Composite Materials

Composite materials are formed from two or more dissimilar materials, each of which contributes to the final properties. Unlike metallic alloys, the materials in a composite remain distinct from each other at the macroscopic level. Most engineering composites consist of two materials: a reinforcement called a filler and a matrix. The filler provides stiffness and strength; the matrix holds the material together and serves to transfer load among the discontinuous reinforcements. The

most common reinforcements, illustrated in Fig. 2–14, are continuous fibers, either straight or woven, short chopped fibers, and particulates. The most common matrices are various plastic resins although other  materials including metals are used. 

Metals and other traditional engineering materials are uniform, or isotropic, in nature. This means that material properties, such as strength, stiffness, and thermal conductivity, are independent of both position within the material and the choice of coordinate system. The discontinuous nature of composite reinforcements, though, means that material properties can vary with both position and direction. For example, an epoxy resin reinforced with continuous graphite fibers will have very high strength and

stiffness in the direction of the fibers, but very low properties normal or transverse to the fibers. For this reason, structures of composite materials are normally constructed of multiple plies (laminates) where each ply is oriented to achieve optimal structural stiffness and strength performance.

High strength-to-weight ratios, up to 5 times greater than those of high-strength steels, can be achieved. High stiffness-to-weight ratios can also be obtained, as much as 8 times greater than those of structural metals. For this reason, composite materials are becoming very popular in automotive, aircraft, and spacecraft  applications where weight is a premium.

The directionality of properties of composite materials increases the complexity of structural analyses. Isotropic materials are fully defined by two engineering constants:

Young’s modulus E and Poisson’s ratio ν. A single ply of a composite material, however, requires four constants, defined with respect to the ply coordinate system. The constants are two Young’s moduli (the longitudinal modulus in the direction of the fibers, E1, and the transverse modulus normal to the fibers, E2), one Poisson’s ratio (ν12, called the major Poisson’s ratio), and one shear modulus (G12). A fifth constant,

the minor Poisson’s ratio, ν21, is determined through the reciprocity relation, ν21/E2 = ν12/E1. Combining this with multiple plies oriented at different angles makes structural analysis of complex structures unapproachable by manual techniques. For this reason, computer software is available to calculate the properties of a laminated composite construction.

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Mechanical Engineering

McGraw−Hill Primis

ISBN: 0−390−76487−6

Text:

Shigley’s Mechanical Engineering Design,

Eighth Edition

Budynas−Nisbett

A Generic Development Process

A process is a sequence of steps that transforms a set of inputs into a set of outputs. Most people are familiar with the idea of physical processes, such as those used to bake a cake or to assemble an automobile. A product development process is the sequence of steps or activities which an enterprise employs to conceive, design, and commercialize a product. Many of these steps and activities are intellectual and organizational rather than physical.

Some organizations define and follow a precise and detailed development process, while others may not even be able to describe their processes. Furthermore, every organization employs a process at least slightly different from that of every other organization. In fact, the same enterprise may follow different processes for each of several different types of development projects.

A well-defined development process is useful for the following reasons:

• Quality assurance: A development process specifies the phases a development project will pass through and the checkpoints along the way. When these phases and checkpoints are chosen wisely, following the development process is one way of assuring the quality of the resulting product.

• Coordination: A clearly articulated development process acts as a master plan which defines the roles of each of the players on the development team. This plan informs the members of the team when their contributions will be needed and with whom they will need to exchange information and materials.

• Planning: A development process contains natural milestones corresponding to the completion of each phase. The timing of these milestones anchors the schedule of the overall development project.

• Management: A development process is a benchmark for assessing the performance of an ongoing development effort. By comparing the actual events to the established process, a manager can identifY possible problem areas.

• Improvement: The careful documentation of an organization's development process often helps to identifY opportunities for improvement.

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References and Bibliography

Many current resources are available on the Internet via

www.ulrich-eppinger.net

Stage-gate product development processes have been dominant in manufacturing firms

for the past 30 years. Cooper describes the modem stage-gate process and many of its

enabling practices.

Cooper, Robert G., Winning at New Products: Accelerating the Process from Idea to

Launch, third edition, Perseus Books, Cambridge, MA, 2001.

Plastics

The term thermoplastics is used to mean any plastic that flows or is moldable when heat is applied to it; the term is sometimes applied to plastics moldable under pressure. Such plastics can be remolded when heated.

A thermoset is a plastic for which the polymerization process is finished in a hot molding press where the plastic is liquefied under pressure. Thermoset plastics cannot be remolded.

Table 2–2 lists some of the most widely used thermoplastics, together with some of their characteristics and the range of their properties. Table 2–3, listing some of the thermosets, is similar. These tables are presented for information only and should not be used to make a final design decision. The range of properties and characteristics that can be obtained with plastics is very great. The influence of many factors, such as cost, moldability, coefficient of friction, weathering, impact strength, and the effect of fillers and reinforcements, must be considered. Manufacturers’ catalogs will be found quite helpful in making possible selections.

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Mechanical Engineering

McGraw−Hill Primis

ISBN: 0−390−76487−6

Text:

Shigley’s Mechanical Engineering Design,

Eighth Edition

Budynas−Nisbett

The Challenges of Product Development

Developing great products is hard. Few companies are highly successful more than half the time. These odds present a significant challenge for a product development team. Some of the characteristics that make product development challenging are:

• Trade-offs: An airplane can be made lighter, but this action will probably increase

manufacturing cost. One of the most difficult aspects of product development is recognizing, understanding, and managing such trade-offs in a way that maximizes the success of the product.

• Dynamics: Technologies improve, customer preferences evolve, competitors introduce new products, and the macroeconomic environment shifts. Decision making in an environment of constant change is a formidable task.

• Details: The choice between using screws or snap-fits on the enclosure of a computer can have economic implications of millions of dollars. Developing a product of even modest complexity may require thousands of such decisions.

• Time pressure: Anyone of these difficulties would be easily manageable by itself given plenty of time, but product development decisions must usually be made quickly and without complete information.

• Economics: Developing, producing, and marketing a new product requires a large investment. To earn a reasonable return on this investment, the resulting product must be both appealing to customers and relatively inexpensive to produce. For many people, product development is interesting precisely because it is challenging. For others, several intrinsic attributes also contribute to its appeal:

• Creation: The product development process begins with an idea and ends with the production of a physical artifact. When viewed both in its entirety and at the level of individual activities, the product development process is intensely creative.

• Satisfaction of societal and individual needs: All products are aimed at satisfying needs of some kind. Individuals interested in developing new products can almost always find institutional settings in which they can develop products satisfying what they consider to be important needs.

• Team diversity: Successful development requires many different skills and talents. As

a result, development teams involve people with a wide range of different training, experience,

perspectives, and personalities.

• Team spirit: Product development teams are often highly motivated, cooperative groups. The team members may be colocated so they can focus their collective energy on creating the product. This situation can result in lasting camaraderie among team members.

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References and Bibliography

A wide variety of resources for this chapter and for the rest of the book are available on the Internet. These resources include data, templates, links to suppliers, and lists of publications. Current resources may be accessed via www.ulrich-eppinger.net Wheelwright and Clark devote much of their book to the very early stages of product development, which we cover in less detail. Wheelwright, Stephen c., and Kim B. Clark, Revolutionizing Product Development: Quantum Leaps in Speed, Efficiency, and Quality, The Free Press, New York, 1992. Katzenbach and Smith write about teams in general, but most of their insights apply to product development teams as well. Katzenbach, Jon R., and Douglas K. Smith, The Wisdom of Teams: Creating the High-Performance Organization, Harvard Business School Press, Boston, 1993. These three books provide rich narratives of development projects, including fascinating descriptions of the intertwined social and technical processes. Kidder, Tracy, The Soul of a New Machine, Avon Books, New York, 1981.

Sabbagh, Karl, Twenty-First-Century Jet: The Making and Marketing of the Boeing 777, Scribner, New York, 1996. Walton, Mary, Car: A Drama of the American Workplace, Norton, New York, 1997.