Thermo-optical modulation of ultrasonic surface waves for NDE

Area of Science:

• Materials Science
• Non-destructive Testing
• Ultrasonics

Background:

• The thermo-elastic effect of laser irradiation generates localized stresses.
• These stresses cause parametric modulation of ultrasonic signals, similar to acousto-elastic effects.
• Fatigue cracks in metallic specimens can be challenging to detect with conventional methods.

Purpose of the Study:

• To investigate the use of optically induced thermo-elastic modulation for detecting small, hidden cracks.
• To enhance the sensitivity of ultrasonic surface wave inspection for fatigue-damaged materials.
• To differentiate between temperature modulation and thermo-elastic stress modulation in ultrasonic signals.

Main Methods:

• Utilizing expanded, long pulse, low-intensity laser beams for irradiation.
• Performing ultrasonic surface wave inspection on aluminum and titanium specimens.
• Conducting numerical and experimental evaluations of temporal and spatial ultrasonic signal variations.
• Separating direct temperature modulation from thermo-elastic stress modulation.

Main Results:

• Optically induced thermo-elastic modulation effectively detects small cracks in fatigue-damaged specimens.
• Cracked specimens exhibit strong parametric modulation due to susceptibility to crack closure.
• The method allows separation of temperature modulation from stress modulation.
• Sensitivity of ultrasonic flaw detection to small fatigue cracks increased by over an order of magnitude.

Conclusions:

• Laser-induced thermo-elastic stress modulation is a viable technique for detecting hidden fatigue cracks.
• This method significantly improves the sensitivity and selectivity of ultrasonic non-destructive testing.
• The approach offers a novel way to enhance the detection of critical flaws in metallic components.