Reliability Engineering Snapshot TM

llustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non-destructive Evaluation




Machine Design - Case No. 58: ASM Tech Paper - Challenging the Thermal Stress Theory

Lifter Tip StressThe FE thermal model showed thermal stresses were present, although the stress pattern was different from that initially assumed. Instead of being along the entire length of the lifter, the highest stresses were located at the ends of the lifters (Fig. 1, left). This pattern was consistent throughout the entire length of the rotary dryer. When the model's deflection mode was turned on, it revealed that the first three rows of lifters closest to the hot discharge end deformed. The tips of the lifters were hotter than the shell and were expanding more than at the lifter/shell junction. This difference caused the lifters to arch (Fig. 2, lower left). Subsequently, the ends of the lifters were digging into the shell in a high compressive three-dimensional state of stress (Fig. 1, left). The shell distortion was verified out in the field (Fig. 3, lower right). The troubling part was that of the first four rows of lifters that were thermally stressed, only one end of the same row of lifters was severely cracking, and those cracks lead into the shell. In terms of numbers, that meant only 10 locations out of 80 were cracking. Those weren't good odds for a thermal stress theory. What was so different about this end of lifters?
FE Thermal Model - Shell Deflection Exxaggerated Shell Distortion Predicted by FE Thermal Model


FE Thermal Model of 3D  State of StressA higher resolution FE model was developed that included just the ends of two lifters (Fig. 4, left). The end of one lifter was from row "A" (the problem row) while the other lifter was from the next row down, row "B" (no cracking). This model showed that there was a difference in the stress level between the two rows. Results indicated that the stresses at row "B" were almost half as much as those at row "A". How could there be a difference in "thermal" stress levels when the ends of each row were right next to each other and on the same circumferential line? The only difference, and a big one, was that row "A" was welded to a thick reinforcing ring underneath the hot discharge end tire. The lifter jutted out past the reinforcing ring by 18". The lifter couldn't bow uniformly. Therefore, the strain went in the path of least resistance, or out into the considerably thinner shell region. This reaction effected only this row of lifters. Row "B" lifters were not tied to the reinforcing ring; they were simply welded to the thinner, and comparatively weaker, shell.


Shell Distortion at Hot-end Lifter (crack highlighted)However, there was still a problem. The FE thermal model showed that the lifter tips were in a high state of compression, a condition in which cracks do not propagate. Something other than thermal stresses had to be opening the cracks up. There was an explanation. It had to do with what was known in pressure vessel design as a "self-limiting" load. In its simplest terms, such a load when first applied will cause a minute degree of plastic deformation, after which, when the load is removed there are residual stresses that remain. As long as subsequent loads are equal to, or less than, the original applied load, the material response will be elastic (Ref 4). In other words, the thermal load caused the lifters to bow, which in turn distorted the shell in compression (Figs. 3 upper right, & 5 above left). This in turn created residual tensile loads in the lifters. Depending upon whether the lifter had a relief groove cut in it or not, the crack either started at the root of the relief groove, or at the end of the lifter (Fig. 5, upper left). The thermal expansion stresses in the lifters were neutralized as evidenced by the yield creep that could be seen (Fig. 3,upper right, & 5 above left). However, the residual stresses had already been established prior to cutting the stress relief grooves.

Hi resolution FE model showing Tensile Stress Pulling on Weldment

The thermal model was turned off with just the mechanical and gravitational loads applied. A closer look at lifter "B" in the 3 o'clock position (loaded) revealed that the lifter was under a tensile stress that was perpendicular to the longitudinal axis of the weld (Fig. 6, left). The highest stress was at the tip. This tensile load opened the cracks. Microstructural examination, field inspections, and the FE thermal and mechanical models supported this failure model.


This ends the Finite Element portion of the paper.....

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