A forged aluminum alloy 2014-T6 catapult-hook attachment fitting (anodized by the chromic acid process to protect it from corrosion) from a naval aircraft broke in service. Spectrographic analysis, visual examination, microscopic examination, and tensile analysis showed minute cracks on the inside surface of a bearing hole, and small areas of pitting corrosion were visible on the exterior surface of the fitting. The analysis also revealed a small number of rosettes, suggestive of eutectic melting, in an otherwise normal structure. These examinations and analyses support the conclusion that the presence of chromic acid stain on the fracture surface proved that the forging had cracked before anodizing. This suggest that the crack initiated during straightening, either after machining or after heat treatment. The structure and composition of the alloy appear to have been acceptable. Ductility was acceptable so rosettes found in the microstructure are believed to have been nondamaging. Had they contributed to the failure, the ductility would have been very low. The recommendations included inspection for cracks and revising the manufacturing process to include a fluorescent liquid-penetrant inspection before anodizing, because chromic acid destroys the penetrant. This inspection would reduce the possibility of cracked parts being used in service.

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.

This report covers case histories of failures in fixed-wing light airplane and helicopter components. In a 2025-T6 or 2219 aluminum alloy propeller blade that failed near the tip, cracks started on the leading edge at surface damage in the critical area-the zone between 4 and 10 in. from the tip of the blade. Incorrect dressing and inadequate pre-flight inspection were the two main causes. Two other types of propeller blade fatigue failures resulted mainly from propeller straightening operations, usually performed after previous blade bending damage. To eliminate blade tip failures, all surface-damaged material should be removed and polished smooth before further flight. The blade should be correctly dressed. Also, the tachometer should be calibrated to ensure the engine/propeller combination is not operated in the critical speed range at normal cruising speeds.