Methods for the design analysis in the past for plastics were based on models of material behavior relevant to traditional metals, as for example elasticity and plastic yield. These principles were embodied in design formulas, design sheets and charts, and in the modern techniques such as those of CAD using finite element analysis (FEA). Design analyst was required only to supply appropriate elastic or plastic constants for the material, and not question the validity of the design methods. Traditional design analysis is thus based on accepted methods and familiar materials, and as a result many designers have little, if any, experience with such other materials as plastics, wood, and glass.

Using this approach it is both tempting and common practice for certain designers to treat plastics as though they were traditional materials such as steel and to apply familiar design methods with what seem appropriate materials constants. It must be admitted that this pragmatic approach does often yield acceptable results. However, it should also be recognized that the mechanical characteristics of plastics are different from those of metals, and the validity of this pragmatic approach is often fortuitous and usually uncertain.

It would be more acceptable for the design analysis to be based on methods developed specifically for the materials, but this action will require the designer of metals to accept new ideas. Obviously, this acceptance becomes easier to the degree that the newer methods are presented as far as possible in the form of limitations or modifications to the existing methods discussed in this book.

Table 4.1 provides examples of mechanical property data of different materials (GN/m2 = kPa). A review is presented concerning the four materials in Table 4.1, where the exact values used are unimportant.

Higher performance types could be used for the metals and plasdcs but those in this table offer a fair comparison for the explanation being presented. This review identifies the need for using design analysis methods appropriate for plastics. It also indicates the uncertainty of using with plastics methods derived from metals, and demonstrates the dangers of making generalized statements about the relative merits of different classes of materials.

Mechanical properties of materials







Mild steel



Tensile modulus (£) 106 GN/m2 (psi)


210 (30)

1.5 (0.21)

15 (2.2)

Tensile strength (a) 103 MN/m2 (psi)

400 (58)

450 (65)

40 (5.8)

280 (40.5)

Specific gravity (5)





Based on the usual data on metals, they are much stiffer and significandy stronger than plastics. This initial evaluation could eliminate the use of plastics in many potential applications, but in practice it is recognized by those familiar with the behavior of plastics that it is the stiffness and strength of the product that is important, not its material properties.

The proper approach is to consider the application in which a material is used such as in panels with identical dimensions with the service requirements of stiffness and strength in flexure. Their flexural stiffnesses and strengths depend directly on the respective material's modulus and strength. Other factors are similar such as no significantly different Poisson ratios. The different panel properties relative to stiffness and strength are shown in Fig. 4.2. The metal panels are stiffer and stronger than the plastic ones because the panels with equal dimensions use equal volumes of materials.

By using the lower densities of plastics it allows them to be used in thicker sections than metals. This approach significantly influences the panel's stiffnesses and strengths. With equal weights and therefore different thickness (f) the panels are loaded in flexure. Their stiffnesses depend on (Epand their strength on (<rt2) where E and a are the material's modulus and its strength. For panels of equal weight their relative stiffnesses are governed by (E/s3) and their relative strengths by (a/s2) where s denotes specific gravity. As shown in Fig. 4.3, the plastics now are much more favorable. So depending on how one wants to present data or more important apply data either Figs. 4.2 or 4.3 or

Figure 4.2 Open bar illustrations represent stiffness and shaded illustrations represent strength with panels having the same dimensions

2 loo


Steel i

200 5 o

Figure 4.3 Open bar illustrations represent stiffness and shaded illustrations represent strength with panels having the equal weights








Sf l

Table 4.1 is used. Thus the designer has the opportunity to balance out the requirements for stiffness, strength, and weight saving.

Recognize that it is easy to misinterpret property data and not properly analyze the merits of plastics. No general conclusions should be drawn on the relative merits of various materials based on this description alone. In comparing materials in Table 4.1 a designer can easily obtain different useful data. As an example the GRP panel has 2.4 times the thickness of a steel panel for the same flexural stiffness. It has 3.6 times its flexural strength and only half its weight. The tensile strength of the GRP panel would be 50% greater than that of the steel panel, but its tensile stiffness is only 17% that of the steel panel.

Similar remarks could be made with respect to various materials' costs and energy contents, which can also be specified per unit of volume or weight. General statements about energy content or cost per unit of stiffness or strength, as well as other factors, should be treated with caution and applied only where relevant. If these factors are to be treated properly, they too must relate to final product values that include the method of fabrication, expected lifetime, repair record, and in-service use.

This review shows what the veteran plastic designer knows; that plastic products are often stiffness critical, whereas metal products are usually strength critical. Consequently, metal products are often made stiffer than required by their service conditions, to avoid failure, whereas plastic products are often made stronger than necessary, for adequate stiffness. In replacing a component in one material with a similar product in another material is not usually necessary to have the same product stiffness and strength.

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