April 2010 Issue

  April 2010
Journal of the IEST

Members Only
Log in for full access
Nonmembers
Complete Tech Talk articles and technical papers’ abstracts (full text by individual sale or subscription)
Subscribe


Peer-reviewed technical papers:
Control of an Over-Actuated Single-Degree-of-Freedom Excitation System
Prognostics and Health Assessment Implementation for Electronic Products
Effect of Resonant Frequency Shifting on Time to Failure of a Cantilevered Beam under Vibration
Novel Guided Head Expander Design Uses Close Coupled Inertial Masses and Hydrostatic Bearings to Minimize Cross-Axis Motion

Otto Hamberg Award-winning technical paper (not peer-reviewed):
Effective Data Validation Methodology for Pyrotechnic Shock Testing

Complimentary
Tech Talk article:
Facts and Fiction in Cleanroom Metrology

The following Tech Talk article is accessible free of charge (access Journal of the IEST).

Facts and Fiction in Cleanroom Metrology
Lothar Gail, Cleanroom Consultancy
Dirk Stanischewski, BIS Prozesstechnik GmbH

This article discusses a number of cleanroom qualification parameters in terms of their proper specification. A critical analysis reveals that the useful operating range of some parameters is not appropriately considered by some early standards and guidelines, which are still used by regulatory authorities (the US Food and Drug Administration (FDA) and the European Union (EU)) and industry professionals. In practice, the windows of safely controlled cleanroom operation prove to be considerably larger than anticipated by existing regulations, especially with regard to unidirectional airflow velocity, pressure difference and other parameters. Many measuring techniques, such as installed HEPA filter integrity testing and recovery time testing, are also regulated more strictly than necessary. Modern cleanroom testing requires more carefully defined targets and more flexibility in using advanced test procedures.

The following paper is the winner of the Otto Hamberg Technical Paper Award from the 25th Aerospace Testing Seminar Proceedings.

Effective Data Validation Methodology for Pyrotechnic Shock Testing
Charles P. Wright, The Aerospace Corporation

Valid test data from explosively or ordnance initiated pyrotechnic shock tests are difficult to acquire. Measurement of these frequency-rich acceleration time histories, a prerequisite to calculation of a valid Shock Response Spectrum, drives the measurement system to its performance limits. Successful acquisition of demonstrably valid acceleration time histories requires a series of performance compromises that must be made with a depth of measurements expertise. Such expertise may not be available from vendors of the various data acquisition systems sold for these tests. All measurement system performance characteristics (transducer mount dynamics, gain, frequency response, phase response, linearity, lead wire effects, sampling rate, etc.) require compromise. It takes professional-level knowledge and experience to make the proper compromises to assure data validity for the measured wave shape. These measurements should never be taken for granted, as often and unfortunately happens. Data validation methods should be used by a test organization to prove the validity of the experimental shock wave shapes and subsequent Shock Response Spectra (SRS). Recent events in the explosively driven, pyrotechnic shock test community show that methods for effective data validation are not in general use. This situation can lead to the problem of invalid shock test acceleration time histories causing invalid shock response spectra with both entering the design verification cycle. This paper defines the requirements for the measurement of valid shock wave shapes. It then defines an ordered series of validity tests that will both identify and quantify a number of detrimental effects in the acquisition of these frequency rich time histories, and resultant shock response spectra. Use of this set of validity checking methods assures the objective identification of invalid shock data to customers of pyrotechnic shock tests.

The Journal is pleased to reprint the Otto Hamberg Award paper and wishes to acknowledge the following Honorable Mention paper:
Extracting Fixed Base Modal Models from Vibration Tests on Flexible Tables
Randy L. Mayes and L. Daniel Bridgers

The April 2010 issue contains the following peer-reviewed technical papers related to the fields of product reliability and design, test, and evaluation. IEST members and Journal of the IEST subscribers have full access to technical papers in the current issue as well as over 20 years of archived technical papers. Others can view free Tech Talk articles as well as full-text abstracts of peer-reviewed technical papers, and have the option to purchase individual papers (access Journal of the IEST).

Control of an Over-Actuated Single-Degree-of-Freedom Excitation System
Michael T. Hale, United States Army Redstone Test Center, Dynamic Test Division
Norman Fitz-Coy, Dept. of Mechanical and Aerospace Engineering, University of Florida

This paper provides results of a laboratory experiment designed to illustrate the theoretical control considerations for an over-actuated excitation system. The experiment is based on control of a beam pinned at one end providing a single rotational degree of freedom and excited by two electrodynamic actuators. Control is achieved through implementation of two different control reference techniques: (1) reference based on linear acceleration autospectral densities (ASD) and cross-spectral densities (CSD) using linear accelerometer feedback and (2) reference based on an angular acceleration ASD using estimates of angular acceleration as feedback. Correlations to the theoretical based predictions were conducted based on common measurements of both linear acceleration and estimates of angular acceleration acquired during each trial.

Prognostics and Health Assessment Implementation for Electronic Products
Jie Gu, formerly a researcher at the Center for Advanced Life Cycle Engineering (CALCE), University of Maryland, College Park
Michael Pecht, Visiting Professor of Electronics Engineering, City University of Hong Kong, and Director of CALCE, University of Maryland, College Park

Traditional handbook-based reliability prediction methods for electronic products are inaccurate and misleading. This paper presents a prognostics and health management (PHM) approach that is more suitable for reliability (remaining life) assessment for electronic products, since it considers actual operational and environmental loading conditions for individual products. The process for implementing PHM in electronics is discussed. Several examples of implementation in industry and defense are also given.

Effect of Resonant Frequency Shifting on Time to Failure of a Cantilevered Beam under Vibration
Mark Paulus and Kirk Doughty, Environmental Test Lab, Naval Undersea Warfare Center–Division Keyport

Many vibration screening programs have implemented the use of repetitive shock (RS) machines (RS). Examination of the RS power spectral density (PSD) shows that there are many high and low points throughout the frequency band. The effect of these sharp peaks and valleys on the time to failure of a test item was unknown. A simple cantilevered beam was subjected to several vibration profiles with peaks and valleys. It was determined that the peaks and valleys had a significant effect on the time to failure due to resonant shifting of the beam during crack propagation.

Novel Guided Head Expander Design Uses Close Coupled Inertial Masses and Hydrostatic Bearings to Minimize Cross-Axis Motion
Douglas Lund, Team Corporation
Robert Crawford, Orbital Sciences

When vertical vibration testing of large test articles is required, it is common to install a head expander on the armature of a shaker. Larger test articles often have a center of gravity relatively far above the mounting surface. When combined with the armature and head expander, these test articles may exhibit multiple structural resonances within the desired test band that do not exist in the intended application. These test configuration-driven characteristics are likely to create unwanted cross-axis excitation during a vibration test.

The difficulty in controlling unwanted cross-axis motion usually increases when testing large items. Excessive cross-axis motion can “over-test” the test item, creating the risk of damaging the test item, or can limit the input in the test axis, thus jeopardizing a successful test.

Orbital Sciences commissioned the design of a guided head expander system that greatly reduces the cross-axis motion at the test article mounting surface of the head expander. The design submitted by Team Corporation couples large inertial masses to the head expander through high-stiffness, hydrostatic, self-aligning bearings. Together, the guided head expander and inertial mass structures have a first resonance higher than the test band of interest and provide high dynamic stiffness. The head expander and inertial masses are supported by a suspension system with a low first resonance, below the test band of interest. It is noteworthy that this design approach exhibits high “dynamic” stiffness and low static stiffness.

Conventional designs for this type of equipment may have relatively high cross-axis load ratings, which might suggest that such designs would provide good cross-axis motion control, but these designs often suffer from structural resonances within the test band of interest that produce unwanted cross-axis motion.

View online April 2010 Journal of the IEST, previous issues, and archive
The following links will take you to our online Journal of the IEST website hosted by EBSCO MetaPress or give you subscription information:

Members Only
Log in for full access
Nonmembers
Complete Tech Talk articles and technical papers’ abstracts (full text by individual sale or subscription)
Subscribe