Investigation of low cycle fatigue performance of diagonal, V-shaped and chevron buckling restrained braced frames

Authors
M. Gerami 1
P. Sedaghati 2
M. Gerami 1
1 Associate professor
2 PHD student of Earthquake Engineering in Semnan University
Abstract
Based on ASTM E1823 standard, fatigue phenomenon is the process of permanent, progressive and localized structural change which occurs to a material point subjected to strains and stresses of variable amplitudes which produce cracks which lead to total failure after a certain number of cycles.
During an earthquake fatigue failure can occur at loads much lower than tensile or yield strengths of material. Therefore material behavior under cyclic loading is an important design criterion.
Fatigue data are obtained from the experiments and are shown in S-N curves which represent stress or strain amplitude versus number of cycles. All fatigue ranges can be included generally in three categories. Ultra Low Cycle Fatigue (ULCF), Low Cycle Fatigue (LCF), and High Cycle Fatigue (HCF). HCF is recognized with low strain amplitude and high frequency, and LCF is a material deterioration which is described as high plastic strain amplitude and low frequency. ULCF involves a few cycles (less than 20) of large plastic strains. ULCF is of great importance for structural and earthquake engineers, because fatigue failure in structural members occurs generally in less than 10 cycles during a seismic event. Fatigue fracture in moment connections, or gusset plates and brace members are examples for ULCF or ductile fracture.
Fatigue life is expressed as the total number of stress cycles required for a fatigue crack to initiate and grow large enough to produce fatigue failure. Currently, two major methods are available for fatigue life prediction of structures. One type is based on material fatigue life curves (e.g., S–N curves or ε–N curves) and a damage accumulation rule. The other is based on fracture mechanics and crack growth analysis.
The Manson–Coffin law is the most widely used procedure to predict material failure under LCF and ULCF. But last researches showed that Manson–Coffin relation overestimates fatigue life in ULCF domain.
Miner’s rule is one of the most widely used cumulative damage models for failures caused by fatigue.
The rainflow method is a method for counting fatigue cycles from a time history. The counting of each load cycle and the relative damage produced must be done with extreme accuracy and care. Rainflow counting has been shown to be most effective. The rainflow method allows the application of Miner's rule in order to assess the fatigue life of a structure.
In this paper low cycle fatigue performance of restrained buckling braced frames with diagonal, V-shaped and chevron configurations are investigated. Last researches and experimental tests results of BRBs usually show very stable hysteresis behavior with an excellent low cycle fatigue life.
In this study For modeling the low cycle fatigue phenomenon, the “fatigue material” model in OpenSees is used. The fatigue material uses a modified rainflow cycle counting algorithm to accumulate damage in a material using Miner’s Rule. Once the Fatigue material model reaches a damage level of 1.0, the force (or stress) of the material becomes zero and the material is destructed completely.
By obtaining the hysteretic loops and also the cumulative damage charts of diagonal, V-shaped and chevron buckling restrained braced frames, the hysteretic behavior and fatigue life of them are evaluated. Buckling restrained braces in three configurations of concentrically braced frames, exhibited stable hysteretic behavior up to failure. Considering area of the hysteretic loops and low cycle fatigue life, V-shaped buckling restrained braced frame showed better low cycle fatigue performance.

Keywords

References
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Modares Civil Engineering journal
Volume 16, Issue 5
November and December 2016
Pages 165-175