Scope

The main aim of cluster 6 "design criteria" is to establish easy to use guidelines to provide a set of relevant design criteria for the application of multi materials.

The current status on design legislation is discussed by introducing methodologies of design and developments in design rules. To get a better understanding of the problems related to design criteria for multimaterial applications various case studies are worked out for five different industry sectors. Basis on these case studies the most essential design criteria for which engineers need further guidance in the use of multimaterials are identified. For these design criteria to use of standards and codes within the five identified sectors of industry is discussed. Finally the lack of available design rules for multimaterial applications is discussed.

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Current status on design legistation

The design of an optimal product is a complex process. Engineers start with the conceptual phase, which will end up with a number of potential outline solutions and the definition of the so called objective functions. The formulation of potential outline solutions is a mixture of experience, insight and creativity by the engineer. It is imperative that the design engineer fully understands the physical / functional requirements of the product as they can put constraints on material selection, design, manufacturing technique etc. After the conceptual phase engineers continue with the optimization phase, which will result in a final design. To show formally that this final design fulfils the design criteria, engineers will make a validation.

To optimise and validate a solution, the engineer has to show that the properties and performances of the product fulfil the prescribed requirements. If this is the case, it is stated that the product fulfils its design criteria.

The term design criterion used in the DOGMA project is defined as: "Property or performance of a product which has to fulfil a prescribed requirement".

In the following sections various multimaterial related products and their applications are reviewed by the cluster 6 partners. The following table presents a general summary of this study.

Various methodologies for product design have been developed over the years. An example of a practical approach is the mix of creative and systematic methods proposed by Nigel Cross (Engineering Design Methods, Second Edition - Strategies for Product Design, John Wiley & Sons, UK, 1994). It is essentially concerned with problem formulation, conceptual and embodiment design, rather than the detail design. The proposed methodology, summarized in table 6.1, involves seven steps in the design process. Each of these steps applies to a widely used rational method.

The case study on cast polymer concrete supplies a good illustration of the potential of this methodology. It also shows that a proper use of design criteria is essential for a successful design of a multimaterial.

Development of design rules

The use of design rules has a long tradition. The paper "Development of Design Rules" presents an overview of these historical developments. It also introduces the nowadays widely accepted limit state concept, and discusses the partial safety factor approach.

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Overview of case studies selected by industrial partners:

This is a kevlar/honeycomb structure which is located at the junction between the fuselage, designed to cover internal components and to provide an aerodynamic surface which shape matches with other structures on the wing and fuselage. It comprises a multilayer lay-up of kevlar oriented at 0°, 90°, ±45° over a Nomex heoneycomb structure. The honeycomb is bonded to the kevlar, and the whole system is cured in a tool in an autoclave. The fairing is bolted to the rest of the structure. The structure is made from kevlar, because this location on the aircraft is likely to sustain bird strike, and so must therefore have good impact performance. The design must balance various requirements, with cost/weight an important design driver, usually in an iterative fashion, and dictated in part by experience gained over the lifetime of the component. As a general rule the design must satisfy the CAA requirement that new materials should not subject an aircraft to higher risk than they accept ith existing materials.

In e.g. Finland sandwich panels are used in big halls, like ice-hockey or football halls. The sandwich structure has the following layering:

a) Epoxy coated steel plate (or some other metal/laminate)

b) Polystyrene or mineral wool insulation

c) Epoxy coated steel plate (or some other metal/laminate)

These steel panels are usually glued with a polyurethae adhesive at room or elevated temperature under pressure.

An example is the front structure of Stena HSS1500 Fast Catamaran. Some details of those fronts are:

- 6.75 m length

- 8.4 m height

- 38 m width

- 15 ton weight (2/3 of the simular size of an aluminium structure)

- 16 kg/m² area weight

- design load: pressure load from weather conditions (no wave loads allowed)

- construction: sandwich shell structure supported with aluminium pillars

- joints: connected to aluminium ship hull via bolted seam

The engineer is confronted with a wide variety of problems during the process of design of this example of a multimaterial product. In "Design of front composite structure of a catamaran" some reflections of a ship builder are given.

This is a sandwich panel between the driver's cab and the passenger area. One design currently in use consists of:

a) Melamine laminate

b) Steel plate

c) Birch plywood, dricon treated (18 mm)

d) Melanine laminate

Relevant standards containing some of the main performance requirements are:

• BS EN 438 (high pressure laminate)

• BS 6853 (fire performance for railway vehicles)

• GMRT 2120

• GM/TT0179

Objective: The new concept allows decrease in the cross section of the beam or we are able to use lower quality wood.

Concept: The concept is that we glue a FRP laminate (Carbon, glass etc.) between the outermost gluelines.

Design criteria:

See case matching sandwich panel structure case study. All joints between the sandwich panels are bonded and overlaminated.

A slot is adhesively bonded in a sandwich panel structure made of:

a) High pressure laminate (0.8 mm)

b) Glass fibre impregnated with phenolic adhesive (0.6 mm)

c) Polyetherimide foam (16.8 mm)

d) Glass fibre impregnated with phenolic adhesive (0.6 mm)

e) High pressure laminate (0.8 mm)

Identification of essential design criteria for which designers need further guidance

Basis on the above stated case studies a large variety of design criteria related to the application of multimaterials has been identified by the cluster 6 partners. The following nine topics have been covered:

1) Cost

2) Reliability

3) Weight

4) Mechanical performance

5) Fire performance

6) Environmental performance

7) Product shape

8) Production

9) Life time

Each of these topics cover various issues. All these issues could be important in certain situations, but the cluster 6 partners had to limit their work to time constrains. For this reason only the issues with the highest priority are selected, as indicated in the following table.

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Review of standards

To identify relevant standards and handbooks related to the above identified six design criteria the cluster 6 partners collected and discussed various documents. This has been done for the aerospace, automotive, building, marine and transport sectors of industry. To limite the number of documents only the case studies mentioned in section 6.3 were considered.

Overview of general gaps of knowledge

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Need for new codes for multimaterial applications

There is a strong need for new codes for multimaterial applications. This is, because the bias of existing codes towards the use of traditional materials making it particularly difficults to introduce new materials. Non-material specific procedures are required to verify new materials and multimaterial combinations. A transfer of knowledge between the different industry sectors might be helpfull to new design rules. This means that generic design criteria have to be developed for multimaterials, which exceed the industry sectors.

Prefered process of developing new codes for design: