PSI - Issue 39

Bineet Kumar et al. / Procedia Structural Integrity 39 (2022) 222–228 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Civil engineering infrastructures are often subjected to repeated mechanical and thermal load cycles during their service life. These loading cycles could be a cause of subcritical crack growth and eventually leads to catastrophic failure. Therefore, the mechanical fracture behavior of structural members under the action of repeated loading is essential to understand. According to the Theory given by Griffith in 1921 for brittle material, fracture process zone (FPZ) present ahead of the crack tip, can be considered as a point, and therefore, the theory of linear elastic fracture mechanics (LEFM) can be applied. Nomenclature Q 0 activation energy at no stress Δ Q activation energy provided by external loading δ nano crack front jump length a n size of nano particle a m micro crack length l m critical length of micro crack k Boltzmann constant h Plank constant T absolute temperature E m modulus of elasticity at micro scale E modulus of elasticity at macro scale ν T characteristic attempt frequency rate ν 1 , ν 2 , ν 3 parametric constant a eff effective crack length D specimen size D 0 transition size (aggregate size) Δσ factored stress amplitude Δ K m stress intensity factor at micro crack tip Δ K stress intensity factor at macro crack tip K If fracture toughness for infinite specimen size K Ic fracture toughness for any specimen size G If c critical energy release rate for infinite specimen size t c cyclic loading period f c cyclic loading frequency C 1 material constant However, in the case of quasi-brittle fracture, (Bazant and Kazemi (1990), Bhowmik and Ray (2019), Otsuka (2000)) the non-linear zone surrounded at the crack tip is filled by the fracture process zone and undergoes progressive softening damage. Moreover, the size of the fracture process zone in quasi-brittle materials is not negligible compared to the crack dimension. The crack path while concrete fracture is very much dependent on the kind of toughening mechanism taking place inside the cyclic fracture process zone. According to the study, (Keerthana and Kishen (2020)) various toughening mechanisms are possible, in which micro cracking covers the significant portion of cyclic FPZ, which guides the actual crack path. Under cyclic loading, initially, random micro-crack gets formed with the breakage of chemical bonds between CSH gels and undergoes loading and unloading, (Keerthana and Kishen (2020), Simon and Chandra Kishen (2017)). However, after certain loading cycles, these random micro-cracks try to coalesce, called toughening. But, not all the micro cracks participate in coalescing; some of its portions go under permanent unloading, and those participating in coalescing lead to the actual crack path, (Le, Manning and Labuz (2014)). So, it can be said that actual macro crack formation occurs after the creation of many small scales cracks, the nano-scale range chemical bonds between CSH gels break and form micro-cracks, and then after these micro-crack coalesce form traction free macro crack, (Zhaodong and Jie (2018)). Therefore, it can be said that the fracture phenomena occurs at different