Damage Self-diagnosing And Finite Element Analysisof Carbon Fiber Composites Under Load

Carbon fiber composite materials have been widely used in aerospace, buildingand sports, etc. for their excellent mechanical properties, fatigue resistance, corrosionresistance and optimal electrical conductivity. The electrical conductivity network canbe formed in carbon fiber composite materials with high volume content electricalconductivity carbon fiber by contacting tightly each other. The internal structure ofcomposite materials will be changed under load, which can causes the changes inconductive network and further lead to the variation of resistance of composite materials.Therefore, carbon fiber can be used as the sensor of external load of compositematerials, which can measure the damage degree by the change of electrical resistancecomposites, and to achieve self-diagnosis function of real-time online warning andintelligent monitoring in long-term work of different environment.In this thesis, G803/5224carbon fabric/epoxy prepreg from Beijing institute ofaerial materials was used as the raw materials of carbon fiber composite materials.Moulding process and load tests for carbon fiber composite materials are conductedreferred to the related standard. The relation between resistance and mechanicalproperties of composite under load was studied, and the basis for damage diagnosing ofcomposites was determined. To prove the effectiveness of using finite element methodin the research on the carbon fiber composite materials damage under load wassimulated with this method. The results are as follows:(1) In the tensile test, resistance and damage degree of composite materials will beincreased as tensile strain increases. In compression and bending test, damage degreewill be increased but resistance of composite materials will be fluctuated ascompression strain and bending stress increases. According to the relationship betweentensile strain, compression strain and bending stress and resistance increase rate ofcomposite materials, resistance increase rate is used to diagnose the damage degree ofthe composite materials in this paper, and the critical resistance increase ratescorresponding to the case of slight damage and serious damage are obtained,respectively.(2) In the cases of impact test, different impact energy can damage composites area and thickness in deffrent degree. Resistance of composite materials will be increased asimpact energy increases, and the corresponding relationship between resistance increaserates and impact energy. Therefore, resistance increase rate is used to diagnose thedamage degree of the composite materials in this paper, and the critical resistanceincrease rates corresponding to the case of slight damage and serious damage areobtained.(3) Simulated results agree well with the experimental results, which prove theeffectiveness of using finite element method to study the damage failure process ofcomposite materials.