Damage Self-diagnosing Of Carbon Fabric Reinforced Epoxy Resin Composites By Electrical Resistance Measurement

Carbon fiber/epoxy-matrix composite materials have been widely used in aerospace, automobile, weapons and equipment, and national economy for their excellent mechanical properties, temperature resistance, corrosion resistance, and fatigue resistance. Due to the optimal electrical conductivity of carbon fibers, the electrical conductivity network of carbon fibers can be formed in carbon fiber/epoxy-matrix composite materials with high volume content carbon fiber by contacting tightly each other. The internal structure of composite materials will be changed by external force, which can causes the changes in conductive network and further lead to the variation of resistance of composite materials. Therefore, carbon fiber can be used as the external sensor of composite materials, which can measure the variation of internal structure by diagnosing the change of electrical resistance composites.In this thesis, carbon/epoxy composite materials were prepared by molding compression with G803/5224 carbon fabric/epoxy prepreg, which were used widely in the aviation industry. The stacking angle of prepreg was [0/90], [0/90/±45],[±45], and carbon fiber volume content was 56%,64%, respectively. Based on channel conduction and tunnel effect conduction, the influence of temperature on the resistance of composites was studied. The resistance change of composites resulted from tension, opening tension,compression, open hole compression, bending, impact conditions was studied, and the basis for damage diagnosing of composites was determined. A new idea of evaluating impact damage in composites was proposed by comprehensive analysis of the effect of impact energy, impact damage (ultrasonic F-scan images), residual compressive strength, resistance change. Computer program of diagnosing damage in composites was compiled. The system of self-diagnosing impact damage in composites was established, which was composed of composite materials, computers, and multichannel data acquisition device, and it was validated by tests.(1) The resistance of different specifications of composites decreased, namely negative temperature coefficient effect can be found in composites when measured in the temperature from-50℃to 160℃. Among these composites, [±45] 8 composites showed a sharp drop with the degree of 9.37%. The resistance of composites declined with different degree in different temperature range of (-50--20)℃, (-20-130)℃, (130-160)℃. In the case of (-20-130)℃, the rate of descent was the maximum.(2) The destruction test of composite materials was measured including epoxy elastic deformation, forced high-elastic deformation of epoxy resin, resin cracks, resin damage, composites delamination, carbon fiber fracture, etc. in the cases of tension, opening tension, compression, open hole compression, and bending test. During the test, the resistance change of composites was caused by the change of conductive channel, the tunnel effect of composite materials, resulted from the contact conditions among carbon fibers and the number change of free electrons.(3) In the cases of tensile and opening tension test, there was tensile damage in composites,when the strain of [0/90],[0/90/±45],[±45]composites was more than 1.8%,1.8%,1.27%, respectively. When the increase rate of resistance in composites was more than the rate of resistance at this time, the tensile damage can be diagnosed in composites. In the cases of compression test, when the resistance increase rate of [0/90]7,[0/90]8 composites was more than 3.6%,2.6%,, respectively, the slight compress damage can be diagnosed in composites. When the rate of was more than 10.2%,3.4%, respectively, the serious compress damage can be diagnosed in composites. In the cases of bending test, when the increases rate of resistance of [0/90]7,[0/90]8 composites was more than 25.8%,31.3%, composites may have been broken. Damage in composites was hard to diagnose by resistance change before they were broken.(4) Impact energy threshold of damage and damage tolerance of different specifications of composites were different. When the value of impact damage energy was less than the threshold, there can make the resistance value of composites between electrode points smaller. When the value of impact damage energy was greater than the threshold, there can make the resistance value of composites between electrode points bigger. The resistance change rate of different electrode points in composites was different under the same impact damage energy. It can be mainly due to the spread of impact energy from the impact point to the surrounding radially, the distance was nearer, and the impact was more obvious. When the composites were impacted by a impact greater than the impact damage tolerance energy threshold, not only the resistance of the composites was significant change, but also the composite materials was destroyed.(5) In the case of impact test, composites can be regarded as "normal" state in the region rounded by 4 electrode points with the length of 6 cm and the width of 5 cm, as long as the resistance increase rate of three electrode points or 2 discrete electrode points was less than the second resistance increase rate of all electrode points of impacted composites arising from the impact damage tolerance energy threshold. When the resistance increase rate was more than the third resistance increase rate, composites can be regarded as "damage" state. When the resistance increase rate was more than the third resistance increase rate of impacted composites by perforation damage, composites can be regarded as "serious damage" state.