PSI - Issue 64

Arij Fawaz et al. / Procedia Structural Integrity 64 (2024) 89–96 Arij FAWAZ/ Structural Integrity Procedia 00 (2019) 000 – 000

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3.4. Mode II fracture toughness calculation (G IIC ) From optical fiber data, the R-curves, which represent the fracture toughness as a function of crack length, can be determined as illustrated in Fig. 8. The R-curves are solely plotted for two data reduction methods, ECM and SBT. These curves exhibit a plateau value for most of the samples.

Fig. 8. Representative R-curves for configurations 1 and 2 using two data reduction methods, ECM and SBT.

The mean values of critical toughness in mode II for samples from both configurations are shown in Table 1, along with the corresponding standard deviation (SD). For configuration 1, nine ELS tests are performed, while for configuration 2, eight tests are carried out.

Table 1. ELS tests’ summary of average GIIC for configurations 1 and 2.

G IIC (N/mm)

ECM

SBT

0.76

0.40

Configuration 1

Average

SD

0.27

0.05

0.80

0.50

Configuration 2

Average

SD

0.13

0.15

4. Discussion The mean values of G IIC calculated using ECM and SBT methods are relatively repeatable and close between configurations 1 and 2 (differences less or equal than 10%), as shown in table 1. For the same sample configuration, the G IIC values differ among the different methods. The G IIC values obtained from the ECM method are significantly different from those obtained from the SBT method. This difference could be attributed to the calculation of the flexural modulus E 1 , particularly for configuration 2 where higher dispersion is observed. Indeed, E 1 is a term in the G IIC formula for the SBT method. The calculation of E 1 is derived from the ISO 15114:2014 standard, which considers several assumptions. The E 1 formula is based on the beam theory and assumes that adhesive thickness is negligible. However, in this study, the adhesive thickness is 1 mm, so it cannot be considered negligible, thereby affecting the values of G IIC derived from the SBT formula. Furthermore, higher dispersion is observed in the configuration 2 samples. Therefore, it is necessary to make modifications not only to the E 1 formula but also to the G IIC formulas for both data reduction methods to account for the unusual geometry of these samples. The results from optical fiber measurements show that the clamping fixture particularly affects crack propagation in the zone closest to the fixed support. Optical fiber allows for tracking the crack propagation and determining its length evolution during the test. An assumption was made to track the movement of the crack tip, which involves considering the maximum strain location in the strain profile as crack tip. However, this assumption may be incorrect