fatigue mechanisms and highlight mitigation strategies that could have reduced the likelihood of failure. Figure 2 shows a crack propagating through a mill shell end flange at the connection between the shell and the mill head. The crack initiates immediately adjacent to the head radial flange split. This is relevant as the head flange creates a stiffness discontinuity, which would not have been captured in an axisymmetric finite element analysis. The left side of the figure shows stress behavior accurately captured in a detailed finite element model. Additional contributors could include the location of the intraflange radial weld seams, and the condition of the flange connection (including fastener preload, flange opening and connection utilization). Figure 2 – Mill shell crack at circumferential flange, adjacent to radial head flange This failure has been seen in several very large ball mills around the world, including some 28ft diameter ball mills. Figure 3. In this case the crack propagates through the cast mill head, with its initiation originating at the termination of the head radial flange near the outer diameter of the trunnion flange. As with the shell failure, this critical stress location would not have been identified if the radial-flange stress concentration had been omitted from the finite element model, significantly increasing the likelihood of fatigue failure. Additional contributing factors may include poor flange performance and casting quality issues. The risk of casting quality issues can be mitigated through appropriate material selection, comprehensive non-destructive testing of all castings, and well-defined NDT acceptance criteria.
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