MODEL MATERIAL AND FABRICATIONProf. V G Idichandy

Dept of Ocean EngineeringIIT Madras

Requirements of models tested in water

Water proofing of model and instrumentation

Commonly used model materials are: Wood Fiber reinforced plastics Plastics Metals Concrete, etc.

Selection of model material

The size of the model and model material are decided based on several practical considerations

Characteristics of test facilitiesAvailable standard sizes of plates,

pipes and other sectionsThe weight of the model

Properties of model materials

For an elastic model, the material should have,

Homogeneity Isotropy, Linear stress-strain relationship and An invariant Poissons ratio

Model materials Materials with high and low elastic modulus Poissons ratio Creep and thermal conductivity Workability When there is complex geometry and irregular

shape, an easily workable material will be the choice.

For shape models a good workable material which can with stand the prolonged exposure to water is preferred.

Metals

High accuracy Satisfies most of the model requirements Easily workable and available in various

shapes and forms Aluminium or its alloys are preferred Best material for hydrodynamic models, if

weight distribution can be satisfied

Plastics Derived by chemical synthesis-contains carbon

compounds Thermoplastics-becomes softer beyond glass

transition temperature-can be formed in to complex shapes and geometries-retains shape on cooling-a good mould is required-examples are acrylic resins (perspex) and poly vinyl chloride (pvc)-available in sheets, tubes, rods-can be machined and joined

Plastics Thermosetting plastics-usually cast into

shape-accurate moulds are required-physical properties can be changes by using suitable fillers-Epoxies are typical examples.

Fiber reinforced plastics-used in making very complex models

Plastics are viscoelastic materials. Care must be taken in strain measurements

Wood

Ideally suited for shape models being workable

Absorbs moisture Wax coating or suitable epoxy coating is

used to prevent water ingression

Polyurethane Foam

Can be easily finished to the required shape and sizes.

Available in slabs-can be machined like any other material

When used in water it requires surface treatment to prevent water ingression

Fabrication of Wood Models

Laminated wood block-each laminate is cut along the water plane lines

The final shape is achieved by removing excess material by planning, shaving and scraping

Numerically controlled machines are increasingly used in model making

Fabrication of Wood Models

No metallic fasteners Final finishing by hand Templates used to check final dimensions Several coats of penetrating wood sealer

is used both inside and outside the model Final finish using epoxy water-resistant

paint

Fiberglass models Preparation of accurate mould Fiberglass is applied on the mould by hand lay-

up Several layers of fiberglass may be required for

the required strength. Each layer is coated with binder usually an epoxy

Mould is made by spraying polyurethane foam to the prepared skeleton of templates

Excess form is removed till the edge of the templates are visible

Plastic Models

By joining machined components Joining is done by capillary welding or

using glues and adhesives Plastic welding Thermal forming of plastics Drape moulding