Tensile and vibration fatigue monitoring of composite laminate bolted joints with local enhancement using buckypaper sensors

It is an urgent problem to evaluate the strengthening effect of composite bolted connection structure after local reinforcement design on aircraft large panel assembly. The purpose of this article is to use buckypaper (BP) sensors to monitor the structural health of composite bolted joints under static tensile and fatigue loads after local reinforcement design. The experimental results show that the BP sensor has a good response to the microcrack monitoring during the tensile pre-experiment and loading/unloading of the bolted joint structure. The tensile failure test of no reinforced (NO-R) joint, glass fiber local reinforced (GFRP-R) joint, and carbon fiber local reinforcement joint were carried out by the BP sensor. According to the change of sensor <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>R</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> , the damage can be divided into I–IV stages, and the damage state of three structures can be visualized. Under the same failure mode of NO-R joint and GFRP-R joint, it is better to evaluate the reinforcement effect of GFRP-R joint according to the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>R</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . The vibration fatigue monitoring of GFRP-R joints is carried out. According to the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>R</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> of the sensor, it is visualized that the structure is in three damage states: microcrack initiation, crack propagation, and structural failure. The BP sensor embedded in the composite bolt connection structure after the local reinforcement design of the aircraft large panel assembly can effectively realize the structural health monitoring.