STATISTICAL EVALUATION OF VARIATION IN ENDURANCE LIMIT AMONG SEVERAL HEATS OF PROPELLER TYPE STEEL

摘要

Samples from three heats of SAE 4330, five heats of SAE 4340, and four heats of SAE 4350 aircraft quality steel were subjected to Prot-type rotating bending endurance tests. All specimens were heat treated to approximately Rockwell C 43 hardness. Separate estimates of the arithmetic average and the standard deviation of the endurance limit were determined for each heat from specimens selected parallel to the direction of mill rolling and from specimens selected perpendicular to that direction。 An evaluation was made of the extent of agreement between the data obtained and a normal frequency distribution of the endurance limits of individual specimens. On the basis of normal distribution, confidence limits of 95% probability were established for each estimate of average endurance limit. The effects of various parameters on the average and on the standard deviation of specimen parameters on the average and on the standard deviation of various parameters on the average and on the standard deviation of specimen endurance limits were studied. These parameters included the carbon strength, type of fatigue fracture, and origin of heat. Additional endurance tests were performed to compares the results of Prot-type tests with conventional endurance tests.