PSI - Issue 42

Mihaela Iordachescu et al. / Procedia Structural Integrity 42 (2022) 602–607 Mihaela Iordachescu et al. / Structural Integrity Procedia 00 (2019) 000 – 000

603

2

also determines the bar microstructure as pearlitic or martensitic, depending on the carbon content of steel. The sensitivity of both bar types to stress corrosion cracking in seawater and in a 20% aqueous solution of ammonium thiocyanate at 50°C - FIP environment (EN ISO 15630-3, 2019) has been previously researched by Valiente et al. (2016) and by Iordachescu and Valiente et al. (2018), as well as by Iordachescu and Perez et al. (2018), with the result that neither type experienced assisted cracking under certain specific threshold conditions of load and pre existing damage. However, once these are exceeded, the pearlitic bars practically did not show any cracking resistance and the resistance of martensitic bars was insufficient to be used as a basis for resilient design. Morris (2011) and Morris et al (2013) showed that actual heat treatment improvements, also incorporated into the manufacture of high-strength martensitic steel bars for structural use, allow lath martensite steels to be produced with a microstructure that provides higher toughness and lowers the ductile-brittle transition temperature. Nonetheless, the microstructure peculiarity given by its three structural components, namely blocks, packets and laths of martensite entails a potential risk of increasing the number of failure micro-mechanisms in assisted damage processes and of influencing the stress corrosion cracking behavior of the steel. This last issue is studied in the present work, together with the corresponding failure micro-mechanisms in a commercial structural bar type made of high-strength lath martensitic steel. The objective of the research is to assess the correlation between the macro- and the micro-mechanisms of failure by performing slow strain rate tensile tests on this steel in the FIP environment and by analyzing thus produced assisted cracking and fracture with scanning electron microscopy (SEM).

Fig. 1. Microstructure of the bars in transverse (a) and axial (a) directions ; sketch of the notched, tensile flat specimens for the stress corrosion tests (c) and notch geometry (d). 2. Material and experimentation 2.1. Material characteristics The material used to carry out the present research is a commercial high-strength lath martensitic steel with the chemical composition given in Table 1. It was supplied by the manufacturer as smooth bars with a diameter of 23 mm. These bars are primarily used as tendon rods and active reinforcement in prestressed structures and are produced by hot rolling, and acquire their mechanical properties by quenching and tempering. The mechanical properties of the steel were determined by tensile testing cylindrical specimens of 5 mm diameter and 35 mm gauge length, machine-cut following the bars longitudinal axis. Table 2 shows the average values of the most indicative mechanical properties.

Table 1. Chemical composition of the studied steel bars.

C

Mn

Si

P

S

Cu

Ni

Cr

Fe

0.47

0.72

1.68

0.011

0.002

0.01

0.02

0.48

Bal

Table 2. Mechanical properties the studied steel bars.

Elastic modulus, [GPa]

Yield strength ͕ [MPa]

Tensile strength [MPa]

[%] Maximum uniform elongation

220

988

1138

8