Influence Of Hygrothermal Environment On The Mechanical Properties Of CFRP-Al Honeycomb Adhesively Sandwich Structures

Using advanced materials to achieve lightweight is one of the effective ways to solve the current problems of automobile energy consumption and pollution.CFRP(Carbon Fiber Reinforced Plastic)has good lightweight effect with high specific stiffness and strength.The CFRP-Al honeycomb adhesively structure can be used on vehicle body due to its efficient lightweight performance,energy absorption and shock resistance.It can effectively improve the stiffness of the structure without significantly increasing the weight.During the long-term service,the mechanical property of CFRP-Al honeycomb sandwich bonding structures will inevitably be affected by high temperature and humidity.CFRP and adhesive in CFRP-Al honeycomb sandwich bonding structures are sensitive to temperature and humidity.The change of the service environment can affect its mechanical properties.At the same time,long-term hygrothermal environment may lead to the performance deterioration of the CFRP-Al honeycomb sandwich bonding structure,which may seriously threaten the safety of the structures.Although scholars have carried out relevant researches on the performance changes of CFRP bonding structures under severe environment,due to the complex composition of the sandwich structure and many factors affecting its overall mechanical properties,there is still a lack of sufficient research to clarify the degradation mechanism of each component(mainly including the influence mechanism of the degradation of CFRP and adhesive)on the overall mechanical properties of the CFRP-Al honeycomb sandwich bonding structure after aging.Therefore,this paper studied the effects of temperature and humidity alone and coupled on the mechanical properties of sandwich structures,quantitatively analyzed the effects of changes in the mechanical properties of CFRP and adhesive on the overall mechanical properties of the sandwich bonding structures after different aging periods.Finally,the progressive damage failure prediction model of CFRP-Al honeycomb sandwich bonding structure after hygrothermal aging was established with great significance.This paper takes the CFRP-Al honeycomb bonding structure as research object.Investigate the effect of temperature and humidity on various mechanical properties of CFRP and adhesive.Analyze the influence of the mechanical property degradation of each component on the overall mechanical properties of the sandwich structure.Reveal the failure mechanism and aging law of the hygrothermal aged sandwich structure under complex stress conditions.Based on the mechanical property parameters of CFRP and adhesive before and after aging,the hygrothermal degradation factor was introduced to modify the material constitutive and failure parameter.Finally,a prediction model for the mechanical properties of the sandwich structure was established to realize the progressive damage simulation and performance prediction of the sandwich structure after hygrothermal aging under complex stress conditions.The degree of influence and mechanism of the mechanical property degradation of CFRP and adhesives on the overall mechanical properties of the sandwich structure after hygrothermal aging were analyzed.The detailed research content was divided into the following four parts:(1)Take CFRP and adhesive in sandwich bonding structure as research objects.The mechanical specimens of woven CFRP laminate and epoxy resin adhesive were fabricated by vacuum-assisted forming method.Then,the In-plane tensile,In-plane compression,Inplane shear,Out-of-plane shear,Interlaminar tensile,Interlaminar shear test of CFRP and the tensile,shear test of adhesive were conducted respectively.The basic mechanical property parameters of each component in the CFRP-Al honeycomb sandwich bonding structures were measured comprehensively.Among them,for the CFRP interlaminar tensile and shear tests,the experimental program and fixture processing were designed specially.In order to verify the effectiveness of the CFRP interlaminar performance test,the stress state of the CFRP specimen during the experiment was analyzed through numerical simulation.Finally,the experimental scheme is proved to measure the CFRP interlaminar mechanical properties effectively.(2)This paper takes two typical automotive service extreme environment “High temperature(80℃/25%RH)” and “High temperature and High humidity(80℃/95%RH)”as aging conditions,which were abbreviated as T80 and TH respectively.The woven CFRP laminates and adhesive were subjected to T80 and TH artificial accelerated aging to obtain the change law of the 6 kinds of mechanical properties of CFRP(In-plane tensile,In-plane compression,In-plane shear,Out-of-plane shear,Interlaminar tension,Interlaminar shear)and the 2 kinds of mechanical properties of adhesives(Tensile,Shear).Combined with Fourier Transform Infrared Spectroscopy(FTIR),Differential Scanning Calorimetry(DSC),Electron Microscopy(SEM)and Moisture absorption test to analyze the mechanical properties degradation mechanism of CFRP and adhesive after aging.According to FTIR and DSC test results,it can be inferred that: during the T80 aging process,the CFRP experienced post-curing in the early stage,followed by thermal oxidative aging in the later stage,however,the adhesive showed post-curing throughout T80 aging process.During the TH aging process,thermo-oxidative aging has always been dominant in CFRP and adhesives.According to the observation results of SEM,it was found that the interface bonding degree between the fiber and resin decreased seriously after TH aging.The performance change of the sandwich structure is the result of the coupling effect of various aging factors.After T80 aging,due to the post-curing effect of the CFRP resin matrix and the adhesive,some mechanical properties of CFRP increased slightly,such as In-plane tensile,Out-of-plane shear,Interlaminar tensile,Interlaminar tensile of CFRP,and tensile and shear of adhesive.While the In-plane compression and the In-plane shear mechanical property of CFRP decreased slightly,which is mainly caused by the degradation of the fibermatrix interface.After TH aging,due to the thermal oxidative aging and water absorption plasticization of the resin matrix,the fiber-matrix interface was deteriorated severely.Therefore,the mechanical properties in CFRP dominated by the resin matrix,such as: Inplane compression,In-plane shear,Out-of-plane shear,Interlaminar shear,Interlaminar tensile decreased significantly.Meanwhile,the tensile and shear mechanical properties of adhesive also decreased significantly.At the same time,it is found that there is a correlation between the degree of moisture absorption and these mechanical properties.When the moisture absorption tends to be saturated,the changes of these mechanical properties are gradually gentle.(3)In order to accurately predict the damage failure behavior of CFRP laminates and reveal its damage failure mechanism.For the woven CFRP laminate in the sandwich bonding structure,this paper proposed a nonlinear fitting method to characterize the Inplane shear stress-strain response before and after TH aging.The nonlinear constitutive was embedded into the established CFRP damage prediction model through VUMAT subroutine.In this model,the intra-laminar damage and inter-laminar damage of CFRP were all considered.Among them,CFRP intra-laminar damage includes shear nonlinear constitutive,strain-based 3D-mixed initial failure criterion,and continuous damage model(CDM)of exponential/linear evolution law;CFRP inter-laminar damage includes the CZM(Cohesive Zone Model)based on the secondary stress initial failure criterion and the B-K damage evolution law.Finally,the numerical simulation of the In-plane shear experiments of CFRP before and after TH aging was carried out,which further clarified the adverse effect of TH aging on the In-plane shear behavior of CFRP.Meanwhile,the accuracy of this CFRP damage prediction model was verified.(4)In order to analyze the influence mechanism of the mechanical properties of CFRP and adhesives on the overall mechanical properties of the sandwich structure,and the various failure behaviors of the sandwich structure under complex stress conditions after TH aging.This part takes the CFRP-Al honeycomb sandwich bonding structure as research object.Conduct periphery shear and three-point bending experiment on the CFRP-Alhoneycomb adhesively sandwich structure before and after T80 and TH aging to obtain the change law of the mechanical properties and failure modes of the sandwich structure.Based on the CFRP progressive damage model and cohesive zone model of adhesive before and after aging,the hygrothermal degradation factor was introduced,then the material constitutive and failure parameters were modified.Finally,a damage prediction model was established to predict the complex mechanical behavior of the CFRP-Al-honeycomb adhesively sandwich structure after hygrothermal aging,which can help us to analyze the influence degree and mechanism of the degraded CFRP and adhesive on the overall performance of the sandwich structure.The numerical simulation results in Abaqus/Explicit show a very good agreement with experimental data.Therefore,it is proved that the proposed method can be used for the prediction of damage failure behavior of CFRP-Al honeycomb sandwich bonding structures before and after aging.Finally,at any time in the aging cycle,failure simulation and performance prediction of sandwich bonding structures under complex stress conditions can be achieved.On this basis,the effect of TH aging on the Periphery shear and three-point bending mechanical property of the sandwich bonding structure at high temperature were further analyzed through comparative research.The results showed that the interlaminar property of CFRP degraded significantly at high temperature,and the mechanical property of the material decreased at high temperature after TH aging,resulting in a greater decrease in the bearing capacity of the sandwich structure at high temperature compared to normal temperature.Compared with Periphery shear property,hygrothermal environment have more obvious effect on the high temperature bending property of sandwich structures.