Research On The Testing Method Of Bar Cracks Based On Modal Analysis

Bar is one of the maximum production and the most extensive application scope metallic materials. Bars are easy to produce various kinds of defects such as cracks, corrosion and artificial damages in different working conditions, and the proportion and danger of cracks are much greater. At present, the widely applied testing methods of defects are limited to known defects and simple in shape structures for they are mostly take rectangular beam as a research model, and using static nondestructive testing methods such as rays, eddy current, ultrasonic and so on. However, modal analysis can acquire dynamic response and reflect local and global defects information, so it can be used for complex structures defects testing. Therefore, the dynamic Non-destructive Testing (NDT) method of bar cracks based on modal analysis has the important engineering application prospects.This paper have studied the method of testing damage degrees and damage locations of bar transverse surface cracks by combination of calculating modal analysis and experimental verification approach. The calculating modal analysis uses the universal finite element software Ansys to do simulative test for bars, including selection of damage index and judgments of damage degrees and damage locations. The experimental verification is to do actual test for bars, and the cracks are simulated by pre-processed rectangular gaps. The effectiveness of this method is confirmed by comparative analysis of these two parts.In the judgment of damage degrees, choose the curvature mode shape that sensitive to local damage as damage degree index. The curvature mode shape of bar with damage will have mutation phenomenon near damage location, and with the increase of damage degree, the mutation phenomenon is more obvious. Through calculating modal analysis, the function expression between them is acquired by the method of least square fitting and realized quantitative analysis of damage degree.In the judgment of damage locations, established the mathematical model for direct index of damage location ISMSD with explicit position coordinates according to strain mode difference curve, through calculating modal analysis, the results show that the direct index of damage location of actual damage position is far outweigh the other positions, and with the increase of damage degree, the calculated value of ISMSD is greater. Therefore,ISMSD can figure out the damage location of bar effectively, and provide theoretical supports and technical references for experimental modal analysis and dynamic NDT research.The results of actual test for bars without damage and with damage degrees (h/D) were0.100,0.150,0.200and0.250show that natural frequencies obtained by experimental modal analysis are lower than calculating modal analysis, and the direct index of damage location calculated by difference curve of experiment mode shape can judge actual damage locations accurately, therefore, verified the correctness of calculating modal analysis.Based on modal analysis, this paper realized dynamic NDT of bar cracks on the research of calculating modal and experimental modal analysis. This method can realize damage localization and primarily quantitative of bar cracks, and provide theoretical supports and practical application foundation for NDT of complex structures.