PSI - Issue 10

K. Kaklis et al. / Procedia Structural Integrity 10 (2018) 129–134

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K. Kaklis et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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In this study the triaxial compression tests were carried out under the confining pressures of 2.09, 3.96, and 6.06 MPa. A cyclic loading scheme with five loops was performed for both the uniaxial and triaxial compression tests. The five loops were executed in the pre-peak region and the axial load was applied under load control with a rate of 200 N/s. The loading increment in each step was about 2.5-3.5 kN. In the final loading the axial load was applied under displacement control with a rate of 0.01 mm/s in order to obtain the complete stress-strain curve in the post peak region. Axial strain values were measured during the uniaxial and triaxial compression tests using the LVDT sensor internal to the MTS loading frame, in order to get comparable strain measurements from these tests. It should be noted that the most accurate strain measurements are achieved by using electrical strain gages, but this was not possible in the case of the triaxial compression tests, due to technical restrictions. In a cycling loading, when a specimen is compressed to a deviator stress level and then unloaded to zero deviator stress and then reloaded, the unloading and reloading branches are in many cases different from the virgin loading curve. The unloading and reloading curves usually differ from each other and form a narrow loop. This narrow loop circumscribed by the unloading and reloading branches may be approximately substituted by a straight line. The slope of this line can be taken as the Young’s modulu s as mentioned by Mogi (2007) and Jia et al. (2018). In this type of loading the strain is partly elastic and partly plastic. The elastic strain ( ε el ) is obtained as a recovered deformation by unloading to zero deviator stress while the plastic strain ( ε pl ) is defined as the accumulated residual axial strain of material after being unloaded to zero stress as presented by Bahn and Hsu (1998) and Mogi (2007). This plastic strain includes various types of unrecoverable deformation due to dislocation, viscous-flow and micro fracturing (Chen (1988)). Fig.2a presents typical deviator stress-strain curves with five loading-unloading cycles under uniaxial compressive and a conventional triaxial compressive loading for confining pressure of 3.96 MPa. The experimental results show that the mortar becomes ductile even under low confining pressure. The initial part of deformation before yielding, the so-called elastic part, is significantly curved and includes appreciable permanent deformation. Two different types of deformation are illustrated in Fig.2a, a strain-softening case for uniaxial compression and a strain-hardening curve for the triaxial compression test. These cases are also observed in Fig.1. At higher confining pressures, failure is less easily recognized as excessive strain on a strain-hardening curve as mentioned by Farmer (1982). In the case of triaxial compression test under a 3.96 MPa confining pressure, the slope of the straight lines for each of the five narrow loops (Fig. 2b) defines the Young’s modulus as described by Mogi, 2007 and Jia et al., 2018. It is obvious that the Young’s modulus decreases with increasing strain. Fig.3 shows the relationship between the plastic strain and the deviator stress (Fig.3a), as well as the total strain (Fig.3b) of mortar specimens subjected to uniaxial compressive and triaxial compressive loading for three different 4. Experimental results and discussion

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E = 15172 MPa 0 1 2 3 4 5 6 7 8 9 10 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 Deniator stress, σ 1 - σ 3 (MPa) Axial strain 1st loop 2nd loop 3rd loop 4th loop 5th loop E = 4710 MPa E = 11354 MPa E = 5877 MPa E = 4879 MPa

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2 Deviator stress, σ 1 - σ 3 (MPa) 4 6 8

Uniaxial 3.96 MPa

0

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

Axial strain

(a) (b) Fig. 2. (a) The complete deviator stress-strain curve for mortar specimen tested in uniaxial and triaxial cyclic loading under a confining pressure of 3.96 MPa. (b) The mean Young’s modulus for each loop of the triaxial cyclic loading under confining pressure of 3.96 MPa.