PSI - Issue 22

Lígia T. Silva, Bruno Mendes, Carlos Oliveira, Cristina Reis, Paula Silva; José Silva / First International Symposium on Risk and Safety of Complex Structures and Components (2019), Porto, Portugal

J. Gonçalves et al. / Procedia Structural Integrity 22 (2019) 181–188

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The L * a * b * colour space (also known as CIE LAB) is one of the most popular colour spaces for colour measurement and is widely used (Konica Minolta, 1998). According to CIE, a colour space is a representation, usually in three dimensions (CIE, 1995). In this colour space, the distance between two points roughly predicts how different colours are in terms of brightness, saturation, and hue (Schwiegerling, 2004). In this, L * indicates the luminosity, which varies from 0 (black) to 100 (white), representing a * and b * the chromatic coordinates, where by the CIELAB standard definition there are no specific limits (Hill and Vorhagen, 1997). Figure 7 shows the chromaticity diagram a *, b *, where a * and b * indicate colour directions: + a * goes red, -a * goes green, + b * goes yellow, and -b * towards blue. The centre is achromatic and with the shift of a * and b * to the periphery the colour saturation increases. ((Konica Minolta, 1998). The colour of the studied granite was determined by using the X-Rite 360 colorimeter, with 45/0 geometry, with the illuminate D65, an 8 mm aperture. These parameters were chosen because they were applied in similar studies (Grossi et al., 2006). The results will be expressed in the CIELAB system. About 20 measurements were made on each specimen, always performed by the same operator, following the collection method that can be observed in Table 3. With the calibrated X-Rite 360 colorimeter, 20 measurements were made. At the end, the equipment's display shows the average of the results. 2.4. Gloss Measurements Brightness can be measured by analyzing images or using specific equipment, brightness meters. In 2002, Erdogan made the procedures established by ASTM D523-89 compatible with image analysis. The preparation of the samples of two ornamental rocks was carried out at different polishing stages, and it was found that in both materials the brightness values were inversely proportional to the surface roughness. Sanmartín et al. (2011). They also noticed that the brightness values were affected by the rock colour, but in different ways depending if the surface is smooth or rough. The brightness of the studied granite was determined by the use of the Novo-Gloss Trio bright meter (Rhopoint Instruments), using the 20º, 60º and 85º geometries and making about 20 measurements in each specimen. These were always performed by the same operator, following the collection method that can be observed in Table 4. This equipment complies with various test standards (such as DIN EN, ISO 2813, ASTM D2457, etc.). Data was taken one by one, and at the end, a statistical study was done. After collecting 10 results the device was recalibrated. According to the manufacturer the data measurement error is 0.5% (which helps to explain the negative results). 3. Results and discussion Colour : As it may be seen in Figure 3 for this physical aspect, there are no significant changes in parameters * a and * b regardless of the finish used. However, as shown in Figure 4, the * L parameter (lightness) varies greatly (which can be seen with the naked eye). It was concluded that the polished surface finish is the darkest, and the grit is the lightest. The colour analysis can be summarized as follows (from darkest to lightest): Polished