Using Laser Technology to Detect Fatigue Cracks


In order to simulate real world conditions turbine blades are vibrated in a sound chamber at a variety of elevated temperatures. A siren directs a blast of pulsed air at the leading edge of the turbine blade creating a tremendous amount of force. This force excites the turbine blade at its resonant frequency, inducing strain in the blade structure. To accurately monitor vibration amplitude and frequency, a non-contact measurement sensor is required. Non-contact eliminates any “mass loading” or dampening of the turbine blade vibration and is not prone to failure or wear. Customarily a CCD camera system is used to view the blade displacement at the leading edge.

blade displacement

Although these work well at determining major displacement or frequency changes associated with catastrophic failures they are not able to detect small changes that signify the formation of a tiny fatigue crack. After reviewing the application details MTI’s Application Engineers selected the Microtrak laser displacement sensor because of the high sampling frequency and sub-micron resolution.

The customer selected the Microtrak laser sensor because of the visible laser spot that provides a means for easy setup and alignment of the sensor and the 6” operating distance allowed the laser head to operate outside the sound chamber. The Vibration results were obtained faster and at a lower cost than with a CCD camera or a Laser Vibrometer. Additionally, the feature of interchangeable laser heads allowed the customer the ability to test and analyze their wide variety of turbine blades and other engine components.