Table of Contents ECNDT '98
D-Sight technique for rapid impact damage detection on composite aircraft structuresJ.H. Heida - NLR, Structures and Materials Division,
A.J.A. Bruinsma - TNO Inst. of Applied Physics, The Netherlands.
Corresponding Author Contact:
|TABLE OF CONTENTS|
The D-Sight technique is an optical double-pass retroreflection surface inspection technique
developed by Diffracto Ltd in Canada. It is a real-time technique for visualizing very small out-of-
plane surface distortions and it is particularly applicable to the rapid and enhanced visual
inspection of large surfaces.
An evaluation of the D-Sight technique for the rapid detection and assessment of impact damage in composite aircraft structures is described. The viability, reliability, limitations and possible improvements of the technique are discussed. The work has been performed within the EUCLID framework, RTP 3.1 "Aeronautical Application Technology", Work Package 2 "Global Diagnostic Damage Detection Methods".
A set of equations has been derived to establish the relation of various parameters of a D-Sight set up. To quantify the inspection results, a D-Sight index has been introduced which represents the contrast of the D-Sight signature of a defect. Image processing routines have been designed for automatic detection of defects from D-Sight images. It is concluded that the D-Sight technique can reliably detect significant impact damage, with a damage area equal to or larger than 5 cm 2 , within a field of view of about 0.25 m 2 . The sensitivity of the technique corresponds to indent depths in the range of 0.01 to 0.05 mm. Inspections on a full scale composite structure show that the D-Sight technique compares favourably with other inspection techniques, for the detection of impact damage and other defects with surface deformation.
The work presented in this paper has been performed within the EUCLID framework, Research
and Technology Project RTP 3.1 "Aeronautical Application Technology". EUCLID (European
Cooperation for the Long Term in Defence) is an European defence research and technology
cooperation initiative in which 13 European countries of the Atlantic Alliance are participating.
EUCLID RTP 3.1 involved industrial partners from 8 different nations with Germany as lead
nation. The work content of the 4 year project RTP 3.1 was divided into 3 research programmes
(Work Packages). The present work has been performed within Work Package 2 "Global
Diagnostic Damage Detection Methods". In this Work Package different damage detection
methods have been considered: shearography, holography, thermography, fibre optics and D-Sight
inspection. The general objective of this programme was the evaluation, verification and
configuration design of global damage detection methods and systems for "on aircraft non-destructive
inspection" of composite structures, which can fulfil the peacetime needs of the
services in a most cost effective way.
A specification of requirements for the damage detection methods evaluated in WP 2 is given in Ref. 1. The most important requirements are:
This paper will give the results of the evaluation of the D-Sight technique. The objective of the work was the assessment of the viability, reliability, limitations and possible improvements for the detection and characterization of impact damage and other surface anomalies in aircraft composite structures. Two companies were participating in this programme: The National Aerospace Laboratory NLR and the TNO Institute of Applied Physics (TPD) in the Netherlands.
D-Sight is an optical double-pass retroreflection surface inspection technique developed by Diffracto Ltd in Canada. It is a patented method for visualizing very small out-of-plane surface distortions such as indentations and protrusions. The principle of the D-Sight technique is well described in the literature (Ref. 2). The application of D-Sight for impact damage detection in composite aircraft structures has been addressed e.g. in Ref. 3.
Fig 1: Schematic diagram of the D-Sight set up
The D-Sight optical set-up consists of a light source, a camera, a retroreflective screen and the specimen (Fig. 1). The light from a standard divergent light source is reflected by the specimen. The surface of the specimen must be specularly reflective (specular reflection can be enhanced, if necessary, by wetting the surface with a thin fluid film). The reflected light then strikes a retroreflective screen which consists of numerous half silvered glass beads (typical diameter 60 µm). This screen attempts to redirect all incident light rays at the same angle to the initial reflection point on the specimen surface. However, the screen is not perfectly retroreflective and actually returns a diverging light cone instead of a single ray at the same angle. It is this imperfection of the retroreflective screen that creates the D-Sight effect. The light is reflected again by the specimen and collected by a camera placed slightly off-set from the light source. When the specimen is perfectly flat the camera sees the specimen with a uniform light intensity over the surface. An out-of-plane surface distortion, on the other hand, will result in local intensity differences. These intensity differences are caused by the imperfection of the screen because the light intensity returned by the glass beads is dependent on the viewing angle of the light cone relative to the incoming light ray. The result is that the surface distortion will appear brighter on one side and darker on the opposite side of the surface distortion, depending on the local slope of the surface. The process hence converts variations in surface slope to changes in light intensity (Ref. 2).
|Fig 2: D-Sight images recorded with the experimental set-up (left) and with the DSIS-500 system (right); composite specimen with 2J and 3J impacts (3J impact below)|
As an example, in Fig. 2 the D-Sight image of a composite specimen with impact damage is shown. The D-Sight technique is in fact a simple reflection surface technique but it has the advantage that a relatively large surface area (flat or moderately curved) can be viewed real-time with a fixed position of the light source, camera and specimen.
A set of equations has been derived in Ref. 5 in order to deepen the understanding of physical aspects and observed phenomena, and to establish the relation of various parameters of a D-Sight set-up. The most important equation is the one derived for the viewing angle þl for the "secondary D-Sight signature" of a local area on the object with a slope difference þ with the surrounding area:
with: 0 the viewing angle for a flat or globally curved object (0 =D /(LL +LR )), the change in viewing angle caused by a difference between the local slope and the slope of the surrounding area, D the distance between the centre of the light source and the centre of the
camera, LL the distance between the centre of the light source (or camera lens) and a point on
the object, and LR the distance between the object and the retroreflector.
Expression (1) allows a prediction of the brightness of the D-Sight signature of an observed point with respect to the surrounding area:
The D-Sight set-up has been modelled using a ray tracing program ASAP which can handle scattered rays. The model gives a good description of the D-Sight phenomenon and of the effect of e.g. the retroreflection diffraction angle, the size of the light source and the lens aperture. It is concluded that an indent with a maximum slope difference of at least 7.2 . 10-3 radians and with a diameter of 15 mm (with a depth of about 0.05 mm) will result in an intensity modulation with a high contrast.
|Fig 3: DAIS hardware configuration and connections (figure 2 from Ref. 4) Fig 4: Optical configuration of the DAIS-500 sensor|
Two inspection systems have been used in the evaluation of the D-Sight technique: an experimental D-Sight system of which the parameters of the optical set-up can be changed (Ref. 5; Fig. 1) and a commercially available DAIS-500 system (Diffracto Ltd) with fixed parameters of the optical set-up (Refs. 4 and 5; Figs. 3 and 4).
The experimental D-Sight set-up ("open" system) was made up of separate components:
Most parameters of the set-up such as LL ,LR , D, and ß (see Fig. 1) can be changed but the default parameters of the set-up were: LL = 1800 mm, LR = 500 mm, D = 50 mm, ß = 27° and the focal length of the imaging lens of the CCD camera: 25 mm. A rectangular inspection field of about 37 x 70 cm (0.26 m2 ) can be selected within the imaged trapezium shaped object field resulting at ß = 27°.
The commercially available DAIS-500 system (D-Sight Aircraft Inspection System) has been developed by Diffracto Ltd in Canada. It is an integral package of hardware and software ("closed system") consisting of the following components (Fig. 3):
For evaluation of the D-Sight technique a range of test specimens was used:
The D-Sight method only operates if the surface of the object to be inspected is specularly
reflective. In practice, surfaces may be finished with different coatings with a visual appearance
varying from highly glossy to mat. To enhance the specular reflectivity of the surface a thin
liquid film, called a highlighter, may be applied. The highlighter decreases the diffuse reflection
and increases the specular reflection. However, macro roughness such as coating thickness
differences and fibre structure in the surface layer will not be reduced by applying a highlighter
and will consequently be visible in the D-Sight images.
D-Sight measurements have been done both with and without highlighter. The applied highlighters were turpentine and M120 (Electron; trademark of Sentry Chemical Co.).
To quantify the inspection results, a D-Sight index was introduced which represents the contrast of the D-Sight signature of a surface defect:
|D-Sight index = 100 . (Imax - Imin )/(Imax + Imin )||(2)|
Automatic detection of defects can significantly decrease the inspection time, as most of the time consuming work during the inspections, such as determining and noting down the positions of defects, can be skipped. Furthermore, the detection of defects would then not be dependent anymore on the skills of the operator to recognize the features of a D-Sight signature of defects. In Ref. 5 a preliminary image processing procedure is presented that was developed to recognize and indicate the features of D-Sight signatures that are specific for indents that result from impact. Furthermore, an attempt was made to ignore D-Sight signatures from all kinds of other disturbances of the surface. In addition, the procedure was designed such that the method is almost insensitive to background intensity and to local variations of the reflection characteristics of the surface to be inspected.
The results of the experiments and measurements with the experimental set-up and with the DAIS-500 system are given in Ref. 5. An example of a D-Sight image of a composite specimen with impact damage is given in Fig. 2. A comparison of the D-Sight index values obtained with the two D-Sight inspection systems is given in Figs. 5 and 6. These figures give an overall picture of the capability of the D-Sight technique for impact damage detection.
Fig 5: D-Sight index (with highlighter) versus impact damage area, determined with the experimental set-up
Fig 6: D-Sight index (with highlighter) versus impact damage area, determined with the DAIS-500 system
D-Sight technique in general
D-Sight inspection systems
Impact damage detection