Issue 49

E.U.L. Palechor et alii, Frattura ed Integrità Strutturale, 49 (2019) 614-629; DOI: 10.3221/IGF-ESIS.49.56 620 Figure 12 : Discretization (s=20 cm elements) of the beam - Experimental tests. Using a spacing “s”, the span length “L” can have (L/s) elements and (L/s+1) nodes. Therefore, for L = 5m, and L = 6m, and s=20 cm, one can generated 26 nodes and 31 nodes for the positioning of the added mass, respectively for Beam-1 (5m) and Beam-2 (6m). E XPERIMENTAL PROCEDURE tep 1: for the model developed in this research, the beam specimens were subjected to impact load produced by Impact Hammer with a force of approximately 400N, applied in the central region near the middle of the beam span – as can be observed in Figure 13. Figure 13 : Excitation force fort the numerical analyses. Step 2: the force and acceleration dynamic signals was acquired by signal conditioner connected to an impact hammer and two accelerometers. The Fig. 14 show programming of acquisition and signal pre-visualization. The acquisition system LabView provide the real time visual of dynamic signals. It was important to avoid double peaks by erroneous impact test. Step 3: the modal impact test was carried out with four repetitions. To reduce the leakage effect, exponential and force windows was applied to acceleration and force signal respectively. The auto-spectrum of dynamic force shows a signal constant (+-3dB) to frequency range choice. Step 4: with an impact hammer test done, the force and acceleration dynamic signals were saved in a hard-disk to be later on treated by a MatLab script to obtain de Frequency Response Function (FRF) between force excitation and accelerometer measurement. Using a resolution greater than that given by Fast Fourier Transform (FFT) and performing a weighted average of the frequencies around a peak detected in the spectrum frequency, the first frequencies were accurately identified. Characteristics and calibration of parameters The frequency resolution is very important considering that there are frequency variations due to the additional mass. Thus, in this research, several configurations for the acquisition or analysis time after excitation (t), number of analyzed data (N), acquisition rate (∆t) and sampling frequency (Fs) are tested. Due to space reasons, it is impossible to present all the results attained. However, the best results were obtained with the following parameters: (a) Fs=1653 Hz, (b) t=9.9 s, (c) ∆t =0.000605 s, and (d) N=16384 (2 14 ). S