| In recent years,with the increasing pressure of global energy conservation and emission reduction,countries have formulated strict laws and regulations on passenger vehicle fuel consumption.Lightweight and intelligent has become a new trend in the technological development of the automobile industry.With the use of a large number of lightweight materials,the materials used in body-in-white gradually developed from the steel plate application stage to the multi-material mixed application stage.As an advanced composite material,Carbon Fibre Plastics(CFRP)is an important part of multi-material body,and its mechanical properties have a significant impact on the body strength.However,CFRP matrix will be affected by hygrothermal in the environment during service,which will produce aging effect and reduce its interlaminar mechanical properties.In addition,the acceleration effect of load on hygrothermal aging is often another important factor affecting the strength design.The hygrothermal aging under load will further degrade the mechanical properties of CFRP interlaminar.Therefore,it is an effective way to accurately predict the residual strength after aging to study the aging failure mechanism of composite materials under the action of hygrothermal and quantitatively evaluate the structural mechanical properties change caused by aging,which has excellent practicability and important engineering practical significance.In this paper,based on the National Natural Science Foundation of China project"Research on aging Life Prediction Method of Adhesive Structure for New Material Body(51775230)",a test method of interlaminar mechanical properties of CFRP under complex stress states is proposed,that is,the self-designed Arcan experimental device is used to load CFRP under different stress states.The effects of temperature,humidity and load on the mechanical properties of CFRP interlaminar under complex stress state were studied.The synergistic mechanism of single factor and multi-factor was analyzed.The variation of residual strength of CFRP interlaminar under different stress state with aging time was summarized.An interlaminar failure strength prediction method of CFRP after aging based on chemical characteristics was established.The specific content can be divided into the following five aspects:(1)The effect of different service temperatures on the mechanical properties of CFRP layers under complex stress state was studied.Five temperature measuring points were selected in the service temperature interval of the vehicle,and the self-designed Arcan test device was used to conduct quasi-static tensile tests under five stress states at each temperature measuring point.The effect of temperature on the interlaminar mechanical properties of composite structures under different stress states was studied.Combined with differential scanning calorimetry(DSC)test and electron microscope analysis,the T_g and failure mode of composite materials were analyzed,and the macroscopic failure mechanism of composite structures under different stress states was discussed.Finally,response surface method is used to establish the secondary stress criterion based on the full service temperature under complex stress state.(2)The mechanical behavior of CFRP interlaminar mechanical properties after temperature aging was studied.Based on the research results in Chapter 2,80℃was selected as the dangerous temperature to evaluate the high-temperature residual strength of CFRP samples under different stress states after aging.FTIR and DSC chemical test methods were used to study the chemical changes of functional groups and _gT of CFRP before and after aging.Combined with the macroscopic and microscopic failure section analysis,the interlaminar failure modes of CFRP under different stress states and test temperatures after temperature aging were discussed,and the action mechanism of high temperature aging on the interlaminar mechanical properties of CFRP was revealed.(3)The synergistic effect of temperature and humidity on the residual strength of CFRP at room temperature and high temperature under complex stress state were studied.The chemical test methods such as FTIR and DSC were used to compare the material components andT_g of CFRP before and after hygrothermal aging.The failure mechanism and action mechanism of hygrothermal aging were discussed in combination with fracture and failure mode.The weibull distribution law based on statistics was established for hygrothermal aging test data to evaluate the probability distribution of aging damage more intuitively.(4)The synergistic effect of hygrothermal environment and load on the mechanical properties of CFRP interlaminar was studied.By means of chemical analysis,the action mechanism of load on hygrothermal aging was studied.Combined with macroscopic fracture morphology observation and microscopic fracture analysis,the influence of aging on the interlaminar failure mode of CFRP under different stress states was discussed.The multi-factor variance analysis was carried out on the hygrothermal and load synergistic aging data to quantitatively study the influence of various environmental factors on the mechanical properties of CFRP interlaminar during synergistic aging.(5)Based on the previous study on the aging mechanism of CFRP interlaminar,an error back-propagation neural network algorithm based on average influence value is proposed to predict the failure strength of CFRP interlaminar after aging.The artificial accelerated aging test of CFRP samples was carried out in the hygrothermal cycle aging environment which was closest to the actual application condition during vehicle service.The function of residual strength of CFRP-Arcan specimens with aging time was established.At the same time,FTIR chemical analysis method was used to test the infrared spectrum corresponding to the aging period.Finally,MIV-BP algorithm was used to analyze the correlation between the residual strength and the change law of chemical properties,so as to achieve quantitative prediction of the residual strength between interlaminar of CFRP-Arcan samples by measuring the chemical properties of CFRP resin matrix.The effect mechanism of chemical properties on interlaminar failure strength is revealed,and the equivalent quantitative relationship between artificial accelerated aging and natural aging is established effectively. |
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