FWD Side Skin Panel — Structural Substantiation

Analysis Overview

The forward side skin panel is substantiated for the maximum upper-compartment demand in the forward 9g condition. The local model combines the sheet panel and surrounding tube frames, then checks the panel using both linear-elastic and nonlinear material response before addressing the rivet pull-out demand separately.

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 resultsPASSLinear, nonlinear, and rivet checks

Model idealization

Thin sheet as plates; surrounding tubes as beams

The skin is represented with plate elements to resolve membrane and bending stress over the sheet, while the slender closed-section frame members are represented efficiently with beam elements using their 1 × 1 × 0.125 in hollow-square section.

Engineering rationale

This preserves the dominant stiffness and load-transfer mechanisms without introducing unnecessary 3D-solid detail into a local static model.

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

I retained the surrounding tube frames in the local FEM to represent the stiffness that supports the skin panel, and I merged the common plate/beam nodes so load can pass directly between both idealizations. For demand, however, I credited no frame sharing: the complete forward-case compartment load is applied to the skin panel. This separates realistic boundary stiffness from a deliberately conservative panel demand and keeps the local model efficient and auditable.

Merged common nodesPlate and beam nodes at shared locations are merged so the local model transfers load directly between the panel and surrounding frame.

Panel-focused restraintLongitudinal translation is restrained along the tube frames except at the compartment opening; lateral and vertical translation are restrained at the model bottom.

Boundary hot spots retainedAlthough restrained-node peaks can be locally irregular, the source assessment intentionally retains them for a conservative strength check.

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 restraint pattern suppresses rigid-body motion while allowing the opening region to deform locally. Because local restraints can amplify nearby stress, those peaks are treated conservatively rather than filtered out.

Load Application

The two upper compartments are rated at 40 lb_f each. The complete 80 lb_f compartment demand is multiplied by the forward 9g load factor and, conservatively, assigned entirely to the skin panel rather than sharing load with the surrounding tube structure.

The forward-case resultant is:

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

The loaded upper-panel area is 206.56 in². The equivalent uniform pressure applied normal to the plate in the positive x-direction 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

Load-Case Selection Rationale

This standalone assessment is intentionally scoped to the forward 9g condition identified in the source as the maximum total upper-compartment load case. The two 40 lb_f compartment capacities are combined, and the full 720 lb_f resultant is applied to the panel. No additional load cases are introduced because they are not part of this source section.

Conservative load-path assumption

Crediting none of the surrounding frame capacity in distributing the 720 lb_f demand forces the full compartment load through the panel idealization. This deliberately biases the panel stress assessment on the conservative side.

Linear Static Analysis

Solver setup

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

Unit-system control

Mass is entered in lb_m, density in lb_m/in³ and acceleration in in/s². WTMASS provides consistent mass-to-force conversion so solver forces are recovered in lb_f and stresses in psi.

Failure metric

Principal-stress comparison

The source methodology compares the maximum tensile principal stress against F_tu and the maximum compressive principal stress against F_cy, using both plate surfaces.

Engineering rationale

Keeping top- and bottom-surface principal stresses visible is important for a thin plate because local bending can reverse the sign and magnitude of 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

Observed behavior: the governing principal-stress peaks occur at elements connected to restrained nodes, where local boundary conditions can produce irregular stress concentrations.

Assessment decision: rather than filtering or excluding those peaks, the source deliberately retains them in both the linear and nonlinear strength checks. This is conservative because the reported margins are based on the local restrained-node maxima, not a smoothed or de-peaked field.

The material allowables used are F_tu = 62 ksi and 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

The same local model and loading are retained, but the material response is changed to a nonlinear plastic formulation in SOL 106. The stress–strain relation is represented with discrete points extracted from the source material curves for the panel and tube-frame alloys.

Why nonlinear material response?

Capture post-yield stress redistribution

A purely linear-elastic solution continues increasing stress in direct proportion to load. The nonlinear material model permits stiffness change after yielding and redistributes local demand while maintaining the same geometry, restraints, and applied pressure.

Model continuity

Because the boundary conditions and loading are unchanged, the nonlinear solution provides a direct comparison of material-model influence rather than mixing changes in load path or restraint.

AL 2024-T3 CLAD panel

Source stress-strain diagram and discrete nonlinear material function for AL 2024-T3 CLAD sheet. — Source stress–strain curve and its discrete nonlinear material representation.

AL 6061-T6 tube frames

Source stress-strain diagram and discrete nonlinear material function for AL 6061-T6 extrusion. — Source stress–strain curve and its discrete nonlinear material representation.

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 upper-compartment panel region to the tube structure. The local FEM does not include the complete adjacent load path needed to resolve rivet shear accurately, so the fastener check is intentionally separated from the panel stress model.

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 panel model fixes the surrounding frame longitudinally and omits adjacent structure, so it cannot reproduce the complete shear load path through the riveted joint.

Conservative substitute

The full 720 lb_f forward-case load is treated as panel pull-out and only ten rivets are credited with sharing it 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
Source method scales the tensile capacity with 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.

The panel passes the linear principal-stress assessment even when restrained-node hot spots are retained.
The nonlinear material solution shows substantial redistribution of the local stress peaks while preserving positive margin.
The CR3212 rivets pass the separate conservative tensile pull-out check used because the local FEM does not resolve the complete joint shear load path.

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