Electrical-mechanical Coupling Behavior Of Three-dimensional Braided Composites Under Cyclic Bending Fatigue

Three-dimensional（3D）braided composites have excellent structural integrity,delamination resistance,damage tolerance,and have been used in the aerospace and weapons industries.Composites are prone to structural damage in long-term service,which can be detected with health monitoring methods to improve the safety of structural components.Carbon fiber composites have well electrical conductivity,and use resistance/ potential itself as sensor of its own damage.We combine model of conductive path with Euler-Bernoulli to establish electromechanical model under three-point bending.The changes of resistance/potential under quasi-static three-point bending load and fatigue load of 3D braided carbon fiber/epoxy resin composites have been measured.The development of resistance method in structural health monitoring system could be obtained from this measurement.The research contents are as follows:（1）The electrical resistance of the 3D braided composites was measured with four-electrode method under direct current electrical field without load,and the correlation between conduction mechanism and the braiding structure was analyzed.The potential of the upper and lower surfaces at different braiding angles under electrodes connected and unconnected,and the correlation between braided unit’s homogenization and potential values was analyzed.（2）Analysis of macroscopic mechanical behavior with different braiding angles using quasistatic three-point bending tests.The four-electrode method was used to monitor the dynamic resistance/potential response of 3D braided composites under bending load,and derivation of electromechanical model based on the three-point bending damage theory.The corresponding relationship between damage and resistance during the quasi-static three-point bending load was revealed,the correlation of braiding angles effect on sensitivity coefficient and conductivity was analyzed,and the fit of electromechanical model and experimental results was verified.（3）The relationship between the number of fatigue life cycles and the stress level was measured under three-point bending cyclic load.Real-time monitoring of braiding angles resistance changes at 70%,80%,and 90% stress levels.It reveals the relationship between the area of the hysteresis loop and the dynamic resistance change under the same stress level during the bending fatigue load,and the interaction between the braiding angles and the stress level.We have found that:（1）The voltage-current relationship of 3D braided composites presents typical Ohmic behavior in the range of 0.01A～0.08 A.The longitudinal resistance increases with the increase of braiding angles.The resistance value depends on the length of single yarn in the internal braided structure.The potential values of the upper and lower surfaces of the samples with large braiding angles tend to be the same in the two electrode modes,showing that the braided structural unit’s homogenization.（2）The bending strength and modulus decrease with the increase of braiding angles under three-point bending load.When the sample has slight resin cracking during the elastic range,the damage has no effect on the surface resistance changes at different braiding angles,while the longitudinal resistance changes of the 15° and 25° decrease.Once the fiber was fractured,the surface resistance changes value reflected the damage degree of the composites and longitudinal resistance changes value reflected the deformation degree of the composites.Longitudinal conductivity of the composites at different braiding angles increases under bending process and the resistance response sensitivity of tensile surface is higher than that of compressive surface.We detected fiber fracture location when damage occurred based on surface electrical potential change.（3）The fatigue limit stress level of 15° sample is 70% under cyclic bending fatigue.Brittle fatigue fracture failure occurred at small braiding angles during the same stress level and the area of proportional fatigue life cycles is positively correlated with resistance changes.Plastic deformation failure occurred at large braiding angles and the area of proportional fatigue life cycles has no correlation with resistance changes.The resistance changes of minimum deflection reflected the global resistance change of the same stress level at different braiding angles.The proportional fatigue life is 0.1～1.0 at low stress levels,the composites stiffness degradation and resistance change rate gradually decrease with the braiding angles increases.The proportional fatigue life is 0.1～0.5 at high stress levels,the first change in resistance gradually increases and then decreases with braiding angles increases,the proportional fatigue life is 0.5～1.0 composites stiffness degradation and resistance change rate decreases.This study reports electrical-mechanical behavior of 3D braided composites under dynamic and static bending.It has been revealed that the law of resistance/potential response under bending load and fatigue load,which provide theoretical basis for the resistance method structure health monitoring system.