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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of 91°C and a Young's modulus (E) of 3100 MPa according to its technical data sheet. The adhesive 2, used in configuration 2 samples only, is a methacrylate adhesive (ADEKIT A 310-1) with Tg = 85°C and E ranging between 1200 and 1700 MPa, as specified in its technical datasheet. For all specimens, the adhesive 1 was cured at 80°C for 16 hours, while the adhesive 2 was cured at 25°C for 24 hours.

2.3. ELS test and instrumentation

The ELS test is carried out utilizing a test rig that follows the specifications outlined in the ISO 15114:2014 standard. At the cracked end of the specimen, where an artificial initial crack is located, the load is applied vertically to induce downward movement. At the opposite end, the specimen is clamped with a block that can slide sideways on bearings, while the point where the load is applied remains fixed. Pre-cracking of the specimen is performed in mode II in order to prevent both the influence of the insert on crack initiation and unstable crack propagation. The pre-crack length is denoted “ap”. To promote stability of the crack propagation, a ratio of (a p /L) > 55% is chosen.

Fig. 4. ELS test setup with a clamping fixture and loading for a sample equipped with optical fiber and inclinometer.

The specimen is equipped with a single mode polyimide coated optical fiber glued in the center of the top surface of the upper adherend (Fig. 4). This high spatial resolution continuous optical fiber allows for continuous monitoring of strain along the upper adherend during the test. Along the entire length of the fiber, several distributed optical fiber sensors exist. A tunable frequency laser is employed to assess the backscattered light along the fiber (Sourisseau et al., 2022). The Optical Distributed Sensor Interrogator utilized is a Luna ODiSI-B. Before crack propagation test, the clamping fixture is calibrated following the procedure detailed in the ISO 15114:2014 standard. A graph of C 1/3 versus the free length L is plotted, and a linear regression of this data is conducted as expressed in equation 1 to obtain a value for the slope. 1 3 = × + × | | (1) With C is the compliance of the specimen, Δ clamp the clamp correction, L the free length, and E 1 the flexural modulus of the specimen. From the value of the slope, the flexural modulus of the specimen E 1 can be calculated by using equation 2, where b the sample width and h the adherend height. E 1 = 2 (ℎ×1 ) 3 (2) To compute the fracture energy values in mode II (G IIC ) for ELS tests, various data reduction methods were employed. Three methods are described in the ISO 15114:2014 standard. All of them are based on Linear Elastic Fracture Mechanics (LEFM) and are the following • The Experimental Compliance Method (ECM), based on the cubic correlation between compliance (equation 4), and the measured crack length, a, (equation 3). = −3 2 2 2 (3) = 0 + 3 (4)