Table of Contents ECNDT '98 Session: Aerospace

New Developments of Pulsed Holography for Industrial Applications O. Petillon*, Aerospatiale *Corresponding Author Contact: Aerospitale, F-91893 Suresnes, France ; ODILE.PETILLON@siege.aerospatiale.fr

TABLE OF CONTENTS

Introduction Developments in holography Qualitative validation in industrial environment Quantitative results and test monitoring Conclusion Acknowledgements References

Introduction

Holographic interferometry has been used for years in the mechanical and non destructive testing (NDT) domains. Classical systems have proved their ability to provide both qualitative and quantitative information on part deformation. Being a global technique and having a high sensitivity, holography is well suited for the analysis of part behaviour under mechanical or vibration loading. The recent developments on pulsed laser, opto-electronic devices, acquisition and processing tools paved the way for the realisation of digital systems, that could operate under any industrial environment. This is the goal of the European Commission (EC) funded project "Pulsed Digital Holography and Shearography". Foreseen applications cover vibration analysis in the automotive industry, NDT, mechanical test monitoring and shock analysis for the aerospace industry, and shock monitoring for the nuclear industry.

Developments in holography

Holography combines fringe pattern obtained on a deformed object using a coherent laser and a recording medium. Most common recording media were photographic plates and later thermoplastic cameras. Those did not facilitate the spreading of the technique, as they did not provide direct quantitative information. This point has been partly resolved with the use of CCD cameras and the technique of Electronic Speckle Pattern Interferometry : the recording process is the same as for classical holography and the reconstruction is carried out in a computer. The applications have been yet reduced to quasi-static measurement, on optical tables.

To get most advantages of the technique in an industrial environment, it was necessary to develop ESPI systems adapted to pulsed laser and dynamic recording. Use of pulsed laser enables indeed to work in a noisy environment and to catch transitory events [1]. This goal is being achieved in the project, with the design of an adapted optical set-up, the development of specific electronic and software for acquisition and reconstruction. A first prototype has been realised, that already proved its performances outside the laboratory on very different types of applications.

The main features of the prototype include : the triggering of a CCD for recording 2 images within a few microseconds, the development of a multi-pulse Ruby laser (up to 4 pulses [2]) and of the associated recording head. Use of the system in various areas will be illustrated below.

Fig 1: loud speaker with t = 40 µs Ø 30 field viewed on the CCD

Fig 2: loud speaker with t = a few s

Fig 3: thermal loading - honeycomb/skin debonding

Fig 4: delamination in a G/E stiffened panel 640 x 480 CCD image

Fig 5: electrical casing

Fig 6: thermal loading - rubber debonding - 1024 x 1024 CCD

Fig 7: Shock loading (0.060 ms after impact)- shearography

Fig 8: Shock loading (0.030 ms after impact) - holography

Qualitative validation in industrial environment

Early validation tests have been based on a qualitative approach, in order to demonstrate the behaviour of the system in a noisy industrial environment. Some of the undertaken tests allowed in addition to cover one foreseen application, NDT.

The first noticeable aspects of a pulsed holography system are its ability to operate in almost any industrial environment. The results obtained are as good when performed in a laboratory, on a optical table, as when realised on industrial premises, with machines in operation in the neighbourhood.

The tests that have been performed during this first step cover the observing of vibrations and the detection of defects in composite parts. The case chosen for vibration was a loud speaker as it enabled an easy choice of the vibration characteristics. Depending on the existing modes, different vibration figures can be observed : they vary according to the eigen modes, their frequency and amplitude.

The results presented have been obtained with a excitation frequency near 10 kHz. In the first case (see Figure 1), the separation between the two shots is small and the high frequency mode is prevalent. The second case (see Figure 2) has been obtained with a pulse separation of a few seconds. Lower frequency modes, with more energy, become significant; the results are much less reproducible and depend on the acquisition time with respect to the different present modes.

Different composite parts have been tested with holography and shearography for the detection of defects, and with different types of loading. A sandwich sample (aluminium skin, honeycomb core) has been tested for skin-core debonding. This kind of defect could be very easily detected with a thermal loading of the part (see Figure 3).

A graphite/epoxy stiffened panel, representative of a wing part, in which several impact damages are present, has also been tested with thermal loading. Due to the stiffness of the part, it is more difficult to detect all delaminations with such a loading and the contrast is not as large as in the previous case. It is nevertheless possible to visualise some damaged areas (see Figure 4).

A electrical casing has also been tested : this small aluminium part has a rubber coating and it is necessary to assure a good bonding of the rubber. The sample chosen for the tests contained several artificial defects (see Figure 5). With a low thermal loading, it is possible to visualise the larger defects (see Figure 6) : the three of them are detected in the view shown below. To detect the smaller flaws, a vacuum loading would be more adequate.

A sandwich panel with glass fibre skin and honeycomb core has been tested with shock loading. This panel contained one identified debonding. The same area where this defect was localised has been tested with both shearography (see Figure 7) and holography (see Figure 8) systems. The inspected zone is around 300 mm x 300 mm. The identified defect (marked on both figures) is clearly highlighted; there is also other information appearing on the holography image that has yet to be confirmed using another method. The various tests performed have all been realised in an industrial site with no precaution taken with regard to vibrations. The qualitative results that were obtained are as good as previous known results on optical benches. Industrial applications of pulsed holography, in the NDT area mainly, look promising and further tests should allow to validate a large scale of use.

The other main interest of holography is its ability to give quantified results on part displacement or deformation. Another scope of the project is to test the applications of a pulsed technique on industrial needs and to evaluate the advantages. Under the foreseen applications were the monitoring of mechanical tests and of shock wave propagation for which the results obtained to date are presented below.

Mechanical test monitoring

In many industrial areas (aeronautics, nuclear casks, automotive), it is necessary to model the behaviour of critical parts and to validate this model through a few mechanical tests, on a restricted scale when possible. The methods used mainly to monitor such mechanical tests are gauges and sensors giving localised data on displacement or strain. The data are used to fit the parameters of the models.

Optical methods present the advantages of providing global information on a part deformation; they allow thus to monitor 100% of the structure behaviour. The most interesting data can then be selected for fitting the model.

Fig 9: sample with inserted flaw submitted to compression test sample size on the CCD : 220 x 350 pixels

To evaluate the application of the pulsed holography, it has been decided to monitor the compression test of representative samples : monolithic and sanwich samples with artificial defects. Due to the high sensitivity of holography, recordings have been made for small compression steps and the out-of-plane displacement of the samples was measured. The results presented have been obtained on a monolithic sample (150 mm x 100 mm). The artificial defect was located on the opposite side of the sample under three composite plies. The displacement map is represented for 200, 400, 600 and 800 N (see Figure 9).

The displacement map shows a major deformation along the 135° axis. This is an effect of the positioning of the defect in the lay-off. It is however difficult to compare such data to conventional results as the range of sensitivity is very different. Pulsed holography appears as a method able to give data in the range where conventional sensors are still unusable.

Shock monitoring

Fig 10: hammer shock (before wave reflection) - 640 x 480 CCD image

Fig11: hammer shock (after wave reflection)

Fig 12: shock wave propagation (23 µs after impact)

Fig 13: shock wave propagation (43 µs after impact)

Different applications were also foreseen that can be labelled as shock monitoring : bird strikes on aircraft canopy, pyrotechnic shock waves on launcher parts. Such applications imply high deformation speed and rate; it was hence necessary to acquire several images within a short time delay, which required the multi-pulse set-up.

The results presented hereafter stand for the increasing complexity steps to evaluate the feasibility of pyrotechnic shock wave monitoring. The first phase was the acquisition of mere hammer shock waves on a plate. The aims were to validate the multi-pulse operation and to define an adequate trigger for the acquisition system. The raw results obtained show the propagation of the wave :

• in the first case (see Figure 10), where the hammer was in the upper left, no reflection of the wave on the sides of the plate has occured yet but the wave front is already gone
• in the second case (see Figure 11), with the hammer up, the initial shock wave is already mixed with reflections and it is possible to observe steady waves.

This enabled to validate the operation of the multi-pulse head but showed that, due to laser fire delay, a triggering by the event was arriving too late and thus was not sufficient to observe the front wave. A pre-trigger system has then been designed and further tests could be realised on a ball drop. The results obtained on a 40 mm ball show clearly the wave front propagation (see Figure 12 and Figure 13). The contrast of the fringes is however decreased when compared to previous results. This is most probably the consequence of the laser pulse duration ( ~ 30 ns) : during the acquisition, the wave front moves around 5000 m/s and the acquired image gives the mean out-of-plane displacement during the pulse duration.

These first results look promising : it was possible to operate and get results with a multi-pulse holography system in a harsh environment (pyrotechnic shocks), without any special adaptation. The limits that appear are essentially linked to the laser characteristics (pulse duration, pulse repetition) and should be solved with the advances in laser technology.

The tests performed in industrial facilities have proved that pulsed digital holography systems can be used on a wide range of applications. The combined use of CCD and pulsed laser gives a new access to holography and shearography. The concerned areas are numerous : visualisation of windshield deformation during car mission, visualisation of vibrations on a bonnet or a gear box for different engine speeds, monitoring of mechanical tests on structural parts, on-site non destructive testing, monitoring of shock waves, etc.

The last drawbacks of conventional holographic methods have been suppressed with the substitution of photographic plates and thermoplastic cameras by CCD cameras. The first results obtained show that current limitations of the technique are the laser and the CCD size. Although the laser technology is constantly improving, it is still difficult to find commercially available pulsed laser suiting holography and industrial requirements : coherence length, acceptable cost, reduced size. In return, there is a high potential for gains in the CCD domain. It should allow in the future the acquisition of larger deformation amplitudes and the study of wider objects.

This work was supported by the European Commission, BRITE-Euram project BRPR-CT96-0152. We thank the team of the Institut für Technische Optik for their support in the operation of the multi-pulse system, and the team of HOLO3 for the realisation of part of the tests.

1. Digital holographic interferometry for shape and vibration analysis
   HJ. Tiziani, G. Pedrini, SPIE conference (1996)
2. Transient vibration measurements by using multi-pulses digital holography
   G. Pedrini, P. Fröning, H. Fessler, HJ. Tiziani, Optics & Laser Technology (1998)

|Top|