Reinforced concrete structures experience nonlinear vibration response because of softening in concrete due to damage, which results in unstable vibration. Thus, considering the nonlinearity in vibration for assessment of damage is imperative to identify the actual damage scenario. Nonlinearity is predominant during the initial stage of damage when cracks start developing in the concrete exhibiting nonlinear breathing-crack mechanism. By suitably extracting nonlinear features from ambient/forced vibration, information on initial stage of damage (primarily breathing-crack) in large concrete structures can be obtained. In the present study, evaluation of nonlinear dynamic features from an experimental model of a reinforced concrete bridge girder-deck system is investigated. The damage scenarios in the bridge girder is experimentally simulated by monotonically applied static load at different levels. Vibration responses are acquired using the accelerometers positioned at regular intervals, so that the global system behaviour can be assessed. Damage features at various damage levels are evaluated based on the signal statistics, amplitude dependency, harmonics, phase-plane information and so on. The breathing-crack behaviour of the bridge girder during initial damage stages is investigated and the existence of sub- and super-harmonic components in the frequency spectra is distinctly evidenced and their amplitudes are quantified. The change of skewness pattern of the frequency spectra with increasing damage level is also evaluated. A distinct variation in phase space portrait is observed with increase in damage levels. It is found that, in spite of presence of inherent material nonlinearity, nonlinearity caused through system damage, even at initial stages of damage, is predominant and could be distinctly captured without any baseline information. The identified nonlinear vibration characteristics showed a remarkable relevance in detecting breathing-crack type damage in weakly nonlinear bridge. The investigated damage features from this study would pave the way for real-time baseline free assessment and identification of damage in concrete structures.

中文翻译：

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用于识别钢筋混凝土桥梁呼吸裂缝型损伤的动态非线性

钢筋混凝土结构由于混凝土因损坏而软化，从而经历非线性振动响应，从而导致不稳定的振动。因此，考虑振动中的非线性来评估损坏对于识别实际损坏情况势在必行。在损坏的初始阶段，当混凝土中的裂缝开始发展时，非线性占主导地位，表现出非线性呼吸裂缝机制。通过从环境/受迫振动中适当地提取非线性特征，可以获得有关大型混凝土结构中损坏（主要是呼吸裂缝）初始阶段的信息。在本研究中，研究了从钢筋混凝土桥梁梁 - 甲板系统的实验模型评估非线性动力特性。通过在不同水平上单调施加的静载荷，对桥梁中的损坏场景进行了实验模拟。使用以固定间隔放置的加速度计获取振动响应，以便可以评估全局系统行为。根据信号统计、幅度相关性、谐波、相平面信息等评估不同损伤级别的损伤特征。研究了桥梁在初始损坏阶段的呼吸裂纹行为，并清楚地证明了频谱中次谐波和超谐波分量的存在，并量化了它们的幅度。还评估了随损伤水平增加的频谱偏度模式的变化。随着损伤水平的增加，观察到相空间图的明显变化。研究发现，尽管存在固有的材料非线性，但由系统损坏引起的非线性，即使在损坏的初始阶段，也是占主导地位的，并且可以在没有任何基线信息的情况下清楚地捕捉到。识别出的非线性振动特性在弱非线性桥梁呼吸裂纹型损伤检测中显示出显着的相关性。本研究中调查的损伤特征将为实时基线自由评估和混凝土结构损伤识别铺平道路。识别出的非线性振动特性在弱非线性桥梁呼吸裂纹型损伤检测中显示出显着的相关性。本研究中调查的损伤特征将为实时基线自由评估和混凝土结构损伤识别铺平道路。识别出的非线性振动特性在弱非线性桥梁呼吸裂纹型损伤检测中显示出显着的相关性。本研究中调查的损伤特征将为实时基线自由评估和混凝土结构损伤识别铺平道路。