The 8620 steel latch tip, carburized and then induction hardened to a minimum surface hardness of 62 HRC, on the main-clutch stop arm on a business machine fractured during normal operation when the latch tip was subjected to intermittent impact loading. Fractographic examination 9x showed a brittle appearance at the fractures. Micrograph examination of an etched section disclosed several small cracks. Fracture of the parts may have occurred through similar cracks. Also observed was a burned layer approximately 0.075 mm (0.003 in.) deep on the latch surface, and hardness at a depth of 0.025 mm (0.001 in.) in this layer was 52 HRC (a minimum of 55 HRC was specified). Thus, the failure was caused by brittle fracture in the hardness-transition zone as the result of excessive impact loading. The burned layer indicated that the cracks had been caused by improper grinding after hardening. Redesign was recommended to include reinforcing the backing web of the tip, increasing the radius at the relief step to 1.5 x 0.5 mm (0.06 x 0.02 in.), the use of proper grinding techniques, and a requirement that the hardened zone extend a minimum of 1.5 mm (0.06 in.) beyond the step.

Presence of transverse marks which were remnant of grinding was indicated in a failed valve spring made from ground rod. The shot-peening pattern was light at this location. A transverse crack was found to grow from one such mark under the influence of local stress fields until it was reoriented to the plane normal to the major tensile axis by sufficient loading. The shot-peening procedure was altered to create adequate surface compression at all stressed points on the springs.

A 230 mm (9 in.) diameter disk attrition mill was scheduled to grind 6.35 mm (0.25 in.) diameter quartz particles to a 0.075 mm (0.003 in.) diameter powder. Due to severe wear on the grinding plates, however, the unit was unable to complete the task of grinding the rock. The mill consisted of a heavy gray cast iron frame, a gravity feeder port, a runner, and a heavy-duty motor. The frame and gravity feeder weighed over 200 kg (440 lb) and, in some areas, was over 25 mm (1 in.) thick. To obtain the operating speed of 200 rpm, a gear system was used to transmit the torque from the 2-hp motor. The runner consisted of a 50 mm (2 in.) diameter shaft and two gray cast iron grinding plates. Investigation (visual inspection, historical review, photographs, model testing of new plates, chemical analysis, hardness testing, optical macrographs, and optical micrographs) supported the conclusion that the primary feed material was harder than the grinding plates, causing wear and eventual failure. Recommendations included reducing the clearance between the flutes and possible material changes.

This report covers case histories of failures in fixed-wing light aeroplane and helicopter components. A crankshaft of AISI 4340 Ni-Cr-Mo alloy steel, heat treated and nitrided all over, failed in bending fatigue. The nitrided layer was ground too rapidly causing excessive heat generation which induced grinding cracks and grinding burn. Tensional stresses resulting from grinding developed in a thin surface layer. On another crankshaft, chromium plating introduced undesirable residual tensile stresses. Such plating is an unsatisfactory finish for crankshafts of aircraft engines. Aircraft engine manufacturers and aeronautical standards require magnetic particle inspection to detect grinding cracks after reconditioning. Renitriding after any grinding is needed also, regardless of the amount of undersize as it introduces beneficial residual compressive stresses.