PSI - Issue 10

Alk. Apostolopoulos et al. / Procedia Structural Integrity 10 (2018) 49–58 Alk. Apostolopoulos and T. Matikas / Structural Integrity Procedia 00 (2018) 000 – 000

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500

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-4

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Fig 2. Impact of buckling on cyclic stress ( a) Representative Hysteresis loop comparison on a 8Φ (blue line) and 6Φ (red line) length bar of ε = ± 2,5 %; ( b) Representative Hysteresis loop comparison on a 8Φ (blue line) and a 6Φ (red line) length bar of ε=±4%.

Figs.1, 2 present representative hysteresis loops for different free lengths of non-corroded rebars for different range of imposed deformation. Fig.1a shows that hysteresis loops of 8 Φ specimens’ present greater buckling phenomena at ±4% strain compared with ±2.5% strain. Fig .1b shows that hysteresis loops of 6 Φ specimens present buckling phenom ena on both categories of str ain levels (±2.5%, ±4%). Fig s.2a, b confirm the above observation. These results lead to low levels of dissipated energy and cycles up to failure for 8 Φ specimens in contrast to 6 Φ specimens. Fig.2a, b and Tables 3-8 present the rapid decrease of maximum stress for 6 Φ and 8 Φ specimens at the same conditions of deformation range. The results which are presented in Tables 3-8 show that increasing free length of the specimens strongly influences on the results of the experimental tests, due to buckling phenomena which lead to premature unexpected failures. The results of Low Cycle Fatigue (LCF) tests show that increasing the buckling length severely reduces the mechanical performance of the steel bar. The negative effect of free length on the mechanical properties of steel bars is greater than the effect of the corrosive action. The number of cycles up to failure [N cycles ] is generally dropped with the increase of the level of imposed deformation and free length of the specimen. Additionally, it is evident that the degradation of steel rebar in terms of dissipated energy values, cycles up to failure and mass loss values. Particularly, the mean values of non corroded Φ 12, B500B steel bar for free length 6 Φ and ± 2 .5% imposed deformation, present 1058.80 MPa dissipated energy and 45 cycles up to failure. After 90 days of exposure time, the mean values of Φ 12, B500B steel bar present 628,57MPa dissipated energy (40.64 % reduction) and 28 cycles up to failure (37.70 % reduction) and 11.06% mass loss. The mean values of non corroded Φ 12, B500B steel bar for free length 6 Φ and ± 4% imposed deformation, present 546 MPa dissipated energy (48.3% reduction) and 13 cycles up to failure (71.1 % reduction). After 90 days of exposure time, the mean values present 385.27 MPa dissipated energy (29.56 % reduction in relation to 546 MPa and 63.6 % in relation to 1058.80 MPa) and 10 cycles up to failure (23.08 % reduction in relation to 13 cycles and 77.7 % reduction in relation to 45 cycles) for 11.39 % mass loss. On the other hand, non-corroded Φ 12, B500B steel bar for free length 8 Φ and ± 2.5 % imposed deformation, the mean values present 391.60 MPa dissipated energy (63 % reduction) and 20 cycles up to failure (55.5 % reduction). It is obvious that the increase of free length from 6 Φ to 8 Φ led to dramatic drop of dissipated energy and life expectancy for Φ 12, B500B steel rebar. After 90 days of exposure time, the mean values present 314.80 MPa dissipated energy (19.62 % reduction) and 17 cycles up to failure (15 % reduction) for 11.43 % mass loss. The mean values of non corroded Φ 12, B500B steel bar for free length 8 Φ and ± 4 % imposed deformation, present 306.92 MPa dissipated energy (71 % reduction) and 9 cycles up to failure (80 % reduction). After 90 days of exposure time, the mean values present 232.49 MPa dissipated energy (24.26 % reduction in relation to 306.92 MPa and 78 % reduction in relation to 1058.8 MPa) and 8 cycles up to failure (11.12 % reduction in relation to 9 cycles and 82.2 % reduction in relation to 45 cycles) for 11.58 % mass loss. Similar results as the above mentioned were presented at mean values of non corroded Φ 12, B500A steel bar for free length 6 Φ and ± 2.5% imposed deformation. 3.3. Failure analysis and fractography of fracture surfaces Ι n order to analyze the type of failure on the cross section surface, a scanning electron microscope (SEM) was used.