Research On Residual Strength Model And Life Prediction Method Of Fiber Reinforced Polymer

Fiber reinforced polymer(FRP)has been widely applied in many important industrial fields,such as aerospace,automobile,wind power,ship,construction and so on,due to its advantages of light weight,high specific strength and stiffness,corrosion and impact resistance,strong designability of structure performance and easy to be integrated into shape.However,due to the influence of alternating loads and environmental factors in the service process of FRP,a variety of damage modes will occur and different damage modes are induced and coupled with each other,which makes it difficult to reveal its strength degradation law and brings great challenges to accurate evaluation of fatigue life of FRP.In this dissertation,the residual strength model and life prediction method of FRP under different loading conditions are studied.The main research contributions and innovative outcomes are summarized as follows:(1)A residual strength model of FRP under constant amplitude cyclic loading is developed.To reveal the strength degradation law of FRP under constant amplitude cyclic loading,firstly,the defects and shortcomings of traditional residual strength models in engineering applications are analyzed and discussed.Then,two typical interference models are developed based on interference analysis theory and Weibull distribution,that is,loading times— fatigue life interference model and cyclic stress—residual strength interference model,and the intrinsic correlation between them is studied.Finally,a probabilistic model of residual strength of FRP under constant amplitude cyclic loading is derived,and the fatigue test data of carbon fiber/epoxy polymer are used to verify the validity of the proposed model.The results show that the proposed model has rigorous theoretical basis and higher calculation accuracy compared with traditional residual strength models,which can not only reduce the dependence on experimental data and subjective experience,but also reflect the inherent dispersion of residual strength of FRP.(2)A residual strength model of FRP under uncertain constant amplitude cyclic loading is developed.Due to the existing studies mainly focus on fatigue reliability assessment and fatigue damage accumulation,the strength degradation of FRP under uncertain constant amplitude cyclic loading has not yet been involved.To solve this issue,firstly,a mapping relationship between uncertain constant amplitude cyclic stress and fatigue life is studied,and a method for inferring probability distribution of fatigue life of FRP based on probability distribution of cyclic stress is presented.Then,a probabilistic model of residual strength of FRP is proposed based on an assumption that residual strength and remaining life depend on the same fatigue damage state in composites.Finally,the proposed model is applied to analyze the strength degradation law of carbon fiber/epoxy polymer under uncertain constant amplitude cyclic loading.The results show that the proposed model can not only characterize the irreversibility of residual strength in the same individual,but also reflect the randomness of residual strength among different individuals.(3)A residual strength model of FRP under random cyclic loading is developed.The application scope of traditional residual strength theory is explored and discussed,and it is pointed out that most of the existing residual strength models are only applicable to simple loading conditions,such as constant amplitude cyclic loading or multi-stage constant amplitude cyclic loading,which are not adequate to characterize the strength degradation law of FRP under random cyclic loading.To solve this issue,firstly,the randomness of external loads is considered,and a fatigue life model of FRP under random cyclic loading is derived based on Miner's rule and full probability formula.Then,the dispersion of material properties is considered,and a probabilistic model of residual strength of FRP under random cyclic loading is proposed based on the probability and statistics theory and the same damage state assumption.Finally,the proposed model is applied to analyze the strength degradation laws of glass fiber/epoxy polymer with different lamination modes.The results show that the proposed model is not only simple in form and convenient for engineering application,but also can reflect the combined effects of randomness in external loads and dispersion in material properties on the evolution law of residual strength of FRP.(4)A residual strength model of FRP under the coexistence of fatigue load and natural aging is developed.To reveal the strength degradation law of FRP under the coexistence of fatigue load and natural aging,firstly,a comprehensive environmental factor is introduced to quantify the combined effects of temperature,humidity and light radiation on natural aging process of FRP.Then,a residual strength model of FRP under natural aging environment is presented,and the natural aging test data of glass fiber/epoxy polymer are used to verify the validity of the presented model.Finally,considering the effect of fatigue load and combining a degradation path model with “fast?slow? fast” trend,a residual strength model of FRP under the coexistence of fatigue load and natural aging is proposed,and the strength degradation laws of glass fiber/epoxy polymer at different test sites are analyzed.The results show that fatigue load has a damage effect on residual strength of FRP,while natural aging has a double-edged effect on residual strength of FRP: it has an enhancement effect in the early stage and a damage effect in the middle and late stages.(5)A life prediction method of FRP under the combined effects of stress amplitude and mean stress is proposed.To accurately predict the fatigue life of FRP under random cyclic loading,firstly,a generalized S-N curved surface that characterizing fatigue properties of FRP is presented based on stress-life equation and Goodman constant life diagram model.Then,the traditional Miner's rule is modified to reflect the combined effects of stress amplitude and mean stress on fatigue damage accumulation law of FRP.Finally,the randomness of stress amplitude and mean stress is considered,a life prediction method of FRP under random cyclic loading is proposed based on the modified Miner's rule and full probability formula,and the test data of glass fiber/epoxy polymer are used to verify the validity of the proposed method.The results show that the laying angle and order of fibers have significant influence on fatigue life of FRP,and matrix plays a major role in fixing fibers and transferring loads between fibers.