Local surface skimming longitudinal wave velocity and residual stress mapping

Area of Science:

• Materials Science
• Non-Destructive Testing
• Acoustic Microscopy

Background:

• Understanding local stress distribution is critical for material integrity, especially around defects like cracks.
• Surface skimming longitudinal waves (SSLW) exhibit velocity changes sensitive to local stress variations.
• Scanning acoustic microscopy offers a potential method for mapping these stress-induced velocity changes.

Purpose of the Study:

• To develop and demonstrate a scanning acoustic microscopy method for imaging local SSLW velocity variations.
• To compare acoustic imaging of SSLW velocity with direct stress measurements around a crack tip.
• To evaluate the capabilities of SSLW velocity imaging for stress analysis in materials.

Main Methods:

• Utilized a scanning acoustic microscope with a narrow electrical impulse to excite the acoustic lens transducer.
• Employed a time-domain separation of SSLW signals from direct reflections.
• Measured time delays at two defocus settings to calculate local SSLW velocity and generated an image of velocity variations around a crack tip in Ti-6Al-4V.

Main Results:

• Successfully imaged local SSLW velocity variations around a crack tip in Ti-6Al-4V.
• The SSLW velocity image qualitatively correlated with stress distribution, serving as a stress map.
• Comparison with X-ray diffraction stress measurements highlighted differences in contrast, spatial resolution, and penetration depth.

Conclusions:

• Scanning acoustic microscopy can effectively image local SSLW velocity variations, providing a representation of stress fields.
• The technique offers a complementary approach to X-ray diffraction for non-destructive stress analysis.
• Further investigation into resolution and penetration depth is warranted for optimizing acoustic stress imaging.