Features Produced by Crack Interactions

The example is for a rapidly moving crack. For a slow moving crack—one moving at only a small fraction of the velocity of sound in the material—the Wallner lines correspond well with the position of the crack front at the moment the stress 

In some instances, a crack will stop growing and will then be restarted under a different stress system. The result is an abrupt change in direction of crack growth that produces a feature like the Wallner fine. This feature is termed an arrest line. 

Hackle is very useful in fracture analysis because it is always oriented parallel to the direction of crack propagation (perpendicular to the crack front). Moreover, fine hackle features tend to coalesce into coarser features and then eventually disappear as the fracture segments converge into a common plane. That coalescence gives an array of hackle features the appearance of tributaries flowing into a river. Because of this appearance, hackle is often referred to as river lines. As the analogy implies, the direction of crack propagation corresponds to the direction of flow of the river. 

Hackle features are also generated by the interaction of the crack front with pores and inclusions in the material. The features are produced when the crack interacts 

A B moving from ieft to right, (b) Cross sections through the fracture surface are generated aiong the iines indicated. A change in stress 

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