PSI - Issue 68

Mihaela Iordachescu et al. / Procedia Structural Integrity 68 (2025) 1147–1152 Iordachescu M. et al. / Structural Integrity Procedia 00 (2025) 000–000

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treatments applied to hot-conformed rods at the end of the manufacturing process. However, in the last decade a less costly method has been developed based on the addition of micro-alloying elements, such as Ti, V and Mo, so that a fine-lath martensite microstructure emerges and the required strength is directly assured by hot-drawing without any additional end-treatments. This singular microstructure entails a significant improvement in the ductile-brittle transition temperature because the crack propagation direction may change when the crack encounters boundaries of martensitic blocks or is arrested at martensitic packet boundaries (Bui et al., 2021, Zhou et al., 2016 and Zhang et al., 2022). Fracture toughness, fatigue cracking resistance and environmentally assisted cracking growth of conventionally manufactured high-strength bars were previously determined and related to microstructure peculiarities by (Iordachescu and Perez-Guerrero et al., 2018, Elices et al., 2017, and Iordachescu and Valiente et al., 2018). These findings are used in the present work as comparative references for the fatigue and fracture resistance assessment of this new type of structural bars. To overcome the limitations imposed by the geometry of the bars, the experimentation was based on testing pre-cracked cylindrical specimens. An engineering failure diagram (FAD) was constructed from the experimental results by plotting the failure loads against the crack size, as respective fractions of the bearing load capacity and the cross section of the uncracked specimen. The diagram was completed by delimiting the regions where toughness and yielding capacity, respectively, control the failure. Nomenclature a major semi-axis of the ellipses to which the fatigue cracks are assimilated b minor semi-axis of the ellipses to which the fatigue cracks are assimilated COD crack opening displacement da/ D N fatigue crack growth rate ∆F load range D K stress intensity factor range K c critical stress intensity factor R radius of cylindrical specimens P 0 bearing load capacity of undamaged cylindrical specimens P m maximum tensile load of pre-cracked cylindrical specimens A 0 cross section of the uncracked cylindrical specimens A f fatigue-cracked area of cylindrical specimens

2. Material and testing procedures 2.1. Steel characteristics

The high-strength martensitic steel sheets used to carry out this research belong to smooth bars of 23 mm diameter intended to form structural tendons. Table 1 gives chemical composition obtained by atomic emission spectrometry and Table 2 the mechanical properties determined by tensile testing cylindrical specimens of 5 mm diameter and 35 mm gauge length, machine-cut along the bar longitudinal axis. Fig.1a illustrates the representative microstructure of the bars, namely lath martensite, grouped in successive levels of laths, blocks and packages of random orientation which determine the crack growth peculiarity.

Table 1. Chemical composition of the bar steel.

C

Mn

Si

P

S

Cu

Ni

Cr

Mo

Ti

V

Fe

0.45

0.65

1.77

0.017

0.004

0.01

0.06

0.15

0.02

0.03

0.003

Bal

Table 2. Mechanical properties the bar steel.

Elastic modulus, [GPa]

Yield strength , [MPa]

Tensile strength [MPa]

Maximum uniform elongation, [%]

220

988

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