Martin S. Annett

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.

Comparison of Test and Finite Element Analysis for Two Full-Scale Helicopter Crash Tests

Finite element analyses have been performed for two full-scale crash tests of an MD-500 helicopter. The first crash test was conducted to evaluate the performance of a composite deployable energy absorber under combined flight loads. In the second crash test, the energy absorber was removed to establish the baseline loads. The use of an energy absorbing device reduced the impact acceleration levels by a factor of three. Accelerations and kinematic data collected from the crash tests were compared to analytical results. Details of the full-scale crash tests and development of the system-integrated finite element model are briefly described along with direct comparisons of acceleration magnitudes and durations for the first full-scale crash test. Because load levels were significantly different between tests, models developed for the purposes of predicting the overall system response with external energy absorbers were not adequate under more severe conditions seen in the second crash test. Relative error comparisons were inadequate to guide model calibration. A newly developed model calibration approach that includes uncertainty estimation, parameter sensitivity, impact shape orthogonality, and numerical optimization was used for the second full-scale crash test. The calibrated parameter set reduced 2-norm prediction error by 51% but did not improve impact shape orthogonality.

Simulating the Response of a Composite Honeycomb Energy Absorber. II: Full-Scale Impact Testing

AbstractThe National Aeronautics and Space Administration (NASA) has sponsored research to evaluate an externally deployable composite honeycomb designed to attenuate loads in the event of a helicopter crash. The concept, designated the deployable energy absorber (DEA), is an expandable Kevlar honeycomb. The DEA incorporates a flexible hinge that allows the honeycomb to be stowed collapsed until needed during an emergency. Evaluation of the DEA began with material characterization of the Kevlar-129 fabric/epoxy and ended with a full-scale crash test of a retrofitted MD-500 helicopter. During each evaluation phase, finite-element (FE) models of the test articles were developed, and simulations were performed using the dynamic FE code LS-DYNA. This paper focuses on simulations of two full-scale impact tests involving the DEA: a mass simulator and a DEA-retrofitted MD-500 helicopter. Isotropic (Mat 24) and composite (Mat 58) material models that were assigned to DEA shell elements were compared. Based on sim...

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.