PSI - Issue 54

Mihaela Iordachescu et al. / Procedia Structural Integrity 54 (2024) 52–58 Mihaela Iordachescu et. al / Structural Integrity Procedia 00 (2023) 000–000

53

2

these conditions, but their brittleness and sensitivity to environmentally assisted cracking entail some degree of risk for structural integrity. These limitations are related to the manufacturing processes strongly conditioned by the required tensile strength of the bars which, according to prEN10138-4 (2005), E722 (2003) and UNE-EN ISO 15630 3 (2019), must exceed 1GPa. The pearlitic or martensitic microstructures obtained from current manufacturing heat treatments lead to the weakness of these bars. Morris (2011) and Morris et al (2013) showed that some heat treatment improvements allow high-strength steels with lath martensitic microstructure to be manufactured, resulting in higher toughness and a lower transition temperature. In this view, previous research performed by Valiente et al. (2016) and Iordachescu et al (2018) regarding the stress corrosion cracking sensitivity of some commercial pearlitic and/or martensitic bars provides insufficient support for these to be used with resilient design purposes because of detected high brittleness and low resistance to assisted cracking. The paper addresses the damage tolerance of high-strength lath martensite steel bars as assessed from slow-rate tensile tests carried out on fatigue pre-cracked SENT specimens simultaneously charged with hydrogen in a standardized 20% aqueous solution of ammonium thiocyanate at 60 0 C (FIP) and compared with tensile fracture tests in air, involving the same material, specimen geometry and initial damage but standard loading rates. The fracture surfaces were analyzed by scanning electron microscopy (SEM) to determine the crack growth resistance and its dependency with the load conditions.

Nomenclature a, ã

crack size, relative crack depth (a/W) limit crack size for small scale yielding regime

a SSY

A 0 , A f resistant area (BW), cracked area B thickness of SENT specimen COD crack opening displacement E´ generalized elastic modulus EDM Electric discharge machining F, F 0

applied load, bearing capacity of SENT specimen

FTA

tensile fracture tests in air stress intensity factor

K

R p0.2

yield strength tensile strength

R m

SEM scanning electron microscopy SENT single edge notched, flat tensile specimen SSY small scale yielding regime SSRT-FIP stress corrosion cracking testing at slow strain rate W width of SENT specimen

2. Material and experimentation 2.1. Material characteristics

The material used for the present research is a high-strength lath martensitic steel which conform smooth bars, used as high-strength tendons in structural engineering. The bars with 23 mm diameter and chemical composition given in Table 1 are produced by hot rolling followed by quenching and tempering heat treatments in order to optimize the mechanical properties. These were measured by tensile testing cylindrical specimens of 5 mm diameter and 35 mm gauge length, given in Table 2 which also includes the Ramberg-Osgood parameters that substantiate the low strain-hardening capacity of the steel. The microstructure of the steel bars is illustrated in Fig 1a, since no difference for discriminating between the transverse and axial directions was observed in the prepared metallographic samples. According to Iordachescu et. al. (2011), the basic phase is the lath martensite, formed within the previous austenitic grains and grouped in