Using Proven Aerospace Technology to Retard Crack Growth for Kiln Applications, 2017PEERS

Metals subjected to cyclic loads experience fatigue cracking.  In recovery operations, cracks in metal kiln shells are a regular occurrence and shorten the service life of the kiln shell.  A common repair is drilling a crack arrest hole (CAH) or stop-drill. The stop-drill is placed on the visible crack to reduce the stress concentration at the crack tip.  This is a temporary repair and with continued cyclic loading, the crack will restart.  A proven aerospace technology for cyclic fatigue durability and damage tolerance, cold expansion, has been adapted for crack repair for other industries.  Cold expansion induces a compressive residual hoop stress around the hole to shield it from the effects of cyclic tensile loads applied in-service.  A clearance fit bushing is cold expanded into the CAH at a high interference level.  This process, known as StopCrackEX, retards or totally arrests cracks in structural steel.  This paper will review the cold expansion processes, describe the residual stresses induced, and show how they enhance the fatigue life of holes.  It will present independent comparative test data and test results for cracks coming from welds.  It describes key parameters, the current kit contents, and process steps.  It concludes with tooling updates for kiln applications and describes a representative kiln repair process. 

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Author: Diana E. Echols and Joy Ransom