Non-destructive testing of elongate composite structures using laser shearography
Publication Date: 2015-Apr-14
The IP.com Prior Art Database
Non-destructive testing (NDT) methods such as for example radiography, ultrasound testing or microscopy or other types of examination can be useful in quality monitoring and control of a variety of performance-critical components, yielding valuable qualitative or constructional data without the testing method causing damage to the component. One optically based NDT method is known as shearography, in which a first interferometric, so-called “shear” image of a component surface is compared with a second interferometric, “shear” image of the same surface. The images may be created from a coherent light source such as a laser. Light reflected from the component material surface may be captured by a CCD camera to form images which may be analysed. The method can allow the detection of small, often invisible or embedded defects in a wide range of components. Shearography relies on a small but observable phenomenon according to which material defects such as voids, cracks, discontinuities, structural or material irregularities or other irregularities cause local distortions at the material surface when the material is in a state of moderate physical strain. The local surface distortions, though small, may be detectable. A first “shear” image is created in an unstressed state of the component material. This provides a kind of reference image. The component is subsequently stressed by any suitable means such as by heating it to induce local thermal stress or by applying a local vacuum or by applying another moderate distorting force to the material. A second “shear” image is created in a consequently strained state of the component material. These first and second interferometric, “shear” images are compared by subtraction to reveal a discernible pattern generated from the component surface in its strained and unstrained states. If the component surface under strain exhibited spots or regions of increased distortion than the surrounding surface, then these spots or regions will become visible from the image comparison. Material defects in the component can thereby be located. When used for quality control of components made from composite material, the shearography NDT method may reveal defects including irregular fibre arrangement such as wrinkles, the presence of which may weaken the composite material component and which may be difficult or impossible to detect using other techniques. Examples of composite components which may be examined using shearography may include high performance structures such as boat hulls, wings, wind turbine blades or components thereof such as reinforcing spars or webs. Any of these may be examined,...