Chapter 2

MATERIALS AND PROCESSES

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Other empirical evidence for this theory comes from the fact that fibers of any material made in very small diameters exhibit much higher tensile strengths than would be expected from stress-strain tests of larger samples of the same material. Presumably, 2

the very small cross sections are approaching a “purer” material state. For example, it is well known that glass has poor tensile strength. However, small-diameter glass fibers show much larger tensile strength than sheet glass, making them a practical (and inexpensive) fiber for use in boat hulls, which are subjected to large tensile stresses in use. Small-diameter fibers of carbon and boron exhibit even higher tensile strengths than glass fiber, which explains their use in composites for spacecraft and military aircraft applications, where their relatively high cost is not a barrier.

 

 

2.8

SELECTING MATERIALS

One of the most important design decisions is the proper choice of material. Materials limit design and new materials are still being invented that open new design possibilities. It would help if there were a systematic way to select a material for an application. M. F. Ashby has proposed such an approach that plots various material properties against one another to form “materials selection charts.”[3] Materials can be roughly divided into six classes, metals, ceramics, polymers (solid or foam), elastomers, glasses, and composites (which include wood). Members of these classes and sub-classes tend to cluster together on a plot of this type.

Figure 2-23 shows such a chart that plots Young’s modulus against density, which is called specific stiffness. By drawing lines of constant slope on such a chart, one can see which materials possess similar properties. A line of specific stiffness E /  = C has been drawn in color on Figure 2-23 and shows that some woods have equivalent specific stiffness to steel and some other metals. The line also passes through the lower range of the engineering composites’ “bubble” indicating that fiberglass (GFRP) has about the same specific stiffness as wood and steel, while the nonreinforced thermoplastics such as nylon and polyester have lower specific stiffness. So if you seek the stiffest/lightest material, you want to move up and to the left on the chart. Other lines are shown that have slopes equal to En /  = C where n is a fraction such as 1/2 or 1/3. These represent loading situations, such as beams in bending, for which the parameter of interest is a nonlinear function of specific stiffness. Since the chart is a log-log plot, ex-ponential functions also plot as straight lines allowing simple comparisons to be made.

Figure 2-24 shows a chart of strength versus density (called specific strength) for a number of materials. In this chart, the particular material strength used varies with the material depending on its character. For example, ductile metals and polymers show their yield strength, brittle ceramics their crushing compressive strength, and elastomers their tear strength. The vertical elongation of a material’s “bubble” indicates the range of strength values that can obtain due to thermal or work hardening, alloying elements, etc. The colored line drawn on the chart represents a particular value of specific strength or  /  = C and shows that the strength-to-weight ratio of some woods are as good as high-strength steel and better than most other metals. It should be no surprise that wood is a popular material in building construction. Note also the high specific strength of engineering ceramics. Unfortunately, their tensile strengths are at best only about 10%

of these compressive strengths, which is why you seldom see them used in structures where tensile stresses are commonly encountered.

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64

MACHINE DESIGN -

An Integrated Approach

 

 

2

 

E / = C

 

F I G U R E 2 - 23

Young's Modulus Plotted Against Density for Engineering Materials (From Fig. 4-3, p. 37 in M. F. Ashby, Materials Selection in Mechanical Design, 2ed, Butterworth-Heinemann 1999, with permission) Ashby’s book[3] is a very useful reference for the practicing engineer. It has doz-ens of charts of the type shown here that plot various properties against one another in a manner that enhances their comparison and develops good understanding.

 

 

2.9

SUMMARY

There are many different kinds of material strengths. It is important to understand which ones are important in particular loading situations. The most commonly measured and

 

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