Papers by Keyword: Crack Damage

Abstract: Structural crack damage will degrade its carrying capacity, and affect the security of the structure. Thus early detection of crack damage is a guarantee of the structure safety. Cracks can change the vibration characteristics of the structure, therefore we proposed a method of identifying the crack damage based on the vibration modal. Take both ends fixed beam as an example, through establish the finite element models of crack-free beam and the crack beam with different location and different depth, we calculate the displacement modal parameters of beam before and after the damage, analyze the variation law of displacement modal horizontal component of change and displacement modal Axial displacement difference rate of change varies with crack depth and location, results show that the variation of displacement modal horizontal component and the change rate of displacement modal Axial displacement difference along crack direction are sensitive to cracks location and depth, these can be used as a basis for identification of beam’s crack damage.

Abstract: Engineering structure often produces the crack in the use of a certain year, the crack will decline the stiffness of structure which affects the safety of the structure and the vibration characteristics of structure. A method of identifying the crack damage from beam by using the vibration modal displacement component to strutting the damage identification was put forward, the finite element models of beam which no crack, different position and different depth were established, the damage displacement modal parameters of beams pre and post were calculated, the change rule that the displacement modals variation of transverse component and displacement modals rate of change of axial displacement difference along with the different of the cracks depth and position was analyzed., The calculations showed that the sensitive to the crack position and depth along the crack direction displacement modal transverse component variation and change rate of the axial displacement difference can be used as the basis for crack damage identification of beam.

Abstract: This paper applies strain frequency response function (SFRF), presenting crack damage identification index based on change amplitude of SFRF. Crack propagation identification of four corners fixed thin plate experiment system based on fiber bragg grating sensors is established, experiments and analysis are carried out. Results show that: this crack damage identification system can identify crack damage occurs and propagation effectively.

Abstract: To recognize whether there exist crack damages in the inner structure of aircraft engine based on the borescope images, a method is presented by estimating the coarse feature of edges extracted from borescope images. The method first decomposes an edge into the approximation and details using wavelet transform at all of the possible scales. For the coefficients of the wavelet transform, the logarithm entropy is presented to represent the respective strength of the approximation and details, and the ratio of the two logarithm entropies is defined as coarseness factor. The coarseness factor can be used to estimate the coarseness of an edge. The greater the coarseness factor is, the coarser the edge is. Experiment results show that using coarseness factor as recognition feature of borescope images, the crack damages of the inner structure of aircraft engine can be recognized successfully.

Abstract: General expressions of crack perturbations are given with perturbation method. With the delta function to express the crack position at the simple beam, the expressions of cross-section rotary inertia and mass per unit length of the whole beam are founded, and then the transverse free vibration equation of the simple beam with a damaged crack is established. The first and second order perturbations are considered as the linear combinations of corresponding mode shapes of intact state, according to the boundary conditions, and theoretical solutions about eigenvalues and mode shapes of the damage state are finally obtained. One simple beam is taken as a numerical example to calculate the modal parameters; the effects of first and second order perturbations on the results are compared. It is believed that the approach proposed in this paper could provide the necessary theoretical background for damage identification in simple beam.

Abstract: Free vibration characteristics of an elastic simple beam with a fatigue notch crack damage located any where of the beam is investigated. The notch crack is modeled by an elastic torsion spring whose stiffness is taken to be finite and is determined from fracture mechanic theory. On the assumption that the crack is always open when the beam vibrates transversely, the motion equation and the boundary conditions of a simple-supported beam with a crack located anywhere of the beam is deduced. The first-order to the sixth-order frequencies varying with the crack depth and the crack location are calculated.

Abstract: An improved method to identify the crack location and size is presented which takes advantages of wavelet finite element (WFE). The important property of wavelet analysis is the capability to represent functions in a dynamic multiscale manner, so solution with WFE enables a hierarchical approximation to the exact solution. WFE has good ability in modal analysis for singularity problems like a cracked beam. The crack in a beam is modeled with WFE and represented as a rotational spring. The additional flexibility caused by crack in its vicinity is evaluated according to linear and elastic fracture mechanics theory. The WFE stiffness matrix of the crack is constructed and the algorithm for crack identification through the use of vibration-based inspection (VBI) is developed. With the accurate natural frequencies obtained from the transient signal measured, graphs of crack equivalent stiffness versus crack location are plotted, by providing the first three natural frequencies as an input. The intersection of the three curves gives the crack location and size. Experimental studies of cracked shafts are presented to demonstrate the accuracy of the method. The error in identification of crack location and size are both less than 2%. This study provides the new method for the diagnosis of incipient small crack.