What is shearography in composite NDT

Shearography is a full-field optical NDT method using laser-speckle interferometry. The surface is imaged under coherent laser light before and after a mild stress (thermal, vacuum, or vibration), and the change in the speckle fringe pattern (the shearogram) reveals subsurface defects (delaminations, disbonds, porosity) by measuring the first derivative of out-of-plane surface deformation.

It is extremely fast over large areas (often greater than 1 square meter) and very sensitive to subsurface damage. The trade-offs: it measures only surface displacement (not volume), requires the part to be stressed, and is sensitive to background vibration. Common applications include honeycomb-core inspection and wind-blade testing where the area to be screened is large and the modality must cover ground quickly.

How shearography works#

Coherent laser light illuminates the part surface. The scattered light from a rough surface produces a granular speckle pattern at the camera, with each speckle's intensity depending on the local interference of light scattered from nearby surface features.

A shearing optic in the camera path (a Michelson interferometer, a Wollaston prism, or a similar shearing element) splits the image into two slightly laterally offset copies and recombines them at the camera. The result is a shearogram: an interference pattern whose fringes encode the spatial gradient of out-of-plane displacement, not the displacement itself.

The part is then stressed. A mild thermal pulse, a vacuum decrement, an acoustic excitation, or a low-frequency mechanical drive induces small displacements at the surface. A second shearogram is captured. The difference between the before-stress and after-stress shearograms (the time-derivative shearogram) reveals where the surface deformed differently in response to the stress.

Subsurface defects (delaminations, disbonds, porosity) cause the surface above them to deform differently than the intact background under the same applied stress. The anomaly appears as a localized fringe pattern (a bull's-eye, a butterfly, a tilted ridge, depending on the defect geometry) against the background of the intact-laminate fringe pattern.

What shearography measures (and what it doesn't)#

Shearography measures the first derivative of out-of-plane surface deformation across the field of view, under a deliberately induced stress.

It does not measure absolute out-of-plane displacement; it measures the spatial gradient. The shearing optic is the source of this distinction. The gradient measurement is what makes shearography highly sensitive to localized anomalies (where the gradient changes sharply) and relatively insensitive to global tilts and rigid-body motions (where the gradient is uniform across the field).

It does not measure depth or volume. The signal is a surface signal. Defect depth, through-thickness extent, and exact morphology require complementary volumetric modalities: ultrasonic for depth-resolved imaging, X-ray CT for full 3-D reconstruction.

It does not measure under stress-free conditions. The induced stress is the modulation that exposes the anomaly. A purely static inspection of an unloaded part is not shearography's regime.

Where shearography fits in the composite NDT stack#

Composite NDT relies on a small number of complementary modalities, each with a defined sweet spot. Shearography's sweet spot is large-area, fast, surface-coupled inspection of skin-to-core bonds and shallow subsurface debonds.

Honeycomb-core panel inspection. Skin-to-core disbonds are the classic shearography target. A vacuum-stress shearogram of a honeycomb panel reveals disbonds as fringe anomalies above the intact-bond background. Wide-area inspection at high cadence; this is where shearography earns its place.

Wind-turbine blade skin inspection. Large blade areas, high inspection cadence, large interest in skin debonds and shear-web defects. Shearography is the workhorse for periodic blade-skin screening, especially for offshore blades where inspection time per blade is expensive.

Tire inspection. Aircraft and high-performance automotive tire belt-edge separations are a major shearography application outside the composite world; the same physics, same instrumentation.

Aerospace composite skin spot-checks. Where speed matters more than sizing precision, shearography is the fast-scan modality before a follow-up PAUT scan localizes and sizes anomalies for disposition.

Confusion points#

Shearography versus holography. Both are coherent-light interferometric methods. Classical holography records the full optical field at the camera, including the absolute displacement information. Shearography records a sheared interference pattern that encodes the gradient of displacement. The shearing simplifies the optical setup, suppresses sensitivity to rigid-body motions, and is the practical method for industrial inspection.

Shearography versus speckle interferometry (ESPI). Both are speckle-based. ESPI measures absolute out-of-plane displacement; shearography measures the gradient. ESPI is more sensitive to global motions and harder to use in industrial environments. Shearography is the field-friendly variant.

Shearography versus thermography. Both are non-contact, full-field, active methods. Thermography records the IR-camera time series after a thermal pulse and maps thermal-impedance anomalies. Shearography records optical fringe patterns under a mild stress and maps surface-displacement gradient anomalies. Different physics, complementary information; both are common in composite NDT.

Shearography versus PAUT. Shearography is fast, full-field, surface-coupled, non-contact, qualitative for sizing. PAUT is slower, point-by-point, contact-coupled, depth-resolved, and quantitative for sizing. The two are complementary: shearography for fast wide-area screening, PAUT for follow-up localization and sizing.

The shearogram is a fringe pattern, not a defect image. Shearographers read fringe morphology to infer defect geometry: a bull's-eye fringe pattern suggests a circular near-surface debond; a butterfly fringe suggests a delamination; a tilted ridge suggests an edge debond. Interpretation is skilled work and improves with operator experience and reference standards. A defect map produced from shearography is the operator's interpretation of fringe patterns, not a direct image of the defect.

Related terms#

• Non-destructive testing (NDT): the broader practice that shearography is one specific implementation of.
• Phased array ultrasonic testing (PAUT): the point-contact, depth-resolved modality that complements shearography's fast wide-area screening.
• Delamination: the subsurface defect class that shearography is well suited to detect in honeycomb-skin and wind-blade applications.
• BVID (barely visible impact damage): the damage class that shearography can flag on large composite skin areas before PAUT sizing.