A Re-Examination of Failure Analysis and Root Cause Determination

31ResultsThe simulation found that preloading (torquing, putting the screws on tightly) the capscrews increased the stress resistance of the system and consequently, the service lifeof the system would lengthen. The table on the following page shows the stressresistance of the 8 cap screws assuming preloading conditions. The best result waswith 90 ksi preloading. The difference can be explained by alternating stresses that arepresent with and without preloading. Sufficient contact pressure prevents alternatingstress and thus, a higher preload can withstand greater stress.IMPORTANT FAILURE ANALYSIS PRINCIPLES REPRESENTEDIN THIS CASE HISTORYFinite element analysis showed that a very simple adjustment (tightening the screws inthe assembly) would solve a vexing problem without a major redesign of the system.Case History #6. Aircraft Engine FailureThe fan section of a turbine aircraft engine contains fanblade separators known as annulus fillers, or simply asspacers. The material specification of an annulus filler isusually 7075 T56 aluminum alloy with a painted coating.The highlighted area in the drawing at left represents the