Kinematic and thermal field measurements can provide rich information in the field of thermo-mechanics of materials and structures (e.g., constitutive model, fatigue behavior, cracking, etc.). Up to now, Up to now, silicon based sensor cameras (CCD and CMOS) have been widely used to perform in situ observation of the kinematic field on the material surfaces, mainly thanks to image correlation or interferometry [1-3]. Nevertheless, the acquirement of thermal fields using a silicon based sensor visible camera is possible and suitable for application at high temperature. In this study, a low-cost, high-resolution and contactless field measurement protocol is proposed to in-situ observe temperature distribution on the surfaces of AISI 304L austenitic stainless steel samples.

Actually, silicon based sensor visible cameras are also sensitive in the near infrared spectral band (0.7-1.1 μm) [4-6]. It makes the acquirement of thermal field possible when temperature exceeds 600°C. In this spectral range, the gray level of image varies with temperature evolution. It is usually considered as issues on the acquired images (saturation or poor dynamic range of gray levels) for visible acquisition. Adjusting the exposure time with temperature evolution is a suitable method to maintain the gray level of image constant.

In this work, we propose an algorithm to precisely and continuously adjust the exposure time to get a constant gray level of images whatever temperature evolution. Each pixel of the camera can be considered as a radiometer which converts the incident flux to pixel gray level values. In order to obtain the thermal field, a radiometric model of the used camera is identified by blackbody experiments: the flux intensity, which is defined as the gray level normalized by exposure time, is evaluated in function of the temperature of the surface. During experiments, the sample surface has an emissivity distribution between 0 and 1, while the emissivity of a blackbody is equal to 1. Thus the estimation of local surface emissivity of the sample surface is also conducted to get the true temperature of sample surface. The method to obtain the true temperature of sample surface can be decomposed in several steps:

(1) the pixel gray level values are obtained from the image acquired by camera;

(2) the corresponding gray level value of blackbody at the same temperature is calculated by the pixel gray level normalized by pixel surface emissivity;

(3) for each pixel, a radiometric model describes the relation between the intensity and temperature, thus the true temperature can be obtained from the intensity based on radiometric model;

(4) the knowledge of the true temperatures of all pixels provides the thermal field of the surface.

The way to evaluate emissivity distribution of the sample surface of AISI 304L austenitic stainless steel is an important issue. The presented technique can provide valuable high resolution thermal fields during high temperature thermo-mechanics experiments.

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[2] Besnard and all, Experimental mechanics, 2006

[3] McCormick and all, Materials today, 2010

[4] J.J. Orteu and all, Experimental mechanics, 2008

[5] Farzaneh and all, Journal of physics D: applied physics, 2009

[6] Teyssieux and all, Review of scientific instruments, 2007