There is an urgent need to develop technologies that inspect the vast variety of critical man-made structures, such as bridges, skyscraper buildings, dams, load-bearing piers, levees, pipelines, etc., and to detect material fatigue and damages that can lead to failure. Condition assessment and characterization of materials and structures by means of nondestructive testing (NDT) methods is a priority need around the world to meet the challenges associated with the durability, maintenance, rehabilitation, retrofitting, renewal, and health monitoring of new and existing infrastructures. Numerous NDT methods that make use of certain components of the electromagnetic and acoustic spectra are currently in use to this effect with various levels of success and there is an intensive worldwide research effort aimed at improving the existing methods and developing new ones. MIT Lincoln Laboratory has been investigating the effects of sound and vibration modulation on laser beams.  In collaboration with the
Department of Civil and Environmental Engineering at MIT, we have observed that this phenomenon can enable a form of non-destructive testing. 

Controlled sound waves are used to cause vibrations in critical structures.  These vibrations are then measured remotely with laser Doppler vibrometry and mapped to form images of the damages and flaws existing at a variety of length scales.  Using the acoustic-laser approach provides several advantages over many current damage detection practices.  The approach is a totally noncontact method that can be used to scan the structure of interest quickly from significant standoff distances while providing location accuracy within millimeters.

At left is the conventional damage inspection method.  It poses many hazards to the inspectors, is very time consuming, and is unable to provide rapid and extensive coverage.   

A collaboration between MIT Lincoln Lab and MIT Department of Civil and Environmental Engineering began several years ago for non-distructive testing of critical load-bearing structures using acoustic-laser vibrometry. Acoustic-laser vibrometry standoff NDT research was initiated through an National Science Foundation award (FY09-13) to investigate acoustic-laser sensing of damages in structures.  At Lincoln Laboratory, we have developed a number of standoff acoustic excitation sources, including a high-powered acoustic array that transmits sound in a narrow and highly directional beam.  Moreover, we have developed a multi-pixel laser vibrometer that can measure the vibration signature of a distant target from a moving vehicle, allowing for practical and rapid coverage rates.  Our ongoing and proposed projects include standoff acoustic-laser method to detect and image unseen damages in fiberglass retrofitted protection (FRP) on critical load-bearing structures,laser vibrometry measurement systems for seismic and acoustic signals, and underwater opto-acoustic system to detect and image hidden damages in submerged man-made structures.