Select face sheet (CFRP, aluminum, steel) and core (honeycomb, foam, balsa) materials, adjust thicknesses, and instantly compute flexural stiffness D, specific stiffness D/m, weight per m², and maximum deflection — with comparison to an equal-weight solid plate.
What exactly is a "sandwich panel," and why is its bending stiffness special?
🎓
Basically, it's a lightweight structure with two stiff, thin face sheets bonded to a thick, lightweight core. It's special because it's incredibly stiff for its weight. Think of an I-beam: the flanges carry most of the bending stress, and the web holds them apart. The sandwich panel works the same way. Try moving the "Core Thickness" slider in the simulator—you'll see that increasing it dramatically boosts stiffness without adding much weight.
🙋
Wait, really? So the core material doesn't need to be super strong? What's the main job of the faces and the core?
🎓
Exactly! The core's main job is to keep the face sheets separated, which is what gives the panel its high bending stiffness. The faces carry the tensile and compressive stresses from bending. In practice, the core just needs to resist shear and keep its shape. A common case is using aluminum or carbon fiber faces with a foam or honeycomb core. In the simulator, try switching the core material from a stiff foam to a very soft one—you'll see the overall stiffness D drop, especially for thicker cores.
🙋
That makes sense. So how do engineers actually calculate if a panel will be stiff enough for, say, a floor or an aircraft wing?
🎓
They use two key numbers: the flexural rigidity (D), which is the panel's resistance to bending, and the resulting deflection under load. For instance, in car crash test barriers or satellite panels, they need high D with minimal weight. The simulator calculates both. Adjust the "Distributed Load" (q) and "Panel Length" (L) to see how a heavier load or a longer span increases deflection. The goal is to design a panel where the deflection is within acceptable limits.
Physical Model & Key Equations
The core idea is that the thin, stiff face sheets carry almost all of the bending moment, while the thick, lightweight core mainly keeps them apart. The total flexural rigidity (D) combines the contribution from the faces and a small contribution from the core itself.
$D$: Flexural Rigidity (N·m²) – The panel's bending stiffness. $E_f$, $E_c$: Young's Modulus of face and core material (Pa). $t_f$, $t_c$: Thickness of face sheet and core (m). $d$ : Distance between the centroids of the two face sheets, $d = t_c + t_f$ (m). Note: The term with $d^2$ is usually dominant. Doubling $t_c$ roughly doubles $d$, which quadruples this term.
For a simply-supported panel under a uniformly distributed load (like a shelf holding weight), the maximum deflection at the center is calculated using this standard beam formula, where the panel's unique stiffness (D) is used.
$$\delta_{max}= \frac{5 q L^4}{384 D}$$
$\delta_{max}$: Maximum deflection at the panel's center (m). $q$: Distributed load per unit width (N/m). $L$: Span length between supports (m).
This shows why stiffness (D) is so critical: deflection is inversely proportional to D. A small increase in D leads to a proportional decrease in sag.
Real-World Applications
Aerospace Structures: Aircraft floors, wing panels, and satellite bodies use carbon fiber faces with aluminum honeycomb cores. The high specific stiffness (D per unit mass) is crucial for fuel efficiency and payload capacity. Engineers use these exact calculations to meet strict deflection limits under aerodynamic loads.
Marine & Transportation: High-speed boat hulls, train interior panels, and semi-trailer walls are built as sandwich panels. They provide the necessary rigidity to handle water pressure or cargo loads while being light enough to improve speed and reduce energy consumption.
Architecture & Construction: Modern building facades, cleanroom walls, and raised access floors often use panels with metal or composite faces and a mineral wool or foam core. They provide thermal insulation, fire resistance, and sufficient stiffness to span between supports without visible sagging.
Consumer & Sports Goods: Snowboards, skis, surfboards, and even high-end doors utilize the sandwich principle. A wood or foam core with fiberglass or carbon fiber faces creates a product that is lightweight, torsionally stiff, and responsive—directly dependent on the calculated flexural rigidity.
Common Misconceptions and Points to Note
First, the idea that "a core just needs to be lightweight" is risky. While weight reduction is certainly important, if the core's shear rigidity is too low, the "maximum deflection δ" calculated by the tool can be significantly underestimated compared to reality. For example, for a 2m long panel under a 1kN/m load, a combination of CFRP facesheets and balsa core might show a small calculated deflection, but the actual panel can deflect noticeably with a "soft" feel due to shear deformation in the core. In practical design, more detailed calculations considering shear deformation or FEM analysis are essential.
Next, do not treat the bonding of the facesheets and core as a "black box". This calculation tool assumes a perfectly monolithic structure, but in practice, delamination of the adhesive layer is a critical failure mode. For instance, in panels used in high-temperature, high-humidity environments, the significant difference in thermal expansion coefficients between the facesheet (e.g., aluminum) and the core (e.g., foam) can cause stress concentration at the adhesive interface under cyclic loading. Even if calculations show excellent performance, adhesive selection and application quality are lifelines for the design.
Finally, note that "highest specific stiffness does not always mean the optimal solution". While CFRP/aluminum honeycomb wins overwhelmingly in specific stiffness, its cost can be tens of times higher. For example, in industrial machine covers, an aluminum/foam combination often provides sufficient stiffness and overall superiority in terms of manufacturability and cost. After comparing performance with the tool, always make your final judgment by considering constraints like cost, manufacturing methods, and environmental resistance.
Enter top and bottom face sheet thickness (tf) in mm—typical values: 1.5–3 mm for glass-fiber composite or aluminum alloy
Set core thickness (tc) in mm—foam cores range 20–100 mm, balsa 15–40 mm
Input panel length (panelL) and distributed load (q in kN/m)
Select material pairs from dropdowns: E-glass/polyester, carbon/epoxy, aluminum, or structural foam
Click Calculate to obtain flexural rigidity D, deflection, weight per unit area, and fundamental frequency
Worked Example
Glass-fiber face sheets (E=10 GPa, t=2 mm each) bonded to PVC foam core (E=15 MPa, t=40 mm), panel length L=1.5 m, distributed load q=2.5 kN/m. Result: D≈450 N·m (moment of inertia-weighted stiffness), maximum deflection δ≈8.2 mm at midspan, weight 4.8 kg/m², f₁≈12.4 Hz. Increasing core to 60 mm raises D to 890 N·m and reduces deflection to 4.1 mm.
Practical Notes
Sandwich panels gain stiffness cubically with core thickness; doubling core height can increase D by 8× while adding only 40% weight
For marine or aerospace applications, verify resin compatibility and moisture absorption of foam or balsa; thin face sheets amplify core shear stress
Use natural frequency output to avoid resonance: f₁ must clear operational machinery frequencies by 15–20% safety margin