Residual Stress Detection Technology for Aramid Reinforced Epoxy Composites Based on Acoustic-Elastic Effect
Li Jin1, Xue Rundong1, Zhao Renyong1, He Jin2, Chen Yun3
1. Key Laboratory of Smart Grid of Education Ministry School of Electrical and Information Engineering Tianjin University Tianjin 300072 China; 2. State Grid Tianjin Electric Power Research Institute Tianjin 300384 China; 3. China Electric Power Research Institute Beijing 100192 China
Abstract：Aramid fiber reinforced epoxy composites have mismatching at the interface between fiber and epoxy resin, and it is easy to produce residual stress in the curing process. The tensile or compressive load under field tests or operation conditions is usually superimposed with the residual stress. When the mechanical stress exceeds the tolerance limit of the insulating material, micro cracks and other damages will be formed, which will lead to mechanical operation failure or insulation breakdown and other accidents. Therefore, it is in urgent need to explore effective detection technology of residual stress. In this paper, a uniaxial stress detection platform based on the acoustic elastic effect of insulated pull rods was built. The relationship between the ultrasonic P-wave velocity and uniaxial stress of aramid reinforced epoxy composites was studied. The acoustic-elastic coefficient of the ultrasonic P-wave propagation direction perpendicular to the compressive stress in the sample was calculated. Then the residual stress of aramid reinforced epoxy composites under different curing conditions was measured and discussed. The results show that there is a highly linear correlation between the ultrasonic P-wave velocity and the vertical compressive stress in the aramid fiber reinforced epoxy composites, which verifies the acoustic-elastic effect. Besides, the ultrasonic path will increase linearly with the increase of vertical compressive stress, so the influence of sample deformation should be considered when calculating the acoustic elastic coefficient. The acoustic elastic coefficient of ultrasonic P-wave in standard epoxy and aramid fiber reinforced epoxy composites is 6.519×10-5 and 10.195×10-5 respectively. The change of curing temperature and curing time will lead to the increase of residual stress in the composite, the maximum of which reaches over 70 MPa. When the curing temperature and curing time increase, the residual stress of the sample increases. The higher curing temperature results in higher the conversion rate of partial cross-linking. In this way, the epoxy material will reach the stability value of curing degree faster and lead to the increase of residual stress of the sample. With the increase of curing time, the final curing degree of epoxy material also increases, resulting in an unbalanced shrinkage during crosslinking reaction. When the curing temperature and curing time decrease, the residual stress of the sample also increases. This is because the reduction of curing temperature and curing time will make the conversion rate of cross-linking reaction lower, resulting in insufficient cross-linking curing reaction and lower curing degree of epoxy materials. There may still remain some unreacted epoxy molecules in the network, and the interaction between them will increase the residual stress of the sample. Moreover, the shortening of curing time will also lead to the uneven curing curve and uneven curing rate, resulting in higher residual stress. Therefore, in order to reduce the residual stress in aramid reinforced epoxy composites, it is necessary to select the appropriate curing process and improve the consistency of process control.
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2023, Vol. 38 
