A metal fan hub was failing in service where the hub shaft support was welded to the flange. Analytical studies predicted low stress levels during the dynamic loading sequence. Strain gage measurements near the weldment supported this prediction. Several of the fan hubs were fabricated for test purposes, and PhotoStress coatings were contoured and bonded over the surface area. After application of the coating, the hubs were sawed through, releasing the internal forces (residual stresses) developed by nonuniform heating during the welding process. The fringe patterns in the PhotoStress coating shown in Photo A revealed locked-in residual stresses, which were of very high magnitude in the welded area. The modest cyclic stresses, superimposed upon the high residual mean stresses, were sufficient to produce field failures. Subsequent test samples were stress-relieved after fabrication, and PhotoStress analysis of the stress-relieved hub showed no evidence of residual stress after cutting, as shown in Photo B.

Back to PhotoStress® Plus seminars