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The paper describes the findings from a damaged propeller blade made from Mn-Ni-Al-bronze, commercially known as Superston 70 (ABS Type 5). The blade had broken at the 0.65 pitch radius location, and the fracture occurred in a brittle mode. The findings reported here point to two potential contributors to the propeller blade failure, viz., the presence of casting flaws at the low pressure side of the propeller blade and service stresses at this surface that reached approximately 400 MPa. This stress value exceeded the yield strength at the corresponding location of the unbroken blade by approximately 40%.

An aluminum alloy propeller blade that had been cold straightened to correct deformation incurred in service fractured soon after being returned to service. Visual examination revealed that crack initiation occurred at the top surface in an area containing numerous surface pits. Macroscopic appearance of the surface was of brittle fracture. X-ray stress analysis did not detect any residual stress in the top surface of the propeller blade adjacent to the fracture. However, a spanwise tensile stress of approximately 51 MPa (7.4 ksi) was indicated in the same surface of the unfailed mating blade at the location of the initial bend. Evidence found supports the conclusions that the residual stress probably originated with straightening, and the apparent absence of stress in the fractured blade was the result of relaxation through fracture. Because no prior crack damage could be attributed to the initial deformation or to straightening, rapid fracture may have been induced by residual stresses contributing to the normal spectrum of cyclic stresses. Recommendations included stress-relief annealing after cold straightening, refinishing of the surface, thus reducing fracturing of propeller blades that were cold straightened to correct deformation experienced in service.

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