Laser-based inspection

Laser shearography
Laser shearography uses an interferometer to detect the out-of-plane derivative of deformation of the subject due to the non-homogeneous strain field caused by subsurface flaws. Creating an out-of-plane stress by applying vacuum, vibration or thermal stress, plays upon the weakest bond of a laminar structure. The combination of correct stressing and shearography provides a rapid non-destructive evaluation of composites, honeycombs and sandwich constructions both in production and in service, to qualify repairs and impact damage. Shearography can map changes in strains to 0.1 microstrain at video frame rates.

Laser ultrasonics
Defect mapping of contoured parts can be rapidly and easily performed by scanning laser beams over the surface of the part to be inspected. Two laser beams are used – one to generate and one to detect ultrasonic waves at the surface of the component.  With this technology, not only is there no need for a couplant, but several other limitations of conventional ultrasonic testing are overcome. For example, there is no critical orientation requirement of the optical beams with respect to the inspected component’s surface. Very complex geometric shapes can thus be inspected and access is only needed from one side.

The system can be used with many different types of materials, including polymer-matrix composites (graphite, Kevlar and glass epoxy), thermoplastics and painted metal and non-metallic structures. Honeycomb structures in, for example, aluminium or Nomex can be inspected with ease.

Digital holography
Complex components ranging from aero engine turbine blades and felt metal engine seals, through to plasma-coated artificial hip and knee joints, are regularly inspected worldwide by digital holography, which provides a real-time inspection that is clean, remote and full field. The systems utilise high-frequency vibration to highlight disbonds or irregular internal structure.

Strain mapping
A significant advance has been made in laser strain measurement in that in-plane, calibrated, linear strain fields can be measured. This significant breakthrough allows engineers to monitor strain concentrations visually instead of calculating stress by finite element modelling. As strain is the material’s reaction to stress, the detection of high strain areas can be used to prevent cracks from forming.

Production equipment and applications
Production shearography equipment has revolved around vacuum test chambers. High-definition shearography systems are able to discern single honeycomb cell failures in a metre-square panel in seconds. These chambers are also used for attenuation where acoustic stressing is used as in ventilated honeycombs for spacecraft. Production shearography systems are configured for numerous applications, ranging from business jet fuselage assemblies to helicopter blades, flaps, honeycomb panels and missile fuselages. Incorporation of shearography into production is proving to reduce inspection time drastically, with 100% coverage and digitally stored images.

On-site equipment and applications
On-site shearography and strain mapping, as in aerospace applications, obviates teardown for C-scans (see Ultrasonics). By vacuum stressing the skin of the repair, unbonded or weakened areas as well as far-side honeycomb debonds will be detected. Repairs in composite honeycomb and solid laminates are also easily evaluated with vacuum loading shearography, as is the ability to distinguish dents (crushed core) in aluminium honeycomb from true unbonds. Vacuum stressing has also found an application in the inspection of the AWACs rotordome, where other techniques have proved unsuccessful in finding defects in the complex composite structure.

Marine applications
Advanced composite constructions such as RNLI Lifeboats are built of FRP with PVC closed cell, foam cores, often 100 mm thick. The laying up of a 17 m hull induces unbonded areas and air pockets even under the most stringent quality regimes. The rapid scanning provided by the portable vacuum hood has the ability to find defects at both the skin to core and intercore bondlines. Using this technique, disbonds and areas of unhardened adhesive have been detected 60 mm below the surface.