Assembly Stresses: Diesel Engine Flywheel Case History:
One of the most frequently ignored situations that can produce unexpected high stresses occurs during the assembly of components which make up the whole part or structure. It is not unusual for local yielding to occur when bolts are tightened, or when parts are pressed into place. And, although such yielding may not necessary impair the safety of the structure, experience has proven in countless instances that fatigue cracks often develop in regions where alternating service stresses are superimposed upon high assembly stresses. If they exist, assembly stresses become immediately apparent when a PhotoStress coating is applied to the part prior to assembly.
A diesel engine flywheel was failing around the bolt circle. Photo A shows a flywheel coated with PhotoStress plastic, and then bolted to the diesel engine for dynamic testing. When the bolts were tightened, very high stresses appeared which were well above the design limit of the material as shown in Photo B. Superposition of forces due to dynamic testing caused premature fatigue failure. The major problem was thus defined by PhotoStress analysis as one of assembly-induced stresses. Redesign of the flywheel (where it mated to the shaft of the diesel engine) significantly reduced the initial
assembly stresses as shown in Photo C.