Threshold level as an index of squeak and rattle performance

A practical approach for evaluating and validating global system designs for Squeak and Rattle performance is proposed. Using simple slip and rattle models, actual sound and vibration data, and the fundamentals of audiological perception, analysis tools adapted from Chaos Theory are used to establish threshold levels of performance and identify system characteristics which are significant contributors to Squeak and Rattle. Focus on system design is maintained by using a simple rattle noise indicator and relating rattle events to levels of dynamic motion (acceleration, velocity, etc.). The threshold level is defined as the level of acceleration at which the system moves from a non-rattling state to a rattling state. The approach is demonstrated with a simple analytical model applied to an experimental structure under dynamic load

Method to predict the shock response spectrum shape from frequency response functions

In an effort to understand the details of why a Shock Response Spectrum (SRS) has a particular shape, a method was designed to predict the shape of the SRS based on Frequency Response Functions (FRFs). Gaining a full understand of the relationship between the FRF of a structure and the SRS shape should prove to be very useful in reducing SRS test time and allow the general shape of an SRS response to be predicted more efficiently using finite element methods or experimentally obtained data. To allow comparisons of different shock response plates and fixtures through the use of FRFs a normalized Shock Response Spectrum (nSRS) was developed. The nSRS is derived directly from the FRF of a structure and when coupled with a library of characterized impactor input spectrums allows an SRS to be predicted without performing any testing. This approach allows modifications to the shock response plate/fixture to be evaluated efficiently and the effect of different impactors to be studied without performing a large number of experimental tests. It is hoped that this approach to understanding and predicting SRSs improves the understanding of how the structural dynamics effects an SRS and efficiency of testing

Sensitivity study of BARC assembly

This paper will present modal analysis results from a systematic study of the assembly of the Box Assembly with Removable Component (BARC). The paper will present results from testing done with both the cut and un-cut version of the BARC and with the different pieces of the BARC both bolted together and attached with a structural adhesive. The boundary condition will be a fixed base excitation. The results will be presented in terms of both Frequency Response Functions (FRFs) and mode shapes and natural frequencies with a goal of showing how the BARC fixture changes with each assembly modification. Upon completion of this testing it is anticipated that a thorough understanding of how assembly methods change the dynamic response of the fixture. This may lead to a suggested assembly method for anyone testing a BARC fixture

DAQ Evaluation and Specifications for Pyroshock Testing

Pyroshock events contain high-amplitude, extreme rise-time accelerations that can be damaging to electronics and small structures. Due to their extreme nature, these events can be difficult to capture, exceeding the performance limits of transducers, signal conditioning, and data acquisition (DAQ) equipment. This study assesses the ability of different data acquisition systems to record quality pyroshock data. Using a function generator and voltage input, different tests were performed to characterize the data acquisition systems’ anti-alias filter, out-of-band energy attenuation, number of effective bits, in-band gain, and slew rate. These tests include a shorted-input noise test, a sine sweep test, and a high amplitude low frequency square wave test. Although the data acquisition systems evaluated have similar specifications, their ability to record quality pyroshock data varied. Some of these data acquisition systems do not appropriately handle the rapid transient content and may have inadequate fidelity to record pyroshock data. Data acquisition system performance for pyroshock testing cannot be evaluated by the specification sheet alone

Frequency based substructuring on resonant plate

Resonant plate pyroshock tests were originally designed to test one component axis at a time, while the qualification pyroshock tests often have multi-axis specifications to meet. Traditionally, one Shock Response Spectrum (SRS) is created for each single axis test record, which is then compared to the specified qualification SRS. There is interest in creating a multi-axis shock test environment using traditional resonant plate test components to save testing time and create a more realistic test environment. As a potential approach to test system design, LaGrange-Multiplier Frequency Based Substructuring (LM-FBS) is used to arrange single-axis resonant plate subsystems in different assembly configurations. LM-FBS uses Frequency Response Functions (FRFs) of the resonant plate parts, virtually assembles the parts, and produces FRFs of the assembly. To estimate potential shock test performance, an inverse Fourier transform is applied to the assembly FRF to get a time domain impulse response, then an SRS is calculated for all three response axes. A least squares regression is used to optimize the SRS produced from different assembly configuration to a multi-axis specification SRS. Preliminary assembly iterations are performed on a finite element model, and the final multi-axis configuration is verified with testing

Effects of variable thickness circular plates on frequency response functions and shock response spectrum

Resonant plates used for shock testing are typically a constant thickness. Prior research demonstrated that circular plates utilize symmetry to limit the number of contributing modes, although more design control is necessary to achieve target shock response spectra (SRS). Analytical modeling results show that variable thickness plates provide more flexibility to meet a target SRS. The first membrane mode of a circular plate correlates with the knee frequency in the shock response spectrum. Higher order membrane modes can cause the SRS to occur outside of the target band. Concave plates decrease the frequency band between first membrane mode and higher order membrane modes, while convex plates show the opposite effect. Using this theory, resonant plate cross section can be altered to tune resonant plate natural frequencies in order to achieve target SRS

Initial modal results and operating data acquisition of shock/vibration fixture

This paper presents the initial experimental and FEA based modal analysis results obtained on a test assembly developed specifically to study the effects of component boundary conditions and excitation techniques on test damage potential during component qualification testing. This assembly was developed as a platform with a simple “component” and “next assembly” that allows the component to be removed and attached via a fixture to shock or vibration test equipment. All data and results will be made publicly available for other groups wishing to study the test assembly in pursuit of insight into how to define appropriate boundary conditions for component testing

Inverse force estimation for resonant shock plate application

Resonant shock plate testing uses a projectile and programmer material to deliver and tune an impulsive force. Typically, the force level is too high to directly measure with conventional force sensors, so the spectral and temporal characteristics of these forces are not well understood. Non-linear simulations of the projectile, programmer, shock plate, and fixture are currently used to predict the results and design a resonant shock plate. A linear model of the resonant shock plate and fixture could be used if a reasonable representation of the applied force was known. This paper explores the use of inverse force estimation to estimate the spectral content of the force applied from the projectile through the programmer material. The process involves de-convolving the resonant plate response and the impulse response of the resonant plate/fixture system. A spectral representation of the force can be obtained by dividing the linear spectrum of the resonant plate/fixture response and its frequency response function

Frequency response inspection of additively manufactured parts for defect identification

The goal of this paper is to evaluate internal defects in AM parts using dynamic measurements. The natural frequencies of AM parts can be identified by measuring the response of the part(s) to a dynamic input. Different excitation methods such as a modal impact hammer or shakers can be used to excite the parts. Various methods exist to measure the parts’ responses and find the natural frequencies. This paper will investigate the use of Doppler lasers, accelerometers and Digital Image Correlation (DIC). The parts evaluated in this work include sets of parts that are still attached to the AM build plate, this makes the identification of a faulty part much more difficult as parts on a build plate interact with each other as well as the build plate complicating the responses. Several approaches to these issues will be presented based on the above listed response measurements

Understanding multi-axis SRS testing results

This paper presents a study done on a round resonant plate fixture used for Shock Response Spectrum (SRS) testing. The goal of this study was to understand the magnitude and character of both on axis and off-axis, with respect to shock input, response of the plate at various locations. The resonant plate was modeled using linear FEA as well as tested experimentally. Tools and approaches based on modal decomposition were developed to understand how the natural frequencies and mode shapes of the structure contribute to the SRS response at a given point and direction on the fixture and/or plate. It is seen that in some instances, the off-axis SRS response can have both a higher amplitude response as well as a different “knee” frequency which can make meeting a designated SRS target very difficult. It is shown that by understanding the modal properties of the plate/fixture assembly, the SRS results can be understood. These results will lead to the capability to predict both the on axis and off-axis SRS response for a given input/output set of locations and eventually the ability to choose the ideal locations to achieve a set of on and off-axis SRS responses to meet a given criteria