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Stressed skin

Stressed skin is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Stressed skin rather than just read about it. In short: 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…

Stressed skin — main illustration
Stressed skin — illustration

Key takeaways

  • Stressed skin belongs to engineering; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Stressed skin to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Stressed skin from memory before moving on to harder problems.

Reference excerpt

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).

Description

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.

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. 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.

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. 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

William Fairbairn documented the development of the Britannia and Conwy tubular bridges for the Chester and Holyhead Railway in 1849; 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. 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". This has been credited as the first instance of stressed skin design, also known as sandwich or double hull.

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. Oswald Short patented an all-metal, stressed-skin wing in the early 1920s. Dr.-Ing Adolf Rohrbach is credited with coining the term "stressed skin" in 1923. By 1940, duralumin sheets had replaced wood and nearly all new designs used monocoque construction. 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.

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.

Dornier-Zeppelin D.I (1918) : first all-metal stressed skin fighter and first with stressed skin wings Zeppelin-Lindau (Dornier) Rs.IV (1918) : first aircraft with an all-metal stressed skin fuselage to fly Zeppelin-Staaken E-4/20 (1919) : first all-metal stressed skin four-engine airliner Short Silver Streak (1920) : first all-metal British stressed skin aircraft Northrop Alpha (1930) : first American all-metal stressed skin aircraft GM New Look bus (1959) : stressed-skin bus, over 44,000 built since 1959, and many still in service

References

External links Stressed Skin Wood to Metal: The Structural Origins of the Modern Airplane

Illustrations

Stressed skin: The Zeppelin-Lindau D.I had stressed skin fuselage and wings.
The Zeppelin-Lindau D.I had stressed skin fuselage and wings.
Stressed skin: 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 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)
Stressed skin: 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
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: Internals of stressed skin construction on Murphy Moose showing frames and supporting skin
Internals of stressed skin construction on Murphy Moose showing frames and supporting skin
Stressed skin: Section from the original Britannia Bridge, showing top and bottom stressed-skin construction
Section from the original Britannia Bridge, showing top and bottom stressed-skin construction

Worked examples

Example 1 — a first encounter with Stressed skin

Start with the simplest possible case. Write down what Stressed skin claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Stressed skin before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Stressed skin ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Stressed skin

In research
Stressed skin appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Stressed skin in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Stressed skin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aircraft skin, Automotive technologies, Structural engineering, so understanding it makes those chapters shorter.
In everyday life
Look for Stressed skin outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Stressed skin in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Stressed skin means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Stressed skin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Stressed skin in simple terms?

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…

Why does Stressed skin matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Stressed skin?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Stressed skin.

Tags

  • Aircraft skin
  • Automotive technologies
  • Structural engineering

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