Laboratory Tests on Composite Beam - Accuracy of Modal Analysis Results

Abstract

The primary aim of this research was to verify accuracy of results of the modal analysis on the composite beam. The research is based on the identification technique, which determines structural characteristics from the measured values of dynamic response. The method uses measured data to optimal update of the stiffness matrix to get numerical model that matched with measured data. This can be used for further calculation to determine the effect of damage to the stiffness of the beam. The experimental beam was made of wooden boards and plasterboards, which were attached to each other by a lot of screws. It was simply supported beam of total length 4m. The stiffness characteristic of beam was determined from experimental measurement of the beam deflection from a load. A total of 24 accelerometers were placed along of the beam. Accelerometers were recording vertical acceleration and the acceleration measurement record lasted for 60 seconds. We gained amplitude spectrum of acceleration from the measured acceleration values by using the fast Fourier transform (FFT). The first eigenfrequencies were determined from the amplitude spectrum. The measured value of first frequencies was f1 = 11.7Hz while the calculated value of first frequency was f1 = 11.6Hz. Other frequencies and shapes were also calculated and measured. When comparing them we need to be careful. The spacing of the transducers very well describes the vertical bending shapes while there is little information about torsional vibration shapes. We are planning extend these results by measurement with a pair of accelerometers located along the beam to verify torsional mode-shapes too. The conclusions and results of this experiment are used in calculations for system identification and determination the change in stiffness value of the damaged beam. The beam corrupted by removing the screws from the beam has changed the stiffness properties of the beam. This change is reflected as increment/decrement of original stiffness matrix - adjusted stiffness matrix.