Leakage was detected at the welds between stiffening plates and the pipe in a transfer line carrying butane and related petrochemical compounds. The line and reinforcing rings were of AISI 316 stainless steel, the pipe being of 508 mm diam and 6.25 mm wall thickness. The design temperature and pressure were 621 deg C and 2.75 kPa, respectively, while the operating conditions were 579 deg C and 1.03 kPa. The line was insulated. Failure occurred after approximately 90,000 h of operation, shutdowns being approximately two per annum. The cracking occurred at the toe of welds between the plates and the pipe. The creep damage failure was attributed to repeated relaxation cycles of very high thermal stresses of resulting from the periodic shutdowns, temperature fluctuations during service, or both. This failure emphasized the information available from an evaluation of the operative creep mechanism, namely grain boundary sliding, relating to the periodic nature of the loading, with high residual stresses being present.

A fan support casting failed unexpectedly while running at 1800 rpm in pulp at 65 deg C (150 deg F). The leading edge of the blade exhibited deep spongy holes leading to reduced section and finally to fracture of the part when the remaining section size was insufficient to support the load. Analysis showed the support casting to be a standard 8620 type composition with a hardness of 311 HRB. The design of the casting was not streamlined. There were several square corners present where great pressure differences could be generated. This was a case of erosion-corrosion with the classic spongy appearance of cavitation. Two changes were proposed: streamlining the part to avoid abrupt changes in fluid flow; and a change in alloy to a more corrosion-resistant material (304 or preferably 316) to increase the tenacity of protective films.

A single-engine aircraft was climbing to 8000 ft when the engine suddenly lost power. The landing gear was torn off during the emergency landing. During the field investigation, the fuel line was found to be separated from the fuel pump outlet due to a failure of the elbow fitting. A bracket which supports the in-line fuel flow transducer also was found broken. Examination of the elbow fracture revealed characteristics of low-cycle fatigue failure. Examination of the support bracket fractures revealed a high-cycle mode of fatigue failure, with the primary fatigue extending along the full length of the 90 deg bend in the bracket. It was concluded that the failure was caused by an incorrectly-installed support bracket. It was recommended that the installation procedure be clarified.

A forged, cadmium-plated electroslag remelt (ESR) 4340 steel mixer pivot support of the rotor support assembly located on an Army attack helicopter was found to be broken in two pieces during an inspection. Visual inspection of the failed part revealed significant wear on surfaces that contacted the bushing and areas at the machined radius where the cadmium coating had been damaged, which allowed corrosion pitting to occur. Optical microscopy showed that the crack origin was located at the machined radius within a region that was severely pitted. Electron microscopy revealed that most of the fracture surface failed in an intergranular fashion. Energy dispersive spectroscopy determined that deposits of sand, corrosion and salts were found within the pits. The failure started by hydrogen charging as a result of corrosion, and was aggravated by the stress concentration effects of pitting at the radius and the high notch sensitivity of the material. The failure mechanism was hydrogen-assisted and was most likely a combination of stress-corrosion cracking and corrosion fatigue. Recommendations were to improve the inspection criteria of the component in service and the material used in fabrication.

This paper summarizes the results of a failure analysis investigation of a fractured main support bridge made of 7075 aluminum alloy from an army helicopter. The part, manufactured by “Contractor IT,” failed component fatigue testing while those of the original equipment manufacturer (OEM) passed. Metallurgical data collected during this investigation indicated that the difference in fatigue life between the components fabricated by IT and by OEM may be attributable to a difference in dimensions at the web where fatigue crack initiation occurred. The webs of the two OEM parts examined had cross-sectional thicknesses significantly larger than the web cross-sectional thicknesses of the IT components. Recommendations included changing the web reference dimension of 0.38 in. to include a tolerance range based upon a fracture mechanics model. Also, the shot peening process should be controlled especially at the critical areas of the web, to assure complete coverage and proper compressive residual stresses.

A support arm on a front-end loader failed in a brittle manner while lifting a load. The arm had a cross section of 50 x 200 mm (2 x 8 in.). Material used for the arm was hot-rolled ASTM A572, grade 42 (type 1), steel, which exhibited poor impact properties in the as-rolled condition and had a ductile-to-brittle transition temperature exceeding 93 deg C (200 deg F). This transition temperature was much too high for the application. It was recommended that a modified ASTM A572, grade 42 (0.15% C max), type 1 or 2, steel be used (type 1, which contains niobium, may be needed to meet strength requirements). The steel should be specified to be killed, fine-grained, and normalized, with Charpy V-notch impact-energy values of 20 J (15 ft·lbf) at -46 deg C (-50 deg F) in the longitudinal direction and 20 J (15 ft·lbf) at -29 deg C (-20 deg F) in the transverse direction.

Despite extensive aircraft landing gear design analyses and tests performed by designers and manufacturers, and the large number of trouble-free landings, aircraft users have experienced problems with and failures of landing gear components. Different data banks and over 200 failure analysis reports were surveyed to provide an overview of structural landing gear component failures as experienced by the Canadian Forces over the last 20 years on more than 20 aircraft types, and to assess trends in failure mechanisms and causes. Case histories were selected to illustrate typical problems, troublesome failure mechanisms, the role of high strength aluminum alloys and steels, and situations where fracture mechanics analyses provided insight into the failures. The two main failure mechanisms were: fatigue occurring mainly in steel components, and corrosion related problems with aluminum alloys. Very few overload failures were noted. A number of causes were identified: design deficiencies and manufacturing defects leading mainly to fatigue failures, and poor materials selection and improper maintenance as the principal causes of corrosion-related failures. The survey showed that a proper understanding of the failure mechanisms and causes, by thorough failure analysis, provides valuable feedback information to designers, operators and maintenance personnel for appropriate corrective actions to be taken.

A supplementary axle, which was used as an extension to a highway-trailer tractor to increase its load-bearing capacity, failed in service. The rolled steel channel extensions that secured the axle assembly to the tractor main-frame I-beams fractured transversely, with the crack in each instance initiating at a weld that joined the edge of the lower flange to the support bracket casting. The cracks propagated through the flange on each side until the effective cross-sectional area had been reduced sufficiently to bring about sudden and complete fracture of the remaining web and upper flange. Fatigue fracture was caused by a combination of high bending stresses in the bottom flanges of the channels due to the heavy load being carried, concentration of stresses due to the rapid change in section modulus of the channel at its point of attachment to the support-bracket casting, and brittleness of the high-hardness HAZ of the weld associated with the abnormally high carbon content in the central part of the channel. Welding of channel edges contributed to harmful gradients in section moduli and should be avoided in future assemblies.