Ma Venkataswamy

Fatigue fracture of crankshaft of an aircraft engine

Abstract The crankshaft of a piston engine of a transport aircraft failed during climb. Cracks were found to initiate at the web radius region of journals 2 and 3 and progressed in the transverse direction of the shaft axis causing it to fracture. The initiation of cracks was by surface contact fatigue due to constant rubbing of bearings against the journal webs. The movement of the bearing was caused by some axial load on the crankshaft. An analysis of the failure is presented in this paper.

Failure of turbine rotor blisk of an aircraft engine

Abstract Cracks were observed at the trailing edge of the blades of a turbine rotor blisk of a gas turbine engine during inspection following an endurance test. The integrally bladed disk was made of Mar-M-247 alloy, cast and hipped. Visual inspection, micro and macro fractography, and metallography of the failed blades indicated that the failure was caused by stress rupture at the trailing edge followed by fatigue crack propagation. A systematic analysis was carried out to ascertain the cause for stress rupture. Microstructural studies revealed that the columnar grains at the blade root airfoil region were oriented unfavorably leading to poor stress rupture property in the transverse direction. Coupled with this, the high operating temperatures were responsible for the failure of the blade by stress rupture. It was recommended that the casting parameters be controlled so that the columnar grains are oriented parallel to the principal stress axis of the blade. Coarse columnar grains with high aspect ratios in the longitudinal direction will improve the stress rupture property. It was also suggested to monitor the operating temperature to see that it does not exceed the specified temperature limit of the alloy.

Fatigue failure of a hollow power transmission shaft

A micro/hairline crack was noticed on a low speed, hollow shaft of a single stage helical gearbox during service. Though this had not resulted in a catastrophic failure, the shaft was withdrawn from service because of leakage of oil. Subsequent investigation revealed that the crack had initiated by fatigue at one of the keyway edges and progressed about 3/4 of the shaft periphery in a helical manner but had not given rise to final fracture of the shaft. The fatigue crack initiation was due to stress concentration arising from a depression mark at the keyway end surface. The problem was further aggravated due to inadequate radius at the keyway edges and rough machining marks. An analysis of the failure, together with recommendations for failure prevention, is presented in this paper.

Failure of reformer tube of an ammonia plant

Abstract The present article describes the failure of a primary reformer tube in an ammonia plant. The failure took place at the stub end of the reformer tube where a SS 321 flange was joined to a HK 40 catalyst tube by welding. Detailed metallurgical examinations indicated that the failure was due to stress corrosion cracking. Both longitudinal and transverse cracks were observed at the region of fracture. The cracks originated at the interface of the weld and HAZ at the inner surface and progressed to the outside of the tube in the transverse (thickness) direction. Microstructural study revealed chromium carbides precipitation at the grain boundaries in the heat affected zones on either sides of the weld. The results clearly indicated that the steel had undergone sensitisation which subsequently led to failure of the reformer tube by stress corrosion cracking.

Effect of span length on micromechanics of fracture under flexure load in CFRP composites

ABSTRACT Unidirectional (UD) and bidirectional (BD) woven carbon fiber reinforced plastic (CFRP) composites were tested under three-point flexure at different span lengths ranging from 20 to 60 mm. Expectedly, flexure load decreases with increasing span length. A sharp change in flexure load is observed at a span length of 30 mm with a transition in failure mechanism. Analysis suggests that crack initiation occurs at compression side and tension side, respectively, in UD and BD woven composites. In case of UD composites, at a span length of 20 mm, the failure was governed by kink band initiation (two fractures per fiber) followed by tension side delaminations, and at other span lengths, microbuckling (multiple fiber fractures) together with compression side delaminations led to failure. On the other hand, the failure in BD composites at a span length of 20 mm is controlled predominantly by thickness crack propagation with tensile fiber pullouts, whereas for other span cases the failure occurs due to the development of delaminations at interlace regions. The stress analysis by finite element modeling (FEM) also supports the experimental observations in an attempt to correlate the failure mechanisms under flexure loads in CFRP composites.

Fractographic analysis of compression failures in unidirectional carbon fiber reinforced plastic composites

Resin infusion process with/without the application of external pressure was employed for fabrication of unidirectional carbon fiber reinforced plastic composites. The prepared samples were tested under compression loads. Three kinds of fracture modes, viz., in-plane shear, complex fracture and through-thickness shear fractures were noticed. It was observed that samples fabricated with application of pressure exhibited in-plane and complex fracture modes whereas other samples displayed through-thickness fracture. Detailed fractographic analysis suggested that in-plane and through-thickness shear fractures resulted due to microbuckling of individual fibers; on the other hand, complex failure is governed by longitudinal splitting followed by microbuckling. Microbuckling directions for both in-plane and through-thickness shear fractures were established. These and other microstructral characterization results were analyzed and an effort was made to identify the reasons for variations in fracture modes and resulting compression properties.