Alternating crack trajectory

Fig. 10. The cros.s-section of the failed specimen with 7 = 35 MPa and alternating crack trajectory. The picture on the right is the scanning electron microscopy (SEM) micrograph of the circled portion of the cross-section.

Fig. 20 shows the failure surfaces of three typical specimens selected from each specimen group of different adherend thickness ratio. The plastic deformation in the adherends introduced before the tests in order to alter the 7-stress was 1.3% for all the three specimens. As a result, the 7-stresses for all the three specimens are positive according the FEA results in Section 2.3.1 and Eq. 7, and their values are shown in Fig. 20. Due to the positive 7-stress level (35 MPa), the crack trajectory in specimen a, in which the fracture is mode I since the specimen is symmetric h/H = 1), is alternating, highly directionally unstable. In specimen b, the 7-stress has increased slightly to 38 MPa due to the low-level fracture mode mixity with G /G = 3% introduced by the asymmetric adherends h/H = 0.75). However, the crack trajectory is predominantly directionally stable except in limited locations where alternating cracks were observed. This effect...

The failure surfaces of the typical specimen selected are shown in Fig. 22. The T-stress in the specimen was 36 MPa and the crack trajectory alternated between the two interfaces, which can be observed in the side-view photograph. To further identify the locus of failure, both the XPS and Auger depth profile analyses were conducted on representative areas of both sides of the specimen as schematically... 

Provided the crack propagation is carried out under near equilibrium conditions, the crack system follows the o-a(c) trajectory given by Eq. 4 from the stable configuration of Eq. 5, (cinit, 0), to the onset of instability characterized by Eq. 6, (cm, Um). Provided, also, that the amplitude terms, x and vj), are invariant during propagation, the extension ratio, Cm/cinit = 4 / 2.52. Under these conditions, an alternative formulation of Eq. 4. 

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