PSI - Issue 45

Yipu Guo et al. / Procedia Structural Integrity 45 (2023) 66–73 Yipu Guo et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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formal loading. Thus, the elastic modulus decreases with increase of confining pressure. In terms of the effects of recycling cycles, the elastic modulus firstly decreases after the 1 st and 2 nd recycling cycles then increases after 3 rd recycling cycle. Interestingly, the NAC has a similar elastic modulus to RACIII when subjected to confined triaxial compression.

9 10 11 12 13 14 15 16 17 18 E (GPa)

NAC RACI

RACII RACIII

0

5

10

15

20

s w (MPa)

Fig. 3. Correlation between elastic modulus and confining pressure.

5 To validate the obtained compressive meridians based on Willam-Warnke failure criterion, Fig. 4 (b) compares the Willam-Warnke fitting curves of NAC and RACI with experiment results from previous literature (Chen et al., 2015; Folino & Xargay, 2014; Yang et al., 2011). The absence of comparisons on RACII and RACIII is due to the unavailability of the database. The comparison illustrates that the compressive meridian of RACI agrees very well with experimental results from other studies. For the compressive meridian of NAC, a higher degree of coincidence is found when / <1.0 , while it underestimates the experimental results from other studies when / > 1.0 . The lower compression meridian under higher confining pressure should be mainly attributed to the lack of 3.4 Failure criterion The Willam-Warnke failure criterion is a five-parameter model that is widely applied in predicting the multiaxial failure characteristics of concrete and cohesive-frictional materials. The confined triaxial compression tests performed on cylinder Multi-RAC specimens in this study correspond to the stress condition 1 > 2 = 3 , in which the Lode angle is equal to 60°. The compression meridian equation can be expressed as: , = 0 + 1 ( )+ 2 ( ) 2 (1) where = 1 3 ( 1 + 2 + 3 ), , = √ 1 15 √( 1 − 2 ) 2 +( 2 − 3 ) 2 +( 1 − 3 ) 2 ; and , denote octahedral normal stress and octahedral shear stress of the compressive meridian, respectively; 1 denotes maximum axial stress; 0 , 1 and 2 are coefficients of the Willam-Warnke failure criterion. By fitting the experimental results, the compression meridian equations for NAC and 3 generations of Multi-RAC are acquired as Eqs. (2) – (5) and illustrated in Fig. 4 (a). NAC , = 0.069 + 0.937 ( )−0.186( ) 2 ( 2 =0.997) (2) RACI , = 0.094 + 0.855 ( )−0.093( ) 2 ( 2 =0.999) (3) RACII , = 0.039 + 1.050 ( )−0.211( ) 2 ( 2 =0.998) (4) RACIII , = 0.061 + 0.969 ( )−0.181( ) 2 ( 2 =0.999) (5)