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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Table 2. Tensile tests of B500A and B500B - non corroded and 90 days in salt spray chamber.

Φ 12

Mass loss ( % ) 90 days

Yield stress, R p ( MPa )

Ultimate strength, R m ( MPa )

Elongation to maximum load, A gt [%]

Energy density, U ( MPa )

Days of exposure

0

90

0

90

0

90

0

90

Mean values B500B 1-4 Mean values B500A 1-4

11.34

532.2

518.05

621.40

536.80

10.07

1.70

61.04

8.95

8.83

530.8

489.10

555.58

490.85

5.91

1.33

36.84

5.96

the drop of the mechanical properties of corroded steel bars is presented in Apostolopoulos et al. (2006). The very low values of U for both steel grades show that their use in construction of seismic areas is strictly contraindicated. Despite the fact that the standards do not require for the U evaluation of the reinforcing steel, the energy density has got a material property which characterizes the potential damage tolerance of the material and may be used to evaluate the material fracture under both, static and fatigue loading conditions, e.g. Sih et al. (1984). Note that the energy density may be directly related to the plain strain fracture toughness value, e.g. Jeong et al. (1995), which evaluates the fracture of a cracked member under plain strain loading conditions. The results of the tensile tests before and after corrosion are in complete accordance with Apostolopoulos et al. (2006) study although is referred to biphasic steel. According to this study, the observed appreciable reduction on tensile ductility may represent a serious problem for the safety of constructions in seismically active areas. As it occurs during the seismic erection, the reinforcement is often subjected to stress events at the region of low cycle fatigue and the need for a sufficient storage capacity of the material is imperative. Therefore, the reduced values of U of corroded steel are expected to have a direct negative impact on the service life of the specimens which come after the LCF tests. Therefore, the selection of high-strength steels (e.g. B500) in seismic areas is insufficient if it is not combined by high ductility characteristics. Such unfortunate choices are frequent when "shortsighted and temporarily" the saving resources factor is dominant in construction. 3.2. Low Cycle Fatigue (LCF) tests Based on mechanical performance of specimens (B500A, B500B) from tensile tests, continuing Low Cycle Fatigue tests (simulating seismic loads) were conducted. The tests took place on corroded and non-corroded B500B and B500A rebars, (A & Β refers to steel grade with low and medium ductility class). Tables 3-8 present the results of LCF tests, in terms of maximum and minimum stress (stress-strain values) corresponding to the different levels of imposed deforma tion, total number of cycles up to failure, dissipated energy (the dissipated energy was evaluated as the sum of the hysteresis loop areas) and mass loss. Table 3. Mechanical properties and life expectancy of Φ 12, B500B – non corroded

Sample

Free length

Strain ( % )

Max stress ( MPa ) 570.0

Stress at min strain ( MPa )

Force at min strain ( kN )

Min stress ( MPa ) -547.1

Min force ( kN ) -61.9

Cycles to failure

Dissipated energy ( MPa )

6 Φ

Mean value B500B 41-45 Mean value B500B 46-50 Mean value B500B 51-55 Mean value B500B 56-60

-540.3

-61.1

45

1058.80

±2.5%

8 Φ

547.8

-475.0

-53.7

-509.1

-57.6

20

391.60

8 Φ

606.9

-401.7

-45.4

-505.7

-57.2

9

306.92

±4%

6 Φ

611.0

-559.3

-63.3

-575.4

-65.1

13

546.90