PSI - Issue 42

António Mourão et al. / Procedia Structural Integrity 42 (2022) 1744–1751 António Mourão / Structural Integrity Procedia 00 (2019) 000 – 000

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Keywords: Crystal Plasticity Finite Element; Bridge steel material; Monotonic response

1. Introduction Material characterization was and remains a relevant topic of study given the phenomenological models presented in literature for static and cyclic structural integrity assessment. Several examples of fatigue induced collapse can be found in literature such as the Kielland offshore platform on the North Sea (France (2019)), the railway accident of Eschede in Germany (Esslinger (2004)) , Chalk’s Ocean Airways Flight 101 out of Miami (NTSB (2005)), Taikoyama wind turbine tower (Liu (2001)), amongst others, all of which with either superficial or near-surface defects leading to the progressive reduction of structural capacity and eventual collapse, more often than not accompanied by the loss of human life. Concerning steel structures, several studies have found that bridges often present sines of damage, fatigue playing a dominant role, either, due to lack of maintenance, high loading demands, environmental conditions, amongst others (Oehme (1989), Imam (2010), Santecchia (2016), Antolovich (2014)). From a holistic perspective into European railway bridges, survey data gathered from all major European railway, quantifies and divides demographic data not only in terms of material but also structural system, with over 2000 composite steel/concrete bridges being over 100 years old and 21% of the total amount of bridges analyzed being purely metallic, totaling just over 47 000 metallic bridges (3% cast iron, 25% wrought iron and 53% steel). As expected from the material variety present in the survey, the European metallic bridge age profile can thus be drawn, with 22% of bridges ranging between the age of 20 to 50 years, hence coinciding with the first wave of steel standards and 68% of bridges falling into the over 50 years old category. Similarly, the American Society of Civil Engineers (ASCE) conducted a similar survey within the United States and arrived at the same conclusion in the annual report card for America’s infrastructure , outlying that the overall classification of the bridge infrastructure is presently classified as mediocre, going further on to describe that over 250 000 bridges are currently over 50 years old and that 7.5% of the total of bridges are considered as structurally deficient, thus raising a concern not only into the structural capacity of the infrastructure in both regions but also the present material conditions given time- and loading-based degradation of original material. Even though microstructural characterization of modern metals has been a topic of intensive study and effects of grain size and morphology impact on the material’s mechanical properties of polycrystalline metals can be found in literature (Keehan (2004), Barraza-Fierro (2014), Wen (2019)), older metallic bridge materials predating the modern standards such as the EN10025 are highly case-study dependent. Moreover, given the emergence of new materials obtained via advanced manufacturing technologies such as 3D printing where micro defects are present in large amounts this aspect becomes once again relevant. When it comes to enhancing the mechanical properties of alloys and metals, namely structural steels, grain size modification is the metallurgical procedure typically followed. Hall (1951), one of the pioneers on the effects of grain size in mild steels where an increase performance of the material under monotonic conditions was observed for specimens with decreased grain sizes, observed a linear relationship between the two variables. Similarly, Petch (1953) proposed a similar theory by correlating grain size and cleavage strength for mild steel, since then extended into other metals and alloys. The grain size effect, either considered by means of empirical data correlation, atomistic modeling or crystal plasticity have been addressed in literature. In this present work an initial study into crystal plasticity finite element of historic bridge material is presented and monotonic material behavior is the focus of this research, regardless, microstructural modeling presents significant advantages for fatigue damage assessment given the ability to translate multiaxial fatigue damage more easily either by the critical plane approach or accumulated strain energy.