An interesting technique that has enjoyed a lot of attention in the last few years and has become a major application for pulsed CO2 laser systems is the non-destructive testing of composite materials. Because the pulse width of the sdilasersTM transversely excited atmospheric (TEA) CO2 lasers is very short, typically 80ns, and the time jitter is very small (~10ns), the pulse can be used to generate an acoustic wave in composite materials that display large absorption coefficients at the wavelengths of TEA CO2 lasers.
The term photo acoustics refers to the generation of acoustic or shock waves by optical radiation in gases, liquids, and solids. Photoacoustics is a form of de-excitation resulting from photo thermal effects after the heating of a material by optical energy. This phenomenon is illustrated by the schematic representation of the possible consequences of optical absorption.
Of these interactions, the most important branch with regards to the generation of photo acoustics signals is the thermal de-excitation branch. This branch leads to the generation of heat that causes an expansion of the irradiated volume. This expansion in turn produces shock or acoustic waves. The percentage energy absorbed depends on inherent characteristics of the material like the absorption coefficient, structural integrity, and the surface and substrate quality. If irradiation should alter any of these characteristics, the amount of absorption would change, affecting the typical photo acoustic waveform.
The illustration here clearly shows the typical profile of a photo acoustic signal vis., the initial pressure wave followed by the subsequent rarefaction pulse that allows the sample to recover to its original state of equilibrium. A list of mechanisms that can generate photo acoustic signals are given below with the efficiency of the process increasing from top to bottom.
- Thermal expansion
- Photochemical changes
- Boiling, ablation
- Plasma formation
The acoustic shockwaves generated by the processes listed above can be used to perform subsurface imaging of defects, irregularities, delamination, etc. The presence of any of these factors will alter the shape of the acoustic waveform given above and by monitoring the waveform a 3D image can be built up of the material being tested.
Once the acoustic shockwave has been generated, the resulting expansion must be quantified to analyze the subsurface features of the material being tested. Conventionally this is done conventionally by monitoring the photoacoustic signal generated by either a piezoelectric transducer or a microphone. However, to image large and complex shapes such as the composite surfaces of aircraft interferometric measurements with a second beam is much more practical.
This technique requires the generation of the photoacoustic pulse by a short pulse length laser with a wavelength suited for the material being tested and the use of a coaxial measurement laser with a long pulse length. The ultrasonic vibrations generated on thesurface of the material being tested can then be monitored by an interferometer that monitors the detection laser output.
Using this method, one can determine the exact stress and damage points within the composite material used in military and commercial aircraft. To date we have sold several TEA CO2 laser systems to major European and American companies in the aerospace industry including the JSF and F22 programs. These lasers were either incorporated in various non-destructive testing systems in the production line or used for research applications focusing on non-destructive testing. SdilasersTM CO2 lasers are used in the PaR Systems LaserUT TM systems for laser ultrasound inspection of composite materials.
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