Applicability Research Of Modal Parameters Identification In Nonlinear Structure

Modal parameter identification describes the process that imposing an excitation to the structure, making response acquisition of key points and calculating the frequency response function between excitation and response, then extracting the natural frequency, damping ratio and modes of the structure. When we get these modal parameters, we can assess the damage condition of structure through their change.Nowadays, modal parameter identification is established based on the deduction of linear system, and has good adaptability to the linear structure. However, when structure has some cracks and other damage, it becomes nonlinear. At this time, the results of these parameters solved basing on linear theory are not fixed. Their value and credibility is associated with the incentive level.To solve this problem, this paper studies the adaptability of current modal parameter identification method for nonlinear structure systems and its variation tendency with different degree of nonlinearity. The main research contents are as follows:(1)Analyzing the common form of noise existing in current project, and choose the true value estimation of frequency response function H1 as the calculation expression of frequency response function data.(2)As an example, this paper chooses the acceleration frequency response function of a single degree system to verify the feasibility of continuous spectrum thinning method. At the same time, this paper also discuss the relationship between the refinement effect of Zoom FFT method and the thinning ratio, finding that the Zoom FFT method is applied to less than 10 times of refinement. When the thinning ratio is greater than 10, the refining effect is not ideal.(3)Giving white noise excitation to the linear and nonlinear numerical structure model, and then calculates their modal parameter using frequency domain modal parameters identification method. Then comparing the result to the corresponding eigenvalue analysis result, found that the modal parameter identification method has a good adaptability and small error for linear structure. For nonlinear structure, the difference between the results is large, and there’s a change trend between the results and the degree of nonlinearity.(4)With linear structure, for example, when it has one or two of Rayleigh damping and the default algorithm numerical damping of fixed step implicit dynamic analysis in ABAQUS, calculating the damping result of modal parameter identification method, finding that the influence of the default algorithm numerical damping of fixed step implicit dynamic analysis can be ignored, for its value is too small compared to the Rayleigh damping.(5)Choosing the overall recognition method to identify the modal parameter of LongPan arch dam model, found that the result has a regular change with the increase of incentive.