In this contribution we show experimental results for combined thermal and DSPI measurements on several fiber
reinforced polymer test samples that have been impacted in a drop tower. Active thermography can give complementary
information with regard to the type and size of the damage, but DSPI is used for assessing the effect of the damage, i.e.
the difference in the displacement or strain field of a damaged and undamaged specimen.
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.
There are no international standards or norms for the use of optical techniques for full-field strain measurement. In the paper the rationale and design of a reference material and a set of standarized materials for the calibration and evaluation of optical systems for full-field measurements of strain are outlined. A classification system for the steps in the measurement process is also proposed and allows the development of a unified approach to diagnostic testing of components in an optical system for strain measurement based on any optical technique. The results described arise from a European study known as SPOTS whose objectives were to begin to fill the gap caused by a lack of standards.
Systematical errors of digital image correlation (DIC) measurements build a limiting factor for the accuracy of the resulting quantities. A major source for introducing systematical errors is the system calibration. We present a 3D digital image correlation system, which provides error information not only of diverse error sources but even more 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.
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.
The need for standards in optical methods of strain measurement has been discussed previously and attention has switched
to the creation of reference materials and standardised tests. Reference materials provide a means of calibrating a
measurement system by comparison to a standard that is traceable to an international standard. In this way an unbroken
chain of comparisons between the measurement system and the international standard with defined uncertainties in each
comparison is created. A standardised test allows the performance of the measurement system to be assessed against a
number of known quantities and such tests should be as challenging as the applications for which the measurement system
has been designed. The preliminary design of a reference material for optical techniques of strain measurement are
presented. Results obtained from the tests of these physical reference materials using digital image correlation, ESPI,
grating (moire) interferometry, photoelasticity, strain gauges and thermoelasticity support the design hypothesis and have
aided the refinement of the design. The first set of results produced with the new design showed remarkable correlation
despite being obtained independently in four different laboratories in four different countries using six different techniques.
Initial designs for a set of standard tests have also been created and some preliminary results will be presented. The concept
of virtual standardised test materials has been introduced to allow the performance of the algorithms within a measurement
system to be assessed so that a standard and comprehensive diagnostic and evaluation framework will be available to system
designers, manufacturers and end-users.
Shearography is a full field interferometric technique for the measurement of small deformation gradients. The main fields of application are non-destructive testing in quality control and material inspection. In both cases the direction of the deformation gradient plays a major role. We present a new image shearing speckle pattern interferometer set-up for simultaneous measurement of two independent deformation gradients equivalent to two shear directions. Herein, we use the polarization effect to separate the two directions. Based on a Michelson interferometer, we placed in one arm an additional polarization beamsplitter (PBS) and an additional mirror. The two orthogonal polarized images can be separated by a further PBS and simultaneously captured by a second camera. Substituting the second PBS with a rotatable polarizator and using only one camera, both sheared images can be recorded in rapid succession. We make use of a liquid crystal cell as a fast rotating polarizator and demonstrate the utility of this new concept with measurements in an industrial environment on a tensile testing machine.
Knowledge about optical and thermal properties of tooth tissue is necessary for application and use of new laser techniques in dentistry. Experiments have been carried out with tooth tissue from human and some species of animals at wavelengths reaching from ultraviolet to visible regions, using different spectroscopic methods which give complementary information about the samples. Photothermal spectroscopy, remittance spectroscopy, and fluorescence spectroscopy provide the possibility for differentiation between tooth constituents.