Issue 27

M.-P. Luong et alii, Frattura ed Integrità Strutturale, 27 (2014) 38-42; DOI: 10.3221/IGF-ESIS.27.05 40 monitor screen (Fig. 1). Temperature differences in heat patterns as fine as 0.1 o C are discernible instantly and represented by several pseudo color grades. Brittle geomaterials often present a very low thermomechanical conversion under monotonic loading. In addition the thermal image of the specimen is often affected by several other factors such as the induced heat of the loading machine system, the undesired effects of the specimen ends, etc. This difficulty can be overcome when using thermal image subtraction or differential thermography . This procedure of thermal image processing readily evidences the manifestation of damage caused by the loading application between the two thermograms. The resulting image is a subtracted image showing the temperature change between two compared images, obtained under nearly identical test conditions, as shown in Fig. 2. This thermal image processing provides quantitative values of dissipation. The damaged areas are precisely located and highlighted by heat patterns in pseudo colors. (2a) Thermogram recorded at reference load level. (2b) Thermogram recorded at a given load level. (2c) = Thermogram (2b) – Thermogram (2a) . Figure 2 : The differential infrared thermography enhances the localization of intrinsic dissipation (temperature changes are given in °C). E XPERIMENTAL RESULTS he proposed technique has been applied in our laboratory on two brittle rock specimens: Massif Central diorite (France) and Viseu granite (Portugal). (3a) Quasi homogeneous diorite D. Grain size 400-800 µm, plagioclase feldspar (> 60%) and few percents of hornblende and biotite. (3b) Heterogeneous granite G. Grain size 100µm-10mm, quartz (≈ 20%), plagioclase feldspar (≈ 20%), orthoclase (≈ 20%), microcline (≈ 10%), biotite and muscovite (≈ 25%), few percents of apatite and altered minerals. Figure 3 : SEM (Scanning Electron Microscopy) images of the 2 specimens The parameter investigated in this work is the heat generation due to the dissipative behavior of the material under cyclic loading. The thermal images are recorded when the compressive cyclic loading is applied on the test specimen (Fig. 4). The contribution of the plasticity term is revealed by the rapid evolution of dissipation, evidencing the occurrence of damage caused by stress concentration in the central part of the specimen. T