Orion MPCV E-STA Structural Dynamics Correlation for NASA NESC Andrew Doan, Brent Erickson, Trevor Owen Quartus Engineering

Transcription

1 Orion MPCV E-STA Structural Dynamics Correlation for NASA NESC Andrew Doan, Brent Erickson, Trevor Owen Quartus Engineering 0

2 Abbreviations Structure E-STA: ESM Structural Test Article MVF: Mechanical Vibration Facility ESM: European Service Module Also abbreviated as SM CM: Crew Module CM-LAS: CM Launch Abort System MPCV: Multi-Purpose Crew Vehicle PSM: Pyramidal Separation Mechanism SAJ: SA Jettisonable panels Also called fairings Test & Analysis SV: Sine Vibration SG: Strain Gauge FRF: Frequency Response Function FEA: Finite Element Analysis FEM: Finite Element Model DOF: Degree of Freedom RBE: Rigid Body Element LLL: Low Load Level HLL: High Load Level CLA: Coupled Loads Analysis 1

3 Background E-STA structural vibration testing performed on mechanical vibration facility (MVF) at NASA Plum Brook Station in Sandusky, Ohio Large nonlinear behaviors observed in test article during vibration testing Significant frequency shifts between low-level random and high-level sine sweep conditions Damping estimates vary widely between test cases Preliminary model correlation shows high sensitivity to the Pyramidal Separation Mechanism (PSM) joints Initial comparison of load share within primary structure indicates FEM distributes too much load to the inner load path during full level testing Load share was not directly measured in the test 2

4 Primary Objective Scope of Assessment Produce correlated FEM(s) to predict nonlinearities Utilize partial tank sine vibration test data; correlate to primary modes (#1-6) Identify and bound key nonlinearities Develop uncertainties in loads due to inherent design nonlinearity To be applied in next load cycle 3

5 Test Setup & Instrumentation E-STA instrumented with accelerometers and strain gauges 12 control accelerometers used to determine test reference 3-axis accelerometer located at each cardinal direction on the MVF as shown East South North West 4

6 Basic load paths shown: Model Overview Dual Load Path CM-SM 4x PSM Joint SAJ Fairings ESM Longerons SA 5

7 Data Processing Summary (1/2) Quartus performed independent processing of raw transient data for partial-fill sine tests (all input levels, no processing of random tests) Reference = average of 4 in-line control channels Validity of reference assumption for primary modes confirmed thru multiple investigations: Low deviation of in-axis controls at resonance Cross-axis inputs significantly lower than in-axis at resonance FEA w/ E-STA + MVF model shows negligible shifts in mode frequencies and low deviation of individual controls relative to E-STA fixed base (no MVF) MVF rigid body rotation inputs result (in FEA) in responses at least an order of magnitude less than responses due to drive inputs Extracted achieved excitation (input) levels for each test Achieved levels differ from test-to-test and mode-to-mode relative to labeled test level (20%, 50%, 80%, 100%) Important to know actual inputs in order to understand nonlinearities 6

8 Data Processing Summary (2/2) Generated FRFs (reference = average of 4 in-line controls) Reduced set of 269 accelerometers + longeron strain gauges Modal extraction performed Modes extracted from drive axis runs with most modal participation Synthesized FRFs used to validate extracted modes Extracted modes back-expanded onto FEM to verify shapes Cross-orthogonality performed with FEM to ensure phase correlation Investigated time histories in attempt to determine location and onset-time of nonlinear events (exhibited as shock events) 7

9 FRF Plots: X-Drive Two example FRF plots are given below Channels shown are in-line with drive direction Responses from all sine input levels are overlayed to illustrate load-dependent nonlinearity Trends for primary modes with increasing input level: Primary Axial (A): Frequency consistently decreases; magnitudes remain fairly stable; nonlinear inflections observed in rampup to resonance CM-LAS ESM Shear Web (+Z) A A 8

10 FRF Plots: Y-Drive Two example FRF plots are given below Channels shown are in-line with drive direction Responses from all sine input levels are overlaid to illustrate load-dependent nonlinearity Trends for primary modes with increasing input level: 1BY: Large frequency shift initially; magnitudes show steady increase from 50% to 100%, variability from 20% to 50% 2BY: Frequency & magnitudes consistently decrease CM-LAS ESM Shear Web (+Z) 1BY 2BY 1BY 2BY 9

11 FRF Plots: Z-Drive Two example FRF plots are given below Channels shown are in-line with drive direction Responses from all sine input levels are overlaid to illustrate load-dependent nonlinearity Trends for primary modes with increasing input level: 1BZ: Large frequency shift initially; magnitudes show increase from 50% to 100%, variability from 20% to 50% 2BZ: Frequency & magnitudes drop considerably from 20% to 50%, more gradually thereafter CM-LAS ESM Shear Web (+Z) 1BZ 2BZ 1BZ 2BZ 10

12 Goal: Approach for FEM Correlation Produce two correlated linear models, one for high-load-levels (HLL) and one for low-load-levels (LLL), in order to bound test results and understand nonlinearities HLL FEM should accurately predict responses for 100% sine tests and similar loading Process: LLL FEM should accurately predict responses for 20% sine tests and similar loading Assemble list of joints with highest likelihood to exhibit nonlinear load-dependent behavior Perform preliminary studies to identify which portions of the model need updating (3 step process) 1) Modal sensitivity analysis, 2) FEM trade studies, 3) FEM vs. CAD review Implement FEM updates based on model review and CAD comparison Tune FEM to prelim LLL configuration by adjusting various properties until responses match 20% sine data LLL model correlated first because this will represent the natural un-slipped state of the structure Correlate HLL model from prelim LLL model to match 100% sine data Create final LLL model from HLL model by only adjusting stiffness of nonlinear joints (to match 20% sine) Tools used to aid correlation: Cross-orthogonality, SAJ partial MAC, mode shape back-expansion to FEM, FRF comparisons, modal sensitivity analysis 11

13 FEM Modifications Summary Performed thorough FEM/CAD comparison and implemented a number of changes High level FEM modification synopsis: Increased interface stiffness at several primary load path joints Added modeling detail to several key interfaces: Several ESM joints SAJ airfoils SAJ PSMs SAJ vertical frangible joints Airfoil elements V2 FEM Updated FEM 12

14 FEM Joint Nonlinearity Table Table below describes the nonlinear properties that differentiate the LLL and HLL correlated models Only 3 joint stiffnesses adjusted to convert from one configuration to the other Airfoil-to-CMA connectivity PSM torsional stiffness ESM spherical bearing axial stiffness Airfoil Springs PSM Springs ESM SB Springs In addition to correlational utility, there is evidence of nonlinear events at these joints High shock loads observed in transients near PSMs and in longerons (near spherical bearings) It is known that the PSMs slipped and lost preload during testing Spherical bearing nonlinearities were previously identified by Thales Alenia Space, Italy (TASI) Wear/gouging was observed in CMA at airfoil interfaces 13

15 Correlated Modes Summary Low-Load-Level (LLL) FEM: High-Load-Level (HLL) FEM: 14

16 Mode Shapes: 1BZ LLL Model HLL Model 15

17 Cross Orthogonality Summary Note: Lockheed Martin (LM) v3 FEM was starting point of correlation effort 16

18 Cross Ortho HLL Best shape correlation for 1BZ, 1BY, 2BZ, & A modes Not much difference between A mode shape HLL vs. LLL Shape correlation not as good for 2BY & T modes Airfoils (AF) / PSMs likely not slipping for 2BY T mode not extracted from high level sine With AF connectivity released, FEM frequency is likely too low Drops below 2BY for HLL FEM Significant effort was made during correlation to de-couple T & BY modes Some coupling remains, but need to let 1B modes drive HLL correlation 17

19 Cross Ortho LLL LLL joint representations are all in the linear, pre-slip configurations 2BY & T mode shapes correlate better for LLL LLL FEM 1B mode shapes correlate better to low level modal test results than to 20% sine test results PSM & AF nonlinearities are exhibited for 1B modes in 20% sine runs 18

20 Damping Due to nonlinear resonant FRF shapes, extracted modal damping values do not always yield correct amplitudes (particularly for Axial and secondary mode responses) The extracted modal damping schedules were modified to better replicate test responses; more break points added to better represent secondary modes Correlated modal damping values are listed in table in uncertainty section Largest scaling required for axial mode damping Local damping study Nonlinear joints likely a significant source of damping in the structure Investigated implementing local damping (viscous, structural) at nonlinear joints Investigation of several different local damping schemes showed no clear benefits for correlation Recommend using modal damping only for linear dynamics Local damping would be required (in conjunction with Rayleigh damping) in any future nonlinear dynamics simulations 19

21 FRF Comparisons FRFs were generated from the LLL & HLL correlated models Separate, correlated modal damping schedules were used for both the LLL & HLL analyses FRF comparison plots created for a handful of key accelerometer and strain gauge locations Many more locations were examined/compared during the correlation process Only CMLAS acceleration responses are shown as an example (good indicator of overall response) Only strain responses from one longeron location (per axis) are shown as an example 20

22 X-Axis CMLAS FRFs Axial mode exhibits nonlinear transition in ramp-up to resonance (both for low and high levels) Inflection point ~ half way to resonance in test Indicative of shift down in frequency once a certain load level is reached in a nonlinear joint Note: high load level test actually follows the LLL FRF very closely until the inflection point Nonlinear inflection leads modal extraction to yield damping that is too low High Loads 100% Sine Test HLL FEM v3 FEM (2%cr) (for reference) Low Loads 20% Sine Test LLL FEM 21

23 Y-Axis CMLAS FRFs Overall 1BY & 2BY response captured well with HLL FEM LLL FEM captures 2BY and ramp up to 1BY well 20% test 1BY exhibits nonlinearity in ramp up to resonance LLL FEM represents pre-nonlinear onset configuration High Loads 100% Sine Test HLL FEM v3 FEM (2%cr) (for reference) Low Loads 20% Sine Test LLL FEM 22

24 Z-Axis CMLAS FRFs High Loads Overall 1BZ & 2BZ response captured well with HLL FEM Both 1BZ & 2BZ modes exhibit nonlinearity in ramp-up to resonance in low level test LLL FEM represents pre-nonlinear onset configuration 100% Sine Test HLL FEM v3 FEM (2%cr) (for reference) Low Loads 20% Sine Test LLL FEM 23

25 Longeron Strain FRFs Example strain comparison shown for each axis General trends are captured but FEM tends to over-predict longeron strain (load in inner load path) X-Axis Y-Axis Z-Axis SCLV Dynamic Note: Environments units in microstrain Workshop, June 28,

26 Characterization of Nonlinear Joints Mode/frequency-specific studies were performed in an attempt to better characterize nonlinear joints Desire to tie nonlinear properties to actual load levels Approach Identify onsets of nonlinearity (example shown at right) Predict joint loads at onset using actual achieved test inputs and correlated LLL FEM Results PSM critical torsion load (for slipping) = XXXX ± XXX in-lb Good agreement across multiple test levels & axes Insufficient data for determining nonlinear onsets for airfoils and spherical joints Modeling of joints in a nonlinear transient would require friction model Slip Event Ramp-up to 1BY 20% Y-Drive No NL Event 2BY Typical Friction Behavior: 25

27 Summary & Recommendations Identified the most significant sources of nonlinearity PSM torsion DOF Airfoil to CMA stick/slip ESM spherical joints Generated and achieved good dynamics correlation with two linear models High and low loads FEMs Unintended load paths can have big effects on structural dynamics Airfoils intended to create aerodynamic seal not to transfer loads Nonlinear dynamics analysis recommended for understanding and bounding responses of nonlinear systems Linear models many not bound responses Cannot decouple response nonlinearity/uncertainty from loads A nonlinear system may exhibit significantly different behaviors to different types of loads (sine vs. random/transient) Tractable approach = reduce bulk of model to superelements, retain joints, correlate nonlinear models for joints, use resulting model in nonlinear direct transient analyses A limited nonlinear analysis study may be able to inform and allow accurate linear CLA 26