Case Studies involving All Elements in All Sectors
Proposed designs for composite wings often consist of a number of spars with laminated skins. A multispar design means that load on the individual spars is low and buckling tends to be the dominant design criterion. A structure of sine wave configuration is ideal for this application.' z The design of the sine wave configuration itself is influenced by a number of factors:
The size and spacing of fasteners through the flanges.
The width of the web.
The critical buckling load.
The ease of processing.
more... The service lives of cast iron components in the cooling water circuits of estuarine power stations are often significantly reduced by a number of factors which include graphitic corrosion, localized corrosion due to mixed metals, pitting of surfaces, seizure of sealing faces during outages and brittle fracture. In order to overcome these problems GRP is being considered as an alternative material of construction and there are a number of units now in service. These are large structures with bore diameters up to 1800 mm. more... CFRP can potentially double, or triple, the span of suspension bridges. Bridges from conventional materials have a theoretical maximum span of around 5000 m. more... The main forces acting on the blade of a wind turbine are primarily aerodynamic and centrifugal. Depending on the type of machine, these may lead to blade bending or axial tension. To achieve power control via blade twisting therefore requires coupling between bending and twisting or between axial extension and twisting. In a vertical axis wind turbine (see figure below), with bending/twisting coupling, centrifugal forces give rise to twist whose magnitude remains constant as the rotor turns. Also generated are aerodynamic forces which act radially inwards on an upwind blade, but radially outwards on a downwind blade. These forces give a blade twist whose magnitude varies cyclically as the rotor turns. more... Intramedullary nails, used to repair fractured femurs, are currently made from stainless steel or a titanium alloy. The nail is slightly curved (typically a 1 cm bow over a 30 cm length) and hollow. Some designs have a longitudinal slit and holes at either end in which to locate fixing screws. Ideally, the nail should have similar mechanical properties to those of cortical bone, be capable of some bending deformation and have sufficient strength to allow the insertion of fixing screws. more... This case study considers the design of a cylindrical storage vessel typical of those used in chemical and process industries to store liquids. Corrosion resistance, strength and ease of fabrication make composite materials particularly attractive for this sort of application. The installed cost of a GRP vessel compares favourably with that of more traditional materials, such as stainless steel and lined carbon steel vessels. The majority of such vessels have diameters in the range 1 to 10 m, with wall thicknesses of between 5 and 50 mm.
In many respects, the process of designing a composite vessel is the same as that facing the designer of metal vessels. The design must take into account the design stress resulting from the pressure and size of the vessel in question. However, the composite designer is faced with the additional task of designing the material to be used. In so doing, they will generally take the opportunity to use a variety of differing layers within the laminate construction in order to achieve the most economical and desirable combination of properties.
The design methodology used in this case study is that developed in BS4994. This requires that the design process is considered in three stages, assessment of allowable strain, calculation of the applied unit loads and the selection of an appropriate laminate configuration. more... Long booms or masts are frequently required on spacecraft for a variety of purposes, such as supports for solar arrays, as antennas, or to locate experiments remote from the spacecraft environment. Prior to, and during, launch the masts must be stored in a compact form on board the spacecraft. There may also be a need to retract and redeploy masts when in orbit, for example to minimize the angular momentum during changes in the attitude of the spacecraft. A variety of designs of extensible or foldable masts have been developed. Some are based on a series of telescopic tubes, others make use of the collapsible tube principle consisting of a thin walled tube of elastic material, split longitudinally, which is opened out flat and coiled for launch. When uncoiled, it reverts to a circular cross-section and in some designs, provision is made for the edges of the split to interlock mechanically to increase the rigidity. The collapsible tube mast (CTM) is a closed tube of lenticular cross-section that can be flattened and then reeled on a drum for storage in a relatively small volume. Collapsible masts made from thin metal sheet were explored in the United States space programme in the 1960s. more... Floating platforms such as tethered leg platforms (TLPs) are the structures which are the most sensitive to weight issues, and buoyancy calculations can be a dominant feature of the structural design. This effect can be most acutely demonstrated in the consideration of riser design. These are the lengths of tubing that convey process fluids from the platform to the drill head on the seabed.
A platform may have up to 50 risers of different diameter. For a given TLP design, and therefore given platform displacement, every tonne saved from the riser top tension (or tether pretension) is an extra tonne of potential payload. If buckling were the limiting design criterion, it would be possible to allow riser tension to fall to a minimum value only slightly greater than the apparent weight of the member concerned. In the case of TLPs this cannot be allowed, since in order to prevent hydrodynamic interaction and possible contact, it is important to maintain compatible profiles between adjacent risers at all times. The principal design factors that control the detailed riser design are the potential blowout pressure (34 MPa, 5 000 psi) and the mean axial top tension (typically 75 tonne /riser).
Consideration of these loadings gives rise to a hybrid design with longitudinal carbon fibres and circumferential glass reinforcement.
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