The torque-arm assembly (aluminum alloy 7075-T73) for an aircraft nose landing gear failed after 22,779 simulated flights. The part, made from an aluminum alloy 7075-T73 forging, had an expected life of 100,000 simulated flights. Initial study of the fracture surfaces indicated that the primary fracture initiated from multiple origins on both sides of a lubrication hole that extended from the outer surface to the bore of a lug in two cadmium-plated flanged bushings made of copper alloy C63000 (aluminum bronze) that were press-fitted into each bored hole in the lug. Sectioning and 2x metallographic analysis showed small fatigue-type cracks in the hole adjacent to the origin of primary fracture. Hardness and electrical conductivity were typical for aluminum alloy 7075. This evidence supported the conclusion that the arm failed in fatigue cracking that initiated on each side of the lubrication hole since no material defects were found at the failure origin. Recommendations included redesign of the lubrication hole, shot peeing of the faces of the lug for added resistance to fatigue failure, and changing of the forging material to aluminum alloy 7175-T736 for its higher mechanical properties.

Two investment-cast A356 aluminum alloy actuators used for handles on passenger doors of commercial aircraft fractured during torquing at less than the design load. Visual examination showed that cracking had occurred through a machined side hole. Fractography revealed that the cracks originated in hot tear locations in the castings. Microprobe analysis of fracture surfaces in the hot tear region indicated a much higher silicon-to-aluminum ratio compared with the overload fracture area. No microstructural anomalies related to the failure were found during metallographic examination. It was concluded that the strength of the castings had been compromised by the presence of the casting defects. Modification of the gating system for casting was recommended to eliminate the hot tear zone. It was also suggested that the balance of the castings from the same manufacturing lot be radiographically inspected.

Two titanium alloy wing attachment bolts from a commercial jetliner failed during the course of a routine service operation. Failure of the bolts occurred during the re-torque process as the wing was being reattached. Metallurgical failure analysis indicated that the fracture mechanism was ductile overload and that the mechanical properties of the bolts were consistent with exemplar bolts that had been supplied. After eliminating other sources of excessive load application, the most probable cause of failure was ascribed to variances between the frictional characteristics of the bolt at the time of re-torque and at the time of initial torque application several years earlier.

Two examples concerning fabricated mild steel rotor spiders which failed due to lack of torsional rigidity, probably supplemented by the presence of high internal stress, are described. The machine concerned in the first case was a 3,000 hp three-phase slip-ring motor. In the second case the machine was a 200 kW alternator, direct-driven by a diesel engine running at 750 rpm. Both the foregoing failures reveal the same basic weakness, i.e., insufficient rigidity when subjected to variations or reversals of torque. In the first case, the bars welded to the arms were inadequately supported in a lateral direction, so that excessive stresses of a fluctuating nature were set up in the welds as a result of the frequent load changes that arose in service. This weakness was eliminated when designing the replacement spider. In the second example, failure also arose as a result of deficient torsional rigidity with the consequent development of excessive stresses in the welds at the junctions of the bars with the sleeve, the torque being of a fluctuating character due to the impulses imparted by the engine.

Fretting and/or fretting corrosion fatigue have been observed on such parts as main rotor counterweight tie rods, fixed-pitch propeller blades, propeller blade clamps, pressure regulator lines, and landing gear support brackets. Microcracks started from severe corrosion pits in a failed control rotor spar tube assembly made of cadmium-plated AISI 4130 Cr-Mo alloy steel. Inadequate design was responsible for the failure. A lower tine of the main rotor blade cuff failed in fatigue. The rotor blade cuff was forged of 2014-T6 aluminum alloy. Initial stages of crack growth displayed features typical of low stress intensity fatigue of aluminum alloys. The fatigue resulted from abnormal fretting owing to inadequate torquing of the main retention bolts. Aircraft maintenance engineers and owners were advised to adhere to specifications when torquing this joint.

Cadmium-plated high-strength steel bolts were used to facilitate quick disassembly of a vehicle. One bolt was found fractured across the root of a thread after being torqued in place for one week. The bolts were made of 8735 steel heat treated to a tensile strength of 1241 to 1379 MPa (180 to 200 ksi) with a hardness of 39 to 43 HRC, followed by cadmium plating. The bolt that failed and several that did not were examined. It was found that failure of the bolts was the result of time-dependent hydrogen embrittlement. Had the remaining bolts been torqued to the normal stress levels, all would have failed within two weeks. The bolts were baked, as specified by ASTM B 242, at 205 deg C (400 deg F) for 30 min. No further failures occurred. Baking for 30 min is the minimum baking time; however, baking times up to 24 h are recommended for greater safety.

A failed tapered-ring sprocket locking device consisted of an assembly of four tapered rings that are retained by a series of cap screws. The middle wedge-shaped rings were pulled closer as the screws were tightened forcing the split inner ring to clamp tightly onto the shaft. One of the wedge-shaped middle rings fractured prior to having been fully torqued, preventing the sprocket from being locked to the shaft. “Woody” fracture features, as a result of decohesion between a high volume fractions of manganese sulfide stringers and the matrix, was revealed during examination. The material was revealed by chemical analysis to be resulfurized grade of carbon steel (SAE type 1144, UNS G11440) which has enhanced longitudinal tensile properties but low transverse properties. It was observed that when the fastening screws were torqued, a significant hoop stress was placed on the middle rings and it caused the failure at the large inclusion present at the minimum section thickness zone of the middle ring. It was concluded that since the material contained a high volume fraction of these inclusions, the material choice was not appropriate for this application. A nonresulfurized grade of low-alloy steel was suggested as recommendation.