Fiber Reinforced Plastic Piping: Actual versus Idealized Envelope

Fiber Reinforced Plastic (FRP) piping is often used as a material for industrial piping. Unlike steel, FRP is an anisotropic material which can accommodate the most load when the loading is orientated with the direction of the fibers. Consequently, FRP piping is assessed by design codes such as ISO 14692 using a stress envelope rather than a single value, as the permissible hoop stress depends on the axial stress and vice versa.

As shown in the image below there are both ideal and actual envelopes. The testing required to determine the actual envelope is impractical as it requires lots of test points with multi-directional loading (different combinations of hoop and axial stresses). For this reason, the idealized envelope is used in ISO 14692 for code conformance checks with FRP piping.

The idealized envelope below can be determined based on 2 data points which are relatively easy to obtain through tests. Point 1 is based on a pressure test where the ratio between hoop and axial stress is 2:1 and point 2 is the result of an axial load test.

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