Design of a sandwich structure
Design of marine application of composite structures is quite demanding. Use of composite materials in marine applications (i.e. commercial large vessels) has been very limited because the weight has not been so critical and the ship building industry is very conservative.
The use of composites has to be decided in the very early phase of the conceptual design of the vessel in order to maximise the use of the favourable properties of composite materials. (In many cases, composites are used when there are no other alternatives.) If structures are first designed in traditional metal materials and during design phase materials are changed for example to reduce the weight of the structure, the end product is not the best possible. The best solution is to design the metal structure at the same time as the composite structure and make some compromises on both areas to ensure that no extra or duplicate structures are needed.
When designing front structure of the fast catamaran there are quite a lot of things to comply with. Loadings of structure are mainly defined in classification society role books, but there might be need for extra analysis to find out the real loadings, which the structure can get during end use. Role books are very conservative and if structure is designed only according to them structure will not become weight optimised. Role book's formulas include various factors (for example hull length) which are not very usable in every structure. Most limitations and requirements (for example minimum thickness) are defined in the same way as for metal structures. Because composite materials behave differently it can very easily happen that structures are going to be too conservative and heavy.
In many cases materials to be used in modern composite structures are not classified by societies and this means a lot of qualification work to be done before new materials (new to classification societies) can be used. They might not be aware of other composites than what are used in sailing or rowing boats.
The selection of materials and structure is a very complicated mess, during which the designer has to balance between what is applicable and what is feasible. In the preliminary design phase decisions about structure type are made: solid laminate shell with stiffeners, sandwich panel with or without stiffeners, sandwich shell structure with bulkheads and pillars. All these types have their good points and weaknesses. Materials are mainly selected according to the following: size of structure, loadings, structural and weight requirements, manufacturing process available, raw material price, operating conditions (for example temperature, fire, water, moisture, ice, other chemicals, etc), classification status and other requirements.
Even large structures can be made in glass fibre sandwich materials if weight is not extremely critical. lf extra stiffness is required carbon fibres are used. Resin selection has to be made according to strength and weight requirements. Epoxy resin is the best choice due to good properties, but it is more expensive than vinyl- or polyesters. lf fire requirements are demanding phenolics can be used. Core material selection is made according to the same kind of selection items. The core material is in many cases the weakest link in sandwich structure design and the selection has to be made very carefully. The materials used in joints, pillars and other supporting structures are selected according the following: stiffness, weight, joining efficiency (easiness and weight of joint manufacturing) and price. Using the material test data it has to be noticed that material properties in extreme conditions are used as design values, not those material supplier's beautiful values measured in +15C dry conditions.
One very demanding requirement for composite applications is fire resistance. Most composite materials do not meet the IMO definition of uncombustible material (steel or the equivalent). This is the strongest barrier against using composite structures in SOLAS ships. In fast vessel regulations the requirements are somewhat easier and composites can be used in certain structures. For example, if there are no crew or passengers in the front structure area during voyage, structure can be classified as a non-structural part so that there might not be any fire requirements. Although structural loadings induced by sea and weather conditions are very demanding. lf the structure is classified as having fire requirements, extra fire insulation is often needed to fulfil the demands.
Design criteria for sandwich structures need to be specified before design is started. In a way criteria can be divided into two sectors: structural requirements and material allowables. In structural requirements are following items:
In material allowables are defined:
- weight and price
- behaviour under loading, structural strength, deflections etc according to role book or equivalent requirements
- joining to other structures, "ship building easiness"
- structural behaviour and breaking modes of materials and structures need to be known and analysed
- certain factors of safety are used in dimensioning and designing the structure to meet the design limit loadings in every loading case
- in many cases structural strength and maximum stresses are not good values as matrix cracking takes place before those stresses are reached in structure
- behaviour and status of core material under loading has to be analysed very carefully
major design criteria are core shear stresses and laminate strains
Design of adhesive bonded joint
of composite structure and metal hull
Design of joint goes on the same way along the design of front structure. You need to have design information of the joint before you design the front and vice versa. Adhesive bonded joint with bolts is often selected because:
Design of joint requires good analysing methods and design guides. Design has to be tested to see real properties and behaviour of joint. Joint design has to be quite conservative because joints can locate easily in places where difficulties appear in construction and assembly phase. For example connecting 1 m wide, 7m height and 8 m long front half part that weighs around 7tonnes into welded aluminium hull is not so easy. Dimensions on joint line can vary quite a lot. Bolting may be used as an additional joining method to keep bonded adherends together during adhesive curing and giving extra reliability in use. A pure bonded joint has to be tested and designed very thoroughly. Design of a bonded joint has to rely on proven materials and concepts.
Design criteria can be specified for example as follows:
Selection of adhesive materials are made according to the following guidelines:
- load carrying capabilities of joint must not be exceeded in any load case
- long term properties of adhesives have to be specified, usually all adhesives have a tendency to creep, reasonable large factors of safety can be used depending on the adhesive
- long term strength properties, in joint design limit loading and stresses have to be very conservative
- requirements for surface treatment of adherends should be reasonably easy to do
applicability in manufacturing process used in joint manufacturing, easiness of use in shipyard conditions, many fancy but tricky aerospace materials need to handled in as- precise conditions that can not be arranged in shipyards
applicability to be used in large joints -price and availability
State of art requirements
Requirements for Catamaran front structure are:
Dynamic behaviour can be tolerated with sandwich structure in front part. Dynamic loads induced by air stream and motions can cause dynamic problems in structure unless they are not taken care of.
Impact resistance is mainly against occasional wave loads, and it should not be the major requirement, since the whole vessel is designed into very tight operating condition limitations. For example waves may not reach the wet deck of vessel. (Unfortunately experience has shown that captains of these vessels can not read the operating manual.) The front loadings are so called weather loads, which are modelled as a static pressure against panels. In real life these pressures are generated by motion of vessel and gusty wind. Naturally some spray water and wave tops may also be included.
Chemical resistance requirements of front structures are concentrated in resistance against water and ice. The front is in contact with moisture all the time. In operating conditions it is also very probable that the water inside front structure (on decks and sumps) will freeze. lf water gets into the sandwich panel it might cause problems in the long run. This is why fronts are covered by painting.
Fire requirements in front structures are neglected, because it is not a crew or passenger area during voyage and it is categorised as a non-structural part. lf fire requirements existed quite a lot of extra insulation would be needed and the material selection would be quite different.
One of the biggest requirement area is assembly. This means that the hole structure is a quite big composite part (38x8x7m) and it can not be manufactured in one or two pieces. Actually the number of separated parts in front is around 150 pieces. So there are quite a lot of joints between panels and other composite parts. The joint design is quite essential to get right. Dimensional accuracy with welded aluminium hull can sometimes be a problem if enough attention is not concentrated on quality matters.