The Analysis Method Of Coupled Hygro-Thermo-Stress Transient Response In Adhesively Bonded Composite Repair

During the service life of aircrafts, the composite structuralcomponents would inevitably contain many defects. In order to restoretheir original properties, to meet the airworthiness requirements and toguarantee safety, it is important to develop repair techniques. Adhesivebonding is of course the main approach for repairing advanced fibercomposite components. However, the performance of composite andadhesive during and after exposure to combined high temperature, highhumidity and external load environment would deteriorate heavily. In orderto guarantee safety after repairing, there is an urgent need to study thecoupled hygro-thermo-stress effects on adhesively bonded compositerepair in coupled hygrothermal and external loads environment. Thestudies here mainly concern the coupled analysis method as follows:(1) Established a coupled moisture diffusion-strain equation.According to the transverse strain-time curve during the moistureabsorption of composite laminate, the relation between internal strain andtime is similar with that between moisture content and time. Both of thestrain and moisture content are linear with time square root. Supposing thathygrothermal strains in laminate also follow Fick’s diffusion law, a socalled―strain diffusion coefficient‖and a strain coupling item wereintroduced into the Fick’s equation, which made the coupled moisturediffusion-strain equation accomplished. The same method was introduced into multi-material system. A coupled moisture diffusion-strain modelapplied to multi-material system was also established. The coupledgoverning equation of moisture diffusion can accurately describe theNon-Fickian diffusion of composite moisture absorption and model theeffect of interactions between complex strain and moisture histories on thediffusion process within composite material. For model validation, themodel predictions are accordance with experimental data. On the basis ofthis coupling model, over-estimate moisture content in the initial stage byFick’s diffusion law is improved obviously.(2) Improved Tsai method. Generally, Tsai method was used tocalculate the effect of temperature and the moisture concentration variationon the mechanical properties. In Tsai method, both of the numerator anddenominator in adimensional temperature are related to moisture content,which made it difficult to distinguish it is the temperature or the moistureabsorption reduce the mechanical properties. In modified Tsai method,glass transition temperature in dry specimen at room temperature was usedas the base value. After modifying, Tsai method highlights the moistureabsorption on properties’ degradation. The modified result is agreed withexperimental result.(3) Proposed a transient coupled moisture-temperature-stress FEanalysis method. The temperature field and moisture field affect the stressfield mainly through two ways. One is that materials may expansion whenthey expose to humid and elevated temperature. The other is that materialproperties are related to temperature and moisture content. In the analysishere, time is a significant variable in study the transient state duringcoupling process. The stiffness matrix in element is deduced from thecoupled temperature field, moisture field and stress field. Thesuperposition of hygrothermal loads and external loads can obtain thehygrothermal stress simultaneously. The result of in-plane shearing test ofstepped lap adhesively bonded composite repair at various temperature andhumidity conditions indicated that the coupled method can be used to predict the mechanical properties of adhesively bonded composite repairwhen the temperature field, moisture field and stress field exist at the sametime accurately.