Issue 50

D. Triantis et alii, Frattura ed Integrità Strutturale, 50 (2019) 537-547; DOI: 10.3221/IGF-ESIS.50.45 539 (stage C), with the same rate with that used in stage A (0.44 MPa/s). Finally, during the last stage (D) the stress level was again kept constant at 63.5 MPa, until the fracture of the specimens. The time evolution of the stress during all four stages, for a characteristic specimen, is depicted in Fig.1, in juxtaposition to the respective evolution of the axial strain (as it was recorded by the strain gauge) for the whole loading procedure until the fracture of the specimen. The respective stress-strain curve is shown in Fig.2. It is noted that for the major portion of stage A the material’s response is perfectly linear (excluding the unavoidable non-linearity accompanying the very early load- ing steps during uniaxial compression) until the stress reaches the level of 57 MPa, corresponding to about 90% of the frac- ture stress (point K in Fig.1)). The modulus of elasticity determined from this linear region of the stress-axial strain curve is equal to about 74 GPa (Fig.2), in good agreement with previous experimental protocols by Kourkoulis et al. [17]. In stage B, i.e., under constant stress of 60.5 MPa, the axial strain appears almost constant. However, thorough study of the respective data indicates that the strain during stage B is actually increasing (although imperceptibly), though at a gradually decreasing rate. In stage D, i.e., under constant stress of 63.5 MPa, the strain is increasing, at rates much higher compared to those of stage B. The time variation of the axial strain rate during stages B and D, namely the stages during which the stress is kept constant, is plotted in Fig.3. It is very interesting to emphasize the significant increase of the axial strain rate during the last steps of stage D, which eventually leads to the fracture of the specimen under constant stress (!), about 60 seconds after the stabilization of the stress level (or, equivalently, 25 seconds before the fracture of the specimen). 0 25 50 75 0 50 100 150 200 250 300 time (sec) stress (MPa) 0.0000 0.0005 0.0010 0.0015 0.0020 strain strain Series3 Series4 stress stage A stage B stage C stage D Axial stress [MPa] Axial strain Time [s] K t o t o strain stress Figure 1: Stress and strain versus time. The four loading stages of the loading scheme are clearly visible. 0 25 50 75 0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 0.00 stress (MPa) stage A stage B stage C stage D slope=74GPa 75 50 25 0 Axial stress [MPa] 0 4 8 12 16 Slope: 74x10 9 stage A stage B stage C stage D slope=74G Sta eA StageB StageC StageD s s s s Axial strain [x10 -4 ] Figure 2 : Axial stress versus axial strain for a typical experiment. The colour code is used to distinguish the four stages of the loading protocol exhibited in Fig.1.