Vibration analysis of a connecting rod using modal test and finite element method

In this study, the vibrational characteristics of a connecting rod that belongs to a fourstroke four-cylinder gasoline engine are examined. According to the literature review, modal analysis is a study that obtains the three main vibrational characteristics as natural frequencies, damping ratios, and mode shapes. In this context, these three main characteristics are obtained with experimental modal analysis and the finite element method. Experimental modal analysis is carried out with the changes in the number of elastic strings and the placement of the connecting rod and then the results are evaluated. However, since the test object is assumed to be hung in space with 6 degrees of freedom and also anything does not restrict its movement, the finite element method does not include an elastic spring binding method. Natural frequency, damping ratio, and mode shape values which are obtained with experimental modal analysis and the finite element method are compared. Mode shapes that belong to the experimental modal analysis results are obtained with a constructed algorithm. Mode shapes that belong to the finite element method are also obtained with the same algorithm and these mode shapes are compared with the ones that belong to experimental modal analysis. For the pair of data from the experimental modal analysis, modal assurance criterion matrixes are obtained with the AutoMAC method. Comparison of the data belong experimental modal analysis and the finite element method is carried out with modal assurance matrixes obtained with the CrossMAC method. In conclusion, the vibrational characteristics of the aforementioned connecting rod are examined with various methods and the results from these results are compared. It is seen that natural frequency values obtained with experimental modal analysis and FEM are reasonably close to each other for the first three modes. Besides, it is determined that obtained natural frequencies are not overlapped with any of the revolution numbers of a running engine. Therefore the resonance in any of the revolution numbers does not occur. The difference between the damping ratios obtained from experimental modal analysis and FEM shows that the damping that occurred in the conditions without gravity or restraining and the ordinary conditions would be significantly different from each other. In practice, damping ratios obtained from the experimental modal analysis ought to be used, since the vibrational characteristics that the connecting rod has in ordinary conditions are close to the characteristics when it is in the running engine. In light of the obtained data and evaluations, it is explained in detail what the significance of these characteristics is. Keywords: Modal Analysis, Frequency Response Function, Finite Element Method, Modal Assurance Criterion