Chapter 46 Accelerated Creep Testing of CFRP with the Stepped Isostress Method J.D. Tanks, K.E. Rader, and S.R. Sharp Abstract 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. Keywords Creep • Creep rupture • Accelerated testing • CFRP • Stepped isostress method 46.1 Introduction Due to its high strength-to-weight ratio and resistance to galvanic corrosion, carbon fiber reinforced polymers (CFRP) are becoming more common in structural components found in aerospace, automotive, pipeline, wind energy, and civil engineering industries as an alternative to steel, aluminum, and titanium alloys. However, as a polymeric material, CFRP is susceptible to higher levels of time-dependent deformation (creep) and failure (creep rupture) at lower temperatures than most metals, requiring in-depth characterization of long-term behavior. Time is too limited for running creep tests at realistic service-life durations, meaning accelerated tests are necessary [1]. According to the time-temperature-stress-superposition principle (TTSSP), the creep of viscoelastic materials is dependent upon time, temperature, and stress in such a way that a time-equivalence can be determined [2–5]. The following Boltzmann integral represents the TTSSP for variable stress: ε tð Þ¼C0σ þð t 0 Ct t τ ð Þ dσ τð Þ dτ dτ ð46:1Þ where ε(t) is creep strain, C0 is instantaneous compliance, σ is applied stress, Ct is creep compliance, andτ is a point in time t when the stress σ(τ) changes. Variable temperature can be incorporated the same way. From this principle an accelerated creep testing methods has been developed, the stepped isothermal method (SIM), and applied with great success to aramid yarns [6–8]. Unlike many TTSSP approaches that use different specimens for each reference temperature, the SIM uses a single specimen for a given reference temperature and the temperature is stepped up in uniform increments until material failure. More recently, the stepped isostress method (SSM) was developed and showcased at Cambridge University, first applied to aramid yarns [7–9] and then to semicrystalline thermoplastics (polyamide-6) [1]. The basic principle of the SSM is the same as the SIM, but instead temperature is held constant and stress is stepped up until material failure. This captures the stress history of a single specimen rather than extrapolate across stress levels between different specimens. This paper describes the first application (to the authors’ knowledge) of the SSM to study creep deformation and creep rupture of unidirectional CFRP laminates; preliminary results are presented here and much more exhaustive experimentation is J.D. Tanks (*) • K.E. Rader • S.R. Sharp Virginia Center for Transportation Innovation and Research, 530 Edgemont Road, Charlottesville, VA 22903, USA e-mail: jdt5ca@virginia.edu #The Society for Experimental Mechanics, Inc. 2016 C. Ralph et al. (eds.), Mechanics of Composite and Multi-functional Materials, Volume 7, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-21762-8_46 397
RkJQdWJsaXNoZXIy MTMzNzEzMQ==