Materials Selection

As stated earlier, the selection of a material for a machine part or  structural member is one of the most important decisions the designer is called on to make. Up to this point in this chapter we have discussed many important material physical properties, various characteristics of typical engineering materials, and various material production

processes. The actual selection of a material for a particular design application can be an easy one, say, based on previous applications (1020 steel is always a good candidate because of its many positive attributes), or the selection process can be as involved and daunting as any design problem with the evaluation of the many material physical, economical, and processing parameters. There are systematic and optimizing approaches to material selection. Here, for illustration, we will only look at how to approach some material properties. One basic technique is to list all the important material properties associated with the design, e.g., strength, stiffness, and cost. This can be prioritized by using a weighting measure depending on what properties are more important than others. Next, for each property, list all available materials and rank them in order beginning with the best material; e.g., for strength, high-strength steel such as 4340 steel should be near the top of the list. For completeness of available materials, this might require a large source of material data. Once the lists are formed, select a manageable amount of materials from the top of each list. From each reduced list select the materials that are contained within every list for further review. The materials in the reduced lists can be graded within the list and then weighted according

to the importance of each property.

M. F. Ashby has developed a powerful systematic method using materials selection charts.16 This method has also been implemented in a software package called CES Edupack.17 The charts display data of various properties for the families and classes of materials listed in Table 2–4. For example, considering material stiffness properties, a simple bar chart plotting Young’s modulus E on the y axis is shown in Fig. 2–15. Each vertical line represents the range of values of E for a particular material. Only some of the materials are labeled. Now, more material information can be displayed if the x axis represents another material property, say density.

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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 six phases of the generic development process

The generic product development process consists of six phases, as illustrated in

Exhibit 2-2. The process begins with a planning phase, which is the link to advanced research and technology development activities. The output of the planning phase is the project's mission statement, which is the input required to begin the concept development phase and which serves as a guide to the development team. The conclusion of the product development process is the product launch, at which time the product becomes available for purchase in the marketplace. One way to think about the development process is as the initial creation of a wide set of alternative product concepts and then the subsequent narrowing of alternatives and increasing specification of the product until the product can be reliably and repeatably produced by the production system. Note that most of the phases of development are defined in terms of the state of the product, although the production process and marketing plans, among other tangible outputs, are also evolving as development progresses.

Another way to think about the development process is as an information-processing system. The process begins with inputs such as the corporate objectives and the capabilities of available technologies, product platforms, and production systems. Various activities process the development information, formulating specifications, concepts, and design details. The process concludes when all the information required to support production and sales has been created and communicated.

A third way to think about the development process is as a risk management system. In the early phases of product development, various risks are identified and prioritized. As the process progresses, risks are reduced as the key uncertainties are eliminated and the functions of the product are validated. When the process is completed, the team should have substantial confidence that the product will work correctly and be well received by the market.

Exhibit 2-2 also identifies the key activities and responsibilities of the different functions of the organization during each development phase. Because of their continuous involvement in the process, we choose to articulate the roles of marketing, design, and  manufacturing. Representatives from other functions, such as research, finance, field service, and sales, also play key roles at particular points in the process.

The six phases of the generic development process are:

o. Planning: The planning activity is often referred to as "phase zero" since it precedes the project approval and launch of the actual product development process. This phase begins with corporate strategy and includes assessment of technology developments and market objectives. The output of the planning phase is the project mission statement, which specifies the target market for the product, business goals, key assumptions, and constraints. Chapter 3, Product Planning, presents a discussion of this planning process.

1. Concept development:

In the concept development phase, the needs of the target market are identified, alternative product concepts are generated and evaluated, and one or more concepts are selected for further development and testing. A concept is a description of the form, function, and features of a product and is usually accompanied by a set of specifications, an analysis of competitive products, and an economic  justification of the project. This book presents several detailed methods for the concept development phase (Chapters 4-8). We expand this phase into each of its constitutive activities in the next section.

2. System-level design: 

The system-level design phase includes the definition of the product architecture and the decomposition of the product into subsystems and components. The final assembly scheme for the production system is usually defined during this phase as well. The output of this phase usually includes a geometric layout of the product, a functional specification of each of the product's subsystems, and a preliminary process flow diagram for the final assembly process. Chapter 9, Product Architecture, discusses

some of the important activities of system-level design. 

3. Detail design: 

The detail design phase includes the complete specification of the geometry, materials, and tolerances of all of the unique parts in the product and the identification of all of the standard parts to be purchased from suppliers. A process plan is established and tooling is designed for each part to be fabricated within the production system. The output of this phase is the control documentation for the product-the drawings

or computer files describing the geometry of each part and its production tooling,

the specifications of the purchased parts, and the process plans for the fabrication and assembly

of the product. Two critical issues addressed in the detail design phase are production

cost and robust performance. These issues are discussed respectively in Chapter 11,

Design for Manufacturing, and Chapter 13, Robust Design.

4. Testing and refinement: 

The testing and refinement phase involves the construction

and evaluation of multiple preproduction versions of the product. Early

(alpha) prototypes are usually built with production-intent parts-parts with the same

geometry and material properties as intended for the production version of the product

but not necessarily fabricated with the actual processes to be used in production.

Alpha prototypes are tested to determine whether the product will work as designed

and whether the product satisfies the key customer needs. Later (beta) prototypes are

usually built with parts supplied by the intended production processes but may not be

assembled using the intended final assembly process. Beta prototypes are extensively

evaluated internally and are also typically tested by customers in their own use environment.

The goal for the beta prototypes is usually to answer questions about performance

and reliability in order to identify necessary engineering changes for the final

product. Chapter 12, Prototyping, presents a thorough discussion of the nature and use

of prototypes.

5. Production ramp-up: In the production ramp-up phase, the product is made using

the intended production system. The purpose of the ramp-up is to train the work force

and to work out any remaining problems in the production processes. Products produced

during production ramp-up are sometimes supplied to preferred customers and are carefully

evaluated to identify any remaining flaws. The transition from production ramp-up

to ongoing production is usually gradual. At some point in this transition, the product is

launched and becomes available for widespread distribution.

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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.

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.