FWD Side Skin Panel — Structural Substantiation

Analysis Overview

Local FEM substantiation of the forward side skin panel for the governing forward 9g compartment load, using linear and nonlinear NASTRAN analysis plus a separate conservative rivet pull-out check.

Compartment demand80 lb_f × 9gForward load case

Panel0.0625 in AL 2024-T3 CLADPlate-element idealization

Tube frames1 × 1 × 0.125 inAL 6061-T6 beam elements

Governing resultsPASSMinimum retained MS = +0.10

Model idealization

Thin sheet as plates; surrounding tubes as beams

The 0.0625 in AL 2024-T3 CLAD skin is modeled with plate elements; the 1 × 1 × 0.125 in AL 6061-T6 frame members use beam elements.

Engineering rationale

This captures panel membrane/bending response and frame stiffness without unnecessary 3D-solid detail.

Finite-element idealization of the forward side skin panel with surrounding tube-frame beam elements and boundary-condition symbols. — Finite-element idealization of the forward side skin panel and surrounding tube frames.

Load-Path Rationale

Tube frames are retained to provide realistic support stiffness and their common nodes are merged with the plate mesh. For demand, no frame load sharing is credited: the full 720 lb_f forward-case load is applied to the skin.

Boundary conditions

Tx: restrained on tube frames except at the compartment opening.
Ty and Tz: restrained at the bottom of the local model.

Engineering rationale

The restraints eliminate rigid-body motion while leaving the opening free to deform. Restrained-node stress peaks are retained rather than de-peaked, making the check conservative.

Load Application

Two upper compartments rated at 40 lb_f each are combined and multiplied by the forward 9g factor. The complete resultant is assigned to the skin panel.

Forward 9g resultant:

F_{9g}=Wn=80\times9=720\,\mathrm{lb_f}

Applied over 206.56 in², the equivalent uniform pressure is:

p=\frac{F_{9g}}{A}=\frac{720}{206.56}=3.486\,\mathrm{psi}

Governing load condition used for the local panel assessment.

Case	Direction	Payload [lb_f]	Load factor [g]	Applied load [lb_f]	Governing basis
1	Forward	80	9.0	720	Maximum total upper-compartment demand identified in the source assessment

Conservative load-path assumption

No surrounding-frame load sharing is credited, so the full 720 lb_f demand passes through the panel idealization.

Linear Static Analysis

Solver setup

SIMCENTER NASTRAN
SOL 101 linear static solution
Static pressure loading
Plate + beam local model

Unit-system control

WTMASS maintains a consistent lb_m–in–s unit system so recovered loads are in lb_f and stresses in psi.

Failure metric

Principal-stress comparison

Maximum tensile and compressive principal stresses from both plate surfaces are compared with F_tu and F_cy.

Engineering rationale

Both surfaces are retained because plate bending can reverse the governing stress through the sheet thickness.

Linear-analysis principal-stress envelope. The highlighted cells represent the governing tensile and compressive principal stresses.

AL 2024-T3 CLAD Sheet		Forward Case		Maximum Tension [ksi]	Maximum Compression [ksi]
Surface	Response	F₁ [ksi]	F₂ [ksi]	Maximum Tension [ksi]	Maximum Compression [ksi]
Top	Tension	23.33	13.58	33.50	−33.50
Top	Compression	−10.70	−33.50
Bottom	Tension	33.50	10.70
Bottom	Compression	−13.58	−23.33

Linear finite-element contours of minimum and maximum principal stress in the forward side skin panel. — Linear-analysis principal-stress contours for the panel.

Boundary Peak-Stress Treatment

The governing peaks occur at restrained nodes. They are intentionally retained in both strength checks rather than smoothed or excluded, so the reported margins use the conservative local maxima.

Allowables: F_tu = 62 ksi; F_cy = 37 ksi.

MS_T=\frac{62}{33.50}-1=0.85

MS_C=\frac{37}{33.50}-1=0.10

Governing linear marginMS = 0.10 · PASS

Nonlinear Material Analysis

SOL 106 reuses the same mesh, restraints and pressure while replacing the elastic material law with discrete plastic stress–strain data for the panel and tube-frame alloys.

Why nonlinear material response?

Capture post-yield stress redistribution

Material nonlinearity captures post-yield stiffness change and local stress redistribution under the same applied load.

Model continuity

Keeping geometry, loading and restraints unchanged isolates the effect of the material model.

AL 2024-T3 CLAD panel

Source stress-strain diagram and discrete nonlinear material function for AL 2024-T3 CLAD sheet. — AL 2024-T3 CLAD source curve and nonlinear material points.

AL 6061-T6 tube frames

Source stress-strain diagram and discrete nonlinear material function for AL 6061-T6 extrusion. — AL 6061-T6 source curve and nonlinear material points.

Nonlinear-analysis principal-stress envelope. The highlighted cells represent the governing tensile and compressive principal stresses.

AL 2024-T3 CLAD Sheet		Forward Case		Maximum Tension [ksi]	Maximum Compression [ksi]
Surface	Response	F₁ [ksi]	F₂ [ksi]	Maximum Tension [ksi]	Maximum Compression [ksi]
Top	Tension	9.59	8.06	17.56	−15.41
Top	Compression	−6.33	−15.41
Bottom	Tension	17.56	5.45
Bottom	Compression	−3.46	−10.25

Nonlinear finite-element contours of minimum and maximum principal stress in the forward side skin panel. — Nonlinear-analysis principal-stress contours for the panel.

MS_T=\frac{62}{17.56}-1=2.53

MS_C=\frac{37}{15.41}-1=1.40

Governing nonlinear marginMS = 1.40 · PASS

Linear peak tension33.50 ksiGoverning tensile principal stress at the restrained-node region.

Nonlinear peak tension17.56 ksiLower local peak after plastic material redistribution.

Nonlinear governing margin1.40Compression remains the governing nonlinear check.

Fastener Assessment

Sixty CR3212 (ARM4) rivets attach the panel region to the tube structure. Because the local model omits the adjacent joint load path, rivet capacity is checked separately instead of extracting shear directly from the FEM.

Layout of CR3212 ARM4 rivets around the upper compartment region of the forward side skin panel. — CR3212 (ARM4) rivet layout around the upper-compartment panel region.

Model limitation

Rivet shear is not extracted from the local panel FEM

The fixed local frame and omitted adjacent structure prevent a reliable joint-shear distribution from being recovered from this model.

Conservative substitute

The full 720 lb_f is therefore treated as pull-out, with only ten rivets credited equally.

Demand per credited rivet:

f_t=\frac{720}{10}=72\,\mathrm{lb_f}

CR3212 (ARM4)

Material: AL 5056 alloy
Reference tensile strength: 285 lb_f for a 0.156 in countersunk sheet
Panel sheet thickness: 0.0625 in
Tensile capacity scaled to the actual sheet thickness

Scaled tensile capacity for the actual panel thickness:

f_{tu,scaled}=\frac{0.0625}{0.156}\times285=114.18\,\mathrm{lb_f}

The source margin calculation applies an additional 1.15 multiplier to the rivet demand term:

MS_{CR3212}=\frac{114.18}{72\times1.15}-1=0.38

Rivet marginMS = 0.38 · PASS

Substantiation Outcome

Linear panel checkMS 0.10PASS using the restrained-node peak stresses.

Nonlinear panel checkMS 1.40PASS with plastic material nonlinearity included.

CR3212 rivet checkMS 0.38PASS under the conservative ten-rivet pull-out assumption.

Conclusion

All evaluated static-strength checks pass.

Linear and nonlinear panel checks retain restrained-node hot spots and remain positive.
The separate conservative CR3212 pull-out check also remains positive.

References

Material Allowables & Constitutive Data

MMPDS-15 — Metallic Materials Properties Development and Standardization

Analysis Solver & Methods

SIMCENTER NASTRAN — SOL 101 linear static and SOL 106 nonlinear static analysis