How carbon-fiber skins and aluminum supports share a panel’s load

Since the previous research note · Earlier expectation not tested

Track continuity

Since the previous research note: How thin carbon-fiber skins and honeycomb can lighten pallet bases

Previous expectation: A lightweight aircraft-pallet base could reduce the dead weight carried in air cargo if its calculated advantage survives operational requirements.

What this study adds: The December 2021 study reports measurements and model comparisons for a different structure with carbon-fiber skins and riveted aluminum stiffeners. It also calculates distinct minimum-mass and minimum-cost laminate choices.

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What changed in this study

How can adding carbon-fiber layers produce the lightest panel while using fewer layers produces the cheapest one? A December 12, 2021 engineering study approached that question with a physical structure, measurements, finite element analysis, and a constrained design calculation.

The object was a sandwich-like panel made from carbon-fiber-reinforced plastic face sheets and aluminum stiffeners. Its purpose was not simply to combine two lightweight materials, but to work out how the resulting structure bends, carries stress, and changes when its dimensions and layered skins are redesigned.

A panel built from layered skins and hollow supports

The manufactured structure used two carbon-fiber face sheets riveted to two aluminum square hollow section stiffeners. The face sheets contained eight layers in the symmetric arrangement [0°, +45°, −45°, 0°]s. The aluminum stiffeners in the described test structure measured 30 × 30 × 2 mm.

This arrangement creates a single-cell interior rather than a solid block. The face sheets form the broad outer surfaces, while the hollow aluminum sections provide support between them. The riveted connection matters because the parts are joined, but they can still experience relative movement under load. A calculation that treats the panel as one perfectly unified piece would miss part of the way it deflects.

The study therefore separated the middle deflection into two contributions. One came from bending calculated with the structural properties of the carbon-fiber sheets and aluminum stiffeners. The other came from relative movement between the face sheet and the stiffener. In the authors’ 500 N example, the two contributions were 1.643 mm and 0.917 mm, giving a calculated total middle deflection of 2.56 mm.

That example makes the connection-level issue visible. The panel’s response was not represented by a single material property or by the bending of the outer skins alone. The movement at the interface was part of the predicted result.

Three ways to check how the structure responds

The physical tests addressed different questions. Displacement measurements showed how far the structure moved under different loading conditions. A displacement meter was placed along the center lines of the aluminum stiffeners and at the center point of the face sheet, allowing the researchers to compare movement in the supports with movement in the skin.

Stress measurements looked at what happened locally within the components. Seven strain gauges were placed at selected points on the stiffener, along the riveting line, in the middle of the face sheets, and near that line. These readings provided measurements from which the stresses in the structural elements could be assessed.

Finite element analysis supplied a third route for comparison. It represented the structure computationally so that calculated deflection and stresses could be compared with both the analytical method and the physical measurements. The authors reported that the results from calculation, experiment, and finite element analysis were near one another for the investigated construction.

That agreement supports the calculation methods for the manufactured test structure with its stated geometry and connections. It does not experimentally validate every configuration later produced by the optimization. In particular, the study’s reported minimum-mass and minimum-cost designs should be understood as calculated outcomes, not as two separately manufactured products demonstrated under service conditions.

The design space was larger than one tested panel

After developing and checking the structural calculations, the researchers varied two geometric features of the aluminum stiffeners: height and wall thickness. They tested the design logic across four carbon-fiber layer arrangements, including the eight-layer [0°, +45°, −45°, 0°]s arrangement, the symmetric [0°, 90°]s arrangement, [0°, 90°, 0°], and [0°, 0°].

The calculation did not seek a thinner or lighter structure without safeguards. Seven categories of constraints shaped the search. Total deflection limited excessive sag. Buckling constraints addressed instability in the face sheets and in the stiffeners’ webs. Stress constraints applied separately to the carbon-fiber face sheets and the aluminum tubes. An eigenfrequency constraint accounted for the structure’s natural vibration behavior, while design-variable constraints kept the chosen dimensions within the allowed range.

The researchers also evaluated two different objectives. One minimized mass. The other minimized cost. The cost calculation included the materials, rivets, heat treatment of the carbon-fiber face sheets, and manufacturing work. Carbon-fiber material was the largest cost component in the study’s assumptions, so adding or removing layers affected more than structural stiffness.

The lightest and cheapest designs went in different directions

The calculated minimum-mass construction weighed 2.498 kg. It used the eight-layer [0°, +45°, −45°, 0°]s carbon-fiber arrangement and aluminum stiffeners measuring 25 × 25 × 1.5 mm. More layers did not make this design heavier in the optimization’s final result because the skin arrangement and stiffener geometry worked together within the full set of constraints.

The minimum-cost result looked very different. It used three carbon-fiber layers in the [0°, 90°, 0°] arrangement and larger, thicker aluminum stiffeners measuring 80 × 80 × 4 mm. The modeled cost was USD 180.958 under the study’s material, heat-treatment, rivet, and manufacturing assumptions.

The dollar figure is therefore an output of a particular cost model, not a verified selling price or proof of production-scale economics. Its value is in showing how an optimization can change when the objective changes. A design that uses more carbon fiber may achieve the lowest calculated mass, while a design with fewer layers can be cheaper when the model accounts for the high material cost of the carbon-fiber skins.

There is also a more specific pattern in the results. For each of the four laminate arrangements, the mass and cost objectives selected the same stiffener dimensions. The overall winning laminate arrangement still differed between the two objectives. In other words, the geometry choice was stable within a given skin arrangement, but the preferred skin arrangement changed when the goal shifted from minimizing mass to minimizing cost.

The measured structure and the optimized structures should remain separate in the reader’s mind. The 30 × 30 × 2 mm stiffeners belonged to the manufactured configuration used for displacement and strain measurements. The 25 × 25 × 1.5 mm and 80 × 80 × 4 mm dimensions were selected by the design calculations. Agreement for the tested panel does not establish that every optimized geometry will behave the same way under every load, connection arrangement, or service environment.

What this could mean for lighter load-bearing panels

A calculation method that accounts for both component bending and movement at the connection could help engineers compare lighter load-bearing panels before committing to a particular material arrangement. For vehicle floors, transport-container panels, building floors, or bridge decks, the practical capability would be to balance structural response, dimensions, and manufacturing cost within one design process.

If the approach extends successfully to application-scale structures and those structures meet their actual service requirements, it could support panels that carry necessary loads with less structural mass. The potential benefit is conditional: the 2021 study established agreement for its investigated construction and calculated alternative designs, but it did not demonstrate a deployed product, operating savings, durability, certification, or a numerical reduction compared with a conventional panel.

Key Sources

• Primary source: Elaboration of Design and Optimization Methods for a Newly Developed CFRP Sandwich-like Structure Validated by Experimental Measurements and Finite Element Analysis.
  DOI or Journal Page · Peer-Reviewed Paper · 2021-12-12