Accelerated Creep Testing of CFRP with the Stepped Isostress Method

Numerous accelerated methods for testing long-term viscoelastic properties of fiber reinforced polymer (FRP) composites such as creep and relaxation have been developed in order to reduce the time needed to characterize the material behavior. Most of them are based on the time-temperature-stress superposition principle (TTSSP), or some variation thereof, which involves the manipulation of temperature, applied stress, or both as a way to reduce the testing duration. This paper reports the application of a test called the stepped isostress method (SSM) to study tensile creep of CFRP laminates used in rehabilitating prestressed concrete structures, which experience sustained loads above 60 % of ultimate strength for decades. The SSM employs a load-stepping approach, typically with three to five steps for a single specimen resulting in creep rupture. A key feature of this method is the independence of test results on step size or duration. A simple, repeatable SSM protocol is presented and it is shown that this method is also useful for studying creep rupture of CFRP.