Nathanael C. Yoder

Vibro-Acoustic Modulation Utilizing a Swept Probing Signal for Robust Crack Detection

One practical issue that must be addressed prior to the implementation of a vibration-based structural health monitoring system is the influence that variations in the structure’s environmental and boundary conditions can have on the vibration response of the structure. This issue is especially prominent in the structural health monitoring of aircraft, which operate in a wide variety of different environmental conditions and possess complex structural components connected through various boundary conditions. However, many types of damage introduce nonlinear stiffness and damping restoring forces, which may be used to detect damage even in the midst of these varying conditions. Vibro-acoustic modulation is a nondestructive evaluation technique that is highly sensitivity to the presence of nonlinearities. One factor that complicates the use of vibro-acoustic modulation as a structural health monitoring technique is that the amount of measured modulation has been shown to be dependent on the frequency of the probing signal. The frequency dependence of the modulation was investigated and the magnitude of modulation was found to be correlated with the underlying vibration characteristics of the structure, which are influenced by environmental and boundary condition variations. To facilitate the use of nonlinear vibro-acoustics for the health monitoring of complex aerospace components in varying environments, a vibro-acoustic modulation technique utilizing a swept probing signal has been developed. The developed method was demonstrated on a steel beam in varying operational conditions. The presence of a crack in the beam was detected both through an increase in the amount of normalized modulation and without the use of historical data by utilizing generalized extreme value statistics.

Transmissibility Analysis for State Awareness in High Bandwidth Structures Under Broadband Loading Conditions

Under broadband loading, structures that are comprised of multiple stiff sub-components with interfaces exhibit complex dynamic responses. These responses can affect both the reliability and functionality of the structure. For example, if sensors within the structure are utilized to monitor its operational response for the purpose of estimating the loading state, it is essential to understand the relationship between those sensor signals and the dynamic complexity across the assembly interfaces. This paper describes experiments that were performed on a massive and stiff structural housing containing lower mass sub-components, which are assembled through lower stiffness, nonlinear threaded interfaces. It is shown that the lower frequency range of the multi-component specimen exhibits primarily rigid body motion and synchronous motions of the internal canister oscillating on a threaded interface. The mid-frequency range exhibits complex asynchronous motions as these various internal components oscillate out of phase with one another. Transmissibility functions, which are the ratios of measured frequency response functions, are used to analyze the transfer path through the structural housing and subcomponents. It is demonstrated that under various preloads, the contribution of each interface to the overall transmission of load/motion varies. These variations in transmission due to differences in preload are shown to be of the same order as the softening nonlinear variations due to a change in the excitation force amplitude.

Near Real-Time Monitoring of Bead Area Damage in Rolling Tires Using a Rotating Wheel Model and Multi-Directional Vibration Data

In current tire durability tests, technicians must stop tests periodically to inspect each tire, resulting in a time consuming, expensive, and relatively subjective process. Online tire monitoring would thus be a dramatic improvement over the current methodology. If such a system could be extended to on-line vehicle use, it could dramatically increase safety, reduce downtime, and lead to better fuel efficiency in commercial and passenger cars and trucks. A near real-time system to monitor the initiation of bead area damage in rolling tires was developed using vibration data collected from the wheel end spindle. To generate an accurate model of the forced response of the wheel end spindle, a tire on a fixed spindle was impacted with a modal punch while the tire was preloaded against a plate. The frequency response functions acquired from this system were then used to develop a rotating tire model consisting of time-delayed forcing functions applied to the tire patch at the rolling frequency; this model was used to assist in interpreting the results from the near real-time monitoring system. The near real-time continuous monitoring system has been deployed in a manufacturing test environment and utilizes frequency data from three mutually orthogonal acceleration measurements. These data are combined to create a highly sensitive composite index that identifies when the initiation of bead area damage has occurred.