Laser Shearography is an optical, Non-Destructive Testing (NDT), surface measurement technique that works on the principle of laser-speckle shearing interferometry. Through the application of a load or excitation to an object‘s surface, a shearography sensor can observe surface bending in the form of an out-of-plane strain field. The measurement attained is imaged as a phase map that presents the relative difference in the two (reference/deformed) states as fringes. Local weaknesses in the structure, caused by discontinuities, are indicated as isolated fringes. Since the sensor is sensitive to laser light interference, it can detect bending within the sub-micrometer range.
Laser Shearography NDT can detect discontinuities down to 40 mm (1.5 “) below the surface. Detection depth is dependent on the bending stiffness of the material, amount of loading, and shearing sensitivity. Examples of applications for Laser Shearography inspection include:
- honeycombs & sandwiches for disbonding, cracked cores, crushed cores, node bond spits, kissing bonds, and (aluminum) corrosion between skin/core.
- laminates & overwraps for delaminations, ply wrinkling, fluid ingresses, and dry spots.
- bondings & coatings for disbonds, dry spots, and kissing bonds.
- compounds & ceramics for voids, cracking, and abrasions.
- spray foams & sealants for porosity and voids.
Since the technique retrieves surface bending information, additional structural information can also be detected including; ply drops, bulkheads, overlaps, splices, stringers, and ribs.
The ultimate goal of any NDT system is to deliver reliable measurement results as economically as possible. The economic advantages of using Laser Shearography include; high inspection rates (i.e. m2/sec), low sample preparation times, and simple automatability. Laser Shearography can detect defects that other NDT methods cannot, including; disbonding, kissing bonds, node bond splits, and ply wrinkling. In many cases the reliability of the technique, as quantified through the smallest detectable defect size detection at 90% Probability of Detection (PoD) and low False Call Rates, is unmatched.
Laser Shearography is currently being used for a large array of NDT applications in various industries including; Aerospace, Aviation & MRO, Wind Power, Marine, Automotive, and Manufacturing. Testing systems can be manual, semi-, or fully automated within post-production (quality control) or in-field (in-service) NDT inspection. The technique is incorporated in the following standards; EN 4179, NAS 410, and ASNT SN-TC-1A.