Stressed skin

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The Zeppelin-Lindau D.I had stressed skin fuselage and wings. Zeppelin-Lindau (Do) D.I.jpg
The Zeppelin-Lindau D.I had stressed skin fuselage and wings.

In mechanical engineering, stressed skin is a rigid construction in which the skin or covering takes a portion of the structural load, intermediate between monocoque, in which the skin assumes all or most of the load, and a rigid frame, which has a non-loaded covering. Typically, the main frame has a rectangular structure and is triangulated by the covering; a stressed skin structure has localized compression-taking elements (rectangular frame) and distributed tension-taking elements (skin).

Contents

Description

A rectangular box can be distorted with a small load (left), but adding a diagonal member to form a triangular truss results in a more rigid structure (right) Triangulation of a truss.svg
A rectangular box can be distorted with a small load (left), but adding a diagonal member to form a triangular truss results in a more rigid structure (right)

A simple framework box with four discrete members is not inherently rigid as it will distort from being square under relatively light loads; however, adding one or more diagonal element(s) that take either tension or compression makes it rigid, because the box cannot deviate from right angles without also altering the diagonals. Sometimes the diagonal elements are flexible like wires, which are used to provide tension, or the elements can be rigid to resist compression, as with a Warren or Pratt truss; in either case, adding discrete diagonal members results in full frame structures in which the skin contributes very little or nothing to the structural rigidity.

The square frame (black members) resist compression while the skin (translucent pink), which is fixed to the frame (blue rivets), resists in-plane shear that would distort the frame from square Stressed Skin with rivets.svg
The square frame (black members) resist compression while the skin (translucent pink), which is fixed to the frame (blue rivets), resists in-plane shear that would distort the frame from square

In a stressed-skin design, the skin or outer covering is bonded or pinned to the frame, adding structural rigidity by serving as the triangulating member which resists distortion of the rectangular structure. [1] The skin provides a significant portion of the overall structural rigidity by taking the in-plane shear stress; however, the skin provides very little resistance to out-of-plane loads. [2] :1

Internals of stressed skin construction on Murphy Moose showing frames and supporting skin MurphyMooseUnderConstruction.JPG
Internals of stressed skin construction on Murphy Moose showing frames and supporting skin

These types of structures may also be called semi-monocoque to distinguish them from monocoque designs. There is some overlap between monocoque, semi-monocoque (stressed skin), and rigid frame structures, depending on the proportion of the structural rigidity contributed by the skin. In a monocoque design, the skin assumes all or most of the stress and the structure has fewer discrete framing elements, sometimes including only longitudinal or lateral members. [3] :175 In contrast, a rigid frame structure derives only a minor portion of the overall stiffness from the skin, and the discrete framing elements provide the majority.

This stressed skin method of construction is lighter than a full frame structure and not as complex to design as a full monocoque.

History

Section from the original Britannia Bridge, showing top and bottom stressed-skin construction Britannia Bridge wrought iron section.jpg
Section from the original Britannia Bridge, showing top and bottom stressed-skin construction

William Fairbairn documented the development of the Britannia and Conwy tubular bridges for the Chester and Holyhead Railway in 1849; [4] in it, Fairbairn describes how Robert Stephenson enlisted his aid to revise Stephenson's original concepts, which would route rail traffic inside riveted steel tubes, supported by chains, with a circular- or egg-shaped cross-section. [4] :2 Experiments with scale models led Fairbairn to suggest a hollow rectangular beam instead, with longitudinal stringers on top and bottom fixed firmly to structural coverings: "two longitudinal plates, divided by vertical plates so as to form squares, calculated to resist the crushing strain in the first instance, and the lower parts [...], also longitudinal plates, well-connected with riveted joints, and of considerable thickness to resist the tensile strain in the second". [4] :16 This has been credited as the first instance of stressed skin design, also known as sandwich or double hull. [5]

Worker carrying partially finished Deperdussin Monocoque fuselage, c. 1912 Deperdussin-general-construction-monocoque-fuselage.jpg
Worker carrying partially finished Deperdussin Monocoque fuselage, c.1912

The first aircraft from the early 1900s were constructed with full frames consisting of wood or steel tube frame members, covered with varnished fabric or plywood, although some companies began developing monocoque structures which were built by bending and laminating thin layers of tulipwood. [6] :8 Oswald Short patented an all-metal, stressed-skin wing in the early 1920s. [7] :97 [8] Dr.-Ing Adolf Rohrbach is credited with coining the term "stressed skin" in 1923. [7] :169 By 1940, duralumin sheets had replaced wood and nearly all new designs used monocoque construction. [6] :8

The adoption of stressed-skin construction resulted in improved aircraft speed and range, accomplished by reduced drag through smoother surfaces, elimination of external bracing, and providing internal space for retractable landing gear. [9] :25

Examples

Examples include nearly all modern all-metal airplanes, as well as some railway vehicles, buses and motorhomes. The London Transport AEC Routemaster incorporated internal panels riveted to the frames which took most of the structure's shear load. Automobile unibodies are a form of stressed skin as well, as are some framed buildings which lack diagonal bracing.

Related Research Articles

<span class="mw-page-title-main">Monocoque</span> Structural design that supports loads through an objects external skin

Monocoque, also called structural skin, is a structural system in which loads are supported by an object's external skin, in a manner similar to an egg shell. The word monocoque is a French term for "single shell".

<span class="mw-page-title-main">Cantilever</span> Beam anchored at only one end

A cantilever is a rigid structural element that extends horizontally and is unsupported at one end. Typically it extends from a flat vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a cantilever can be formed as a beam, plate, truss, or slab.

<span class="mw-page-title-main">Fuselage</span> Main body of an aircraft

The fuselage is an aircraft's main body section. It holds crew, passengers, or cargo. In single-engine aircraft, it will usually contain an engine as well, although in some amphibious aircraft the single engine is mounted on a pylon attached to the fuselage, which in turn is used as a floating hull. The fuselage also serves to position the control and stabilization surfaces in specific relationships to lifting surfaces, which is required for aircraft stability and maneuverability.

<span class="mw-page-title-main">Truss</span> Rigid structure that consists of two-force members only

A truss is an assembly of members such as beams, connected by nodes, that creates a rigid structure.

<span class="mw-page-title-main">Airframe</span> Mechanical structure of an aircraft

The mechanical structure of an aircraft is known as the airframe. This structure is typically considered to include the fuselage, undercarriage, empennage and wings, and excludes the propulsion system.

<span class="mw-page-title-main">Space frame</span> Rigid three dimensional load bearing truss structure

In architecture and structural engineering, a space frame or space structure is a rigid, lightweight, truss-like structure constructed from interlocking struts in a geometric pattern. Space frames can be used to span large areas with few interior supports. Like the truss, a space frame is strong because of the inherent rigidity of the triangle; flexing loads are transmitted as tension and compression loads along the length of each strut.

<span class="mw-page-title-main">Semi-monocoque</span> Type of vehicle structure

The term semi-monocoque or semimonocoque refers to a stressed shell structure that is similar to a true monocoque, but which derives at least some of its strength from conventional reinforcement. Semi-monocoque construction is used for, among other things, aircraft fuselages, car bodies and motorcycle frames.

<span class="mw-page-title-main">Seismic retrofit</span> Modification of existing structures to make them more resistant to seismic activity

Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. With better understanding of seismic demand on structures and with our recent experiences with large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior to the introduction of modern seismic codes in the late 1960s for developed countries and late 1970s for many other parts of the world, many structures were designed without adequate detailing and reinforcement for seismic protection. In view of the imminent problem, various research work has been carried out. State-of-the-art technical guidelines for seismic assessment, retrofit and rehabilitation have been published around the world – such as the ASCE-SEI 41 and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines. These codes must be regularly updated; the 1994 Northridge earthquake brought to light the brittleness of welded steel frames, for example.

<span class="mw-page-title-main">Geodetic airframe</span> Type of aircraft structure

A geodetic airframe is a type of construction for the airframes of aircraft developed by British aeronautical engineer Barnes Wallis in the 1930s. Earlier, it was used by Prof. Schütte for the Schütte Lanz Airship SL 1 in 1909. It makes use of a space frame formed from a spirally crossing basket-weave of load-bearing members. The principle is that two geodesic arcs can be drawn to intersect on a curving surface in a manner that the torsional load on each cancels out that on the other.

<span class="mw-page-title-main">Shear wall</span> A wall intended to withstand the lateral load

In structural engineering, a shear wall is a two-dimensional vertical element of a system that is designed to resist in-plane lateral forces, typically wind and seismic loads.

The term structural system or structural frame in structural engineering refers to the load-resisting sub-system of a building or object. The structural system transfers loads through interconnected elements or members.

<span class="mw-page-title-main">Spar (aeronautics)</span> Main structural member of the wing of an aircraft

In a fixed-wing aircraft, the spar is often the main structural member of the wing, running spanwise at right angles to the fuselage. The spar carries flight loads and the weight of the wings while on the ground. Other structural and forming members such as ribs may be attached to the spar or spars, with stressed skin construction also sharing the loads where it is used. There may be more than one spar in a wing or none at all. Where a single spar carries most of the force, it is known as the main spar.

<span class="mw-page-title-main">Sandwich-structured composite</span> Material composed of two thin, stiff skins around a lightweight core

In materials science, a sandwich-structured composite is a special class of composite materials that is fabricated by attaching two thin-but-stiff skins to a lightweight but thick core. The core material is normally low strength, but its higher thickness provides the sandwich composite with high bending stiffness with overall low density.

<span class="mw-page-title-main">Lattice girder</span> Truss girder where the load is carried by a web of latticed metal

A lattice girder is a truss girder where the load is carried by a web of latticed metal.

<span class="mw-page-title-main">Motorcycle frame</span> Frame of a motorcycle

A motorcycle frame is a motorcycle's core structure. It supports the engine, provides a location for the steering and rear suspension, and supports the rider and any passenger or luggage. Also attached to the frame are the fuel tank and battery. At the front of the frame is found the steering head tube that holds the pivoting front fork, while at the rear there is a pivot point for the swingarm suspension motion. Some motorcycles include the engine as a load-bearing stressed member; while some other bikes do not use a single frame, but instead have a front and a rear subframe attached to the engine.

<span class="mw-page-title-main">Vehicle frame</span> Main supporting structure of a motor vehicle

A vehicle frame, also historically known as its chassis, is the main supporting structure of a motor vehicle to which all other components are attached, comparable to the skeleton of an organism.

<span class="mw-page-title-main">Bristol M.R.1</span> Type of aircraft

The Bristol M.R.1 was an experimental biplane with an aluminium monocoque fuselage and metal wings, produced by Bristol during the First World War. Two were built to government order.

<span class="mw-page-title-main">Zeppelin-Staaken E-4/20</span> Type of aircraft

The Zeppelin-Staaken E-4/20 was a revolutionary four-engine all-metal passenger monoplane designed in 1917 by Adolf Rohrbach and completed in 1919 at the Zeppelin-Staaken works outside Berlin, Germany. The E-4/20 was the first four-engine, all-metal stressed skin heavier-than-air airliner built.

<span class="mw-page-title-main">Zeppelin-Lindau D.I</span> 1918 WW1 German stressed-skin semi-monocoque strutless fighter biplane

The Zeppelin D.I, or Zeppelin-Lindau D.I or Zeppelin D.I (Do), as named in German documents, also sometimes referred to postwar as the Dornier D.I or Dornier-Zeppelin D.I, for the designer, was a single-seat all-metal stressed skin monocoque cantilever-wing biplane fighter, developed by Claude Dornier while working for Luftschiffbau Zeppelin at their Lindau facility. It was too late to see operational service with the German Air Force (Luftstreitkräfte) during World War I.

This glossary of structural engineering terms pertains specifically to structural engineering and its sub-disciplines. Please see glossary of engineering for a broad overview of the major concepts of engineering.

References

  1. Sood, Pallika (3 March 2022). "Stressed Skin Floors". Blue Engineering. Retrieved 28 November 2023. When a sheet of ply[wood] is fixed on the top or bottom of a timber joist, we effectively create a T-shape beam with a large mass, away from the centroid; thus creating a stiffened floor and ultimately, more efficient structural depths. [...] the connection between the elements, provided by the glue and screws, is sufficient to consider the T-section as one full section.
  2. Bryan, E. R. (1973). The Stressed Skin Design of Steel Buildings . London: Crosby Lockwood Staples. ISBN   0-258-96858-3.
  3. Standards and Curriculum Division, Training, Bureau of Naval Personnel (1945). Navy Training Courses: Aircraft Metal Work. Government Printing Office. Retrieved 27 November 2023.{{cite book}}: CS1 maint: multiple names: authors list (link)
  4. 1 2 3 Fairbairn, William (1849). An Account of the Construction of the Britannia and Conway Tubular Bridges, with a complete history of their progress, from the conception of the original idea, to the conclusion of the elaborate experiments which determined the exact form and mode of construction ultimately adopted. London: John Weale and Longman, Brown, Green and Longmans. Retrieved 28 November 2023.
  5. Meyboom, AnnaLisa; Correa, David; Krieg, Oliver David (October 21–26, 2019). Stressed Skin Wood Surface Structures. ACADIA 19: Ubiquity and Autonomy (Proceedings of the 39th Annual Conference). The University of Texas at Austin School of Architecture: Association for Computer Aided Design in Architecture (ACADIA). pp. 470–477. doi: 10.52842/conf.acadia.2019.470 . ISBN   978-0-578-59179-7.
  6. 1 2 Gunston, Bill (1988). The Anatomy of Aircraft: Ninety years of development from the Wright Flyer to the B-2 'Stealth' Bomber . Stamford, Connecticut: Longmeadow Press. ISBN   0-681-40714-X.
  7. 1 2 3 Howard, Frank; Gunston, Bill (1972). The Conquest of the Air . New York: Random House. LCCN   72-3818.
  8. USPatent 1451059A,Hugh Oswald Short,"Wing and like member constructed of metal for aeroplane flying machines and other aircraft",published April 10, 1923
  9. Haddon, J. D. (1945). An Introduction to Aeronautical Engineering. Vol. II: Structures (Fifth ed.). London: Sir Isaac Pitman & Sons, Ltd. Retrieved 28 November 2023.
  10. "The NACA and the Modern Airliner - America by Air". airandspace.SI.edu. Retrieved 5 May 2019.