Fatigue design of CFRP strengthened steel members

Fatigue failure is brittle and sudden and is one of the main problems with steel members and connections. Carbon fiber reinforced polymer (CFRP) sheets and laminates have been shown to be effective and practical for strengthening steel under fatigue loading regardless of the existence of initial cracks. Many studies have examined the fatigue behaviors of CFRP strengthened steel, but fatigue design guides or available programs for designers and engineers are limited. Thus, based on existing design codes and guidance for pure steel under fatigue loading (e.g., Design Guide for Circular and Rectangular Hollow Section Welded Joints under Fatigue Loading and Recommendations for Fatigue Design of Welded Joints and Components), this paper proposes fatigue design guides and programs for CFRP strengthened steel structures. First, for steel without initial fatigue cracks, Classification method is adopted along with a related calculation method for obtaining the reduced stress range of steel after strengthening. Then, a classification table for hybrid CFRP-steel members is given to illustrate where to glue CFRP sheets or laminates and the correct fiber orientation. Second, for steel with initial fatigue cracks, fracture mechanics are adopted to obtain the reduced stress range. This paper considers debonding at the crack tip using the finite element method (FEM) and introduces a coefficient d to enlarge the range of the stress intensity factor (SIP). Then, a program called "EasyFatigueforFSS" (Easy Fatigue design for FRP Strengthened Steel) is developed to calculate the available life or allowable stress. Finally, typical design examples are given for reference.