Selecting Functional Requirements - P152.0

 

Selecting Functional Requirements
Practice document
Document Type	Practice
Document Identifier	152
Tags	Integrated product development Functional Requirements
Objective functions	Cost Maintain Rate Maintain Quality Maintain
MSTE workflow	Development
Prerequisites	Integrated Product Development Practice Practice for Developing a Product

Introduction

MSTEP "House" illustrating the framework and interactions between all the different components that make up the development process, with the function on top.

The first step to creating a composite product is to establish the framework and constraints to work within. This page will describe some of the functional requirements that a product may have and how that may affect the continuation of the development process. This list aims to briefly describe some common design and manufacturing requirements that should be considered and quantified before beginning further development steps. There are many considerations outside of this list more specific to your application, this page is meant to provide a starting point and highlight some of the important requirements.

Requirements

Production Volumes

When planning and initializing a factory it is important to predict and build for the correct production volume. If the factory is too small, you may not be able to meet demand, but if it is too large it is a source of a big loss in capital.

Geometry and Dimensional Accuracy

The requirements for geometry may be very confined or relatively open depending on application, to define these requirements early will guide the rest of the development process for the composite product. The dimensional accuracy of the final part will play a part in deciding tooling type, resin and material system as well as post cure processing. If the dimensional accuracy requirement is set to be very accurate, the post cure machining can rapidly drive up the cost of the production.

Assembly Requirements and Interactions with Other Parts

If the composite is to be a part of a much larger structure or a product is composed of many smaller composite parts, the methodology of how to join the various components together must be considered. In addition to this, the interaction between different material systems must be characterized and considered for the development process.

Surface Finish

The surface finish requirements will have an influence on tooling and bagging choice, if a gel coat is needed and if the part needs to be painted before it is completed.

Load Cases

The load cases requirements come from the predicted loads that the part will see during its lifetime. Attention should not only be given to the magnitude of load, but also the direction of load as this will influence fibre direction in the part. . This will have a heavy influence on the material and resin system, as well as the shape design. To read more about strength calculations, see Micro-Mechanics and Macro-Mechanics.

Deflection Limits

A subsidiary of load cases, the deflection limits is how much the part is allowed to elastically deform under service loads. Some applications will allow for a large amount of deflection for shock absorption, whilst others will require next to no deflection for stability.

Lifetime Requirements

The requirements for how long the part is to be in service is a vital to determining Fatigue Limits and damage tolerances.

Environmental Conditions

The predicted environmental conditions that the part will be operating in is vital when selecting material and resin system. This involves temperature concerns as both high and low temperatures can adversely affect composites and wet conditions can cause moisture absorption into the composite. Other factors such as UV exposure and chemical contact should also be considered.

Weight

Weight is often one of the most important objectives, and a major factor when choosing composites with a high specific strength and stiffness. It is important to also consider additional weight due to joints, damage tolerance, coating, and moisture gain, among other factors, during service of the product.

Regulatory Requirements, Laws and Standards

An important aspect of the development process is not only to satisfy the structural requirements for a part, but also the relevant laws and regulations for where the product is to be used. This is also the case for certification of the components if the end application requires it. These types of requirements are important in the development process and they also come into play later as the need for testing will arise, whether it is through coupon testing or full-scale prototype testing.

Cost

A vital aspect of developing a part is that the price of manufacturing must must be viable. The cost of manufacturing includes the cost of labor and overhead, materials and tooling as well as equipment and other capital costs. Costing will guide the feasibility of choosing a manufacturing process; if a high initial investment is economically viable, if there is already equipment that is available and the product must be developed for a specific process or some requirements need to be altered to conform to the costing requirement. See AIM event recording on Costing composite parts.

Reparability

A challenge with composite parts is the low potential for repairs. The component design needs to take into account if repair is an option and to what extent a part can be repaired.

Handling

The handling refers to the fibre and matrix system during production and how it is to be applied to the tool. For example, if choosing a pre-impregnated (pre-preg) fibre and resin system, the conformability and tack are important to consider. The conformability dictates how easily the material is to drape over contours and applied into corners. The tack, is how sticky the pre-preg is and how well it sticks to the tool and subsequent plies. If the handling of your material system is too difficult or takes a lot of time, the production costs will increase rapidly and rates will decrease.

Generic Development Guidelines

The following is a list of generic rule-of-thumb, compiled through years of experience with composites product development.

• Plies which do not extend to the edge of the part should be inside the ply stack and should not end on the laminate surface. A minimum ratio of 10:1 should be maintained for the taper and plies are typically dropped off in pairs. Be wary of the effects ply drop offs can have on the surface contour of the part.
• There should be an off-set distance between a ply drop off and a mating surface.
• The structural thickness of the material should also be designed to withstand impact from maintenance, tool drop and handling accidents.
• For carbon composites, if any other materials are to come into contact with the laminate, especially aluminum, a glass fibre ply in the contact are should be considered to prevent galvanic corrosion.
• The final surface ply of a laminate should be oriented in a direction that is the least structurally critical. For woven fabric it is generally ±45°. If the last ply is a unidirectional tape, two plies should be used perpendicular to each other, usually +45° and -45°.
• If a part is made from mainly unidirectional tape, it is recommended to apply a layer of carbon woven fabric or a thin glass fibre scrim ply to the surfaces. This is to prevent handling damage and machining damage such as splintering.
• To reduce chances of matrix microcracking in unidirectional laminates, a few plies should be placed with the same orientation. For quarter inch laminates, four plies of 0.008 inch plies in the same direction should suffice.
• If a load bearing part is in contact with a moving part, prevent abrasion by applying a wear resistant material.
• Plies can be spliced in order to create a ply that is large enough for the layup. The two types of splices are butt joints and overlap joints. Plies with dissimilar orientations should not be spliced.

Next Steps

Continue the product development process with Material Selection and Shape Development.

Related pages

Page type	Links
Introduction to Composites Articles
Foundational Knowledge Articles
Foundational Knowledge Method Documents
Foundational Knowledge Worked Examples
Systems Knowledge Articles
Systems Knowledge Method Documents
Systems Knowledge Worked Examples
Systems Catalogue Articles
Systems Catalogue Objects – Material
Systems Catalogue Objects – Shape
Systems Catalogue Objects – Tooling and consumables
Systems Catalogue Objects – Equipment
Practice Documents	Composite Production Part Drawing - P169 Material Selection - P153 Practice for Developing a Deposition Step - P157 Practice for Developing a Product - P111 Practice for Identifying Process Steps - P102 Process selection for production scale-up - P171 Selecting Functional Requirements - P152.0 Shape Development - P151.0
Case Studies	Developing for Future Production Scale Up - C113 Lack of Requirements in Product Development - C104 Use of Right Sized Simulation to Aid Decision Making in Thermoplastic Composite Manufacturing Systems - C114
Perspectives Articles	CKN’s Approach to Developing Products with Composite Materials - A332

References

About	Help

Material and process (M), Shape (S), Tooling and consumables (T) & Equipment (E) - all have an interlinked effect on the Process step (P). (See MSTE factory ontology)

Engineered materials (designed to have specific properties) made from two or more constituent materials with different physical or chemical properties. The constituents remain separate and distinct on a macroscopic level within the finished structure.

For polymer matrix composites (PMCs), resin refers to the matrix; the continuous material phase that binds the reinforcement together, maintains shape, and transfers load. Resins are divided into two main groups: thermosets and thermoplastics.

Application and Impact Mobilization Events (AIM).

The continuous material phase that binds the reinforcement together, maintains shape, transfers load, protects the reinforcement from environment and damage, and provides the composite support in compression.

Desirable characteristics:

• Moisture/chemical resistance
• Low density
• Processability