PSI - Issue 42

Rafael Magalhães de Melo Freire et al. / Procedia Structural Integrity 42 (2022) 672–679 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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temperature, strain hardening, or process and events that causes precipitations in the microstructure. Many authors have given importance to the effect of plastic deformation on toughness degradation, as done by Kosuge et al. 2020. The present studies also analyze the material behavior for steels applying plastic deformation, as Kosuge et al. 2020. However, a different methodology is employed to carry out the pre-strain cycles that can simulate earthquakes or other events that plastically deform the material. It is known that the transition temperature changes when steel is strained once, Miki et al. 2000, and it can be inferred that a structure, which was carefully designed, can start operating in the ductile-brittle transition or brittle zone. Therefore, the assessment of cyclic pre-strained materials under low temperatures plays an important role in safety conditions. Assuming that many events that cause plastic deformation of a structure in operation can be classified as low cycle fatigue, the material damage estimation rule, introduced by Coffin (1954), Manson (1965), and Minner (1945), can be used to calculate the material damage degree in order to assess cyclic pre-strained steels. Also, CTOD values and Weibull stress can be related to the corresponding damage degree to investigate the impact of cyclic pre-strain on material toughness. Based on these considerations, the present study uses these approaches to analyze the fracture toughness degradation of SM490AN steel under various pre-strain patterns, carrying out four-point bending tests, CTOD experiments, and Finite Element Method simulations. This research used a new proposed equation of Weibull stress that can account for the difference between the performance for pre-strained materials with the last pre-strain cycle by compression or tensile load, and it was coined based on experimental data and the application of low cycle fatigue and damage degree given by Miner’s Law. After calculating the Weibull stress values, the experimental data was analyzed using the new approach for a minimum of three equivalents (MOTE) as a parameter to present the relevant values of critical CTOD, introduced by Ozawa et al. (2022). In addition, a new CTOD equation is proposed with the objective to simplify the analysis for fracture toughness reduction due to plastic deformation. The new approach of the MOTE of critical CTOD, Ozawa et al. (2022), was employed to obtain the critical CTOD values for the experiments and calibrate constants of the proposed CTOD equation obtained from the Finite Element Method simulation.

Nomenclature CTOD Crack tip opening displacement σ 1 m Maximum principal stress α 1

The largest of the principal components of the back stress

d α 1 Back stress rate δ cr,MOTE Minimum of three equivalent of critical CTOD δ cr,u Critical CTOD δ sat Saturation value of fracture toughness reduction Δ Stroke amount P Load from CTOD test Vg Notch mouth opening displacement

2. Methodology The degradation of fracture toughness can be evaluated by manufacturing the specimens and conducting pre-strain procedures and CTOD tests from pre-strained specimens. The material used in the experiments is JIS G3106 SM490AN, and its properties are shown in Table 1. T he material’s transition temperature is -75°C and CTOD tests from the “as - received” steel plate at various temperatures were carried out to obtain it. For evaluation of the pre strained material’s performance, the main CTOD tests of this study were executed at this temperature.

Table 1 – Material properties

Chemical compositions [mass%]

Tensile property*

C

Si

Mn

P

S

Nb

YP 364

TS

EL

0.17 0.32 1.37 0.008 0.002 0.003

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