elongation
% reduction of area
98.509 87.2 13.0 35.96
Microstructure
A sample from the root was polished, etched with Keller’s re-agent, and observed in a microscope. The microstructure showed well dispersed, duplex ␣ and b structure with no abnormalities (Fig. CH32.3). No oxygen stabilized casing was observed, which rules out oxygen absorption during forming operations.
Discussion and Conclusion
The preceding observations show that the blade had an accept-able microstructure and that its strength and ductility were normal.
It can, therefore, be concluded that the blade failed due to a sudden impact load acting on its concave side.
Fig. CH32.1 Fracture surface of the blade showing chevron marks
DOI:10.1361/faes2005p141 www.asminternational.org
142 / Failure Analysis of Engineering Structures: Case Histories
20 m
Fig. CH32.3 Microstructure of the blade material at the root showing well-dispersed␣ and b structure
20 m
Fig. CH32.2 SEM fractograph of the fracture surface showing dimpled rupture
CASE 33
Failure of a Low-Pressure Turbine Rotor (LPTR) Blade
Summary
A low-pressure turbine rotor (LPTR) blade failed during a test run causing damage to all the blades. The blade had failed by fatigue and the fatigue crack had initiated at surface coating cracks caused by mechanical damage.
Background
A low-pressure turbine rotor blade failed within hours of a test run causing extensive damage to the engine. Strip examination revealed that one of the LPTR blades had fractured near the blade root. The failed blade was examined for analyzing the cause of failure.
Pertinent Specifications
The LPTR blade was made of directionally solidified cast nickel-base superalloys with platinum aluminide surface coating.
Visual Examination of General Physical Features
Figure CH33.1 shows the failed blade. The blade had fractured above the blade root platform in the airfoil section. Macroscopi-cally, the fracture surface showed two distinctive regions (Fig.
CH33.2). In region I, the crack had propagated parallel to the blade root platform and the fracture surface was found discolored due to oxidation. The maximum oxidation was observed near the leading edge (LE), and the extent of oxidation decreased gradually toward the interior of the blade. The color changes could be noticed from dark blue at the LE to light yellow at the end of region I. This is indicative of progressive crack propagation. Under the stereo-binocular microscope, a half-moon-shaped region consisting of beach marks, typical of fatigue, was observed on the fracture sur-face at the LE (Fig. CH33.3). The beach marks were found to have emanated from the LE, indicating that the crack initiation was at the LE. Detailed examination revealed two dent marks on the lead-ing edge at the crack origin region (Fig. CH33.4).
The remaining portion of the fracture surface, region II (Fig.
CH33.2), had a fresh and rough fracture surface and a coarse crys-talline appearance. The gross fractographic features were typical of overload failure.
5 mm
Fig. CH33.1 Photograph of the failed LPTR blade
5 mm Region I
Region II
Overload
Fatigue
Fig. CH33.2 Fracture surface showing two fracture zones
DOI:10.1361/faes2005p143 www.asminternational.org
144 / Failure Analysis of Engineering Structures: Case Histories
Testing Procedure and Results
Scanning Electron Microscopy and Fractography
Figure CH33.5 shows a low-magnification fractograph of the fatigue-failed region. Beach marks, typical of progressive failure, can be seen. From the orientation of the beach marks, it was found that there were two fatigue crack origins (Fig. CH33.6). Both the fatigue cracks were found to have initiated at the mechanically damaged region, and they were separated by a distance of about 300lm.
At higher magnifications, striations typical of fatigue were ob-served (Fig. CH33.7). Examination revealed wide variations in striation spacing as well as localized change in the direction of the crack front depending on the grain orientation. The coating thick-ness at the crack origin was measured to be about 73lm, with no abnormalities in the coating.
The SEM photograph of the mechanically damaged regions (dents) is shown in Fig. CH33.8. Cracks were seen in the coating around the dent mark. Examination also revealed that the fatigue cracks had initiated from these coating cracks. The surface of the damaged regions was found oxidized and found to be similar to that observed elsewhere in the blade.
Leading edge
Fig. CH33.3 Photograph showing beach marks emanating from the lead-ing edge of the blade
Dent marks
Fig. CH33.4 Photograph showing dent marks at the crack origin
100 m
Fig. CH33.5 SEM fractograph at the fatigue crack origin
A
Fig. CH33.6 SEM fractograph showing two fatigue cracks
Discussion
Fractographic examination suggests that the LPTR blade had failed by fatigue. The fatigue crack had initiated at the leading edge above the blade root platform and propagated progressively over a period of time before culminating in an overload fracture at the trailing edge. From the orientation of the beach marks, two fatigue crack origins were identified at the leading edge.
Examination revealed that there were two shallow dent marks on the leading edge of the blade at the fatigue crack origin region.
A network of cracks was observed in the coating surrounding these dent marks. The surface of the dented regions was found oxidized
similar to that seen elsewhere on the blade surface. This indicates that the dents were not caused freshly due to secondary damages, and they were present in the blade before the failure took place.
This suggests that the cracks in the coating caused due to me-chanical damage were responsible for the fatigue crack initiation in the blade.
Conclusion
The LPTR blade failed by fatigue due to the presence of coating cracks created as a result of mechanical damage.
10 m
Fig. CH33.7 SEM fractograph showing fatigue striations at location A shown in Fig. CH33.6
30 m
Fig. CH33.8 SEM picture showing network of crack around the mechan-ical dent