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  Metallurgy  
  Turbine Blade Failure  |  Sector Shaft Failure   |   
 

Turbine Blade Failure

 
 

BEAR engineers determine the root cause for failure, not just the failure mode. The turbine blade in the photograph at right, for example, cracked due to creep (creep is deformation of a material subjected to tensile stresses at high temperature).

 
 
SEM Micrograph of Turbine Blade
Our SEM micrograph of the crack surface, at left, shows the intercrystalline fracture mode and voids typical for creep failures. However, additional examination by optical microscopy showed that during service, the blade had been subjected to extreme temperatures, which were severe enough to cause melting.

 
 
Optical Micrograph of Turbine Blade
An optical micrograph of the blade, at left, shows a void that resulted from melting of the metal underneath the protective coating. Centrifugal forces during service caused the melted metal to flow toward the edge of the blade, where it displaced the coating, at the arrow. The coating then spalled off, leaving the blade unprotected and susceptible to creep damage.

We can also determine whether a set of turbine blades has experienced high temperature degradation by examining their microstructures. Changes in the microstructure of blades that have been in service can signal the onset of creep damage. By sacrificing one or two blades for examination, we can determine whether an entire set may continue in service.

 
     
 

Sector Shaft Failure

 
  Other recent investigations conducted at BEAR have focused on interpreting fractures to eliminate certain suspected causes for failure. In this case, BEAR was asked to determine whether shaft failure was the cause of an accident.

Fracture Surface of Sector Shaft
The photograph at left shows the fracture surface of the sector shaft from the steering mechanism on a truck that was involved in an accident while transporting a full load. The fracture surface had circumferential smearing and a slightly off-center final fracture zone.

 
 

Side View of Sector Shaft
The side view of the broken shaft, at right, showed twisting deformation from torsional forces during fracture. The fracture showed no indications of fatigue cracking, which would possibly point to a defect in the shaft as the cause for failure.

By interpreting the fracture features, we were able to determine that the failure was due to a single, torsional overload event. In other words, the shaft was subjected to a force during the accident that resulted in its failure.

 
     
 

 

 
     
     
 
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