Study On Fatigue Damage Evolution And Life Controllable Design Method Of FRP Cables

Steel cables in many bridges have serious corrosion under the coupling of environmental and fatigue loads.Fiber reinforced polymer(FRP)has the advantages of light weight,high strength,corrosion resistance,strong design,and can reduce the overall life-cycle cost of structures.During the last few decades,fiber-reinforced polymer(FRP)composites have become widely accepted as competitive alternatives to steel for structural retrofitting and as structural components in new constructions.The basalt fiber-reinforced polymer(BFRP)composite is an emerging FRP material.Basalt fibers are environmentally friendly and non-hazardous fibers that are produced from basalt rock by drawing fibers from melt basalt stones.In addition to the mentioned highly desirable characteristics of FRP composites,the most significant advantages of BFRP lie in its superior mechanical properties,corrosion resistance and creep rupture behavior compared with glass FRP(GFRP)as well as its low cost compared with carbon FRP(CFRP).However,although BFRP composites exhibit high performance like better fatigue and durability than traditional structural steels,it is still lack of quantitative experimental results,evaluation models and design theories for BFRP composites.Compared with structural steel,fatigue and durability properties of FRP are affected by more parameters.Various kinds of fibers and matrix,the coupling effect of environment and load and many other effects will accelerate the fatigue/creep damage of FRP structures.In order to introduce BFRP in fatigue sensitive structure components like cables,it is essential to investigate fatigue behaviors and develop new fatigue design methodology for BFRP composites.In this paper,the fatigue damage evolution and fatigue performance of BFRP are mainly studied,and the corresponding fatigue design method is established.In order to get a better understanding of the the fatigue damage evolution and performance of FRP,an in-situ scanning electron microscopy(SEM)fatigue test device is introduced and used in this paper.The whole test system can simultaneously perform the fatigue loading test and SEM observation,which allows the fatigue damage propagation observed by SEM during the fatigue loading without unloading the specimens.Considering the characteristics of anchorage zone of stress transfer and the anisotropy of FRP material,a slow transition anchorage form of composite materials was designed based on the requirements of in-situ tensile fatigue test device and FRP,to avoid anchorage failure mode.Based on the above test device and method,the fatigue characteristics and damage modes of BFRP composites under cyclic loading of 107 times(50-100 years’ design life are usually 106 times)are studied.The fatigue characteristics and damage modes of BFRP in short life cycles(105 cycles),medium long life cycles(106 cycles)and long life cycles(107 cycles)were compared,and the change of damage modes was revealed.The results show that under high stress levels(short life)the critical fiber breaking failure was the dominant damage,while the matrix cracking and interfacial debonding were main damage patterns at the low(long life)and middle(medium long life)fatigue stress level for BFRP.Due to this damage mode change,fatigue strength predicted by fatigue data less than 2×106 cycle was lower than the fatigue strength fitted by the data below the 1×1O7 cycle.The fatigue characteristics and damage modes of BFRP composites under different stress ratios(RF=0.1,0.5,0.8)were studied,and the cumulative damage rule of BFRP under different load spectra was determined.The results show that the fatigue life decreases and the fatigue life degradation rate increased with the decrease of stress ratio for examined BFRP composites.The stiffness degradation was also sensitive to different stress ratios,showing a greater stiffness loss before failure at lower stress ratio.From the SEM images,it is indicated that the micro-damage mode shift from interface debonding and matrix cracking into fiber breaking with decreasing stress ratios.However,the cumulative damage law of BFRP under different stress ratio loads can obey the linear damge cumulative law.The fatigue properties and damage evolution laws of BFRP composites under multi-field coupling environment like saltwater environment,different temperatures and fatigue were studied.Tension-tension fatigue tests of basalt fiber-reinforced polymer(BFRP)composites were carried out after exposure to a salt solution and under different environment temperatures.From the SEM images of fatigue damage for different aging times,it can be seen that the fiber/matrix interface region becomed a controlling factor in the fatigue behavior with early interface debonding occurring in the aged specimens.Based on the fatigue damage modes of BFRP(matrix cracking and interface debonding are main damage patterns at the low and middle fatigue stress level for BFRP),BFRP fatigue life improvement methods based on resin toughening and interface modification was put forward.The effects of toughening and interfacial modification on the fatigue life and damage patterns of BFRP were investigated.It was found that the ductility of the toughened vinyl resin was greatly improved,and the cracking of the toughened vinyl BFRP was obviously reduced under static and fatigue.Although the static strength decreases after toughening,the fatigue life increase rate increased.After the interface modification,the strength of interface to a certain extent increased and reduced the fiber pulling-out during the fatigue failure.Although the static strength of epoxy based BFRP decreases after the interface modification,the fatigue life increase rate also increased.In the same low stress level under long fatigue life,the fatigue life of BFRP specimen can be larger after toughened or modified than that of control group.Finally,based on the above test and damage mode data,a series parallel model was applied to study the reliability based static and fatigue performance prediction methods from impreganated yarn to FRP single cable to FRP cable.The fatigue strength reduction coefficient based on medium long life(106 cycles),long life(107 cycles)and probability distribution was established.Meanwhile,considering the FRP fatigue life improvement methods and life cycle cost,a life controllable design theory of FRP cable structures was established.