Session: 02-05-02: Experimental Studies in Structural Dynamics - II

Paper Number: 110823

110823 - Effects of Accelerometers’ Positions on the Dynamical Responses of Complex Systems 

There are many engineering benefits that can be applied from determining the physical responses of different testbeds. In order to collect these responses, researchers use several different transducers that measure the dynamical responses of systems. This study aims to investigate the effects that one of those transducers, an accelerometer, has on the measured responses of a complex structure, more specifically on a common testbed studied in the literature, the Box Assembly with Removable Component (BARC). The aim of this structure is to characterize non-rigid boundary conditions found in a working environment. The BARC structure is an example of substructuring that is seen in literature. The BARC is a system that was introduced by Sandia National Laboratories and Kansas City National Security Campus to examine the results collected from said substructuring and how the boundary conditions affect results in environmental testing. The BARC has many boundary conditions that can be varied to greatly impact the dynamical responses of the system.

For the purpose of simplifying the system as well as the discrepancies observed in the literature, the Removable Component (RC) is solely utilized when conducting the following experimental investigations. Prior to the experiments, a series of finite element analysis (FEA) is carried out on the removable component to determine locations of high and low displacements. To determine a range for our experimental efforts, two different cases of bolted connections are computationally modeled. The first is the stick case, which couples the entire surfaces of the two bodies in contact in the simulation. The second is the unrestrained case which unlike the stick case only couples the underside of the bolt head with the top and bottom of the other two bodies. These simulations are then reiterated with the inclusion of modeled accelerometers to determine the influence they might produce on the mode shapes as well as the natural frequencies of the complex structure.

Following the finite element analysis, a series of free vibration testing is run to analyze the fundamental frequency and damping over time for an array of accelerometer’s locations. These tests yield multiple time histories which through logarithmic decrement made it possible to compute the variations of frequencies and damping over time. It becomes evident that the accelerometer placed on the left position has the largest effect on the results collected as it has the largest variance in frequency over time. When considering the damping, it is found that the left accelerometer has the highest damping of all three locations due to its influence on the effective mass of the system causing the largest displacement. Alongside this, random vibration is performed to determine the nonlinear characteristics of the RC with respect to accelerometer’s locations and direction of excitation. In the RC’s first mode of vibration, both nonlinear softening and damping are more evident when the accelerometer is placed in a particular region on the structure. It is concluded that the accelerometer’s position plays a critical role in determining the nonlinear characteristics and has a direct relation to the results collected with respect to direction of excitation and reference axis. Until a less intrusive method is created that can measure both absolute and relative responses effectively, accelerometers will continue to be utilized in the laboratory setting. It is because of this that choosing an optimal location for them during tests must be considered. This study demonstrates that with respect to displacement from a certain vibrational mode, an accelerometer can be placed in a more reliable orientation. However, placing an accelerometer on the node of a particular mode shape may not capture the nonlinearities present in the structure and assume linear behavior. It has been shown that the location of the accelerometer can greatly affect the responses collected and should be deeply considered during experimental setup. This will allow more uniform testing to be conducted when utilizing the RC structure.

Presenting Author: Jonah Madrid New Mexico State University

Presenting Author Biography: Jonah is a rising senior at New Mexico State University. His expertise is in computational simulations and experimental techniques. He has interned with Los Alamos National Laboratory and collaborated on numerous structural dynamics-based projects. His plans are to pursue graduate studies and continue his research.

Authors:

Jonah Madrid New Mexico State University
Ezekiel Granillo New Mexico State University
Adam Takeshita New Mexico State University
Abdessattar Abdelkefi New Mexico State University