Charts for yield strength Strength

The big picture: charts for yield strength Strength can be displayed on material property charts. Two are particularly useful. The strength–density chart Figure 6.6 shows the yield strength σy or elastic limit σel plotted against density ρ. The range of strength for engineering materials, like that of the modulus, spans about six decades: from less than 0.01 MPa for foams, used in packaging and energy-absorbing systems, to 104 MPa for diamond, exploited in diamond tooling for machining and as the indenter of the Vickers hardness test. Members of each family again cluster together and can be enclosed in envelopes, each of which occupies a characteristic part of the chart. Comparison with the modulus–density chart (Figure 4.7) reveals some marked differences. The modulus of a solid is a well-defined quantity with a narrow range of values. The strength is not. The strength range for a given class of metals, such as stainless steels, can span a factor of 10 or more, while the spread in stiffness is at most 10%. Since density varies very little (Chapter 4), the strength bubbles for metals are long and thin. The wide ranges for metals reflect the underlying physics of yielding and present designers with an opportunity for manipulation of the strength by varying composition and process history. Both are discussed later in this chapter. Polymers cluster together with strengths between 10 and 100 MPa. The composites CFRP and GFRP have strengths that lie between those of polymers and ceramics, as one might expect since they are mixtures of the two. The analysis of the strength of composites is not as straightforward as for modulus in Chapter 4, though the same bounds (with strength replacing modulus) generally give realistic estimates. 
Materials
Engineering, Science,
Processing and Design
Michael Ashby, Hugh Shercliff and David Cebon
University of Cambridge,
UK
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Butterworth-Heinemann is an imprint of Elsevier

MATERIALS AND PROCESS SELECTION

At the same time form is being developed, it is important to identify  materials and production techniques and to be aware of their specific engineering requirements.
Anexperienced designer has a short list of materials and processes in mind even with the earliest concepts.
In developing an understanding of the product, we may have set requirements on materials, manufacturing, and assembly. At a minimum we did competitive benchmarking on similar devices, studying them for conceptual ideas and for what they were made of and how they were made. All this information influences the
embodiment of the product in several ways:
First, the quantity of the product to be manufactured greatly influences the selection of the manufacturing processes to be used.
For a product that will be built only once, it is difficult to justify the use of a process that requires high tooling costs. Such is the case with injection molding, in which the mold cost almost exclusively determines the component cost for low-volume production (see Section 11.2.4). In general, injection-molded plastic components are only cost-effective if the production run is at least 15,000.
A second major influence on the selection of a material and a manufacturing process is prior-use knowledge for similar applications. This knowledge can be both a blessing and a curse. It can direct selection to reliable choices, yet it may also obscure new and better choices. In general it is best to be conservative, and heed the axiom below.
When studying existing mechanical devices, get into the habit of determining what kind of materials were used for what types of functions. With practice, the identity of many different types of plastics and, to some degree, of the type of steel or aluminum can be determined simply by sight or feel.
AppendixAprovides an excellent reference for material selection. It includes
two types of information: a compendium of the properties of the 25 materials
most often used in mechanical devices and a list of the materials used in common
mechanical devices. The 25 most commonly used materials include eight steels
and irons, five aluminums, two other metals, five plastics, two ceramics, one wood,
and two other composite materials. The properties listed include the standard
mechanical properties, along with cost per unit volume and weight. This list is
intended to serve as a starting place for material selection. Detailed information
on the many thousands of different materials available can be found in the list of
references given at the end of Appendix A. Additionally, the appendix contains
a list of materials used in common products. Since many different materials can
be used in the manufacture of most products, this list gives only those most
commonly used.
Knowledge and experience are the third influence on the choice of materials
and manufacturing processes. Limited knowledge and experience limit choices.
If only available resources can be utilized, then the materials and the processes are
limited by these capabilities. However, knowledge can be extended by including
on the design team vendors or consultants who have more knowledge of materials
and manufacturing processes, so the number of choices can be increased.

The Mechanical
Design Process
Fourth Edition
David G. Ullman
Professor Emeritus, Oregon State University

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  • Density, stress, strain and moduli

    Density
    Many applications (e.g. sports equipment, transport systems) require low weight and this depends in part on the density of the materials of which they are made.
    Density is mass per unit volume. It is measured in kg/m3 or sometimes, for convenience, Mg/m3 (1 Mg/m3 1000 kg/m3).
    The density of samples with regular shapes can be determined using precision mass balance and accurate measurements of the dimensions (to give the volume), but this is not the best way. Better is the ‘double weighing’ method: the sample is first weighed in air and then when fully immersed in a liquid of known density. When immersed, the sample feels an upwards force equal to the weight of liquid it displaces (Archimedes’ principle1). The density is then calculated as shown in Figure 4.1.




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  • Materials Engineering, Science, Processing and Design
    Michael Ashby, Hugh Shercliff and David Cebon
    University of Cambridge,
    UK AMSTERDAM


    for STEP BY STEP GUIDE solidwork simple tutorial please visit.........
    www.solidworksimpletutorial.blogspot.com

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  • The strategy: translation, screening, ranking and documentation

    Selection involves seeking the best match between the attribute profiles of the materials and processes—bearing in mind that these must be mutually compatible—and those required by the design. The strategy, applied to materials, is sketched in Figure 3.6. The first task is that of translation: converting the design requirements into a prescription for selecting a material. This proceeds by identifying the constraints that the material must meet and the objectives that the design must fulfill. These become the filters: materials that meet the
    constraints and rank highly in their ability to fulfill the objectives are potential candidates for the design. The second task, then, is that of screening: eliminating the material that cannot meet the constraints. This is followed by the ranking step, ordering the survivors by their ability to meet a criterion of excellence, such as that of minimizing cost. The final task is to explore the most promising candidates in depth, examining how they are used at present, how best to design with them, case histories of failures and a step we call documentation.


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  • Materials Engineering, Science, Processing and Design
    Michael Ashby, Hugh Shercliff and David Cebon
    University of Cambridge,
    UK AMSTERDAM


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  • The interaction between design requirements, material, shape and process

    The final stage of detailed design requires a still higher level of precision and detail, but for only one or a very few materials. Such information is best found in the data sheets issued by the material producers themselves and in detailed databases for restricted material classes. A given material (polyethylene, for instance) has a range of properties that derive from differences in the ways different
    producers make it. At the detailed design stage, a supplier must be identified and the properties of his product used in the design calculations; that from another supplier may have slightly different properties. And sometimes even this is not good enough. If the component is a critical one (meaning that its failure could, in some sense or another, be disastrous) then it may be prudent to
    conduct in-house tests to measure the critical properties, using a sample of the material that will be used to make the product itself. The process is one of narrowing
    the materials search space by screening out materials that cannot meet the design requirements, ranking those that remain and identifying the most promising choice (Figure 3.4).
    The materials input does not end with the establishment of production.
    Products fail in service and failures contain information. It is an imprudent manufacturer who does not collect and analyze data on failures. Often this points to the misuse of a material, one that redesign or re-selection can eliminate.
    The selection of a material cannot be separated from that of process and of shape. To make a shape, a material is subjected to processes that, collectively, we shall call manufacture. Figure 2.5 of Chapter 2 introduced them. The selection of process follows a route that runs parallel to that of material (Figure 3.1, right-hand side). The starting point is a catalog of all processes, which is then narrowed by screening out those that fail to make the desired shape or are incompatible with the choice of material. Material, shape and process interact (Figure 3.5). Process choice is influenced by the material: by its formability, machinability, weldability, heat treatability and so on. Process choice is influenced by the
    requirements for shape—the process determines the shape, the size, the precision and, to a large extent, the cost of a component. The interactions are twoway: specification of shape restricts the choice of material and process, but equally the specification of process limits the materials you can use and the
    shapes they can take. The more sophisticated the design, the tighter the specifications and the greater the interactions. The interaction between material, shape and process lies at the heart of the selection process. To tackle it we need a strategy.




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  • Materials Engineering, Science, Processing and Design
    Michael Ashby, Hugh Shercliff and David Cebon
    University of Cambridge,
    UK AMSTERDAM

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