Dimples generally form in ductile materials from loads that exceed the yield stress of the material. In these materials, there are inclusions, weak grain boundaries, particles, and cleavage planes. When the material starts to yield, or plastically deform, it starts to form cracks around these particles, inclusions, or at weak grain boundaries. As the material continues to deform, these cracks form into microvoids in the material. The material around these voids continues to stretch creating a cupping effect.
These cupping microvoids are what give the fracture morphology dimples (Figure 1). When the material starts to yield these microvoids expand laterally until they coalesce with other microvoids and form a central crack in the material.
In a torsional load, the microvoids elongate when the material starts to yield (Figure 2). The matching faces of the failure will show the elongated dimples in opposite directions.
The other way they can form is from tearing as shown in Figure 3. The matching faces of the failure will show the elongated dimples in the same direction.
Figure 1 - Microvoid forming froma tension load around an inclusion and what it looked like in the failed state under a Scanning Electron Microscope
Figure 2 - Microvoid forming from a torsion load around an inclusion and what it looked like in the failed state under a Scanning Electron Microscope
Figure 3 - Microvoid forming from a tearing load around an inclusion and what it looked like in the failed state under a Scanning Electron Microscope
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