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.

The propeller from a small private airplane came off in flight. The head ends of all six attachment bolts remained in the propeller hub when it was found. Two threaded shanks with nuts remained with the engine, while the remaining four shank ends with their nuts were missing. Parts available for examination, in addition to the hub and attachment bolts, were the two propeller blades and the engine crankshaft. The purpose of this examination was to determine the nature and probable cause of failure in the six attachment bolts. Indications of fatigue failure and wear were the major findings in visual and low power microscopic examination. Fracture surfaces indicated failure was initiated in the threads in four bolts and in the shanks in two. The group of four bolts failed primarily due to tensile loads, while the other two bolts failed primarily due to bending loads. It was concluded that failure was due to improper installation torqueing of the bolts.

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%.

A large four-engine aircraft was on a cargo flight at night when a loud bang was heard, accompanied by a loss of power from both engines on the left side. After an emergency landing, it was discovered that the propellers from both left side engines were missing. The initial investigation determined that the four-bladed propeller from the left inboard engine had separated in flight, subsequently impacting the left outboard engine, causing its propeller to separate also. Three years later, the left inboard propeller hub was recovered. All four blades had separated through the shank area adjacent to the hub. Detailed SEM examination confirmed a fatigue mode of failure in this area with a primary single origin on the inside surface of the shank. The main fatigue origin site was coincident with one of the defects on the inner surface of the blade shank. The most probable source for creating the defects on the ID bore of the shank was the blade tip chrome plating process, which was carried out during the last overhaul prior to the failure.

An accidental overspeed condition during wind tunnel testing resulted in the destruction of a propeller rotor The occurrence was initially attributed to malfunction in the collective pitch control system. All fractured parts in the system were inspected. Highly suspect parts, including the pitch control thrust bearing set, head bolts, hub fork, and actuator rod end, were examined in more detail The thrust bearing set (52100 steel) was identified as the probable source of the uncommanded pitch angle change. A complete failure analysis of the bearing indicated that failure was precipitated by excessive heating, causing cage disintegration, plastic flow of the races and balls, and eventual separation of inner and outer races. It was recommended that the bearing set be resized to accommodate the large thrust as and that a thermocouple be added to monitor the condition of the bearing during testing.

An aluminum bronze propeller tap bolt from a twin-screw vessel fractured just below the bolt head. Liquid penetrant testing revealed a large network of cracks that extended radially from sites in and just below the bolthead. Metallographic analysis indicated that the tap bolt failed by stress-corrosion cracking. It was surmised that seawater or some other corrosive substance was present in sufficient quantity to induce intergranular cracking at regions of high stress concentration. It was recommended that all tap bolts be replaced with new bolts made from an alloy with a higher copper content and at least the same yield strength. Steps to exclude seawater and any possible source of ammonia from the bolt shank were also suggested.

A controllable pitch propeller (CPP) on a dynamic positioning ship failed after eight months of operation. The CPP design consists of a hollow propeller shaft and a concentrically located pipe that operates inside. The pitch of the propeller blades is controlled hydraulically through the longitudinal displacement of the inner (concentric) pipe. Fractography, microstructural, microhardness, and chemical analyses revealed that the concentric pipe failed due to fatigue. Fatigue cracks initiated along longitudinal welds where wire spacers attach to the external surface of the pipe. The effect of crack-like defects, stress concentration at the weld toe, residual tensile stress, and lack of penetration contributed to a shorter fatigue crack initiation phase and premature failure.

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.

Two shafts that transmit power from the engine to the propeller of a container ship failed after a short time in service. The shafts usually have a 25 year lifetime, but the two in question failed after only a few years. One of the shafts, which carries power from a gearbox to the propeller, is made of low alloy steel. The other shaft, part of a clutch mechanism that regulates the transmission of power from the engine to the gears, is made of carbon steel. Fracture surface examination of the gear shaft revealed circumferential ratchet marks with the presence of inward progressive beach marks, suggesting rotary-bending fatigue. The fracture surfaces on the clutch shaft exhibited a star-shaped pattern, suggesting that the failure was due to torsional overload which may have initiated at corrosion pits discovered during the examination. Based on the observations, it was concluded that rotational bending stresses caused the gear shaft to fail due to insufficient fatigue strength. This led to the torsional failure of the corroded clutch shaft, which was subjected to a sudden, high level load when the shaft connecting the gearbox to the propeller failed.

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.

The gun mount used in two types of self-propelled artillery consists of an oil-filled recoil cylinder and a sand-cast (MIL-I-11466, grade D7003) ductile-iron piston that connects to the gun tube through a threaded rod. The piston contains several orifices through which oil is forced as a means of absorbing recoil energy. During operation, the piston is stressed in tension, pulled by oil pressure on one end and the opposing force of the gun tube on the other. The casting specification stipulated that the graphite be substantially nodular and that metallographic test results be provided for each lot. Investigation (visual inspection, fatigue testing, 0.25x/0.35x/50x magnifications, 2% nital etched 60x/65x magnifications, and SEM views) showed that most of the service fractures occurred in pistons containing vermicular graphite. Recommendations included ultrasonic testing of pistons already in the field to identify and reject those containing vermicular graphite. In addition, metallographic control standards were suggested for future production lots.

Cavitation damage of diesel engine cylinder liners is due to vibration of the cylinder wall, initiated by slap of the piston under the combined forces of inertia and firing pressure as it passes top dead center. The occurrence on the anti-thrust side may possibly result from bouncing of the piston. The exact mechanism of cavitation damage is not entirely clear. Two schools of thought have developed, one supporting an essentially erosive, and the other an essentially corrosive, mechanism. Measures to prevent, or reduce, cavitation damage should be considered firstly from the aspect of design, attention being given to methods of reducing the amplitude of the liner vibration. Attempts have been made to reduce the severity of attack by attention to the environment. Inhibitors, such as chromates, benzoate/nitrite mixtures, and emulsified oils, have been tried with varying success. Attempts have been made to reduce or prevent cavitation damage by the application of cathodic protection, and this has been found to be effective in certain instances of trouble on propellers.

The locking collar on a machine failed suddenly when the shaft it restrained was inadvertently subjected to an axial load slightly higher than the allowable working load. The locking collar fractured abruptly, producing four large fragments. This allowed the shaft to be propelled forcefully in the direction of the load, causing substantial damage to other machinery components in the vicinity. The failed component, which was 43 cm (17 in.) in diameter, was machined from 4140 plate and heat treated to 34 to 36 HRC. Analysis (visual inspection, composite micrographs, scanning electron microscopy, and mechanical-property analysis) supported the conclusions that the alloy steel plate used in this application contained significant brittle microstructural fibering or banding. This condition produced considerable anisotropy in ductility and toughness as revealed by mechanical testing. Unfortunately, the potential effects of anisotropy were apparently neglected when this component was designed and manufactured from the plate stock, because the loading was applied in a direction that stressed the weakest planes in the material, that is, a direction normal to the fibering. No recommendations were made.

Military aircraft use a cartridge ignition system for emergency engine starts. Analysis of premature failures of steel (AISI 4340) breech chambers in which the solid propellant cartridges were burned identified corrosion as one problem with an indication that stress-corrosion cracking may have occurred. A study was made for stress-corrosion cracking susceptibility of 4340 steel in a paste made of the residues collected from used breech chambers. The constant extension rate test (CERT) technique was employed and SCC susceptibility was demonstrated. The residues, which contained both combustion products from the cartridges and corrosion products from the chamber, were analyzed using elemental analysis and x-ray diffraction techniques. Electrochemical polarization techniques were also utilized to estimate corrosion rates.

Cartridge-pneumatic starter systems are used on military aircraft. In the cartridge mode used for alert starts, the starter turbine is driven by hot gases produced through the controlled burning of a solid propellant cartridge within a closed chamber (the breech chamber/cartridge chamber assembly). Premature failures of steel breech chambers have been prevalent enough to cause serious concern. The breech chamber is fabricated from a 4340 Ni-Cr-Mo steel forging heat treated to a hardness in the range HRC 40 to 45. The failures have taken several forms, including fracture and unzipping of the chamber dome, burn-through of the dome, and shearing of bayonet locking lugs. Factors identified as significant in the failures are the pressure developed in the chamber and internal corrosion of the chamber in an environment that can produce stress-corrosion cracking. The interior configuration of the chamber and the stress distribution also have a bearing upon the failure modes. Several failures are reviewed to illustrate the problems.

Pinhole defects were found in a main combustion chamber made from NARloy-Z after an unexpectedly short time in service. Analysis indicated that the throat section of the liner had been exposed to very severe environmental conditions of high temperature and high oxygen content, which caused ductility loss and grain-boundary separation. The excessive oxygen content in the liner was attributed to diffusion from an oxygen-rich environment that had resulted from nonuniform mixing of propellants. The internal oxygen embrittled the alloy and reduced its thermal conductivity, which resulted in a higher hot-gas wall temperature and associated degradation of mechanical properties.

Several large-diameter type 304L stainless steel impeller/propeller blades in a circulating water pump failed after approximately 8 months of operation. The impeller was a single casting that had been modified with a fillet weld buildup at the blade root. Visual examination indicated that the fracture originated near the blade-to-hub attachment in the area of the weld buildup. Specimens from four failed castings and from an impeller that had developed cracks prior to design modification were subjected to a complete analysis. A number of finite-element-method computer models were also constructed. It was determined that the blades failed by fatigue that had been accelerated by stress-corrosion cracking. The mechanism of failure was flow-induced vibration, in which the vortex-shedding frequencies of the blades were attuned to the natural frequency of the blade/hub configuration. A number of solutions involving material selection and impeller redesign were recommended.

A bearing cup in a drive shaft assembly on an automobile was found to have failed. A detailed analysis was conducted using the QC story approach, which begins by proposing several possible failure scenarios then following them to determine the main root cause. A number of alternative solutions were identified and then validated based on chemical analysis, endurance and hardness tests, and microstructural examination. The investigation revealed that carbonitriding can effectively eliminate the type of failure encountered because it prevents through hardening of the bearing cup assembly.

In January 1965, a Reaction Control System (RCS) pressure vessel (titanium alloy Ti-6Al-4V) on an Apollo spacecraft cracked in six adjacent locations. It used N2O4 for vehicle attitude control through roll, pitch, and yaw engines, and was protected from the N2O4 by a Teflon positive expulsion bladder. Investigation (visual inspection, pressure testing of 10 similar vessels, and chemical testing of the N2O4) supported the conclusion that the failure was due to stress corrosion from the N2O4, and specifically from a specification change in the military specification MIL-P-26539. Recommendations included revising the specification to require a minimum NO content of 0.6%.

During an acceptance test of the Apollo spacecraft 101 service module prior to delivery, an SPS fuel pressure vessel (SN054) (titanium Ti-6Al-4V, approximately 1.2 m (4 ft) in diam and 3 m (10 ft) long) containing methanol developed cracks adjacent to the welds. The test was stopped. This acceptance test had been run 38 times on similar pressure vessels without problems. The methanol was a safe-fluid replacement for the storable hypergolic fuels (blend of 50% hydrazine and 50% unsymmetrical dimethyl hydrazine). Investigation (visual inspection and 65X images) showed similarities to stress-corrosion resulting from contamination during misprocessing of the vessels. However, another vessel underwent a more severe testing procedure and failed catastrophically. Further investigation supported the conclusion that the failure cause was SCC of titanium in methanol. Attack is promoted by crazing of the protective oxide film. It was learned that minor changes in the testing procedures could inhibit or accelerate the reaction. Recommendations included replacing the methanol with a suitable alternate fluid. Isopropyl alcohol was chosen after considerable testing. This incident further resulted in the imposition of a control specification (MF0004-018) for all fluids that contact titanium for existing and future space designs.