Stiffness degradation of GFRP pipes under fatigue loading

This research on the stiffness of glass fiber reinforced plastics (GFRP) pipes constructed according to various types of winding patterns and under varied loading conditions, proposes a model of stiffness degradation for GFRP pipes and the parameters for a model of stiffness degradation. Based on a hoop winding GFRP pipe fatigue test, it was found that the stiffness reduction rate of the first half a million cycles is 0.67 times that of the last 500,000 cycles when maximum alternating displacement is 5 % of the diameter of the GFRP pipe. For the same type of winding GFRP pipe, the stiffness reduction rate at a maximum alternating displacement of its diameter'of 10 % and 15 % yields 1.34 and 1.59 times the stiffness reduction rate of a GFRP pipe with an alternating displacement maximum of its diameter of 5 %, respectively. For GFRP pipes of varied types of winding modes, when the maximum alternating displacement is 5 % of the diameter of a GFRP pipe, the rate of stiffness degradation of the helical winding GFRP pipe and hoop-helical combined winding GFRP pipe is 1.50 and 1.28 times that of a hoop winding GFRP pipe, respectively.