Influence Of Bending Load On The Properties Of CFRP Under Hygrothermal Environment

Carbon fiber reinforced resin matrix composites (CFRP) is widely used in theaircraft’s main load-bearing components, secondary load-carrying member as well asspecial function components of special parts, because of its excellent comprehensiveproperties, but its mechanical properties are easily affected by environmental factorsand external load, especially the common hygrothermal circulation and bending load.The focus of the current study is put in the performance of CFRP under the simple wet,heat or wet-heat environment, few reports could be found about the influence ofhygrothermal circulation and bending load-wet-thermal coupling on CFRP, whichmakes it of great significance to research on the influence of bending load on themechanical properties of CFRP under hygrothermal environment.This paper investigated the effect of bending load on the hygroscopic properties,moisture diffusion mechanism, degradation of static and dynamic mechanical properties,desorption properties and retention of mechanical properties after dehydration of CFRPunder two different kinds of hydrothermal environments named constant temperatureand humidity, hydrothermal circulation. FTIR was used to analyze the change offunctional group in polymer after absorption, while SEM, metallurgical microscope andDMA tests were chosen to find the change of microstructure of CFRP after aging. Amodel about the degradation of mechanical properties of CFRP after hygrothermal andcyclic hygrothermal aging was given, as well as a method of predicting the mechanicalproperties. Finally according to the related life prediction model and the correlationbetween the normal temperature test and accelerated test, the service life of CFRP waspredicted under wet-thermal and bending load-wet-thermal environment.The experimental study indicates that the absorption curve was not changed bybending load of different levels (0%,30%,60%), so is true for the moisture diffusionmechanism of CFRP. However, the rate of moisture absorption and the equilibriummoisture content was increased with the higher level of bending load. The fitting resultsbased on the Langmuir and Fick diffusion model showed that the former model wascloser to the experimental data, which also indicated that the water in CFRP includedfree diffusion phase and combination phase. It could be found from the tests ofmechanical properties that moisture can resulted in the degradation of mechanicalproperties of CFRP and it could get worse under higher level of bending load. As can beseen from the infrared spectrum, the polymer didn’t undergo any chemical reaction likehydrolysis or chain scission, which meant the degradation of mechanical propertiescouldn’t be attributed to chemical aging. By the observation of SEM and metallographicmicroscope, interfacial debonding and crack in matrix have been caused by absorption and the bending load made this two kinds of irreversible damage deteriorated. DMAtests indicated that physical aging and mechanical aging were responsible for the severedegradation of mechanical properties of CFRP, which had nothing to do with chemicalaging. The moisture absorption characteristics and mechanical properties of five typicalmaterials (bend specimen、compression specimen、bending-bearing specimen、bendspecimen with a hole and impact specimen) showed that the absorption historyinfluenced the resorption and redesorption of CFRP and frequent hygrothermal shockcould cause the degradation of mechanical properties of composite specimen, especiallythese with damage like bending-bearing specimen. It can also be concluded from theprediction of CFRP service life under wet-heat and bending load-wet-heat environmentthat the bending load has a significant influence on the reliability of composite aircraftcomponent, which can’t be ignored.