Digital Shearography and Infrared Thermography (IRT) techniques were employed to test non-destructively samples from
aircraft structures of composite material nature. Background information on the techniques is presented and it is noted that
much of the inspection work reviewed in the literature has focused on qualitative evaluation of the defects rather than
quantitative. There is however, need to quantify the defects if the threshold rejection criterion of whether the component
inspected is fit for service has to be established. In this paper an attempt to quantify induced defects on a helicopter main
rotor blade and Unmanned Aerospace Vehicle (UAV) composite material is presented. The fringe patterns exhibited by
Digital Shearography were used to quantify the defects by relating the number of fringes created to the depth of the defect or
flaw. Qualitative evaluation of defects with IRT was achieved through a hot spot temperature indication above the flaw on
the surface of the material. The results of the work indicate that the Shearographic technique proved to be more sensitive than
the IRT technique. It should be mentioned that there is "no set standard procedure" tailored for testing of composites. Each
composite material tested is more likely to respond differently to defect detection and this depends generally on the
component geometry and a suitable selection of the loading system to suit a particular test. The experimental procedure that
is reported in this paper can be used as a basis for designing a testing or calibration procedure for defects detection on any
particular composite material component or structure.
The paper presents results of non-destructive testing of composite main rotor helicopter blade calibration specimens using the laser based optical NDE technique known as Shearography. The tests were performed initially using the already well established near real-time non-destructive technique of Shearography, with the specimens perturbed during testing for a few seconds using the hot air from a domestic hair dryer. Subsequent to modification of the shearing device utilized in the shearographic setup, phase stepping of one of the sheared images to be captured by the CCD camera was enabled and identical tests were performed on the composite main rotor helicopter blade specimens. Considerable enhancement of the images manifesting or depicting the defects on the specimens is noted suggesting that phase stepping is a desirable enhancement technique to the traditional Shearographic setup.
Optical interference techniques such as electronic speckle pattern interferometry (ESPI) and Digital Shearography have been shown to be of great value for the Non-destructive Evaluation of composite materials. The recent signing of a contract to purchase numerous Airbus aircraft for commercial use in South Africa as well as the purchase of the Grippen military aircraft has resulted in the capabilities of optical interference techniques for NDE purposes receiving increased attention.
The NDT Laboratory in the Department of Mechanical Engineering at the University of Cape Town has for a number of years been involved with the research, development, and applications of the optical NDE techniques of ESPI and Digital Shearography. This has led to the development of a portable inspection unit, based on Digital Shearography and the latest addition of a portable ESPI prototype.
In order to compare the capabilities of the developed prototypes, industry acceptable test specimens of composite aircraft components are subjected to tests using both systems. The results are presented and comparisons are drawn highlighting the advantages and disadvantages of these two optical NDE techniques.
Electronic Speckle Pattern Interferometry (ESPI) and Digital Shearography are optical interference techniques, suitable for non-destructive inspection procedures. Due to the stringent vibration isolation conditions required for ESPI, the technique is mainly suited for laboratory based inspection procedures, which cannot be said for Digital Shearography. On the other hand, the interference patterns obtained using ESPI exhibit better fringe definition and contrast than those obtained using Digital Shearography. The image quality of Digital Shearography can be improved by introducing phase stepping and unwrapping techniques, but these methods add a level of complexity to the inspection system and reduce the image refresh rate of the overall process. As part of a project to produce a low cost portable ESPI system suitable for industrial applications, this paper investigates various methods of minimizing the impact of environmental vibration on the ESPI technique. This can be achieved by effectively 'freezing' the object movement during the image acquisition process. The methods employed include using a high-powered infra-red laser, which is continuously pulsed using an electronic signal generator as well as a mechanical chopper. The effect of using a variable shutter speed camera in conjunction with custom written software acquisition routines is also studied. The techniques employed are described and are applied to selected samples. The initial results are presented and analyzed. Conclusions are drawn and their impact on the feasibility of a portable ESPI system discussed.
Currently, the NDE procedure for Alouette helicopter rotor blades consists of a visual inspection followed by a manual acoustic inspection procedure by trained personnel using purpose manufactured tapping hammers. The former inspection is aimed at detecting surface cracks and corrosion whereas the latter is intended to inspect the rotor blade for possible areas of delamination between the alloy skin and the leading edge spar or blade root reinforcing strip. This paper investigates the feasibility of using either the authors locally developed Portable Digital Shearography or Electronic Speckle Pattern Interferometry in conjunction with Mechanical Impedance Analysis in order to determine the possible presence and extent of defects more accurately and reliably. Both optical inspection techniques are discussed, their theory and apparatus presented and the inspection procedure described. The principle of Mechanical Impedance Analysis is also outlined and the inspection method described. The successful results of the study as well as defects detected are presented and discussed. Outlining the potential of using this alternative NDE method as an on-site, in-situ inspection procedure concludes the paper.