Designing with Composites

Abstract

Understanding how to design with composites, especially fiber reinforced composites, is very important because composite materials do not represent just another new class of materials. Although there have been, over the years, ongoing efforts by researchers to improve the properties of different materials such as new alloys, composite materials, especially fiber reinforced composites, represent a rather radical departure. Schier and Juergens (1983) analyzed the design impact of composites on fighter aircraft. The authors echoed the sentiments of many researchers and engineers in making the following statement: “Composites have introduced an extraordinary fluidity to design engineering, in effect forcing the designer-analyst to create a different material for each application….” A single component made of a laminated composite can have areas of distinctively different mechanical properties. For example, the wing-skin of an F/A-18 airplane is made up of 134 plies. Each ply has a specific fiber orientation and geometric shape. Computer graphics allow us to define each ply “in place” as well as its relationship to other plies. The reader can easily appreciate that storage and transmission of such engineering data via computer makes for easy communication between design engineers and manufacturing engineers. In this chapter, we discuss some of the salient points in regard to this important subject of designing with composites.

Problems

• 14.1. Following are the data for a 60 % V _f , unidirectionally reinforced, carbon fiber/epoxy composite:

  $$\begin{array}{llllllll} {\hbox{Longitudinal}}\,{\hbox{tensile}}\,{\hbox{strength}} = 1,200\,{\hbox{MPa}} \\{\hbox{Longitudinal}}\,{\hbox{compressive}}\,{\hbox{strength = 1,000}}\,{\hbox{MPa}} \\{\hbox{Transverse}}\,{\hbox{tensile}}\,{\hbox{strength = 50}}\,{\hbox{MPa}} \\{\hbox{Transverse}}\,{\hbox{compressive}}\,{\hbox{strength = 250}}\,{\hbox{MPa}} \end{array} $$

  Calculate F ₁₁, F ₂₂, F ₆₆, F ₁, F ₂, and $F_{{12}}^{*}$. Take $F_{{12}}^{*} = - 0.5$. Compute the focal points plot the failure envelope for this composite.

• 14.2. For a carbon/epoxy composite, the strength parameters are:

  $$ \begin{array}{cccccc} {F_{{11}}}{\left( {\hbox{GPa}} \right)^{{ - 2}}} = 0.45 \\{F_{{22}}}{\left( {\hbox{GPa}} \right)^{{ - 2}}} = 101 \\{F_{{12}}}{\left( {\hbox{GPa}} \right)^{{ - 2}}} = - 3.4 \\{F_{{66}}}{\left( {\hbox{GPa}} \right)^{{ - 2}}} = 215 \\{F_1}{\left( {\hbox{GPa}} \right)^{{ - 1}}} = 0 \\{F_2}{\left( {\hbox{GPa}} \right)^{{ - 1}}} = 21. \end{array} $$

  Compute the off axis uniaxial strengths of this composite for different θ and obtain a plot of σ _x vs. θ.