Issue 41

A. Mardaliazad et alii, Frattura ed Integrità Strutturale, 41 (2017) 504-523; DOI: 10.3221/IGF-ESIS.41.62 507 This study is aimed to investigate the mechanical response of the rock based on both the maximum loading and the displacement. The applied load of the compressive platen was measured automatically by the load cell of the apparatus. It is not mentioned how to measure the displacement of the specimen on the ASTM standard. It was decided to measure the displacement of the specimen by means of a displacement gage contacting extensometer. As can be seen in Fig. 2, the flexible tip of the extensometer is located at the center of the lower face of specimen. However, since the extensometer, itself, is fixed to the fixture of the testing machine, the displacements of all the components in between are measured. Therefore, in the configuration expressed in Fig. 2, the experimental data provided by the extensometer contains the displacements of the specimen, the fixed rollers and the steel blocks between the rollers and the fixture of the apparatus. This set of data therefore doesn’t represent the actual displacement of the specimen, but since the mechanical properties of the rollers and the steel blocks are known, they can be replicated in the numerical simulations as well. This set up is thus a convenient way to record the experimental data of the flexural test in terms of displacement. The broken specimens after performing the flexural test are indicated in Fig. 3. In all of the specimens, cracks initiate at the lower part, which is subjected to tension stresses and then propagate in an upward manner through the depth. Also, the cracks are located under (and close to) the section of the specimens which are in contact with the moving rods. Figure 3 : The broken specimens after performing the flexural test; (a) front view; (b) isometric view. The maximum load, displacement and the flexural strength are reported in Tab. 1, separately for each specimen. Average values, 95% Confidence Interval (“95% CI”) of the average value and standard deviations has been also calculated from experimental data. CI is an interval that try to estimate range of the average value of the population from the sample data. Due to the large variability of the mechanical behavior of rock this range is more representative than the single average value obtained from the 5 experimental tests. Fig. 4 also expresses the load-mid span displacement diagram of the all specimens. Maximum Load [kN] Maximum displacement Flexural Strength [MPa] Specimen K1 3.59 0.502 8.3124 Specimen K2 4.258 0.662 10.024 Specimen K3 2.963 0.313 6.831 Specimen K4 4.16 0.669 9.7927 Specimen K5 4.206 0.628 9.9576 Average value 3.8354 0.5548 8.9836 “95% CI” of average value [3.1433; 4.5275] [0.36737; 0.74223] [7.2530; 10.7140] Standard deviation 0.55736 0.15095 1.3937 Table 1 : The experimental results of the flexural test. (a) (b)