PSI - Issue 71

Karthik K R. et al. / Procedia Structural Integrity 71 (2025) 210–217

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u = Displacement vector Variation of J with respect to crack extension is shown in Figure 4. Initially, upon loading, the crack tip will start blunting because of the plastic deformation. After a certain load, a small crack will start to initiate from the blunted crack as shown . Further, there will be a large increase in crack size with a small increase in J, this is caused by unstable crack growth. Up to this state, crack growth will be stable. J Ic is defined as the J at which initiation of unstable crack growth happens.

Figure 4: J R (resistance) curve for a ductile material (Anderson (2017))

J Ic can be evaluated from the ASTM E1820 standard (2020). It is evaluated for through the thickness crack (Broek (1984)). For surface cracks, critical J can be evaluated from surface crack tension tests with the help of finite element (FE) simulation. NASA has conducted surface crack tension (SCT) studies on AA2219 T87 parent and TIG weld metals at room temperature. Welding was accomplished using a direct current straight polarity (DCSP) gas tungsten arc (GTA) welding process. During the course of the experimental program, three distinctly different types of tests were conducted (load/unload, fracture, cyclic). The load/unload (LUL) tests consisted of monotonically loading to a predetermined load in approximately one minute and then unloading rapidly. In this test, the specimen undergoes only a single cycle of loading. The hold time at maximum load was essentially zero and the unloading time was generally less than 15 sec. Fracture tests consisted of monotonically loading a specimen until it had fractured. The loading rate for the fracture tests was programmed so that the fracture would occur in one to two minutes. Fracture tests are monitored with pressure cups to find out the load at which the surface crack becomes a through crack. The failure mode for most of the conditions tested was break-through before failure (leak-before-break). The limited number of conditions for which the failure mode was anticipated to be fractured was confined to the thicker base metal specimens having the lowest aspect ratio (a/2c) flaws. Failed specimens were inspected, and the initial crack size as well as direction of crack growth were reported. Depending upon the crack aspect ratio, the growth behaviour varies as shown in Figure 5. Crack initiation happened at the deepest point for cracks having aspect ratio less than 0.3 and for others near to the surface.

Figure 5: Growth on loading for various flaw shapes (Finger (1976)).