PSI - Issue 83

Sze Pei Tan et al. / Procedia Structural Integrity 83 (2026) 28–40

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mechanics using S-version FEM. Furthermore, based on the fatigue crack propagation testing, the relationship between crack propagation rate da /dN and Δ K can be found. Δ K is the range of stress intensity factors during the fatigue cycle, which is Δ K = K max - K min . The test is performed in two stages, first the pre-cracking and the Paris region where it is conducted using the Instron 8801 servo-hydraulic fatigue testing machine. Fig. 5 (b) shows the crack opening displacement (COD) attached to the tip of the knife edges at the specimen. In addition, Fig. 5 (c) shows the crack growth observance during the fatigue crack propagation testing. Notice the crack initiated at the notch of the CT specimen and the crack propagated at straight line. To perform the experiment, testing parameters is required. Table 2 is presenting the fatigue crack propagation testing control parameters following the experiment conducted by Siddique et al. [23] . The 10 mm initial crack length is already introduced for the specimen following the ASTM E647 standard. However, this 10 mm crack length is not the real crack on the CT specimen. Thus, pre cracking is needed to obtain physical cracks on the CT specimen. The 2 mm pre-crack is introduced at 4 MPa √ m, constant Δ K until the crack length reached 12 mm. The constant Δ K is used to ensure the initiation of the physical crack is even. This is due to the K value does represent the tendency of the crack to grow. Then, to acquire the Paris’ region, the test parameters are changed for the fatigue crack propagation where instead of constant Δ K , constant amplitude loading is used. The stress ratio, R used during the testing is 0.1.

Table 2. Test control parameters for fatigue crack propagation testing.

Pre-cracking

Initial crack length Final crack length Mode: Constant Δ K

10 mm 12 mm

4 MPa √ m

Paris Region

Mode:

Constant Load (K-increasing)

Load

2000 N 50 mm

Final crack length

In addition, the evaluation of the critical stress intensity factors ( K c ) can be acquired from the fractography of the fatigue crack propagation specimen. The K c is a parameter that contains the dimensionless geometrical correction factor, f( α ) where it estimates the occurrence of the fracture. Therefore, not to be confused with the common SIF ( K ) where it represented the crack propagation whilst the K c is defining the fracture. Once the K c value is reached by the K , it will initiate a rapid crack propagation on the structure prior to the fracture. The K c can evaluated using the following Eq. (1) as mentioned by Andreson [24] . (1) where the applied loading on the CT specimen is denoted as P , with B representing the thickness and W the width from the centre of the load point to the end of specimen. Subsequently, f( α ) can be expressed in Eq. (2) where, the α is represented by the ratio of a c /W . The a c as shown in Fig. 5 (d) is defined as the critical crack length. The critical crack length is measured based on the fractography of the specimen after failure. Further discussion on the evaluation of K c based on the fractography will be presented later.

(2)