Among the different optical measurement techniques for full-field analysis of displacements and strains, digital image correlation (DIC) has proven to be very flexible, robust, and easy to use, covering a wide range of different applications. DIC measurement results, among others, are influenced by the systematic errors of the measurements system, a major source of which is due to imaging system calibration. We present a 3-D DIC system that provides online error information concerning diverse error sources, and even more important, the propagation of errors throughout the calculations to the resulting contours, displacements, and strains. On the basis of this system we discuss error sources, error propagation, and the impact on correlation results. Performance tests for studying the impact of calibration errors on the resulting data are shown for static and dynamic applications.
Digital speckle correlation techniques have already been successfully proven for accurate displacement analysis. With the use of two cameras, three dimensional measurements of contours and displacements can be carried out. The principle of this technique is pretty easy to understood and realized, opening a nearly unlimited range of applications. Rapid new developments in the field of digital imaging and computer technology, especially for very much dynamic applications, opens further applications for these measurement method up to high speed deformation and strain analysis, e.g. in the fields of, material testing, fracture mechanics, high speed testing, advanced materials and component testing. The dynamic range is combined with the capability to measure very large strains (up to more than 100%). The resolution of the deformation in space and time opens a wide range of applications for vibration analysis of objects. Since the system determines the absolute position and displacements of the object in space it is capable of measuring high amplitudes and even objects with rigid body movements, which is a big advantage against full field ESPI systems. The absolute resolution depends on the field of view and is scalable. Calibration of the optical setup is a crucial point which will be discussed in detail. Examples of the analysis of high speed harmonic vibration and transient events out of material research and industrial applications are presented. Results of measurement performed on a vibrating membrane and a tensile test sample are show typical features of the system.
Shearography has been validated as fast and reliable inspection technique for aerospace components. Following several years phase of evaluation of the technique, meanwhile, shearography has entered the industrial production inspection.
The applications basically range from serial inspection in the production line to field inspection in assembly and to applications in the maintenance and repair area. In all applications, the main advantages of shearography, as very fast and full field insection and high sensitivity even on very complex on composite materials have led to the decision for laser shearography as inspection tool.
In this paper, we present some highlights of industrial shearography inspection. One of the first industrial installations of laser shearography in Europe was a fully automatic inspection system for helicopter rotorblades. Complete rotor blades are inspected within 10 minutes on delaminations and debondingg in the composite structure.
In case of more complex components, robotic manipulation of the shearography camera has proven to be the optimal solution. An industry 6-axis robot give utmost flexibility to position the camera in any angle and distance. Automatic defect marking systems have also been introduced to indicate the exact position of the defect directly on the inspected component.
Other applications are shearography inspection systems for abradable seals in jet engines and portable shearography inspection systems for maintenance and repair inspection in the field. In this paper, recent installations of automatice inspection systems in aerospace industries are presented.
Full field optical measurement techniques have already entered into various fields of industrial applications covering static as well as dynamic phenomena. The electronic speckle pattern interferometry (ESPI) allows the non contact, sensitive and three dimensional measurement of displacements in the sub micron range of objects with dimensions from mm2 to m2. For dynamic and transient phenomena, the use of pulsed laser have already been reported for various applications and successfully proven for the determination of the structural response of different components. In this paper we would like to present recent developments in the field of pulsed ESPI applications where emphasis is put onto the full field measurement result. The use of a completely computer controlled system allows easy access to mode shape characterization, deformation measurements and the characterization of transient events like shock wave propagation. Recent developments of the 3D-PulseESPI technique led to a very compact and complete system with improved characteristics regarding robustness and operation. The integrated design of the illumination laser and sensors for image acquisition allows easy aiming and adjustments with respect to the object of inspection. The laser is completely computer controlled which is advantageously used in a completely automatic brake squeal inspection system, which captures the squealing signal, automatically fires the laser and provides the complete deformation map of the component under test. Examples of recent applications in the field of dynamic structure response, with an emphasis in the field of automotive applications are given.
Shearography has been validated as fast and reliable inspection technique for aerospace components. Following several years phase of evaluation of the technique, meanwhile, shearography has entered the industrial production inspection.
The applications basically range from serial inspection in the production line to field inspection in assembly and to applications in the maintenance and repair area. In all applications, the main advantages of shearography, as very fast and full field inspection and high sensitivity even on very complex composite materials have led to the decision for laser shearography as inspection tool.
In this paper, we present examples of recent industrial shearography inspection systems in the field of aerospace. One of the first industrial installations of laser shearography in Europe was a fully automatic inspection system for helicopter rotorblades. Complete rotor blades are inspected within 10 minutes on delaminations and debondings in the composite structure.
In case of more complex components, robotic manipulation of the shearography camera has proven to be the optimum solution. An industry 6-axis robot gives utmost flexibility to position the camera in any angle and distance. Automatic defect marking systems have also been introduced to indicate the exact position of the defect directly on the inspected component.
Other applications cover the inspection of abradable seals in jet engines and portable shearography inspection systems for maintenance and repair inspection in the field.
In structural testing the determination of the dynamic structure response is one of the most important and difficult tasks. Double pulse holography interferometer has been widely used for analysis of the dynamic behavior of the structures. Recently PulsESPI techniques are increasingly used to replace conventional double pulse holography interferometer, not only taking over the main benefits of holography towards conventional measuring techniques, such as full field, non contact and sensitive, but also uses modern video and computer techniques for image capturing and processing. Recent developments have extended the capabilities of PulsESPI techniques to 3D measurement of dynamic structure response and modal analysis of vibrations.
A shearographic measurement system, named ILIAS, is presented which allows the non-contact large area defect detection and structural analysis of composites and other lightweight structures (e.g. in the field of aerospace) with respect to faulty materials and structural defects. Of main interest are defects which are situated below the surface and are not detectable using visual inspection. The use of pulsed illumination additionally allows the vibrational analysis and detection of constructive weak points. In contrast to conventional methods such as ultrasonic, X-ray or eddy current, the ILIAS system allows a fast and large area survey inspection in industrial environments. Applications are presented for composite structures and materials of aircrafts.
Multilayer coatings manufactured from metallic hafnium and silica sources by reactive electron beam deposition, are being developed for high fluence optics in a fusion lasers with a wavelength of 1053 nm and a 3 ns pulse length. Damage threshold studies have revealed a correlation between laser damage and nodular defects, but interestingly laser damage is also present in nodule-free regions. Photothermal studies of optical coatings reveal the existence of defects with strong optical absorption in nodule-free regions of the coating. A variety of microscopic techniques were employed to characterize the defects for a better understanding of the thermal properties of nodular defects and role of thermal defects in laser damage. Photothermal microscopy, utilizing the surface thermal lensing technique, was used to map the thermal characteristics of 3 mm X 3 mm areas of the coatings. High resolution subaperture scans, with a 1 micrometers step size and a 3 micrometers pump beam diameter, were conducted on the defects to characterize their photothermal properties. Optical and atomic force microscopy was used to visually identify defects and characterize their topography. The defects were then irradiated to determine the role of nodular and thermal defects in limiting the damage threshold of the multilayer.
KEYWORDS: Laser beam diagnostics, Optical coatings, Reflectors, Absorption, Copper vapor lasers, Signal detection, Thermography, High power lasers, Temperature metrology, Thin film coatings
Surface thermal lensing is an alternate configuration of a photothermal deflection system that was used to measure low levels of optical absorption. The thermal lensing configuration facilitated the alignment of the pump and prove laser beams by using a larger diameter probe beam. This technique was applied to high performance optical coatings, specifically high reflectors at 511 nm, zero degrees angle of incidence. The absorptance of these coatings was previously measured using a high power copper vapor laser system. A high power copper laser beam is focused onto an approximately 2 mm diameter spot. A thermal camera senses the temperature rise with respect to the rest of the coating. The temperature change, power density and beam diameter were used with an empirical formula that yields optical absorption. The surface thermal lensing technique was able to resolve absorption levels lower than that achieved with the copper laser method.
Photothermal deflection measurements on optical components for 10.6 micrometer are presented. This technique is used to measure the photo induced deformation of the coated surface. The experimental results are compared to finite element calculations which allows the determination of the temperature and deformation profile at every point on the surface and within the sample. This comparison allows the determination of the thermal conductivity of the coatings. Although in the finite element calculations the coatings are approximated as only one single layer, we find a good agreement with the experimental results. The thermal conductivity of typical coated copper mirrors for 10.6 micrometer is found to be up to an order of magnitude lower than those of the corresponding bulk material.
Contamination-induced degradation of optics often leads to enhanced absorption, which presents a serious limit for many important applications, especially those associated with high power laser systems and/or large aperture components. For this reason many different techniques were developed during the last decade for weak absorption studies, which include laser calorimetry, photoacoustic spectroscopy, as well as various photothermal techniques. In this paper recent progress is presented for the surface thermal lensing (STL) technique, a novel photothermal method which is demonstrated to be an ultra-sensitive tool for monitoring contamination effects on optical components. Compared with the various conventional photothermal methods, such as the photothermal deflection technique, STL drastically reduces the experimental complexity but retains the advantages of being sensitive, accurate and non-contact, and capable of in-situ monitoring of optical absorption down to the sub-ppm level. Experimental data with emphasis on absorption measurements and contamination studies of optical thin films are presented.
Up to now theoretical calculations of the photothermal response of optical components have only been performed using analytical approximations of the adequate thermo-mechanical equations. In contrast, this paper presents finite-element (FE) calculations for the description of the photothermal behavior of optical components at 10.6 micrometers . The FE-analysis allows the calculation of the surface temperature- and deformation-profile for bare substrates and two additional coating layers with respect to their thermo-optical properties. Also the behavior of the interfaces (substrate-coating, coating layer1 - coating layer2) can be calculated. These FE- model calculations of the photothermal response were performed with respect to the thermal properties and absorption characteristics of the coating and substrate respectively. The results are compared to experimental results obtained using the photothermal deflection technique (PDT). Additionally, the FE-model used allows the calculation of the time resolved response of the optical component to pulsed irradiation. The calculations of the temporal behavior are studied for a pulsed PDT situation and are compared to measurements. The results can also be used to determine the heat conductivity of the coatings of optical components. Some measurements and comparisons with the calculations are shown and discussed in detail.
The approved Draft International Standard 11551 on test methods for absorptance of optical laser components recently passed the international voting procedure. The utility of this standard practice document is the subject of the present round robin test. In order to cover a broad range of CO2-laser optical components, different types of metal mirrors and transmissive ZnSe-optics were included in the master sample set. After an initial inspection, this set passed through a series of optical laboratories in the United States, the United Kingdom, and Germany. The absorption of the samples was measured by calorimetric methods according to ISO DIS 11551, and the measurement results were compiled by the coordinating institute. The evaluation of the experiment was not started until all tests were completed. The results of the round robin test are discussed, compared and evaluated with respect to ageing mechanisms in optical coatings for CO2-laser systems. Although a great variety of different test facilities was employed by the round robin partners, a good agrement of the absorption values was observed for the wavelength of 10.6 micrometers . This demonstrates the versatility of the approved Draft International Standard 11551 for the calorimetric measurement of optical absorption in CO2-laser components.
The photothermal deflection technique (PDT) is applied to the optical and thermal characterization of optical components for high-power CO2 lasers. Besides bare copper and silicon substrates with different metallic and dielectric coatings, ZnSe optics were investigated at 10.6 micrometers using the laser-induced sample surface doformation to deflect a HeNe probe laser beam. The measured deflection signal is compared to laser-calorimetrically measured absorption whereas the surface deformation profile is used to investigate the thermal behavior of the different coating and substrate types. A finite-element-analysis (FEA) has also been performed, which allows the calculation of the surface temperature and the deformation profile for bare substrates and two additional coating layers with respect to the thermo-optical properties. These results are used as additional information for the sample characterization. The accuracy of the measurements of the thermal conductivity using the PDT technique is discussed in detail.
Single shot laser damage studies have been performed using an RF-excited long pulse laser and a TEA-CO2-laser with pulse durations of 1.2 ms and 100 ns, respectively. Besides bare diamond turned copper mirrors with different metal and dielectric coatings, ZnSe-optics with selected coating types were tested. The temporal damage behavior in the long pulse regime was investigated on the basis of a damage detection system with a time resolution of 10 microsecond(s) . The dependence of the damage threshold on the intensity is discussed in consideration of the integral absorptance of the coatings. The measured damage thresholds of this detection system are compared to those obtained by Nomarski/darkfield microscopy. The local variation of the laser induced damage threshold is correlated to the corresponding photothermal deflection signal, reflectance, and defect density of the coated surfaces.
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