The conditions determined above give us insight into when dynamic effects can significantly alter the mechanisms of deformation and the resultant bridging phenomena. For instance, if the crack tip velocity exceeds the condition prescribed in Eqn. 8a, oscillatory displacement fields will be introduced in the wake of the crack, and these multiple oscillations could lead to crack face interpenetration. When such oscillations are present, the mechanics of bridging and the efficacy of through thickness bridging ligaments on the energetics of crack growth will be considerably altered. For example, stick-slip propagation modes would appear to be possible, as contacting fracture surfaces bounce. The complex details of such a possibility will be considered elsewhere. Here, we model the arms of the DCB specimen as an EB beam and study propagation characteristics up to the point of fracture surface contact, which is a simpler problem. (Constants b, b and d are given in Eqn.6b) Wedge-Loaded Double Cantilever Beam Figure 1: Schematic of through thickness reinforced DCB specimen loaded with a flying wedge The double cantilever beam (DCB) specimen loaded dynamically by a flying wedge, of constant velocity v, offers a relatively simple experimental approach to studying the mode I dynamic delamination problem (Figure 1). The test is especially attractive for studying the bridging effects supplied by through-thickness reinforcement (e.g., stitches or rods) in laminates. In figure 1, 2a is the wedge angle, l is the distance between the wedge and the crack tip and a0 is the length of the bridging zone. In non-dimensional form, l h L º , and 0 0a h A º . The role of the bridging on the crack energy release rate is determined in this section. We assume that the crack propagates under steady state conditions and confirm the possibility of steady state propagation by finding consistent solutions. Further, we assume the bridging zone size is invariant and translates with the crack tip. For the unbridged portion, the deflection profile ( u uw hW º ) is obtained by setting b = d = 0. Therefore: 0 2 2 2 4 4 = ¶ ¶ + ¶ ¶ x b x u u W W for ) ( 0A L- £ £- x (9a) 2a a0 a v 2h l Bridging zone Delamination crack tip
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