Laser Shearography uses the coherent and monochromatic properties of laser light to illuminate the surface of a component under investigation. If the surface is optically rough e.g. not a mirror surface, the lights reflected by the component surface will generate a speckle pattern. This speckle pattern is then recorded by a digital camera.
When the component is stressed e.g. by a mechanical load, thermal heating etc., the speckle pattern will change accordingly as the component deforms. By recording the new speckle pattern, and subtracting it from the original speckle pattern, a shearograpic fringe pattern will be produced and can be displayed on a computer screen.
This fringe pattern contains information about the relative deformation of the component between its two states (before and after stress), and is made up of a series of characteristic black and white fringes. When there are no features within the component, a regular fringe pattern (usually in the form of uniformly distributed fringes) will be obtained. When there is a subsurface feature such as a defect (e.g. a void, crack, or delamination), or a stringer, the regular fringe pattern will be disturbed.
This enables the defect to be identified by the operator. In addition, the shearographic fringe pattern can also be used to quantitatively measure the deformation of the component, rendering shearography a powerful non-contact, full field strain/stress measurement technique.
TWI Resource
TWI owns 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.
TWI has also established an optical lab, which contains two vibration isolated optical tables, a range of laser sources (class 3B and class 4), several high definition digital cameras, and a high speed camera. A team is dedicated to developing optical techniques for NDT and strain/displacement measurement, which includes four Marie Curie Fellows since 2007.
Benefits
- Rapid, full-field inspection
- Non-contact
- High sensitivity
- Able to distinguish between in and out of plane strain
Applications
For more information about TWI's NDT services, please email contactus@twi.co.uk.