Laser NDT methods based on interferometric imaging, primarily holography and shearography, have seen growing acceptance since the mid 1980's. With the large increase in the use of composite materials and sandwich structures, the need for high speed, large area inspection for fracture critical, sub-surface defects such as disbonds, delaminations, sheared core or non-visible damage in aircraft, missiles and marine composites led to broad acceptance of laser based NDT methods. Laser NDT Methods employing holography and shearography imaging interferometers compliment UT, Thermography and other NDT methods as highly developed, mature and cost effective technology.
Laser interferometric imaging NDT techniques such as holography and shearography have seen dramatic performance improvements in the last decade and wide acceptance in industry as a means for high-speed, cost effective inspection and manufacturing process control. These performance gains have been made possible by the development of the personal computer, high resolution CCD and digital video cameras, high performance solid-state lasers and the development of phase stepping algorithms. System output images show qualitatively pictures of structural features and surface and subsurface anomalies as well as quantitative data such as defect size, area, depth, material deformation vs. load change and material properties. Both holography and shearography have been implemented in important aerospace programs providing cost effective, high-speed defect detection.
Holography images test part responses to changes in load showing the, as well as part movement. Holography using continuous wave lasers and video frame rate data acquisition require vibration isolation usually in the form of air supported isolation tables. Coupled with ultrasonic vibration excitation of the test part, holographic systems in production provide very high-resolution images of disbonds in small complex shaped components, such as turbine aircraft components and medical devices.
Shearography NDT systems use a common path interferometer to image the first derivative of the out-of-plane deformation of the test part surface in response to a change in load. This important distinction is responsible to two key phenomena. First, shearography is less sensitive to the image degrading effect of environmental vibration. Shearography systems may be built as portable units or into gantry systems, similar to UT C-Scan systems, for scanning large structures. Second, the changes in the applied load required to reveal subsurface anomalies frequently induce gross deformation or rotation of the test part. With holography, several important test part stressing techniques, such as thermal and vacuum stress, create gross part deformation. Defect indications may be completed obscured by these translation fringe lines. Shearography, on the other hand is sensitive only to the deformation derivatives and tend to show only the local deformation on the target surface due to the presence of a surface or subsurface flaw.
Shearography, in particular offers unique and proven defect detection capabilities in aerospace composites manufacturing. Shearography images show changes in surface slope, in response to a change in applied load. Shearography whole field, real-time imaging of the out-of-plane deformation derivatives is sensitive to subsurface disbonds, delaminations, core damage, core splice joint separations as well as surface damage. Secondary aircraft structures have long used composite materials. The drive for better vehicle performance, lower fuel consumption and maintainability are pushing the application of composites and sandwich designs for primary structures as well. Faster and less expensive inspection tools are necessary to reduce manufacturing costs and ensure consistent quality.