·Table of Contents
·Methods and Instrumentation
Temperature Measurements with Two-Wavelength Pyrometer for Material Testing
H. Madura, T. Piatkowski, H. Polakowski, Z. Sikorski, M. Dabrowski
Institute of Optoelectronics, Military University of Technology,
2 Kaliskiego St. 00-908 Warsaw, Poland,
fax. (+48 22) 6668950,
The paper presents comparison of results of surface temperature measurements obtained with thermovision camera and fast pyrometer during tensile test of steel samples leading to necking and damage. The thermovision camera Inframetrics 760 (frame rate - 50 Hz) with the system of high speed data storage and the new elaborated fast operating two-wavelength pyrometer with sampling time of 0.2 ms, were independently applied for temperature measurements. It was shown that an adequate construction of two-wavelength pyrometer ensures more accurate temperature measurements due to compensation of surface emissivity changes of an investigated material. The data obtained using pyrometer enable also better interpretation of surface temperature distributions.
Investigations of mechanical properties of metals and their alloys can be supplemented with temperature measurements. Typical example of such investigations is material testing, where temperature changes accompanying the process of deformation carry information about transformation of material microstructure and distribution of stresses. During breaking, thermal effects occur rapidly in a relatively short time. It requires application of temperature meter of short measurement time. Typical thermovision cameras and pyrometers do not provide a complete data set needed for proper interpretation of temporal characteristics of physical processes. Thus, it is necessary to design new instruments measuring temperatures in a very short time .
The paper presents comparison of results of surface temperature measurements obtained during tensile test of steel samples leading to necking and damage. The termovision camera Inframetrics 760 (frame rate - 50 Hz) with the system of high speed data storage and the new elaborated fast operating two-wavelength pyrometer with sampling time of 0.2 ms, were independently applied for temperature measurements. Two-wavelength pyrometer was chosen because its measurements are independent of changes of a sample surface emissivity. Moreover, there is a possibility of selection adequate measuring area, optimal in relation to dimensions of the examined sample.
The results of temperature measurements obtained by means of thermovision camera have been compared with the results from two-wavelength pyrometer. A scheme of the pyrometer is shown in Fig. 1. It is a device adapted for very fast measurements. During the experiment the frequency of sampling was two orders of magnitude higher than frequency of a thermovision camera and amounted about 5 kHz. The number of the obtained data allows their additional processing, e.g., averaging with no loss of information on fast changes . Optical working bands were chosen in respect with the predicted range of the measured temperatures, i.e., 50 °C ¸ 150°C. Separation of determined radiation bands is ensured due to application of the following interference filters, Filter I - 5.46 m
m and Filter II - 4.5 mm. Moreover, the detectors with spectral characteristics ensuring maximal signal were used. The Detector I is of PDI type  and it is optimised for a wavelength of 5.5 mm.
Fig 1: Diagram of two-wavelength fast pyrometer.|
The Detector II is a PbSe photoreceiver. For the required signal-to-noise ratio, the interference filters of relatively wide band were applied, e.g., for Filter II of l
= 5460 nm this band has the Half Width of 500 nm . A measuring area of the pyrometer was 1´1 mm2.
Fig 2: Sample temperature measured by two-wavelength pyrometer ( ° ) and thermovision camera.(·)|
The results obtained during the initial phase of the experiment from the thermovision camera are consistent with those from the two-wavelength pyrometer but they differ for higher temperatures. The difference results from the fact that emissivity value for the camera was stated as a constant one while the two-wavelength pyrometer measures temperature independently of emissivity value. The sample was non-grey body within the considered range of temperatures and its emissivity is slowly varying function of numbers of parameters. The change in the sample emissivity results among others from temperature changes (for metals (~ÖT) and dynamic changes of a sample surface. For example, for the class of materials to which belongs the investigated material, increase in roughness rms from 3 to 4 mm causes the change in emissivity of 20% . Additional virtue of two-wavelength pyrometer is its low sensitivity to the change of a state of the measured surface. Due to that fact, temperature measurements made with the pyrometer are more representative for the final phase of an experiment than using thermovision camera.
It was shown that an adequate construction of two-wavelength pyrometer ensures more accurate temperature measurements due to compensation of surface emissivity changes of an investigated material. The data obtained using pyrometer enable also better interpretation of surface temperature distributions.
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