Local damage identification of high-strength circular concrete-filled steel tubes under low cycle fatigue

This paper investigates the low cycle fatigue (LCF) induced damages of high-strength circular concrete-filled steel tubular (CFT) beam-columns subjected to nonstationary loading history. The test specimens are fabricated by ultra-thin walled steel tubes with 2 mm in thickness to clarify the vulnerability concerning fatigue failure, and to evaluate the fatigue life of slender and over-design circular CFT beam-column members. Experimental results indicated that the specimens with a large D/t ratio together with thin-walled steel tubes tend to trigger highly-concentrated local buckling at column bottom where sequentially experienced premature fractures due to low cycle fatigue and the fragmentation of concrete infill. This complex LCF-induced failure mode can be identified by developing a new damage detection technique using polymer optical fiber sensors with distributed placement. It is found that the sequence of variable amplitudes during LCF history plays significant influences on the accumulation of plastic deformation which is not necessarily correlated to the sequence of nonstationary loading history induced by earthquake ground motions. Moreover, the epistemic uncertainty of variable amplitude of displacement demonstrates considerable influences on fatigue life with variable displacement amplitudes.