NDT-tests of blades provide the designers with increased knowledge on the structure.
The NDT-tests may be thermo graphic inspection of blades in fatigue test or stress wave analysis on blades in both static and fatigue test.
Thermo Graphic inspections of blades in test give the designer information of hot spots in the structure where small imperfections can initiate damages.
Thermo graphic view of the same part of the blade at an early stage of the test. The main beam is seen as the “luminous” part in the middle of the blade.
At the end of this accelerated test a redistribution of the heat emission is seen. This is caused by a damage in the main bar due to very high fatigue loads. The increase in the emission at the trailing edge is caused by the redistribution of load.
Acoustic Emission Detection Stress wave analysis or acoustic emission gives the possibility of locating small cracks and imperfections in the structure. By use of the acoustic emission detection system during fatigue test and static test it becomes possible to stop tests before damages extend to a size that is fatal to the blade.
The acoustic emission detection system is a set of piezo-electric transducers positioned on the blade in a pre-defined pattern. The sensors are connected to a data acquisition system that samples the transducer signal. Knowing the position of the sensors, it becomes possible to locate the area of the signal source (the damage). Hence the system is set up to register “first hit” among all sensors, any signal will be determined to originate from the zone of a single sensor.
With a close pattern of sensors it becomes possible to make accurate determinations of the origin of the damage.
The initiating damage in a fatal test can be determined and surveyed before and up till the collapse of the structure. Through these method critical areas of the blade i.e. specific changes in the structure, details from manufacturing or bond lines can be focused on during test.