Mercedes C. Reaves

Test Cases for Modeling and Validation of Structures with Piezoelectric Actuators

A set of benchmark test articles were developed to validate techniques for modeling structures containing piezoelectric actuators using commercially available finite element analysis packages. The paper presents the development, modeling, and testing of two structures: an aluminum plate with surface mounted patch actuators and a composite box beam with surface mounted actuators. Three approaches for modeling structures containing piezoelectric actuators using the commercially available packages: MSC/NASTRAN and ANSYS are presented. The approaches, applications, and limitations are discussed. Data for both test articles are compared in terms of frequency response functions from deflection and strain data to input voltage to the actuator. Frequency response function results using the three different analysis approaches provided comparable test/analysis results. It is shown that global versus local behavior of the analytical model and test article must be considered when comparing different approaches. Also, improper bonding of actuators greatly reduces the electrical to mechanical effectiveness of the actuators producing anti-resonance errors.

Ground and Flight Test Structural Excitation Using Piezoelectric Actuators

A flight flutter experiment at the National Aeronautics and Space Administration (NASA) Dryden Flight Research Center, Edwards, California, used an 18-inch half-span composite model called the Aerostructures Test Wing (ATW). The ATW was mounted on a centerline flight test fixture on the NASA F-15B and used distributed piezoelectric strain actuators for in-flight structural excitation. The main focus of this paper is to investigate the performance of the piezoelectric actuators and test their ability to excite the first-bending and first-torsion modes of the ATW on the ground and in-flight. On the ground, wing response resulting from piezoelectric and impact excitation was recorded and compared. The comparison shows less than a 1-percent difference in modal frequency and a 3-percent increase in damping. A comparison of in-flight response resulting from piezoelectric excitation and atmospheric turbulence shows that the piezoelectric excitation consistently created an increased response in the wing throughout the flight envelope tested. The data also showed that to obtain a good correlation between the piezoelectric input and the wing accelerometer response, the input had to be nearly 3.5 times greater than the turbulence excitation on the wing.

Orion crew module landing system simulation and verification

NASA Langley Research Center (LaRC) has developed a comprehensive test and analysis program to evaluate the ability of LS-DYNA to model the materials and the phenomena involved in soil and water landing impacts of the Orion crew module. 12Elemental, scale boilerplate, and full-scale prototype testing is being conducted in support of the simulation verification and validation approach. Aspects of the simulations evaluated against test data include soil constitutive properties, water equations of state, and contact algorithms. Subsystems tested include airbags, crushable energy absorbing honeycomb materials, and energy absorbing seat support struts. The procedures, instrumentation, and general observations from each test series are presented. Plans for a series of swing tests of a full-scale boilerplate into a purpose-built water basin are described. Further plans for swing tests of flight-like prototypes into the water basin are noted.

Finite Element Model Calibration Approach for Ares I-X

Ares I-X is a pathfinder vehicle concept under development by NASA to demonstrate a new class of launch vehicles. Although this vehicle is essentially a shell of what the Ares I vehicle will be, efforts are underway to model and calibrate the analytical models before its maiden flight. Work reported in this document will summarize the model calibration approach used including uncertainty quantification of vehicle responses and the use of nonconventional boundary conditions during component testing. Since finite element modeling is the primary modeling tool, the calibration process uses these models, often developed by different groups, to assess model deficiencies and to update parameters to reconcile test with predictions. Data for two major component tests and the flight vehicle are presented along with the calibration results. For calibration, sensitivity analysis is conducted using Analysis of Variance (ANOVA). To reduce the computational burden associated with ANOVA calculations, response surface models are used in lieu of computationally intensive finite element solutions. From the sensitivity studies, parameter importance is assessed as a function of frequency. In addition, the work presents an approach to evaluate the probability that a parameter set exists to reconcile test with analysis. Comparisons of pre-test predictions of frequency response uncertainty bounds with measured data, results from the variancebased sensitivity analysis, and results from component test models with calibrated boundary stiffness models are all presented.

Ares I-X Launch Vehicle Modal Test Overview

The first test flight of NASA’s Ares I crew launch vehicle, called Ares I-X, is scheduled for launch in 2009. Ares I-X will use a 4-segment reusable solid rocket booster from the Space Shuttle heritage with mass simulators for the 5th segment, upper stage, crew module and launch abort system. Flight test data will provide important information on ascent loads, vehicle control, separation, and first stage reentry dynamics. As part of hardware verification, a series of modal tests were designed to verify the dynamic finite element model (FEM) used in loads assessments and flight control evaluations. Based on flight control system studies, the critical modes were the first three free-free bending mode pairs. Since a test of the free-free vehicle is not practical within project constraints, modal tests for several configurations in the nominal integration flow were defined to calibrate the FEM. A traceability study by Aerospace Corporation was used to identify the critical modes for the tested configurations. Test configurations included two partial stacks and the full Ares I-X launch vehicle on the Mobile Launcher Platform. This paper provides an overview for companion papers in the Ares I-X Modal Test Session. The requirements flow down, pre-test analysis, constraints and overall test planning are described.

Multi-Dimensional Calibration of Impact Dynamic Models

NASA Langley, under the Subsonic Rotary Wing Program, recently completed two helicopter tests in support of an in-house effort to study crashworthiness. As part of this effort, work is on-going to investigate model calibration approaches and calibration metrics for impact dynamics models. Model calibration of impact dynamics problems has traditionally assessed model adequacy by comparing time histories from analytical predictions to test at only a few critical locations. Although this approach provides for a direct measure of the model predictive capability, overall system behavior is only qualitatively assessed using full vehicle animations. In order to understand the spatial and temporal relationships of impact loads as they migrate throughout the structure, a more quantitative approach is needed. In this work impact shapes derived from simulated time history data are used to recommend sensor placement and to assess model adequacy using time based metrics and orthogonality multi-dimensional metrics. An approach for model calibration is presented that includes metric definitions, uncertainty bounds, parameter sensitivity, and numerical optimization to estimate parameters to reconcile test with analysis. The process is illustrated using simulated experiment data.

Multi-Dimensional Calibration of Impact Models

As computational capabilities continue to improve and the costs associated with test programs continue to increase, certification of future rotorcraft will rely more on computational tools along with strategic testing of critical components. Today, military standards (MIL-STD 1290A (AV), 1988) encourage designers of rotary wing vehicles to demonstrate compliance with the certification requirements for impact velocity and volume loss by analysis. Reliance on computational tools, however, will only come after rigorous demonstration of the predictive capabilities of existing computational tools. NASA, under the Subsonic Rotary Wing Program, is sponsoring the development and validation of such tools. Jackson (2006) discussed detailed requirements and challenges associated with certification by analysis. Fundamental to the certification effort is the demonstration of verification, validation, calibration, and algorithms for this class of problems. Work in this chapter deals with model calibration of systems undergoing impact loads. The process of model calibration, which follows the verification and validation phases, involves reconciling differences between test and analysis. Most calibration efforts combine both heuristics and quantitative methods to assess model deficiencies, to consider uncertainty, to evaluate parameter importance, and to compute required model changes. Calibration of rotorcraft structural models presents particular challenges because the computational time, often measured in hours, limits the number of solutions obtainable in a timely manner. Oftentimes, efforts are focused on predicting responses at critical locations as opposed to assessing the overall adequacy of the model. For example (Kamat, 1976) conducted a survey, which at the time, studied the most popular finite element analysis codes and validation efforts by comparing impact responses from a UH-1H helicopter drop test. Similarly, (Wittlin and Gamon, 1975) used the KRASH analysis program for data correlation of the UH-1H helicopter. Another excellent example of a rotary wing calibration effort is that of (Cronkhite and Mazza, 1988) comparing results from a U.S. Army composite helicopter with simulation data from the KRASH analysis program. Recently, (Tabiei, Lawrence, and Fasanella, 2009) reported on a validation effort using anthropomorphic test dummy data from crash tests to validate an LS-DYNA (Hallquist, 2006) finite element model. Common to all these calibration efforts is the use of scalar deterministic metrics. One complication with calibration efforts of nonlinear models is the lack of universally accepted metrics to judge model adequacy. Work by (Oberkampf et al., 2006) and later (Schwer et al., 2007) are two noteworthy efforts that provide users with metrics to evaluate nonlinear time histories. Unfortunately, seldom does one see them used to assess model

Composite Fuselage Impact Testing and Simulation: A Model Calibration Exercise

Results from a model calibration study of a composite fuselage front frame test article subjected to impact loading are presented. The effort, undertaken over a 2 year time period, involved both modal and impact testing of the front frame section of a composite fuselage test article. Data from both types of tests were used with multi-dimensional calibration metrics to assess model adequacy. Specifically, two metrics are used: (1) a metric to assess the probability of enveloping the measured displacement during impact, and (2) an orthogonality metric to assess model adequacy between test and analysis. Because the impact test is destructive, vibration test data were also collected for model assessments in terms of modes of vibration. To justify the use of vibration data for calibration of the impact model, the paper discusses how to assess the relevancy of vibration data for calibration of impact models. Finally, results from the impact test are discussed and compared to model predictions and model uncertainty bounds.

Evaluation of Two Crew Module Boilerplate Tests Using Newly Developed Calibration Metrics

The paper discusses an application of multi-dimensional calibration metrics to evaluate pressure data from water drop tests of the Max Launch Abort System crew module boilerplate. Specifically, three metrics are discussed: (1) a metric to assess the probability of enveloping the measured data with the model, (2) a multi-dimensional orthogonality metric to assess model adequacy between test and analysis, and (3) a prediction error metric to conduct sensor placement to minimise pressure prediction errors. Data from similar (nearly repeated) capsule drop tests show significant variability in the measured pressure responses. When compared to expected variability using model predictions, it is demonstrated that the measured variability cannot be explained by the model under the current uncertainty assumptions.

Characterization of Dynamics Behavior of an Inflatable/Rigidizable Structure

Recently, a 3m -diamet er hexapod structure was designed and built to conduct research on modeling and vibration control of lightweight inflatable space structures. Modeling such structures is a challenging task due to the structures flexibility and the substantial number of vi bration modes. This paper presents an investigation of the dynamics characteristics of this structure and it is shown that the identified natural frequencies vary as the gain of input excitation changes, demonstrating the nonlinear dynamics of these modes. To investigate and quantify parameter variations, experiments with various levels of sine -sweep excitation at specified narrow frequency bands are conducted to obtain FRF data. To model the variation of the identified parameters, a singular value decompos ition technique is applied to represent the parameter change via an optimal linear interval model.