Identification of breathing cracks in a beam structure with entropy

Abstract

During vibration of engineering structures, fatigue cracks may exhibit repetitive crack open-close breathing like phenomenon which ultimately result in a distinct crack type, breathing cracks.This breathing phenomenon generates bi-linearity and irregularities in vibration signals of the cracked structure which carry useful information about the crack occurrence. In this thesis, the concept of entropy is employed to quantify this bi-linearity/irregularity of the vibration response so as to evaluate crack severity. To increase the sensitivity of the entropy calculation to detect the damage severity, sample entropy and quantized approximation of sample entropy are merged with wavelet transformation (WT) which is capable of amplifying the weak irregularities in vibration signal caused by small and initial breathing cracks. A cantilever beam with a breathing crack is studied to asses proposed crack identification method under two vibration conditions with sinusoidal and random excitations. An iterative numerical model is established to generate accurate time domain vibration responses of the cantilever with a breathing crack. Through both numerical simulations and experimental testing, the breathing crack identification with entropy under sinusoidal excitation is studied first and proven to be effective. Then, the crack identification sensitivity under lower excitation frequencies is further improved by parametric optimization of sample entropy and WT. Finally, effective breathing crack identification under general random excitations are experimentally studied and realized using frequency response functions (FRFs) which adapts the proposed crack identification technique to the incurred extra complexity due to random nature of the excitation and structural response.iii

Detection of Closing Cracks in Beams Based on Responses Induced by Harmonic Excitation

“…J. Prawin et al [21] proposed a method for the localization of breathing cracks using single sensor measurement in both single and multiple breathing cracks. Wimarshana et al [22] use entropy measures to detect cracks. M. Cao et al [23] propose a nonlinearity-sensitive approach for the detection of "breathing" cracks.…”

Damage identification for nonlinear fatigue crack of cantilever beam under harmonic excitation

“…Andreaus et al [20][21][22] conducted experimental and numerical researches about nonlinear forced response of cantilever beam with fatigue cracks. Wimarshana et al [23] realized the breathing crack detection of cantilever beam using entropy method, which could identity the early crack damage of 3% beam thickness. Zhang et al [24] modeled the fatigue crack as two elastic, frictionless half space and investigated the threshold behaviors of subharmonic responses.…”