Fatigue Study

  
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Fatigue Finite Element Analysis:
A large, 18 ton, ribbed, rotating drum (110 rpm) used in the bulk-plastics, processing industry was experiencing what was thought to be fatigue related cracking at certain fastener locations along the longitudinal bars. The project objective was to determine the cyclic load mechanism and suggest fastener design changes that would ameliorate the cyclic loading effect.

Special features:
3D model setup using 3D plates and bricks, spherical bearing constraints at the shaft drive and driven locations, and rotational and gravity loading effects.

Output:
Modal frequency and mode shapes with displacements, gravity effected longitudinal displacement of fastener components, and cyclic stresses impacting fatigue susceptibility assessment.

Comments:
Several load scenarios were analyzed in an effort to find the "smoking gun," that cyclic load capable of generating fatigue related cracking (those analyses are not included in this summary). It was determined that the cyclic load was gravity induced. The machine drum would sag due to gravity. As it rotated, the longitudinal bars would alternately go into tension and compression, generating a sufficient fatigue related load. The graphics above (click on the thumbnails to access larger graphics) summarize some of the setup and findings. An animation of a 1/4-symmetry sub-model of the fastener hole in a section of the longitudinal bar is provided that indicates the greatest cyclic stresses, coupled with a stress concentration factor due to the fastener thread-form (not explicitly modeled), did indeed correlate to known locations of cracking. The client subsequently implemented a suggested fastener and hole design change that lowered the cyclic stresses substantially.