structural analysis and model validation for the jwst...

MSC Software Confidential 2012 Aerospace Users Symposium

Structural Analysis and Model Validation for

the JWST ISIM Structure Using MSC

NASTRAN 2012 Aerospace Users Symposium

Presented By: John Johnston

September 18, 2012


• This presentation describes the structural modelling, analysis,

and model validation completed to verify the performance of the

James Webb Space Telescope (JWST) Integrated Science

Instrument Module (ISIM) metering structure.

• Topics:

– JWST and the ISIM Structure

– Structural Modelling, Analysis, and Model Validation Approach

– Structural Model Architecture

– Distortion Analysis

– Dynamics Analysis

– Strength Analysis

Overview

2


James Webb Space Telescope (JWST)

3

• JWST is a large, infrared-

optimized space telescope

consisting of an Optical

telescope element (OTE),

Integrated science instrument

module (ISIM), a Spacecraft, and

a Sunshield.

• ISIM consists of the JWST

science instruments, fine

guidance system, ISIM Structure,

and thermal and electrical

subsystems.

• JWST's instruments are designed

to work primarily in the infrared

range of the electromagnetic

spectrum, and operate at

cryogenic temperatures (~35 K).


Integrated Science Instrument (ISIM) Structure

4

• The JWST ISIM Structure is a

precision optical metering structure

that supports the JWST science

instruments and guider.

• Needs to meet stringent

performance requirements at both

ambient temperatures for launch and

cryogenic operating temperatures

on orbit.

• Structural analysis was completed

for both these environments using

MSC NASTRAN to simulate key

aspects of structural performance,

including stress, dynamics, and

distortion.


• ISIM Structure design, analysis, and fabrication development employed an

incremental building blocks approach starting with coupons, then joints,

followed by a sub-assembly, and finally the flight space hardware.

• The development of new analysis approaches followed this same approach with

incremental model validation for strength, dynamics/stiffness, and thermal

distortion at each level of assembly.

ISIM Modeling, Analysis, and Model Validation

5

• These efforts recently culminated

in the successful completion of

qualification testing for the flight

hardware and model correlation

studies demonstrating that our

finite element analyses accurately

simulate the complex behavior of

the flight hardware.

• This presentation will describe the

innovative analyses completed

during the development and

qualification of the ISIM Structure.


• ISIM structural models have been developed for generating performance predictions

at the system and detailed component levels.

• An integrated ISIM structural model is used for both system performance predictions

and derivation of load sets for detailed stress analysis.

• Detailed stress models are used for component and joint analysis.

Structural Model Architecture

6


Integrated ISIM System Model

7

• High fidelity NASTRAN structural

model.

– 1.5 million nodes

– ~ 5M DOFs

• Composite frame structure is modeled

using solid elements to capture fine

details such as bond lines and bond

shapes.

• Model is linked to a materials database

specifically generated for the program,

and utilizes temperature-dependent

CTE and stiffness properties to

accurately predict thermal distortion

and stiffness behavior at both

cryogenic and room temperatures.

• Full physical model used for static

analysis, and reduced Craig-Bampton

model used for dynamic analysis.


Dynamics Analysis

8

• The integrated ISIM model is used for

dynamic analysis to predict:

– Normal modes (frequencies, mode shapes) via

MSC NASTRAN SOL 103

– Response during vibration testing and launch

via coupled loads analysis (loads, accelerations,

and displacements) using normal modes results

in custom application software.

• Model Reduction:

– All subsystems and component models are

reduced to Craig-Bampton models on

NASTRAN DMIG. Modes to 400 Hz are

retained for the subsystem and component

models.

– DMIG Craig-Bampton models are incorporated

into the ISIM structure model and an ISIM

Element Craig-Bampton model is generated

with 36 boundary degrees-of-freedom (6 DOFs

at base of each KM) and cantilevered modes

retained to 200 Hz.

1 23.86 3.81 33.30 721.98 30.16 1090.77 57.24

Mode # Freq (Hz) V1 (kg) V2 (kg) V3 (kg) Rv1 (kg-m2) Rv2 (kg-m2) Rv3 (kg-m2)


Dynamic Model Validation

9

• Dynamic model validation was

completed through correlation with

test results from two modal survey

tests:

– ISIM Breadbox subassembly

– Flight ISIM Structure with payload mass

simulators

• The subassembly test provided early

validation of the modeling approach

and led to the adoption of the high

fidelity distortion model as the

baseline for dynamics analysis.

• The flight test provided final

validation of structural modes to 100

Hz.

• Pretest analyses were completed to

determine instrumentation locations,

shaker drive points for excitation, and

target modes.

Modal Fixture

ISIM Modal Test

ConfigurationModal Facility Platform

with T-Slots


Dynamic Model Validation - cont

10

• Model validation objectives were successfully completed for significant

modes for both the subassembly and flight structure tests:

– Frequency match within 5%

– Cross-orthogonality between model and test modes:

• Absolute value of diagonal terms between 0.9 and 1.1

• Absolute value of off-diagonal terms less than 0.2

Analysis Mode 1 2 3 4 5 6 7 8 10

Mode Description 23.44 26.44 27.82 30.38 32.68 35.88 39.51 45.01 51.73

Test Mode freq % diff 1.0% 2.9% -0.5% 2.1% 2.1% 1.9% 1.1% 3.0% 0.8%

ISIM V3 1 23.2 0.98 -0.05 0.15 0.01 -0.05 -0.04 -0.03 0.01 0.00

ISIM V2 2 25.7 0.05 0.99 -0.07 0.11 -0.02 0.01 0.00 0.02 0.00

ISIM V2, MIRI & NIRSpec V3 out of phase 3 28.0 0.11 -0.10 -0.96 0.17 -0.13 -0.07 -0.05 0.02 -0.01

ISIM RV3 4 29.8 -0.04 -0.09 0.18 0.98 0.00 -0.02 0.00 -0.02 -0.02

FGS V1 5 32.0 0.06 0.00 -0.09 0.05 0.99 -0.05 -0.04 0.01 0.00

NIRCam V1 6 35.2 -0.04 0.03 0.06 -0.03 -0.04 -0.99 0.03 -0.05 0.05

ISIM Rv1 7 39.1 -0.04 0.00 0.04 -0.01 -0.03 -0.01 -0.99 0.00 0.03

NIRSpec V1, ISIM RV2 8 43.7 -0.02 -0.01 0.01 -0.01 0.01 -0.05 0.00 0.99 0.01

NIRCam V2, MIRI V1 & V2 11 51.3 0.02 0.00 -0.01 -0.01 0.03 0.06 -0.01 -0.03 0.96

ISIM Final Cross-Orthogonality: Test modes > 5% Modal Effective Mass

Flight Structure Modal Survey Correlation Results


Distortion Analysis

11

• The integrated ISIM model is used for distortion analysis to predict:

– Thermal distortion for cooldown to cryogenic temperatures and cryogenic thermal stability

– Gravity distortion under 1 G loading

• Linear static analysis using MSC NASTRAN SOL 101.

• In addition to standard analyses using nominal material properties, stochastic

analysis is completed to quantifying the error bounds and uncertainties in

predictions due to variations in model parameters associated with material,

manufacturing, and spatial variability.

• This effort is presented in greater detail by Emmanuel Cofie at this symposium.

Deformation

Plots for

Ambient to

Cryogenic

Distortion


Thermal Distortion Model Validation

12

• Two major cryogenic environmental

tests were then completed as part of the

ISIM Structure verification program:

– The Cryoset Test was completed in May

2010 for thermal distortion performance

characterization.

– The Cryoproof Test was completed in

October 2010 for demonstration of cryogenic

strength.

• During each of these tests, the hardware

under test was thermal cycled between

ambient and cryogenic temperatures

with metrology performed via

photogrammetry (PG) at the warm and

cold states.

• Thermal distortion predictions were

validated through comparison with PG

data.


Thermal Distortion Model Validation – cont

13

• Detailed structural models were

completed including the flight

hardware and associated

mechanical ground support

equipment in the test setup.

• Temperature sensor

measurements are used to

generate a temperature value for

each node in the flight hardware

and MGSE structural models via

temperature mapping.

• Model validation criteria satisfied:

– Nominal predictions with MUF=1.6

bound the measured performance

– Stochastic model predictions

including 2-sigma uncertainty

bandwidth with MUF=1.4 envelop

measured performance


Strength Models and Analysis - Overview

14

• Detailed stress analysis was

completed to verify the structural

integrity of the ISIM Structure.

• Development and validation of

methodologies for the analysis of

the bonded joints, particularly at

cryogenic temperatures, was a

major focus of the analysis effort.

• Bonded joint analysis is based on a

semi-empirical method anchored in

joint model validation.

– Strength allowables derived from

extensive coupon test program

– Modeling and analysis approach

utilizes common mesh size for

derivation of allowables from

coupon tests, validation of method

through joint level tests, and flight

joint analysis.

Phase-2 Test Program – Development Joints

Phase-1 Test Program – Allowable Coupons

Bonded Joint Analysis

Procedure

Basic Coupon Testing

• FWT Coupons for Peel

• DSJ Coupons for Shear

• RT and Cryo

• Coupons Modeled/analyzed

per Analysis Procedure

Develop Failure Criteria

(Allowables) from Coupon

Test Results

Development Joint TestingDev Joint Test/Analysis

Correlation and Validation

ISIM Flight Structure

Design and Analysis


Strength Models and Analysis - Ambient

15

• Ambient bonded joint strength

analysis was completed using global

joint models:

– Global models for allowable coupons,

development joints, and flight joints are

all modeled per the same methodology.

– Eight noded hex elements at critical

bond areas with 2.5mm X 2.5mm in-

plane dimensions.

– Smeared laminate orthotropic properties

for composite elements.

– Analysis layers at bonded interfaces for

critical interlaminar failure modes.

• Failure modes assessed include:

– Composite interlaminar failure

– Composite in-plane failure

– Metallic ultimate/yield failure

• Linear static analysis using MSC

NASTRAN SOL 101.

Gusset

Tube

Plug

Adhesive

Adhesive

2.5 mm

X

2.5mm

Composite Analysis Plies

at Bonded Interfaces

RBE3

Global Joint FEM

Approach for Ambient Strength


Strength Model Validation - Ambient

16

• Model validation for ambient bonded

joint strength was completed through

correlation of analysis predictions

with test results from the destructive

testing of seven joint configurations.

• Basic plug joint model and analysis

results shown in figure at right.

– Predicted failure load (B-basis strength) =

48.8 kips

– Predicted failure load (average strength) =

60.5 kips

– Average failure load = 58.5 kips (0.1%

COV)

• All RT development joints failed above

flight predictions demonstrating that

the flight analysis methodology for

bonded joints at RT is validated and

conservative.

X

Y

Z

Global FEM

Interlaminar Stress from Global FEM


Strength Models and Analysis - Cryogenic

17

• Cryogenic bonded joint strength

analysis was completed using global-

local joint models:

– Global model approach is the same as

ambient case

– Local models based on refined mesh for

cryogenic adhesive failure mode.

– Loading of local models includes thermal

loads plus boundary displacements mapped

from global joint models to local models.

• Failure modes assessed include:

– Composite interlaminar failure (global)

– Composite in-plane failure (global)

– Metallic ultimate/yield failure (global)

– Adhesive maximum principal stress failure

(local)

• Linear static analysis using MSC

NASTRAN SOL 101.

Global FEM displacements

applied at cut boundaries

Global

FEM

Local FEM

Global FEM

Cut Boundary

Cut Boundary

Cut Boundary

Cut Boundary

Cut Section of Critical Bond Area

Clip

Tube

Plug

Global-Local Joint FEM

Approach for Cryogenic

Strength


Strength Model Validation - Cryogenic

18

• Model validation for cryogenic bonded

joint strength was completed through

correlation of analysis predictions with

test results from the destructive testing

at cold survival temperature (27 K) for

three joint configurations that cover the

basic bonded joint types.

• Basic plug joint model and analysis

results shown in figure at right.

– Predicted failure load (B-basis strength) =

15.7 kips

– Predicted failure load (average strength) =

18.6 kips

– Average failure load = 21.4 kips (8% COV)

• All cryo development joints failed

above flight predictions demonstrating

that the flight analysis methodology for

bonded joints at cryo is validated and

conservative.

Global FEM Local FEM

Adhesive Stress from Local FEM

Invar

Adhesive cracks

Cut fiber

Turning the corner and propagating along interface

Cross section view of Invar plug-composite tube sample (S/N002) tested at 19K

To the Invar

interface

To the tube

interface


Summary

19

• The development and validation of

modeling and analysis capabilities for

predicting dynamic, distortion, and

strength performance of the JWST

ISIM Structure was successfully

completed.

• The approaches were grounded in

initial constituent materials testing,

benchmarked to test results at the

bonded joint/subassembly level, and

verified for the flight hardware.

• Integration and test at the next level of

assembly (ISIM Element) is underway

and will culminate with additional

cryogenic thermal-optical

performance tests and a full suite of

ambient mechanical environmental

tests.

Fully Verified Flight ISIM Structure

Ready for Integration and Test at

the next level of assembly

structural analysis and model validation for the jwst...

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