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Is it a casting crack or is it a brittle failure? Grain growth pattern versus crack path, which came first?
The hammerhead to the left broke off after only one week of operation. It came out of a rock crusher. This type of failure is very rare within this family of hammerheads. If it were a brittle failure it would have had to have hit something very hard. This happens all of the time when metal roofing bolts inadvertently come up out of the mine along with the rock and slip past the magnet that usually catches them and separates them out from the rock. However, there is usually a large dent in the hammer that indicates where the roofing bolt, or other non crushable object, hit the hammer. Roofing bolt impacts never take out a hammer. If the hammer did indeed hit something monumental, the impact would be from one side, and all of the chevron marks would lead to that initiation point. The somewhat confusing information on the fracture surface shows a uniform inward crack growth all of the way around the stem of the hammer, and toward the center, instead of from just the point of impact. A drawing of the crack growth pattern is shown in the upper right hand side of the picture to the right. The chevron marks start from the outside and are perpendicular to the outside surface. |
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What we are really seeing here is the way in which grains grow as they cool. The grains nucleate along the outer edge where it is the coldest and grow inwards as the core temperature cools down. The shiny portion of surface that runs along the top (in the picture above right) is the region that did not crack through upon cooling and held the hammer together for one weeks service. In the picture to the right, you can see faint parallel lines that are close to each other in the shiny portion, and are illustrated in the drawing. These marks are called beach marks and delineate where the crack stopped in its travel for a period of time before continuing to the next point where it stopped. Close examination will show that they started from inside and traveled outwards. The very outer edge on the right side of the hammer in the picture to the right is rough. This is the final fracture point and there is evidence of yielding. It isn't quite clear why the casting cracked upon cooling, but it more than likely happened. Notice the direction of the chevron marks in the picture to the right and far right. They start along the outer edge and travel inwards where they meet along the lines indicated in the small drawing on the right side of the picture to the lower left.
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Chevron marks are perpendicular to, and always follow, the crack(s) path. It's actually the location where two parallel running cracks join up haphazardly. The orientation of the chevron marks in both pictures above and the pictures to the left and right begin at the surface and travel inwards perpendicular toward the center. When they all meet they form a boundary line that looks like the drawing in the right hand picture. An impact would have never made a pattern such as this. All of the chevron marks would have pointed to just one side, one location.
Revisiting this failure several months later... It was a casting crack. The foundry apologized for the slip in their inspection procedure. |
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