Experimental Study On Fatigue Property Of Ultra High Performance ECC And High Strength Lightweight ECC Under Normal And Chloride Penetration Environment

In recent years,the application of engineered cementious composite(ECC)in coastal traffic infrastructure and other structures subjected to environmental erosion and cyclic fatigue loading has become a hot issue,but the strength and toughness of traditional ECC is limited(anti-The compressive strength is about 50 MPa,the tensile strength is less than 5 MPa,and the ultimate tensile strain is less than 5%.Therefore,it is limited in the application of coastal traffic infrastructure structures that need to withstand high stress.By carrying out continuous optimization design of ECC ratio and performance,the research team used ultra-high molecular weight polyethylene(PE)fiber with high modulus of elasticity and strong fiber strength to design ultra-high performance strain hardening cement-based materials(Ultra)-high performance engineer cementious composite(UHP-ECC)and high strength light-weight engineer cementious composite(HSLW-ECC).The former has a compressive strength of nearly 120 MPa,a tensile strength of 12 MPa,and a ultimate tensile strain of 8% or more;the latter has a compressive strength of approximately 60 MPa,a tensile strength of 8 MPa,and a ultimate tensile strain of approximately 8%.The emergence of ultra-high-performance ECC,on the one hand,is beneficial to improve the fatigue performance of the transportation infrastructure,and on the other hand,it can be used for the repair of coastal structures,thereby greatly improving the durability of the structure.In this paper,the fatigue performance and degradation mechanism of ultra-high performance ECC and the fatigue performance after chloride salt erosion are studied.The main contents include:(1)The tensile fatigue properties of UHP-ECC under different stress levels were tested,that is,tensile fatigue properties at different stress levels,after different fatigue times,and single crack tensile tests after fatigue.By comparing the stress-strain relationship curve,fracture section,crack form and fiber failure morphology,it can be seen that the number of fatigue cycles has a significant effect on the degradation mechanism of UHP-ECC specimens,and the toughness decreases with the increase of the number of fatigue cycles..Based on the pseudo strain hardening criterion(PSH),the toughness after different tensile fatigue loading times was evaluated.The results show that with the increase of fatigue loading times,the PSH value is significantly reduced,and the reduction rate reaches 51%.(2)The bending fatigue properties of UHP-ECC and HSLW-ECC under different stress levels were studied experimentally.Comparing the fatigue life under different stress levels,the mid-span deflection and the deformation of the bottom pure bending section,the mid-span deflection during the bending fatigue loading of the specimen increases with the increase of the fatigue stress level,and its development has gone through three stages.That is,rapid deformation growth(Phase I),deformation stability(Phase),and instability failure(Phase III).The deformation of the second stage is mainly caused by the expansion of the original crack and the generation of new cracks.The fatigue life depends mainly on the length of the second stage and the magnitude of the deformation,which is closely related to the number and width of the cracks.Secondly,using Digital Image Correlation(DIC),the crack propagation process under static load and stress level S=0.8 is observed,and the crack development map is obtained.Finally,the environmental SEM was used to observe and analyze the fiber morphology at different stress level sections,and the fatigue life S-N curve and fatigue life prediction equation were established.The fatigue damage law and failure mode of UHP-ECC under different cyclic loading were obtained.The fractured sections after cyclic loading of the specimens showed different fracture characteristics.(3)Based on the study of the bending fatigue properties of UHP-ECC and HSLW-ECC,the fatigue life equations of UHP-ECC and HSLW-ECC are established.The current research has adopted the development stage model with inflection point to predict the fatigue life of UHP-ECC.However,the data to accurately quantify the inflection point is very limited,and all existing development stage models are not continuous at the inflection point.Therefore,this paper proposes a continuous nonlinear equation to describe the S-N curve of UHP-ECC.The results show that the established fatigue life model has a good prediction effect.In this paper,the linear regression method is adopted for the lightweight ECC fatigue life equation.The fatigue life equation of HSLW-ECC is consistent with the fatigue equation in the existing research,but the bending strength is twice as high as the 6.4 MPa in the existing research.,indicating that HSLW-ECC has more excellent fatigue performance.(4)The fatigue performance and chloride ion corrosion resistance of UHP-ECC and HSLW-ECC under chloride attack were tested.The test showed that the free chloride ion content showed different characteristics.As a whole,with the increase of penetration depth,The free chloride ion content drops significantly.However,at the same penetration depth,the free chloride ion content of UHP-ECC increases first and then decreases with the increase of soaking time.The free chloride ion content of HSLW-ECC always increases,which is compared with the self-porosity of HSLW-ECC.Big related.With the increase of immersion time,the number of cracks in the pure bend section of UHP-ECC and HSLW-ECC caused by direct static load and fatigue loading test increased,and the toughness increased.In addition,the mode of fatigue failure is related to the number of cracks in the pure curved section.The more the number of cracks in the pure curved section,the larger the proportion of the area of the fiber extraction area to the area of the entire section,and the stronger the toughness of the test piece.