Research On Damage Identification Of Beam Structure Based On Dynamic Characteristics

The method of dynamic characteristic damage identification is a kind of global,noninvasive and rapid monitoring method,which has outstanding advantages,and becomes the hot spot of research at home and abroad.Although a series of progress has been made theoretically,there are still many problems.Most of the damage identification methods can only be used for damage localization and qualitative judgement of damage degree,and few damage identification methods can reflect the actual damage degree,which have the characteristics of large quantitative errors and complex calculations,and some have to use the benchmark model.In addition,due to the neglect of some problems in practical engineering application,which makes it impossible to apply to practical engineering.Therefore,it is an urgent problem to be solved in the damage identification technology using the dynamic characteristics to study the method that can correctly reflect the position and damage degree of structral damage.The selection of sensitivity parameters determines the success or failure of the damage degree identification,on the basis of the existing theories,this paper starts with the selection of sensitive parameters and establishes the relationship between the damage degree and the sensitivity index,damage quantitative analysis of the total damage form and the local damage form between the measured points of simply supported beam and continuous beam are studied,and the main contents include:1.Established a simple supported beam model corresponding to different beam theories under different ratios of length to height.Comparing the solution obtained by different beam theories with the solution obtained by the elastic beam theory.The paper analyses the relative error of natural frequencies of different beam theories and obtains the applicable range of different beam theory,and provides reference for the numerical simulation verification of structural damage identification in model selection.2.In order to achieve the quantitative structural damage degree,based on the definite relationship between displacement curvature and stiffness,the quantitative formula of structural damage degree is deduced,and the display relationship of structural damage degree with respect to displacement curvature change was established.Since the displacement curvature is a static parameter,there are many disadvantages.Therefore,the approximate expression of the dynamic characteristic index of the displacement curvature is sought,and the display expression of the damage degree of the dynamic characteristic index is established.3.For the actual engineering damage is arbitrary.based on the series spring principle and the line stiffness idea to propose the damage degree of the equivalent line stiffness of the measuring point,and the relationship between the detection point damage degree identification value and the local actual damage degree is established.Based on the principle of linear interpolation,a formula for the middle position of the damage zone is proposed.Based on the principle of displacement superposition,the formula for determining the length of any damage length is proposed to realize the damage degree recognition of any damage length of a simply supported beam.For the effect of a continuous beam due to the redistribution of bending moments,the identification of any damage length will need further study.4.For the case that the damage is in the form of a crack,the equivalent line stiffness is difficult to correctly reflect the actual damage.Therefore,based on the analysis of the crack's vibration,the stiffness of the torsion spring is reduced by the concentricity model to simulate the damage,and the series equivalent line stiffness is proposed.The model establishes the relationship between the crack height and the damage degree of the equivalent line stiffness at the measurement point,so as to achieve crack height quantification.5.Duing to the uniform load surface curvature difference(ULSC)and curvature model difference has some limitations in the positioning and quantification of the damage,based on the existing indicators,surrogate indexes of ULSC and curvature model difference are studied.(1)It is difficult to identify the damage at the displacement inflection point in the statically indeterminate structure by ULSC,and there is a leakage judgment phenomenon,and when the damage is quantified,there is a peak interference at the inflection point of the displacement,and the index IULSC is proposed.By absorbing the idea of the modal flexibility curvature matrix difference and modal flexibility difference curvature matrix,which used 2 times difference of row and column could overcome the characteristics of unrecognized damage at the inflection point of the displacement and the idea of ULSC is added by line,and the modal flexibility matrix curvature norm difference index is proposed.The existing flexibility curvature index(ULSC,the flexibility curvature matrix difference,the flexibility difference curvature matrix],the improved uniform load surface curvature difference(IULSC)and flexibility matrix curvature norm difference index proposed in this paper are used to identify the damage of the simple supported beam and the continuous beam.The iterative damage degree method considering the change of bending moment was proposed for the situation that the damage value caused by redistribution of moments in continuous beams was too small.(2)For the curvature mode difference index,it is unable to identify the damage at the mode shape node,and it is unable to correctly and accurately reflect the defect of the actual damage degree,but the low-order curvature mode difference can better reflect the characteristics of the actual damage position,while the superimposed curvature modal change rate index can correctly indicate the damage position,and absorb the advantages of the superimposed curvature modal change rate,introduce frequency weight to make the lower-order modes occupy the major proportion,and propose the frequency weight superimposed curvature modal change rate index.Using the existing curvature index(curvature mode difference,superimposed curvature modal change rate)and the frequency weight superimposed curvature modal change rate index,which proposed in this paper,to carry out damage identification of a simply supported beam and a continuous beam.(3)The results shows that the indexes that can achieve quantitative damage of simply supported beams include ULSC,the modal flexibility matrix curvature norm difference,the frequency weight superimposed curvature modal change rate index,and can realize quantitative damage of continuous beams include IULSC,the modal flexibility matrix curvature norm difference,the frequency weight superimposed curvature modal change rate index.(4)Gaussian white noise is applied to the mode shape to analyze the anti noise performance of each index.The study shows that the modal flexibility matrix curvature norm difference index proposed in this paper has strong anti-noise performance,and the anti noise performance of IULSC is the next,and the frequency weight superimposed curvature modal change rate index has poor anti-noise performance,but the frequency weight superimposed curvature modal change rate index has the advantage of not having to normalize the quality of the mode shape.6.The simple supported beam is taken as an example to analyze the effect of different length-to-height ratios on the theoretical damage identification of different beams by using ULSC,and the applicable scope of modal analysis of different beam theories is obtained.7.Different simple beam models with length-to-height ratio of 20,taking into account the different types of damage,support constraints position,the location of measurement points,curvature curve continuity of the damage degree of quantitative impact,were analyzed by using ULSC.8.Aiming at the lack of reference data in a simply supported beam model,a stepwise polynomial fitting data reconstruction method is proposed to realize the damage location and quantification of the structure using only the modal parameters in the damage state.