Fatigue life and damage behaviors of GFRP pipes under circumferential bend loading

The fatigue life and damage behaviors of glass fiber reinforced polymer (GFRP) pipes were investigated under periodically varying deformation during circumferential bending fatigue test. The fatigue life and ring stiffness decline of GFRP pipes were measured under different initial (average) deformation ratios <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <m:mrow> <m:mover accent="true"> <m:mrow> <m:mtext> </m:mtext> <m:mi>λ</m:mi> </m:mrow> <m:mo>‾</m:mo> </m:mover> </m:mrow> </m:math> 0.17emλ of 6.5 %, 9.0 %, and 11.5 %, respectively, and different loading frequencies of 2 Hz or 3 Hz. Comprehensive analyses on stiffness degradation, typical damage morphologies, variations of strain and temperature under different fatigue cycles have been conducted by utilizing digital image correlation (DIC), infrared thermography (IRT) and optical microscope. It is observed that the entire fatigue process of GFRP pipes under circumferential bending roughly includes two distinct stages. As <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <m:mrow> <m:mover accent="true"> <m:mi>λ</m:mi> <m:mo>‾</m:mo> </m:mover> </m:mrow> </m:math> λ ̄ = 6.5 %, 9.0 % and higher loading frequencies are applied on GFRP, the ring stiffness of GFRP pipes reduces more apparent, accompanying with more significant energy dissipation, decrease in strain and temperature rise. The higher <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <m:mrow> <m:mover accent="true"> <m:mi>λ</m:mi> <m:mo>‾</m:mo> </m:mover> </m:mrow> </m:math> λ ̄ and higher loading frequency accelerate delamination between the sandwich layer and the structural layer of the GFRP pipe, and lead to the decrease in the fatigue life. This study can provide valuable references for the designs and applications of GFRP pipes under circumferential bending fatigue loads.