Sensors and Instrumentation, Aircraft/Aerospace and Dynamic Environments Testing, Volume 7

The Structural Validation Branch (AFRL/RQVV) provides test services for internal and external customers covering a wide range of research and development programs. Calculating and documenting test data uncertainty is a necessary capability for a test organization. Test data uncertainty analysis provides error limit values based on the combined effects of random and systematic error sources. Error source examination can also be used to determine if corrective actions are necessary and/or possible to eliminate or reduce the measurement errors. This project was initiated to review uncertainty calculation methods and develop the internal processes and procedures that will be used to calculate, document, and report measurement uncertainty values.

This work is the next of a series on vibrations of non-homogeneous structures. It addresses the lateral harmonic forcing, with spatial dependencies, of a two-segment damped Timoshenko beam. In the series, frequency response functions (FRFs) were determined for segmented structures, such as rods and beams, using analytic and numerical approaches. These structures are composed of stacked cells, which are made of different materials and may have different geometric properties. The goal is the determination of frequency response functions (FRFs). Two approaches are employed. The first approach uses displacement differential equations for each segment, where boundary and interface continuity conditions are used to determine the constants involved in the solutions. Then the response, as a function of forcing frequency, can be obtained. This procedure is unwieldy, and determining particular integrals can become difficult for arbitrary spatial variations. The second approach uses logistic functions to model segment discontinuities. The result is a system of partial differential equations with variable coefficients. Numerical solutions are developed with the aid of MAPLE® software. For free/fixed boundary conditions, spatially constant force, and viscous damping, excellent agreement is found between the methods. The numerical approach is then used to obtain FRFs for cases including spatially varying load.

Multiple single-axis vibration tests are commonly linearly superposed as a multi-axis test. However, due to cross-axial responses in single-axis, the net response of multiple single-axis vibration tests results in an overestimation of the severity of the service environment. In practice, over-testing has been a result of the limitations associated with physical experimentation, which indicates a need to improve simulation methods and accuracy of current lifetime test strategies. In this chapter, single- and multi-axis experimental vibration tests of the Box Assembly with Removable Component (BARC) model are simulated with implicit and explicit methods using the commercial FEA tool ABAQUS, where the multiple observed outputs are vibrational responses in the three translational degrees of freedom. The model looks to validate physically obtained experimental data by replicating certain boundary conditions and service environments.

Resonant plate and other resonant fixture shock techniques were developed in the 1980s at Sandia National Laboratories as flexible methods to simulate mid-field pyroshock for component qualification. Since that time, many high severity shocks have been specified that take considerable time and expertise to setup and validate. To aid in test setup and to verify the shock test is providing the intended shock loading, it is useful to visualize the resonant motion of the test hardware. Experimental modal analysis is a valuable tool for structural dynamics visualization and model validation. This chapter describes a method to perform experimental modal testing at pyroshock excitation levels, utilizing input forces calculated via the SWAT-TEEM (Sum of Weighted Accelerations Technique—Time Eliminated Elastic Motion) method and the measured acceleration responses. The calculated input force and the measured acceleration data are processed to estimate natural frequencies, damping, and scaled mode shapes of a resonant plate test system. The modal properties estimated from the pyroshock-level test environment are compared to a traditional low-level modal test. The differences between the two modal tests are examined to determine the nonlinearity of the resonant plate test system.

Environmental testing is very useful for qualifying components for their real-world use. To that end, random vibration testing is performed on component(s) in the laboratory in such a way as to mimic their real-world environment. This is particularly useful when testing a component in its real-world environment is time-consuming or costly. To accurately control, a random vibration test is not a simple challenge to meet, with issues arising such as lightly damped modes, signal noise, poorly conditioned frequency response functions (FRFs), and others. This chapter presents a parameter study on the performance of a random vibration control method called the Matrix Power Control Algorithm (MPCA). Additionally, this chapter shows that a simple modification to MPCA can result in improved stability and convergence. In particular, a proportional gain controller is used to change the control parameter for MPCA as a function of the error. Two simulation environments are used in this chapter: a single-input single-output (SISO) fixed-free cantilever beam with base excitation and a multiple-input multiple-output (MIMO) fixed–fixed beam with base excitation at both ends of the beam. Additionally, the Box Assembly with a Removeable Component (BARC) was used as a laboratory example illustrating the performance of MPCA and our modification in the laboratory.

Acoustic resonance testing (ART) is a method for efficient and objective serial testing of workpieces. It nondestructively detects smallest variations in geometry, weight, and bonding and can identify holes, cracks, and other structural defects. In ART, a fully automated impact excitation is fundamental. Fully automatic, in this context, means an excitation point search and an adjustment of the excitation force controlled directly in the hammer. Moreover, it is essential to perform this excitation in a precisely reproducible process and in large series. No manual configuration steps are necessary for the user. For nondestructive ART on production lines, changes in the position of the test object must also be compensated.

To demonstrate the suitability and the advantages of the first smart impulse hammer in series testing, an ART was carried out on floor tiles on production line. For evaluation, the force sensor signal of the hammer and additionally a measuring microphone were used. Artificial intelligence and signal processing methods were used for classification. The system and results of the ART are presented in this chapter.

While research in multiple-input/multiple-output (MIMO) random vibration testing techniques, control methods, and test design has been increasing in recent years, research into specifications for these types of tests has not kept pace. This is perhaps due to the very particular requirement for most MIMO random vibration control specifications – they must be narrowband, fully populated cross-power spectral density matrices. This requirement puts constraints on the specification derivation process and restricts the application of many of the traditional techniques used to define single-axis random vibration specifications, such as averaging or straight-lining. This requirement also restricts the applicability of MIMO testing by requiring a very specific and rich field test data set to serve as the basis for the MIMO test specification. Here, frequency-warping and channel averaging techniques are proposed to soften the requirements for MIMO specifications with the goal of expanding the applicability of MIMO random vibration testing and enabling tests to be run in the absence of the necessary field test data.

Structural health monitoring (SHM) is gaining more attention, particularly for bridge structures to continuously evaluate their structural performance and assure their serviceability and safety throughout their service life. To assure reliable SHM systems, optimal sensor placement (OSP) is a cornerstone since it directly influences the quality of obtained data and, thus, the accuracy of the diagnosis. Hence, determining the optimal sensor configuration is a critical step to acquire the maximum information on the structural behavior. Of the available OSP methods, the effective independence method (EFI), which aims to assure that the mode shapes identified from the recorded vibrations are orthogonal to each other, is popular and widely used in literature. Despite its popularity, there are no clear guidelines on how to select the modes that will be included in the OSP application. This chapter evaluates the impact of various parameters including the selected number of mode shapes and sensors on the results of OSP using EFI.

The majority of data acquisition in experimental structure dynamics are conducted using impulse excitation and acceleration sensors. As the shape of the impulse in time domain is the crucial basis for the frequency content calculated from the impulse, mechanical parameters of the automatic modal impulse hammer (AMimpact) are discussed that have an influence on the impulse function. Using minimal models, the individual effects of the parameters are theoretically discussed and shown in experiments. Mounting the sensors to the structure is a critical step towards achieving good measurement quality. This investigation presents an overview and compares several, feasible methods for fixing acceleration sensors to metal (aluminum) structures. The methods are discussed regarding their effort, their positional precision, and their repeatability. The various methods are discussed in terms of their positional precision and their repeatability. The experiments are conducted and analyzed on an academic aluminum test structure and the automatic modal hammer (AMimpact). One difficulty is the lack of any ground truth, for which reason two sensors are stacked together to compare frequency response functions. These FRFs are obtained in two directions to enable the sensor fixation to be loaded in normal and tangential direction. The conclusion of the paper states that the fixing methods used for acceleration sensors are not critical in the normal direction but can have a non-negligible influence on measurements in the tangential direction. This effect should therefore be considered at an early stage of a measurement campaign.

A critical task involved with being able to predict flight loads accurately in aerospace finite element models (FEMs) is the prior verification of the FEMs by conducting modal survey testing (MST). Experience comparing dynamic response of initial FEMs to MST data tends to demonstrate that FEMs can have unacceptable accuracy even when best modeling practices are followed. One inherent source of inaccuracy in linear dynamic FEMs is the modeling of nonlinear joints with mechanisms such as spherical bearings. These joints are usually designed to freely translate or rotate under the high levels of loading experienced in flight. Engineers who create linear FEMs conventionally model these joints without any stiffness in the mechanism degrees of freedom to meet this design intent. However, inaccuracy is observed during test validation of these FEMs, which usually relies on low-level force excitation orders of magnitude below flight load levels. This low-level modal test rarely overcomes the joint friction that is present, and thus the mechanism joints are able to react loads. This divide between the test results and the FEM creates a significant challenge to the engineer who is performing the correlation, in that the engineer has no basis for what stiffness value should be used to make the FEM match the test results. A compounding challenge is that complicated built-up aerospace structures commonly have multiple joints through a load path where each joint will “stick” and “slip” at different levels of force input. Explicitly matching the dynamics of a system containing these nonlinear mechanisms would require a nonlinear FEM, which is prohibitively costly for dynamic simulations of most aerospace systems. The objective of this chapter is to present a workflow that can efficiently cycle through many iterations of a FEM, allowing a Monte Carlo–style examination of the design space to identify candidate stiffness values for nonlinear mechanism joints. The outlined approach is specific to MSC Nastran and utilizes MSC Nastran’s symbolic substitution capabilities, coupled with the IMAT™ and Attune™ software packages developed by ATA Engineering, Inc. (ATA). The workflow is demonstrated with a case study from the correlation effort for The Boeing Company’s Crew Space Transportation (CST)-100 Starliner FEM.

Unlike traditional base excitation vibration qualification testing, multi-axis vibration testing methods can be significantly faster and more accurate. Here, a 12-shaker multiple-input/multiple-output (MIMO) test method called intrinsic connection excitation (ICE) is developed and assessed for use on an example aerospace component. In this study, the ICE technique utilizes 12 shakers, 1 for each boundary condition attachment degree of freedom to the component, specially designed fixtures, and MIMO control to provide an accurate set of loads and boundary conditions during the test. Acceleration, force, and voltage control provide insight into the viability of this testing method. System field test and ICE test results are compared to traditional single degree of freedom specification development and testing. Results indicate the multi-shaker ICE test provided a much more accurate replication of system field test response compared with single degree of freedom testing.

One concept in smart dynamic testing is to match the impedance that a component experiences between test and the environment of interest, but this begs the question: how much of an impedance match is needed, and could there be too much? In a prior work, the authors performed MIMO testing with a small component connected to various assemblies, each of which had a differing degree of similarity to the actual flight boundary conditions. The results showed that the fidelity of the response at locations away from the control accelerometers was highly sensitive to the impedance. This work presents further case studies to explore these ideas. Subsequent tests are presented for an assembly that presumably matched the impedance even better, and which was also much more flexible, and the results obtained are even worse than when no attention was given to the impedance. Hence, the work presented here suggests that one should seek a balance between (1) matching the impedance and (2) improving the controllability of the component of interest. The concepts are explored using both test data of a benchmark component, for which the environment of interest was recorded as the component flew on a sounding rocket.

As test articles become dimensionally larger, more complex, and massive in weight, combined with the need to excite them to higher than traditional levels in order to identify their nonlinear characteristics, modal shakers that can generate significantly higher force levels, have longer stroke lengths, and possess higher velocity limits are required. While large-scale modal tests may be performed with electrodynamic modal shakers, hydraulic modal shakers become attractive since they can generate higher force levels at lower unit cost with a smaller spatial footprint. While test engineers familiar with electrodynamic modal shakers are familiar with the challenges of displacement and velocity limits and the relatively mild shaker nonlinear distortion due to amplifier gains and shaker flexure structural geometric nonlinearities, they probably are not as familiar with the unique set of challenges hydraulic modal shakers present. These unique challenges include significant nonlinear distortion in the shaker force, issues with the setup of the hydraulic power supply and the associated hydraulic hosing, velocity limits as they relate to potentially damaging the hydraulic actuator piston, and safety issues with operating high-pressure hydraulic systems. This chapter addresses these unique challenges to help the test engineer to better utilize hydraulic modal shakers on large-scale modal tests.

Efficient and reliable detection of damages is of critical importance in the aerospace industry. Within the context of non-destructive testing (NDT), advanced non-contact measurement techniques can be leveraged to obtain full-field responses of a target structure, facilitating the development of structural health monitoring (SHM) methodologies. The motivation of this chapter is to compare several types of measurement techniques, excitation signals, and post-processing methods applied to an Airbus airplane panel with sections of damaged and loose rivets. Its response is obtained using both digital image correlation (DIC) and laser Doppler vibrometry (LDV) techniques. The panel is excited with modal shakers or PZT patches using chirp or pseudorandom signals, for different frequency ranges. Full-field measurements were obtained, along with other more localized measurements on the area containing defects. On a later stage, methodologies inspired on nonlinear analysis and machine learning (ML) techniques were applied for damage detection. A description of the obtained results from these different experimental settings is shown in this chapter, together with a successful structural characterization and detection of damages on the airplane panel.

Violins are probably known among the most technically complex musical instruments. Their dynamics is usually studied through structural vibration methods. However, these methods present a few limitations. Therefore, this chapter focuses on the qualitative identification of violin sonic areas generating different musical notes. For such a purpose, a recently introduced tool by Siemens, the Simcenter Sound Camera, which is an array-based sound source localisation technique, was implemented. A contemporary violin, based on a Guarneri del Gesù model, was used as a test instrument. Stationary excitation was induced by an expert violin player to generate different notes. The locations of the sound sources were identified for several configurations and frequency bandwidths, inside an anechoic chamber, i.e. open-field conditions. The identified areas were qualitatively compared with the analysis of vibrations in global and local modes. The comparison confirmed the strong importance of “f” holes and lungs of the instrument, which were clearly identified as the main sound sources and signature vibration modes of the violin.