Shape memory alloy-carbon fiber reinforced polymer system for strengthening fatigue-sensitive metallic structures

This paper provides an overview of the basic elements, course of development, experimental evaluation, and numerical simulation of a thermally activated shape memory alloy (SMA) and carbon fiber reinforced polymer (CFRP) composite system for fatigue repair or retrofit of metallic structures. Nickel titanium niobium (NiTiNb) SMA wires, which are able to generate 400 MPa recovery stress upon thermal loading and maintain that stress level at a wide range of temperatures, was adopted to apply compressive stresses near the crack. Monotonic bond behavior of single and multiple SMA wires to CFRP was investigated; the debonding onset load and maximum capacity were quantified. The fatigue behavior of patches consisting of multiple wires bonded to CFRP was studied. Results indicated that the system could maintain 80% of the recovery stress, after up to 2 million load cycles, so long as the maximum applied stress was below the debonding onset level. A fatigue strengthening system, using such multiple SMA wire system as underlay and CFRP patch as overlay, was applied to fatigue sensitive steel plates for fatigue life improvement evaluation. The average fatigue life of the patched steel plates was over 26 times longer than that of the unpatched plates tested at the same load range. Finally, a numerical framework was developed to simulate the fatigue crack growth in steel plates patched with such strengthening system and was validated by the experimental data. The findings suggest that the proposed system could be a promising alternative to traditional techniques for fatigue crack repair.