for a parameterized FEA in a Design of Experiments (DoE). The measurement results are shown in Table 2. Table 2. Recorded strain gauge distances and offsets from notch. Specimen Distance x Side 1 (mm) Offset y Side 1 (mm) Distance x Side 2 (mm) Offset y Side 2 (mm) OB1 2.05 0.41 1.61 0.01 OB2 1.90 0.21 1.72 0.13 OB3 1.87 0.01 1.90 0.13 OB4 1.64 0.33 1.68 0.06 OB6 1.60 0.18 1.80 0.20 2.3 Strain gauge positions