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Equipment requirements for ground vibration testing of assemblies, sub-assemblies, and components are dependent on several factors. These include test configuration (e.g., size and mass), equipment performance parameters (e.g., stroke limit), and test architecture (e.g., single-axis or multi-axis). Equipment is historically selected based on similitude to past testing, engineering judgement, and/or rudimentary calculations based on Newton’s second law of physics. From these methods alone it can be challenging to know if all desired test configurations and environments are achievable. As an alternative method to determine if a system is capable of running specific tests, a relationship can be developed between the electrical inputs and physical outputs to the system and used to predict if specific testing can be achieved. This paper explores and begins to quantify the relationship between the physical response of an electrodynamic shaker and DUT with the electrical signals of the power amplifiers and the data acquisition systems. Critical parameters of the system that impact shaker performance are identified using experimental data from three different ground vibration shaker tests. Understanding the relationship between the electrical and mechanical responses of an electrodynamic shaker system can provide test engineers with valuable information on the performance limitations of system, as well as knowledge to better utilize testing equipment resources and predict the testability of a proposed experiment. Objectives for this effort were fulfilled largely through experimental means, including post-processing of experimental data and reporting of findings to develop a knowledge database and technical approach, with an end-goal of developing metrics or tools upon which to base equipment requirements for future testing.

Vibration qualification testing verifies and quantifies a system’s longevity in its proposed service environments. Service environments a system could encounter can impart many ranges of excitation in all directions; however, multi-axis excitation testing capabilities for simulating realistic environments are rare and costly. Therefore, multiple, single-axis vibration tests are commonly used to qualify a system and its components to a lifetime of service environments. Quantifying the equivalent amount of time a component has been tested can be difficult when limited to single-axis tests. Further complications arise due to the fact that real-world service conditions are often measured at a system level without instrumentation on each component. In addition, many mechanical systems include joints and contact surfaces that, if altered, can significantly change the component’s vibration characteristics. This makes replicating the boundary conditions of each component difficult. Therefore, another crucial part of single-axis vibration testing is determining boundary conditions to replicate best the real-world environment onto each component. This paper aims to analyze the effects on lifetime estimates using single-axis vibration testing of components under variations in boundary conditions, testing strategies, control locations, and other configuration options. Methods such as power spectral density (PSD), fatigue damage spectrum (FDS), and Miner’s Rule, with quantities such as fatigue cycles, peak response, and RMS response are used to evaluate boundary conditions, study the response of the components, and determine the severity of various test strategies as it pertains to the overall lifetime of the system.

Structural health monitoring refers to the process of making an assessment, based on nondestructive, in-situ, autonomous measurements, about the ability of a structure to perform its intended function. This paper presents work done on a bolted connection in carbon-fiber reinforced polymer composite materials. A composite specimen is bolted in a double lap joint configuration to a test apparatus that applies an increasing tensile load. Ultimately, the load results in bearing failure of the material around the bolt hole. To monitor the progression of damage, macro fiber composite sensors are bonded in a circular array around the bolt hole. These sensors are then used to generate ultrasonic guided waves, a popular technique in nondestructive evaluation because of the favorable combination of propagation distance and sensitivity to damage. As the specimen is subjected to increasing load levels, measurements are taken repeatedly and compared with one another. Because damage will change the local mechanical properties of the material, the ultrasonic waves passing through the damaged region will be scattered differently in each direction, resulting in a different waveform arriving at the other surrounding sensors. By applying appropriate signal processing techniques, these changes may be interpreted as indicating the extent of damage that has occurred in the specimen. Preliminary analysis is presented demonstrating the correlation between changes in received strain signals and increasing damage levels.

This document describes, Echo, a comprehensive suite of tools for data wrangling, management, and analysis, and its application.

This article introduces a novel approach to damage detection in the context of a structural health monitoring system. A statistical model of the ultrasonic guided wave interrogation process was developed and used to formulate a likelihood ratio test. From the likelihood ratio test, the optimal detector for distinguishing damaged and undamaged states of the structure was derived and found to be a metric related to signal energy. That result was confirmed using a data-driven approach based on using receiver-operating characteristic curves to compare the energy metric to other metrics found in the literature. The data in this study were generated from instrumenting two separate, geometrically complex structures with ultrasonic sensor–actuators. A three-story bolted-frame structure was constructed in the laboratory to test the approach for connections that produce highly uncertain wave paths, with damage being introduced through local impedance changes and bolt loosening. The second test structure was a section of a fuselage rib taken from a commercial aircraft in which holes and cracks were introduced to provide a testbed with a high degree of realism. The detection performance in both structures was quantified and presented. Finally, different sensor fusion strategies were implemented and the ability of such techniques to increase the statistical distinction between damaged and undamaged cases was quantified.

The nature of some high-value systems requires that damage be detected without disassembly of the test article. Spin-down testing, one such nondestructive evaluation method, involves spinning a test article and observing its angular deceleration. An increase in angular deceleration compared to an identical test of a healthy system indicates damage has occurred. However, current testing is limited, providing only a binary result of “damaged” or “undamaged”. More sophisticated techniques are required to identify the location, extent, and type of damage. To achieve this, non-contact and surface-mounted sensors were used to monitor the test article in this experiment. A spin-down unit was used to test both a healthy specimen and a specimen with simulated damage. The signals from each sensor were compared across tests. Correlation of anomalous signals with rotational position and multi-sensor triangulation techniques were used to localize damage. Analysis of the frequency content and amplitude of output signals allowed for a degree of characterization of the damage. Through these techniques, spin-down testing was enhanced from a binary detector to a more thorough testing method giving detailed information about the type and location of damage.

Ultrasonic guided waves represent a promising technique for detecting and localizing structural damage, but their application to realistic structures has been hampered by the complicated interference patterns produced by scattering from geometric features. This work presents a new damage localization paradigm based on a statistical approach to dealing with uncertainty in the guided wave signals. A bolted frame and a section of a fuselage rib are tested with different simulated damage conditions and used to conduct a detailed comparison between the proposed solution and other sparse-array localization approaches. After establishing the superiority of the statistical approach, two novel innovations to the localization procedure are proposed: an approach to sensor fusion based on the Neyman–Pearson criterion, and a method of constructing simple models of geometrical features. Including the sensor fusion and geometrical models produces a substantial improvement in the system׳s localization accuracy. The final result is a robust and accurate framework for single-site damage localization that moves structural health monitoring towards practical implementation on a much broader range of structures.

Using full-field ultrasonic guided wave data can provide a wealth of information on the state of a structure through a detailed characterization of its wave propagation properties. However, the need for appropriate feature selection and quantified metrics for making rigorous assessments of the structural state is in no way lessened by the density of information. In this study, a simple steel bolted connection with two bolts is monitored for bolt loosening. The full-field data were acquired using a scanning-laser-generated ultrasound system with a single surface-mounted sensor. Such laser systems have many advantages that make them attractive for nondestructive evaluation, including their high-speed, high spatial resolution, and the ability to scan large areas of in-service structures. In order to characterize the relationship between bolt torque and the resulting wavefield in this specimen, the bolt torque in each of the bolts is independently varied from fully tightened to fully loosened in several steps. First, qualitative observations about the changes in the wavefield are presented. Next, an approach to quantifying the wave transmission through the bolted joint is discussed. Finally, a method of monitoring the bolt torque using the ultrasonic data is demonstrated.

Bearing damage in composite bolted connections (such as those commonly used in aerospace applications) is investigated through the application of ultrasonic guided waves. Specifically, identical macro-fiber composite sensor arrays were bonded to each of two identical composite plates. One plate was then inspected ultrasonically in the unloaded condition with bearing damage being introduced through successive uniaxial tensile tests, while the other plate was assessed while under load in the tensile testing machine. A scattering matrix approach is employed to characterize the interaction of the guided waves with the target bolt hole. The effects of applied load and the bolt fixture on the ultrasonic results are explored. A parametric study is carried out to determine the optimal actuation frequency and interrogation angle for this application. The results demonstrate that the system is capable of detecting bolt bearing damage as well as monitoring of the applied load in the elastic region.

Assessing the robustness of a sensor system and the related predictions is a key step in structural health monitoring (SHM). In this paper, the SHM system under consideration uses macro fiber composite (MFC) sensors to generate ultrasonic guided waves for inspection of a composite bolted joint. Bolt bearing failure is introduced to a target hole through tensile loading. The MFC sensor-actuators are configured in a circular array around the target hole and used to send and receive the ultrasonic waves from many different directions. This strategy facilitates a scattering matrix approach to identifying the most effective actuation and sensing angles for damage detection. Multiple interrogation wavelengths are also utilized to study the effect of the wavelength relative to the size of hole being monitored. However, various other factors are expected to impact the ultrasonic signal and therefore the damage detection results. Particularly, the position and orientation of each sensor is precisely measured through image processing techniques in order to quantify the effects of sensor misalignment. Finally, the sensitivity of the ultrasonic inspection technique to variation in each phenomenon is compared to better understand the most significant effects on damage detection performance.

This paper presents the development of a Bayesian framework for optimizing the design of a structural health monitoring (SHM) system. Statistical damage detection techniques are applied to a geometrically-complex, three-story structure with bolted joints. A sparse network of PZT sensor-actuators is bonded to the structure, using ultrasonic guided waves in both pulse-echo and pitch-catch modes to inspect the structure. Receiver operating characteristics are used to quantify the performance of multiple features (or detectors). The detection rate of the system is compared across different types and levels of damage. A Bayesian cost model is implemented to determine the best performing network.

This paper presents an uncertainty analysis for a new methodology to predict reaction forces in weapon store connections of naval aircraft. The proposed methodology utilizes a strain-gage-based measurement technique in which a series of sensors are affixed to the connector and calibrated with a set of known loading configurations. The calibration matrix relating the measured strains to the loads can then be used to estimate unknown loads from measured strains. In this way, the system can be used to monitor the force through the connector when the aircraft is in service.

Recent studies by the Department of Energy suggest that wind energy could make up as much as 20% of total U.S. power generation by 2030 [1]. By better understanding the load transmission in wind turbines, subsequent improvements in design could result in more efficient turbines and reduced maintenance costs. The objective of this study is twofold. First, a low-cost monitoring system to assess the condition of the bolted joint connections used to attach the blades to the hub is devised in order to characterize the force transmission into the hub and to detect when bolt loosening for in-service wind turbines. A section of the CX100 blade is used for the joint monitoring study. Second, a method to estimate the tip deflection of the blade is developed to better understand the dynamic loads acting on the rotor hub. A one-meter-long blade is used to estimate the tip deflection under laboratory conditions. Using the results from these tests, this study demonstrates how a better understanding of load transmission from the blade to the hub may be achieved.