For those of you that like to work with numbers, we have compiled the following chart to illustrate the salient features of various constructions of reinforcements. While you should not consider these figures as design parameters, they are ball-park numbers applicable to both polyester and epoxy hand lay-ups. All of the numbers refer to the resin/reinforcement composite and not the reinforcement by itself. Keep in mind that if a reinforcement has a tensile strength of 100,000 psi, when combined in a 50/50 ratio with a resin of perhaps 8,000 psi tensile strength, the composite laminate will have a tensile strength of about 54,000 psi.

Column 1 - specifies the type of reinforcement and the orientation of the fibers relative to the load.

Column 2 - gives the density of the composite in pounds per cubic inch.

Column 3 - specifies the ultimate tensile strength in pounds per square inch. This is the static load required to damage the composite. Fiberglass will generally withstand an infinite lifetime of a dynamic (alternating) tensile load of 30%of its ultimate tensile strength. One might use this as a conservative design value.

Column 4 - specifies the tensile modulus.

Column 5 - provides the specific tensile modulus (tensile modulus divided by density) in units of millions of inches. We have included this column as a guide for those wishing to select a material based upon inherent stiffness to weight ratio.

The last three entries of the chart are included so that you may compare the properties of the various composites with those of more conventional materials.

Material Density
(lb/cu in)
Tensile Strength
Modulus 106
Specific Modulus
Veil Surfacing Mat .045 8,000 .7 15
Chopped Strand Mat .058 22,000 1.4 24
Woven Roving .060 40,000 2.0 33
10 oz. Fabric .060 33,000 2.4 40
Tooling Fabric .065 31,000 2.0 31
Kevlar® .050 49,000 5.0 100
Graphite, parallel to fiber .057 170,000 18.0 300
For Comparison
Steel .280 160,000 22.0 78
Aluminum .100 70,000 10.0 100
Douglas Fir, parallel .020 8,000 1.5 75