ISO/NP 8203-4

Fibre-reinforced plastics — Non-destructive inspection techniques — Part 4: Laser shearography

Scope

1. Principal method
This draft specifies the principal method for laser shearography NDE inspection of FRP composites. Here, out-of-plane surface displacement gradients are monitored, using coaxial illumination and observation, both effectively perpendicular to the part surface. Defect detection relies on local influences on the global surface deformation from sub-surface anomalies under the selected loading regime.

Laser shearography resolves some of the practical issues associated with traditional speckle interferometry by using self-referencing, rather than a separate reference beam, to minimise vibration sensitivity and using common optical paths to relax the coherence length requirements of the illumination.

Variations on this basic configuration are available but not covered here. For example, the nature of the surface deformation detected depends on the measurement configuration thus, by using wide angle symmetrical illumination of the part surface with perpendicular observation, the in-plane surface displacement gradients could be determined. It is also possible, under fully collimated illumination, to estimate the absolute change in the displacement field between stressed and reference conditions by integrating the shearogram results with respect to the shear vector. As the effect of the finite shear offset is still present in the raw data, the out-of-plane surface displacement determined is only an approximation of the true surface displacement.

2. Measurements
For this method of operation, three different measurements are considered:

A) Magnitude map of the phase shift of reflected light from the inspected surface resulting from stress-induced deformation changes. This measurement is approximately related to the out-of-plane displacement gradient of the inspected surface. This type of measurement may be used for defect detection and characterisation.
Measurements are ideally performed by comparison with a reference standard measured before or after the test component to verify system operation.

B) Estimation of the depth of the defect.
Note: This may require a series of interval loading steps of the component, either increasing or decreasing.

C) Effective defect size.
Note: The size observed may be influenced by material properties, defect type and defect depth, e.g. for a given defect size the apparent size is smaller near the surface than deeper within the part.

The measurement is performed with single-sided access where illumination and interrogation are applied on the same inspected surface. Stress excitation, suitable for generating deformation on the measurement side, may be introduced via the optimum route for the component and application (see sections 1.3, 5.2 and 7.7).

3. Stress excitation methods
Suitable excitation methods are required to induce the required changes in surface displacement of the composite part. Several proven approaches are available:
Vacuum – for large, flat or gently curving structures where close contact allows a transparent vacuum hood to be sealed onto the surface to create a local partial vacuum to generate surface flexure.

Pressure – for pipes or closed vessels where internal pressure changes generate global surface bending.

Thermal – for any shape of part where three-dimensional thermal expansion gradients occur in the region of heating producing bending.

Direct – for any part where small mechanical loads can be readily applied in either tension, compression, flexure, shear or torsion.

This draft does not currently cover dynamic/periodic or otherwise modulated excitation, i.e. sinusoidal heating or vibration loading.
Note: Novel or targeted excitation methods, e.g. microwave heating to highlight water ingress, are not covered specifically here though the same measurement principles may apply.

4. Materials and structures
This draft is applicable to both monolithic and sandwich FRP laminate constructions, with or without curved surfaces.

Fibre-reinforced thermoset and thermoplastic matrix composites incorporating uni- or multi-directional reinforcements in either a continuous or discontinuous format; including but not limited to woven fabrics, stitched fabrics, short fibre or particulate filled, honeycomb or foam cores, as well as combination or hybrid reinforcements.