Blades cracks

Helicopter blades cracked by unauthorized flight maneuver (overload pilot not part of system). 

The developed method is used in eddy current defectoscopes like Zond VD used for detecting corrosion spots in the body of the plane through aluminium cover, cracks detecting in helicopter blades under dielectric covers up to 8-10 mm thick, in pipelines under protective covers up to 10 mm thick, etc... 

Cracks detection in welds, mounting hardwares, rivets, on blades and vanes, turbine disks, rotors blades, etc. 

Cracks depth measurement in the airframe, lending gear, turbine blades etc. 

Cracks depth measurement in pipe - lines, boilers, rotor duct, on turbines blades, in tread rings, welds etc. 

This special inspection problem concerns the detection of cracks in the root of turbine blades. The customer required a fast and reliable inspection on the wings", i.e. without dismounting the blades from the engine. 

Figure 1 presents a turbine blade and shows the critical zone where cracks have to be detected within an aircraft inspection. 

Fig. 1 Turbine Blade with critical crack in the root...

Figure 4 presents such a crack indication in the A-Scan-image. The respective A-Scan for a flawless blade is shown in figure 5. In every case the echo from the comer serves as an indication of a good coupling. 

Up to now it was demonstrated, that the probe design enables a fast positioning and that the acoustical parameters of the probe ensure a reliable crack detection implying even a coupling control. Now, the final customer s requirement was the inspection of the blades without demounting them from the engine. 

However, this probe design is dedicated to a special turbine blade type and a special inspection task on this blade. But similar probe types and probe holders can be provided also for other engine types and for other inspection tasks like wall thickness measurement and crack detection in other zones of a blade. 

Variable nozzles produce a series of jets of gas entering the rotor, and these impulses add up to form a frequency equal to the blade-passing frequency the number of revolutions per second multiplied by the number of nozzle vanes, which is of the order of thousands of cycles per second. Frequently the rotor will resonate at this frequency, and if it does, it will be fatigued and crack and break up thus these frequencies must be avoided, and the manufac turer should be asked to supply information to the customer on this subject. 

Because oxides are usually quite brittle at the temperatures encountered on a turbine blade surface, they can crack, especially when the temperature of the blade changes and differential thermal contraction and expansion stresses are set up between alloy and oxide. These can act as ideal nucleation centres for thermal fatigue cracks and, because oxide layers in nickel alloys are stuck well to the underlying alloy (they would be useless if they were not), the crack can spread into the alloy itself (Fig. 22.3). The properties of the oxide film are thus very important in affecting the fatigue properties of the whole component. 

Some manufacturers have experienced die above mentioned Ni3S2 scale formation phenomenon under certain gas conditions, which led to die failure of a rotating blade. One such experience involved a fracture dial was distinctly intergranular with evidence of secondary intergranular cracks or grain separation across die fracture. Intergranular facets of die fracture were sharp and distinct with little evidence of any ductile mode. The fracture appeared to have occurred in a brittle intergranular mode. 

In support of the power recovery expander market for fluid catalytic cracking units in refineries, some turboexpander manufacturers have an ongoing program to improve the solid particle erosion characteristics of the machine. Improved erosion characteristics will result in longer blade life, less downtime, and consequently greater profits for the users. 

Turbine rotating blades cannot be field-repaired if they are cracked. If one or two blades are damaged mechanically, the manufacturer may recommend field repair or replacement of the damaged blades. However, if several blades are fatigue cracked, it is recommended that the entire set be replaced, since the remaining blades have been exposed to the same operating conditions and, therefore, have little fatigue life left. 

Centrifugal compressors can have problems at the inducer and blade tips. These parts of the blade can be excited by aerodynamic forces. Blade discs can have stresses at the rotor tips, which lead to cracks. To solve this problem, the offending part is removed and a scalloped disc results as seen in Figure 21-20. This type of disc experiences some efficiency loss (about 2 %). 

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