The case and stiffener of an inner-combustion-chamber case assembly failed by completely fracturing circumferentially around the edge of a groove arc weld joining the case and stiffener to the flange. The assembly consisted of a cylindrical stiffener inserted into a cylindrical case that were both welded to a flange. The case, stiffener, flange, and weld deposit were all of nickel-base alloy 718. It was observed that a manual arc weld repair had been made along almost the entire circumference of the original weld. Investigation (visual inspection, 0.5x macrographs, and 10x etched with 2% chromic acid plus HCl views) supported the conclusions that failure was by fatigue from multiple origins caused by welding defects. Ultimate failure was by tensile overload of the sections partly separated by the fatigue cracks. Recommendations included correct fit-up of the case, stiffener, and flange and more skillful welding techniques to avoid undercutting and unfused interfaces.

One of the connecting rods of a vertical, four-cylinder engine with a cylinder diameter of 5 in. failed by fatigue cracking just below the gudgeon-pin boss. Failure took place in line with the lower edge of a deposit of weld metal. The fracture surface was smooth, conchoidal, and characteristic of that resulting from fatigue. The origin of the major crack was associated with a crescent-shaped area immediately below the weld deposit. This showed brittle fracture characteristics and appeared to be an initial crack that occurred at the time of welding and from which the fatigue crack subsequently developed. The rod was made from a medium carbon or low-alloy steel in the hardened and fully tempered condition. Evidence indicated that, following modification to the oil feed system, the rod that broke was returned to service with fine cracks present immediately below the weld deposit, which served as the starting points of the fatigue cracks. Following this accident, the remaining three rods (which had been modified in a similar manner) were replaced as a precautionary measure.

Shaft fracture of a 10 hp squirrel cage motor took place at the driving end just outside the roller bearing and not at an abrupt change of section behind the bearing where it might be expected to occur. A portion of shaft to the right of the fracture was deeply grooved. About a year prior to failure the inner race of the roller bearing became slack on the shaft and the seating was built up by the metal-spray process. The shaft was machined to form a rough thread to provide the requisite mechanical key for the sprayed-on metal. Part of this sprayed-on layer became detached after the fatigue failure occurred. The quality of the welding was poor. Slag inclusions were present adjacent to the sides of the keyway, which had been re-cut shorter than the original one after the welding repair. Failure at the unusual location was caused by the presence of the weld deposit.

The crankshaft of a compressor fractured through the web remote from the driving end after about three years of service. The fracture ran diagonally across the web into the crankpin. It passed through the centers of two screwed plugs inserted into the web from opposite faces approximately in line with the crankpin center line. The fracture was of the fatigue type, slowly developing cracks having started from opposite sides of each tapped hole and crept across the section. Microstructure of the crankshaft indicated the material was a plain carbon steel, the carbon content being of the order of 0.3%. The failure resulted principally from the stress-raising effects of the screw holes combined with the cracks in the welds. If the screw holes had been left unfilled or if some form of mechanical locking had been used if plugged, failure would have been postponed if not averted.

A fluorescent liquid-penetrant inspection of an experimental stator vane of a first-stage axial compressor revealed the presence of a longitudinal crack over 50 mm (2 in.) long at the edge of a resistance seam weld. The vane was made of titanium alloy Ti-6Al-4V (AMS 4911). The crack was opened by fracturing the vane. The crack surface displayed fatigue beach marks emanating from the seam-weld interface. Both the leading-edge and trailing-edge seam welds exhibited weld-metal expulsions up to 3.6 mm (0.14 in.) in length. Metallographic examination confirmed that metal expulsion from the resistance welds was generally present. The stator vane failed by a fatigue crack that initiated at internal surface discontinuities caused by metal expulsion from the resistance seam weld used in fabricating the vane. Expulsion of metal from seam welds should be eliminated by a slight reduction in welding current to reduce the temperature, by an increase in the electrode force, or both.