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Recent PA-46-600TP NL Modelling Applications
FEMAP SYMPOSIUM 2016January 17, Vero Beach, FL
Simone Fevola, Lead Structures ODA Unit Member
Contact information: [email protected]
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FEMAP Symposium 2016
FAA Accepted Means of ComplianceAnalyses Supported By Test : Lightweight Aircraft Structures
Limited number of Certification Full Scale Test Articles: fully compressed schedule
Required Minimum Efforts for Risk Reduction
DEVELOPMENT STRUCTURAL TESTING (SUBCOMPONENT / FULL SCALE)
Linear Loads FEM(Internal Loads
Validation)
Non-Linear FEM(Stability and Strain Gage
validation)
CLASSICAL
ANALYSIS(NACA, Bruhn, Niu)
Time to Cert, $$
CERTIFICATION STRUCTURAL
TESTING (FULL SCALE)
• Removal of ANY block effort risks re-design, re- certification efforts and ultimately risks program cancellation.
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FEMAP Symposium 2016
PA-46-600TPCase Study: Vertical Tail Stability Driven Analysis
• Linear Analysis
• NL-FEM of Test Configuration
• Development Test
• Design Changes Incorporated
• NL-FEM of Revised Configuration
• Spar, Rib, and Attachment Margins
• Joint Analysis Using NL-FEM Results
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FEMAP Symposium 2016
– linear analysis strength margins
1 Aft Spar Crippling 29 0.00 62 / 79
3 Rib 2 Buckling 29 0.15 52 / 79
4 Aft Spar Web Buckling 29 1.49 49 / 79
Loc.
IDComponent Critical Check
Load
CaseMSLoad Description Page
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FEMAP Symposium 2016
Vertical Tail development NL model 38,622 nodes, 41158 elements
Includes representative tail-cone to pressure dome
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FEMAP Symposium 2016
Convergence plot – test configuration
Final time step = 0.66(118.5%)
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FEMAP Symposium 2016
Non-linear fem results
• development configuration FEM (AVI_lds_baseline) fails to continue finding a solution at time step 0.66
0.66 × 180% = 118.8%
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FEMAP Symposium 2016
Development Test
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FEMAP Symposium 2016
Development Test-
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Lateral Deflection versus video time
Lateral Spar Tip Deflection
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FEMAP Symposium 2016
Cause and Analysis Observations (Ref. VB-25XX)
Observations:
Non-linear large displacement analysis can be predictive of overall instability for shell type structure
Deformed shape review is critical for determining whether non-convergence is driven by an overall instability or something structurally bounded to allow for higher loading levels.
Stress and Strain levels are just as important at any time step but may take a back-seat to stability driven failure and these levels can substantially change over very few time steps.
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FEMAP Symposium 2016
83401-012 (Tested Design) 83401-013 (Revised Design)
* Replaced skin panel breakers with
full intercostal
* Extended front spar to tip rib
* Added skin doubler in critical areas
Approximate instability location
Key design changes
* Various fastener
type, size, and pitch changes
Ribs 3 and 4 split to accommodate front
spar
* Changes required per non-linear analysis
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FEMAP Symposium 2016
Revised design – front spar 83414-002 46V550012-001
Revision Reason
Extendedspar to tip rib
Full spar needed to complete torque box for high torsion load cases
Increasedflange width near Rib 2
To further distribute fastener loads at the joint
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FEMAP Symposium 2016
Revised design – skin Doubler46V550027
Revision Reason
Created .020” bonded doubler (both sides)
Required for peaking stresses due to buckling at rib-to-spar interface
Effective skin thickness increase required for fastener loads
Locally stiffens skin panels that are subject to shear buckling
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FEMAP Symposium 2016
Revised design – intercostal
46V550008
Revision Reason
Created intercostal to replace skin panel stiffeners
Previous testing revealed that panel buckles spread across existing stiffeners around limit load-nodal line not enforced (see photo)
Intercostal offers greater resistance to shear panel buckling
46V550025
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FEMAP Symposium 2016
Vertical Tail production design NL model 41959 nodes, 44310 elements
Includes representative tail-cone to pressure dome
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FEMAP Symposium 2016
Convergence plot – revised design for production
Final time step = 1.00(>150 DLL%)
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FEMAP Symposium 2016
Non-Linear Rem ResultsRevised configuration at ~151% Limit Load
• Revised configuration FEM converges through time step (151.2% DLL and beyond)
• Revised VT is expected to be stable at 150% Limit Load
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FEMAP Symposium 2016
Non-Linear FEM Results
Revised configuration at ~151% Limit LoadRevised configuration at ~119% Limit LoadTest configuration at ~119% Limit Load
model for production design with skin edge doubler shows reduced Plate Top Major Prin Stresses at 151% compared to Test model at 119%
Revised configuration at ~151% Limit LoadRevised configuration at ~119% Limit LoadTest configuration at ~119% Limit Load
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FEMAP Symposium 2016
Development Test model and Revised production model show similar stresses near VT-fuselage attachments at 119% (as expected)
Non-Linear FEM Results
Revised configuration at ~119% Limit LoadTest configuration at ~119% Limit Load
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P I P E R A I R C R A F T I N C.
JOINT ANALYSIS USING VALIDATED NL FEM
Fx
Fy
Fxy
Spar flange elements
Skin only elements
• NL-FEM can disclose structural strength and stability, but does not demonstrate joint margins.
• Fastener loads may revise skin thickness, rivet pitch, and rivet type/size.
• Inter-rivet buckling may revise attachment skin thicknesses and spacing.
• “Tear out” may revise attachment skin thicknesses and spacing.
• “Pull-Thru” may revise attachments, skin thicknesses and spacing.
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FEMAP Symposium 2016
-70
-60
-50
-40
-30
-20
-10
0
0 200 400 600 800 1000 1200 1400
Dis
tan
ce f
rom
Tip
Rib
, in
Running Load, lb/in
Fastener Running Load
Ny → Nxy
Fastener shear – rear spar
RIB 1
RIB 2
RIB 3
RIB 4
RIB 5
A
B
C
D
A. YAA4 S = 1.000” t = 0.032” → Pall = 250 lb(Ny → Nxy)max = 99 lb/in
B. YAA4 S = 1.000” t = 0.052” → Pall = 350 lb(Ny → Nxy)max = 223 lb/in
C. ATZ4 S = 0.750” t = 0.052” → Pall = 558 lb(Ny → Nxy)max = 518 lb/in
D. ATZ5 S = 0.644” t = 0.052” → Pall = 854 lb(Ny → Nxy)max = 1307 lb/in
{Total load over 4 ATZ5’s = 2622 lb, or 656 lb/rivet}
Note: discreet running load determinations for joints can be determined with the refined NL model and include skin buckling effects. Transverse (pull-thru loads) can be much more realistic than classical text would require.
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FEMAP Symposium 2016
Rear spar to fuselage joint
Minimum bearing margin - Spar
Minimum bearing margin – Fuselage structure
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FEMAP Symposium 2016
Vertical –redesign resists critical design ultimate load
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FEMAP Symposium 2016
PA-46-600TPComplex Fuselage Considerations • Longeron Type Fuselage (Bruhn section C11.35)
• Textbook panel compression + DT analysis limitation : per Bruhn section C11.36 para. 2. (no such thing as pure shear)
• Non-circular (not covered by textbook theory)• Multiple openings (windshields, windows, doors)• Tail-cone and windshield transitions• Pressurization + Primary bending (not independent)• Discreet loads from wing, tail and seats• Stress limitations from thin sheet bending-permanent
buckling • Testing Required for Validation• Buckled Shape Correlation • NL-FEM of Revised Configuration• Joint Analysis Using validated NL-FEM Results
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FEMAP Symposium 2016
Legacy Longeron Type Fuselage (Bruhn section C11.35)
Paragraph C11.4 limitation
Paragraph C11.36.
conservatism
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FEMAP Symposium 2016
Fuselage NL stability gross stress model
38,622 nodes, 41158 elements
Includes representative tail-cone to pressure dome
Used for all required analyses for tested and non-tested conditions
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FEMAP Symposium 2016
NL FEM stability gross stress model solution Required for accurate for internal load distribution FAR 23.301 (c)
Free from Textbook limitations and conservatisms
Valid for forensic analyses
Material Non-linearity can be exploited for thin sheet yield margin
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FEMAP Symposium 2016
Buckled skin panel analyses
More accurate (post-buckled) model based panel buckling stress
Accurate shear buckling stress in the in the presence of compression
Model based compression to shear ratio
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FEMAP Symposium 2016
No permanent buckling < design limit load
Low permanent buckling strength is a challenge and can drive a thickness and weight,
Alleviated with less conservative NL model determined combined fscr
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FEMAP Symposium 2016
Model derived nodal loads feed joint analyses with accurate (post-buckled) pull-off loads versus Bruhn design load ( 0.15 x t x Ftu ) C11.24
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FEMAP Symposium 2016
Leverage automated gross stress max/min load-case surveys for critical gross stress locations
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FEMAP Symposium 2016
Model fidelity important for gross stress survey-peak model stresses may be conservative yet acceptable for cut-outs feature locations-or additional analyses may be required
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FEMAP Symposium 2016
Complex Section analysis with accurate (post-buckled skin) free body loads
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FEMAP Symposium 2016
Test versus Theory Aft fuselage
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FEMAP Symposium 2016
Test versus Theory: Forward Fuselage