Laser Shearography uses the coherent, monochromatic properties of laser light to generate speckle patterns. The component to be inspected is illuminated by the laser. The surface reflects the light creating a speckle pattern at the viewing plane, which can be processed to provide information such as the presence of defects, material degradation or residual stress. The system records the speckle pattern from an unstressed component surface. The image is recorded using a video camera, digitised and stored on a computer.

The surface is then stressed and a new speckle pattern generated, recorded and stored. The computer subtracts the speckle patterns from each other, thus forming an image made up of series of characteristic black and white fringes, representing the surface strain in the area of interest.

If a defect such as a void or disbond exists, this will affect the surface strain and the defect can be revealed by the fringe pattern developed. See Fig.1. This can be processed further by the computer to make the fringe pattern easier to interpret by the user (Figs. 2 and 3).

The NDT Validation Centre uses a Laser Optical Engineering SM 1200 Strain Mapper. This is unique in that it can separately resolve in-plane and out-of-plane strain through the use of a novel dual laser system. This is especially useful when it is necessary to differentiate between faults that produce mainly out-of-plane strain, such as skin to core disbonds, and those that produce mostly in-plane strain, such as cracks.



The equipment is able to obtain the in-plane strain field of a loaded component rapidly and without contact with the surface, which gives the following capabilities:

• Test hot, cold and electrified components.
• Detect all types of defects including cracks.
• Can be used to validate FEA models.
• Can be used to inform design during rapid prototyping.