## 3 Experimental Investigation and Results
### Modal Test and Model Validation
Other values will be calculated based on these two parameters. For the trigger configuration, select the trigger type and use Force Channel 1 (connected to the acquisition device, DT9837B) as the reference port. Choose the +slope of the trigger and set the trigger lines and lock lines to observe and adjust the time signal from the hammer impact, as shown in Figure 4a. In this test, use 0.4% of the channel voltage and set a pre-trigger delay of 10 samples. In the setup for the channels spreadsheet, activate four channels and select AC Coupling for one of the piezoelectric force hammers and for the other three channels, connect them to accelerometers. Set the force unit as lbf and the acceleration unit as g. Enter the sensitivity constant of 50 mV/unit for all the transducers (using the instrument's datasheet). During impact testing, use the force window for the hammer channel and the Hanning window for the acceleration channel. Figure 3: Structural testing refer of the 3D wireframe model of CBF. Precautions to be taken during signal acquisition: It is essential to carefully examine the response results each time an impact is applied to ensure that the signal patterns are suitable for subsequent signal processing, as shown in Figure 4b. This is particularly important because 208 data points are collected for the FRF, and all three directions of response are measured using accelerometers, resulting in a total of 624 functions. Furthermore, it is not feasible to collect response data in the time domain. Therefore, it is necessary to observe the response results only during the data acquisition process. For instance, let us consider the measurement at point 80 in the -Y direction (denoted by a negative sign, indicating the opposite direction of the reference installation of the accelerometer) compared to point 131 in the +X direction. The response results in the time domain are depicted in Figure 4b. In this specific test, point 80 in the -Y direction is selected as the impact location using a hammer, and roving accelerometers are employed to measure the response at the designated position, which is point 131 in the +X direction on the structure. 5. The responses of the CBF are acquired in the tri-axial accelerometer and hammer data acquisition system and then transferred into MEscope software. For parameter estimation of the CBF, the experimental modal analysis (EMA) software [26] is used. The CBF tests are conducted under predetermined environmental conditions. The frequency span of 0-25 Hz is chosen for the model validation. 6. The modal identification is performed using a technique of extraction in the peak picking method from the power spectral densities. This technique provides estimations of the natural frequencies and mode shapes. The first six mode shapes obtained from this method are shown in the next part to compare with the simulation, and Figures 5-7 illustrates the demonstration of the motion snapshot of the CBF at 4.31 Hz to 23.8 Hz, respectively. If a CBF is excited near one of its frequencies, the corresponding mode shape will dominate. However, there will still be a small contribution from all other modes present in this response. The road excitation from the transmission system has typical values varying from 0 to 100 Hz. In practice, diesel engines are known to have an operating speed of about 8 to 33 Hz [19]. In the low-speed idling condition, the excitation frequencies are about 8 to 10 Hz. The main excitation is at low speeds, when the truck is in the first gear during the idling condition of engine. At higher gears or speed, the excitation to the chassis is much less. Figure 4: (**a**) Setting up the processing using acquisition; (**b**) force and acceleration signal. Figure 5: Motion snapshot from relative maximum deviation behavior of EMA: (**a**) at 4.31 Hz; (**b**) at 8.87 Hz. Figure 6: Motion snapshot from relative maximum deviation behavior of EMA: (**a**) at 10.6 Hz; (**b**) at 10.9 Hz.