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(4) Local Stress and Strain Intensification
The hoop and bending stresses acting on the cracked wall produce a plastic zone surrounded by an intense stress field at the crack tip. The COD (crack opening displacement) is a measure of the plastic strains within the zone and this depends on the flow strength of the pipe and the stress field intensity.
(5) Plastic Deformation
The plastic deformation of steels is a rate-sensitive process. The flow strength of material within the plastic zone, and consequently the COD, therefore depend on the velocity of the crack.
(6) Ductile Cracking
The extension of a ductile crack by either the shear or the fibrous mode is associated with a critical COD, i.e., a critical crack opening displacement and Kc, a critical stress field intensity, which correspond with a critical level of plastic strain in the locale of the crack.
These 6 processes are discussed and formulated in the following sections. The predictions of the models are compared in Figures with crack speed measurements on 100％ shear cracks performed by Duffy and coworkers. The comparisons illustrate that the models have many features in common with full-scale pipe failures. The change in crack speed accompanying the change in fracture toughness associated with the shear-to-cleavage transition is similar. At the same time, it should be clear that the descriptions used are all more or less approximate, and while some of the errors may cancel out, discrepancies remain. For one thing, it seems likely that the rate sensitivity of the flow strength is overestimated by the linear expression used. There is some evidence that the stress intensification produced by long cracks is also overestimated. The effects of higher strength and higher stress thus tend to cancel. Several discrepancies are revealed by the comparisons with full-scale test data. The calculated influence of pipe radius does not seem to match the existing test experience and neither of the two calculated trends with thickness appear to be good representations of the data. The fact that Models 2J and 3J give contradictory results is a sign that the exact forms of the expressions for B and Cs are quite critical. In addition to these problems, there are two other shortcomings that stand in the way of calculating crack arrest: (1) the models only provide for the axial crack path and (2) the description of Cs, does not take into account the rate at which pressure is lost through the ruptured wall. Finally, it should be noted that while inertia forces have been neglected, these and other dynamic effects need to be considered in further treatments of the problem.
受压管壁的不稳定裂纹的速度和路径影响着裂缝的长度。为了了解更多类似问题，麦克卢尔，达菲和Eiber对管道的大小和钢材根据管道研究委员会赞助的天然气输送管道类似的全面测试。他们的研究表明，100％的受剪裂缝传播速度的范围从400-800 FPS到800-1200 FPS为了更高的强度淬火回火等级。最初，类似的失败遵循的直链，轴向路径，但转向成一个螺旋形轨迹这大大降低速度的轴向和实际路径分量和管的损坏的长度。令人惊讶的是，该行的压力（或环向应力）似乎对速度和轨迹没有明显可见的影响;既不是分配给回填或管道的几何形状（半径壁厚之比），论文网尽管后者进行了检查仅在一个有限的方式覆盖仅在为气体输送管线一般使用直径和壁厚有系统的效果。而全面的测试，暴露在管道裂纹扩展的主要特点，他们不这样做，本身，解释所观察到的速度和轨迹。在这个方向至今只取得了有限的进展。McClure和同事已经展现了已解析的某一类的弹性波，可在气缸传播，具有速度和轨迹类似的切割裂纹的弹性波。支配特定波的选择原则并非如此。这似乎是不可能的弹性波将与慢行剪切裂缝强烈的相互作用，并没有其他的解释提出为控制剪切裂缝的机制。然而，该剪切模式是重要的技术，因为剪切裂缝已观察到不稳定传播。