Monocoque

Monocoque (pronounced /ˈmɒnɵkɒk/ or /ˈmɒnɵkoʊk/) is a construction technique that supports structural load by using an object's external skin, as opposed to using an internal frame or truss that is then covered with a non-load-bearing skin or coachwork. The term is also used to indicate a form of vehicle construction in which the body and chassis form a single unit. The word monocoque comes from the Greek for single (mono) and French for shell (coque). The technique may also be called structural skin, stressed skin, unit body, unibody, unitary construction, or Body Frame Integral. Pure monocoques lack internal longitudinal stiffening but are heavier as a result.^{[citation needed]} A semi-monocoque differs in having longerons and stringers. Structures built up from hollow tube frames, such as bicycles are not usually monococques as the primary stresses are not applied to the surface of the tube, but rather to the ends.^{[citation needed]}

Monocoque construction was pioneered in boats, such as with the Viking Longship and the Amerindian Canoe where it offered the low structural weight neccessary in vessels that were frequently expected to be beached or carried or portaged overland.^{[citation needed]}

Modern use of moncocoque structures in other applications began in aviation where pioneers used their experience in boatbuilding to provide the necessary structures. Early designs appeared in 1912 such as the Deperdussin Monocoque, and had found widespread use by the late 1930s when it was applied to metal structures such as on the Douglas DC-3. Automobiles used monocoque designs as early as 1923 but widespread adoption did not begin until the second half of the 20th century. Today, a welded unit body is the predominant automobile construction technique. Monocoque designs also appear (albeit extremely rarely) on motorcycles and in architecture - such as with geodesic domes.^{[citation needed]}

Aircraft

View of the inside of the tail of a Murphy Moose homebuilt aircraft under construction, showing the semi-monocoque design

Early aircraft were constructed using internal frames, typically of wood or steel tubing, which were then covered (or skinned) with fabric such as irish linen or cotton. Aircraft dope was then applied which tightened the fabric and provided a smooth sealed surface necessary to prevent excessive drag.

This fabric could provide no strength in compression and little in tension so to resist buckling these aircraft relied on the rigidity of the framework. Some early aircraft designers began to use sheet metal or plywood but only to provide access to the inside of the airframe or for non-structural aerodynamic fairings. Due to the alloys available, this provided little additional strength however the experience in working with molded laminates would prove useful.^{[citation needed]}

Design and development

In 1912 Deperdussin introduced their monocoque racer using a fuselage made up of three layers of laminated strips of glued poplar veneer, which provided both the external skin and the main load-bearing structure. This reduced drag so effectively it was able to win most of the races it was entered into. This style of construction was copied, with some variations, in Germany by Albatros and others however it was prone to damage from moisture.

Initially used only for skinning over an existing structure due to poor structural qualities, new aluminium alloys useful for structural purposes first appeared during World War I. By the late 1920s further improvements in its strength and durability allowed duralumin to be adopted extensively for internal framing members and the skin.^{[citation needed]}

It was realised that if the skin were strong enough it could eliminate the need for internal framing but this would be heavy. Thin sheet metal gauges could easily resist tension and shear loads but buckled under bending and compression. However, if curved, corrugated or rolled into pipe, sheet metal could be made strong against bending and compression loads as well. Stressed skins began to be combined with greatly reduced internal stiffening and came to be what is now known as semi-monocoque. The first aircraft known to have used this new method of construction came from Dornier, the Dornier D.I of 1918 for example, but, although of considerable official interest, it was ignored by other aircraft manufacturers.^{[citation needed]}

In the meantime, the Junkers F.13 and the Ford Trimotor, which loosely copied the Junkers structural design, retained an internal aluminium frame but covered these with a thin skin of corrugated aluminium sheeting. The corrugations allowed the aircraft's skin to take compression and bending loads, replacing ribs and stringers and could be regarded as a stressed skin structure augmented by an internal frame or semi-monocoque structure. The skin itself was now a significant structural element and it was to become even more important when airframes were required to take ever increasing loads.^{[citation needed]}

In the 1930s huge increases in engine power, speed, and operating altitudes required monoplanes and streamlined airframes with stiff, strong skins which semi-monocoque construction was ideal for. Torsional (twisting) stiffness was essential to avoid aerolastic deformation under the rising aerodynamic loads and structures that did not make use of the skin as a structural element could not handle the loads without being excessively thick. An outstanding example is the Douglas DC-3. World War II was a major catalyst for aircraft development. At the beginning of the war many aircraft still used mixed construction and relied on internal frames but by the end of the war all high-performance airplanes used semi or full monocoque construction. Sheetmetal is strongest when curved in more than two directions, but this requires expensive stamping machines or the laborious use of English wheels. The cost of these machines delayed the introduction of this technique in many countries, such as Italy, Holland and France.^{[citation needed]}

Modern technologies

First-stage view of the Falcon I rocket

Composite structures in aircraft date back to the Deperdussin of 1912, however during World War II, new materials were discovered that would change everything. By impregnating layers of glassfiber with a synthetic resin, it was found that a material of great strength and lightness could be made. It would face considerable opposition and many years before it became an acceptable alternative however structural tests carried out on boats showed that rather than weakening with age, the material got stronger, and it allowed the construction of a true monococque that didn't have the problems that wood had. Like wood laminates, its use was initially limited to trim parts but as experience was gained, eventually it became common for structural components as well.

The use of composite materials in monocoque skins allows strength, stiffness and flexibility to be controlled in different directions. Careful design of the direction of the grain of successive layers of materials coupled with the use of carbon fibre or other non isotropic composites can produce different mechanical properties in different directions while optimising for weight. Composite materials can be readily built up into complex three-dimensional shapes making them ideal for many aircraft components. They can be built made to be flexible such as on helicopter blades that are longitudinally rigid but capable of being twisted transversally to adjust the cyclic pitch instead of being mounted on a pivot.

Various rockets also use a flight pressure-stabilized monocoque design, including the Atlas II and Falcon I.

Automobiles

Cutaway drawing of the 1934 Citroen

Cutaway drawing of the 1942 Nash 600 body design

Similar to aircraft, automobile designs originally used body-on-frame construction, where a load-bearing chassis consisting of frame, powertrain, and suspension formed the base vehicle, and supported a non-load-bearing body or coachwork. Over time, this was supplanted by monocoque designs, integrating the body and chassis into a single unit. The external panels may be stressed, in such cases as the rocker panels, windshield frame and roof pillars, or non-stressed, as is often the case with fenders. Today, spot welded unit body is the dominant technique, although some vehicles (particularly trucks and buses) still use body-on-frame.^{[citation needed]}

Early designs

1934 Citroen Traction Avant
Steel monocoque construction

The first automotive application of the monocoque technique was 1923's Lancia Lambda, but it was not a true monocoque because it did not have a stressed roof, it was akin to a boat and has been described as 'punt' type construction. In 1928 German motorcycle manufacturer DKW launched their first car, the P15 wood and fabric bodied monocoque car. The Airflow and Traction Avant steel partially monocoque cars (stressed panels on internal frames) were both launched in 1934. General Motors subsidiary Opel then followed with the Olympia in 1935. In 1936, Lincoln introduced the Zephyr, a monocoque design which was as strong as the Airflow yet much lighter.

A halfway house to full monocoque construction was the 'semi-monocoque' used by the 1930s designed Volkswagen Beetle. This used a lightweight separate chassis made from pressed sheet steel panels forming a 'platform chassis', to give the benefits of a traditional chassis, but with lower weight and greater stiffness. This chassis was used for several different models. Volkswagen made use of the bodyshell for structural strength as well as the chassis - hence 'semi-monocoque'.

Nash Motors introduced this type of construction in 1941 with the new 600, generally credited^{[citation needed]} with being the first popular mass-produced unibody construction automobile made in the United States. The all-welded steel with sturdy bridge-like girders that arched front to rear made for improved strength, safety, and durability. Nash engineers claimed that about 500 pounds of excess weight was cut out (compared to body-on-frame automobiles) and the body's lower air drag helped it to achieve better fuel economy. The company's 1942 news release text attached to the X-ray drawing describes how "... all auto bodies will be built ... as this some day..."

Developments after 1945

After World War II the technique came into wider use. The Alec Issigonis Morris Minor of 1948 featured a monocoque body, as did the Hudson Motor range of the same period. General Motors-Holden in Australia built the monocoque-bodied Australian Holden of 1948. Other automakers incorporated this type of construction, and the terms unit body and unibody became more common in general use. The Ford Consul was the first Ford built in England using a unibody.

In 1960, a major breakthrough^{[citation needed]} in unibody construction was reached in mass-produced Detroit vehicles with over 99% of Chrysler vehicles produced that year being fully unitized; some of the basic designs surviving almost untouched through the mid 1970s (for example: Valiant, Dart, etc.) with tens of millions eventually produced. Convertible versions needed special supports welded underneath to compensate for the "missing" shape on the top.

American Motors (AMC) continued its engineering heritage from Nash and Hudson, in 1963 combining separate parts into single stampings. The Rambler Classic had "uniside" door-surrounds from a single stamping of steel: this reduced weight and assembly-costs, as well as increasing structural rigidity and improving door fitment.

Hybrid designs

Some American automobiles, such as the 1967-81 Chevrolet Camaro and Pontiac Firebird, 1968-79 Chevrolet Nova and many larger Chrysler Corporation RWD automobiles from 1965 until 1989, used a compromise design with a partial monocoque combined with a rubber-isolated subframe carrying the front end and powertrain. The intention was to provide some of the rigidity and strength of a unibody while easing manufacture. Results were mixed, in large part because the powertrain subframe contained the greatest single portion of the vehicle's overall mass, and thus movement of the subframe relative to the rest of the body could cause distortion and vibration. Subframes or partial subframes are still sometimes employed in otherwise monocoque construction, typically as a way of isolating the vibration and noise of powertrain or suspension components from the rest of the vehicle.

Modern monocoques

In automobiles, it is now common to see true monocoque frames, where the structural members around the window and door frames are built by folding the skin material several times. In these situations the main concerns are spreading the load evenly, having no holes for corrosion to start, and reducing the overall workload. Compared to older techniques, in which a body is bolted to a frame, monocoque cars are less expensive, lighter, more rigid, and can be more protective of occupants in a crash when appropriately designed. The use of higher strength steels in panels at points of high stress has increased strength and rigidity without increasing weight.

In sophisticated monocoque designs, the windshield and rear window glass is bonded in place and often makes an important contribution to the designed structural strength of automobiles.

Disadvantages

Unfortunately, when a vehicle with a unibody design is involved in a serious accident, it may be more difficult to repair than a vehicle with a full frame. Rust can be more of a problem, since the structural metal is part of the load-bearing structure (of metal that is much thinner than a conventional chassis) making it more critical, and must be repaired by cutting-out and welding rather than by simply bolting on new parts (as would be the case for a separate chassis). Structural rust of monocoque cars was a serious problem until the 1990s. Since then, more and more car makers have adopted protection techniques such as galvanizing for structural areas or for the whole body.

Armored vehicles

Tanks and other armored vehicles generally use a body or chassis which is built of the armor rather than attaching armor to a body-on-frame design. Though this generally produces a fairly heavy vehicle, it can reduce weight for a given amount of armor compared to soft-skinned vehicles to which armor has been added either as a modification or a kit. For example, the German Fuchs 2 and RG-33 have monocoque hulls rather than a separate body and frame, while the truck-based M3 Half-track and up-armored humvee have separate bodies to which armor has been added.

Two-wheeled vehicles

LCR Sidecar in race paddock

Aluminum monocoque downhill mountainbike frame.

Traditional bicycles are not monocoques; they are classic framed structures. However, carbon fibre monocoque framesets are slowly emerging in high-end competitive bicycles, due to their stiffness and light weight. The American company Kestrel USA pioneered the use of carbon fibre monocoques in bike frame manufacture in the 1980s, and since then the technique has become increasingly widely used. Items such as seat-posts and other components are now employing the same technique.

A Grand Prix motorcycle racing monocoque motorcycle was developed in 1967 by Ossa, a Spanish motorcycle brand. Notable designers such as Eric Offenstadt and Dan Hanebrink created unique monocoque designs in the early 1970s. The 1973 Isle of Man TT was won by Peter Williams on the monocoque-framed Norton John Player Special. Honda also experimented with a monocoque motorcycle in 1979 with its NR500. In 1987 John Britten developed the Aero-D One featuring a composite monocoque chassis that only weighed 12 kg. In 2009 Ducati introduced the Desmosedici GP9 with a carbon fibre semi-monocoque chassis.

Boat hulls

Frameless glass fibre reinforced or moulded plastic kayaks and canoes and reinforced concrete yacht hulls are monocoques, larger ships tend to have frames but may be hybrids with stressed skins over frames. A submarine has a massive tubular pressure hull at its heart designed primarily to withstand water pressure but because this is so strong it essentially forms a massive stressed skin and can be regarded as a monocoque.

Architecture

Architects occasionally take advantage of the increasing sophistication of monocoque technology in their building projects. Using monocoque technology in buildings allows for interior spaces without columns and load-bearing walls; this creates more spatial and programmatic openness inside. Notable examples are reinforced concrete shells.

Many 1950s and 1960s UK underground protected nuclear bunkers were constructed as reinforced concrete monocoque structures for their inherent strength, robustness and protective factors. They were often described as "underground submarines" in that, if they were dug up and placed in water, they would have floated and stayed waterproof.

The Lord's Media Centre at Lord's Cricket Ground is a semi-monocoque aluminium structure designed by London based Future Systems in 1999 and built by Pendennis Shipyard in Falmouth drawing on the company's boatbuilding experience.

The Wichita House designed by Buckminster Fuller used monocoque construction based on the Dymaxion design.

A geodesic dome is a hybrid design, combining monocoque and frame elements as are Quonset huts.

Structural Insulated Panels, or SIPs, are a type of preinsulated modular wall system. Formerly called "stressed skin panels", they are monocoques in and of themselves. Fastened together properly, they can yield a monocoque housing structure.

See also

• Backbone chassis
• Frame (vehicle)
• Chassis
• Coachwork
• Body-on-frame
• Coachbuilder
• Strut bar
• Airframe
• Spaceframe
• Superleggera
• Thin-shell structure