Wallner lines

A crack in a ceramic material upon initiation may accelerate and interact with microstructure, stress field and the generated acoustic vibrations. These interactions may lead to some fractographic features such as fracture mirror, hackle or river patterns and Wallner lines. 

Figure 2.33 Wallner lines (between white arrows) on the fracture surface of an aluminum alloy A356-T6 casting. The black arrow indicates the direction of crack propagation. (Reprinted with permission from ASM International. All rights reserved www.asminternational.org)...

Wallner lines are formed when sonic waves generated during fracture interact with principal stress driving the propagating crack front. Wallner lines appear as a series of arc shaped steps as shown in Figure 2.33. 

The specimen failed after K c was reached. These different stages exist in most cases of tensile brittle fracture, but their duration, their relative importance, and additional structural features (Wallner lines in impact, parabolic markings due to the initiation of secondary cracks, striations in fatigue) depend on the stress-time history (19). 

This type of smooth undulating feature on the fracture surface is exemplified in Wallner lines. In Figure 9.3, the Wallner lines are the curved features on the fracture surface. Wallner lines are often created by stress pulses generated as the crack interacts with inclusions in the body or with surface imperfections. (Those stress pulses usually consist of a compression followed by a dilatation so that the general wave form approximates the O stress in Figure 3.2a.)... 

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... 

Figure 9.3 Fracture surface in glass showing Wallner lines (curved features) and twist hackle (linear features perpendicular to the Wallner lines). The fracture origin (not shown) was in the upper left corner of the figure. 

Dwell Mark. A fracture surface marking resembling a pronounced ripple mark OR WALLNER LINE, indicating a pause in crack propagation at that point. 

Ripple Mark. = wallner line (q.v.). Rippled. See dragged. 

Wallner Line. Wavelike lines on the FRACTURE SURFACE (q.v.) Caused by... 

Figure 15-42. Wallner lines and initial fracture sites. (Courtesy of Eastman Chemical Company.)...

Elastic (sonic) waves are also generated during a fracture event, and the locus of intersections of these waves with a propagating crack front gives rise to another type of surface feature known as a Wallner line. Wallner lines are arc shaped, and they provide information regarding stress distributions and directions of crack propagation. 

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