Torsional Fatigue Failure of Crane Shaft
In a shaft subjected to reversed torsional stresses, failure resulted from the gradual development of fatigue cracks from opposite sides of the shaft. These broke out from origins located adjacent to the fillets at the start of the square section. The remaining uncracked material which fractured at the time of the mishap was in the form of a narrow strip, situated slightly to one side of the center of the shaft. The material was a mild steel in the normalized or annealed condition, having a carbon content of approximately 0.3%. The cracking was characteristic of that resulting from torsional fatigue. Because it occurred on two different planes at 45 deg to the axis of the shaft it was due to reversals of torsional stress rather than fluctuations of unidirectional torque. Following this failure, the shafts of six other similar cranes were tested ultrasonically. Cracks to varying degree were found in all the shafts. Timely replacement was possible and the likelihood of serious accidents removed.