Laser Shearography is a Non-Destructive Testing (NDT) technique used to reveal sub-surface defects in structures. When a load or stress (i.e. thermal, vacuum, vibration, acoustic, or mechanical excitation) is applied to a surface, the surface above a defect will deform comparatively more than the parent material. This is because isolated defects cause a localized weakness in the structure. Laser Shearography measures how the material surface responds (bends) to the applied load.
Discontinuities beneath the surface can be located using Laser Shearography through assessment of the surface (bending) strain field. Measurements are acquired of the relative (qualitative) change in the out-of-plane displacement taken before and after a change in load. The presence of discontinuities can be identified through localized changes in the rate-of-change (gradient) of the out-of-plane deformation, which is identifiable as black & white fringes in a 2D shearogram, also known as a phase map.
When a monochromatic, coherent laser light source is projected on a rough surface, the surface microstructure causes the light source to scatter forming a unique (thumb-print) stochastic speckle pattern. This is caused due to the effect of light interference. The reflecting waves from the structure reach the imaging sensor in different phases, due to the differences in the relative (scattered) travel paths. The interference of each wave contributes to the intensity on each camera pixel, resulting in the perception of a bright or light speckle.
When the surface shape changes (i.e. as the object is loaded), the speckle pattern changes. The changing speckle pattern (Δ) is then analysed via image subtraction between the two states, yielding a fringe display. Practically, this is performed by taking a reference image at one state and then subtracting this from a snapshot (measurement) image at another state.
One fringe consists of one grey/grey transformation, also known as a 2π phase jump. Thus, fringes can be compared to isometric (constant lines) of surface out-of-plane displacement levels. The number of fringes gives a clear indication of how close/far a discontinuity is to the surface or how strong/weak a discontinuity is relative to the parent material. More fringes equals more relative out-of-plane displacement.
Laser Shearography is very sensitive to changes in the out-of-plane displacement (to a sub-micrometer level). Measurement sensitivity is dependent on the laser source, the shearing amount (ie. shear distance) and the amount of out-of-plane bending (i.e. relative applied load). A shearing piezo is used in Laser Shearography to control the sensitivity of the sensor to out-of-plane movement. Most modern systems use automatic software control for this function. For any similar setup, the measurement result is the product of shearing distance and applied load. As such, a common measurement result can be obtained by proportional adjustment of either the shearing or loading parameters.
