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Effect of Applied Load on Guided Wave Monitoring of a Composite Bolted Joint



A structural health monitoring study based on ultrasonic guided wave inspection is presented for the monitoring of a composite bolted joint under applied load. Bolt bearing failure in particular is selected as the type of damage studied. This damage mode is of interest for aerospace applications due to the increased prevalence of bolted connections and the complex mechanics required to predict the onset of damage in composite materials. The phenomenon is investigated experimentally by fabricating a carbon fiber reinforced polymer specimen and instrumenting it with macro-fiber composite sensor-actuators. The bolt fixture is designed to apply a tensile load to the specimen so as to introduce bolt bearing failure. The sensors are arranged in a circular array around the bolt hole to study the interaction of the guided waves with damage for different angles. Measurements are then made on the plate alone, with the plate in the bolt fixture, and finally with the specimen placed in the tensile testing machine at various levels of load. After collecting a sufficient amount of baseline data, more measurements are taken for a single tensile test where load is increased until failure occurs, pausing at three different levels of damage. Because bolt bearing damage changes the mechanical properties of the material locally, guided waves will scatter differently when the damage is present than they do for the undamaged plate. The objective of this work is to characterize the influence of applied load on the ultrasonic measurements made. First, the effect of loading on the recorded ultrasonic data is observed. Physical explanations for the load-dependent behavior are proposed. A technique for monitoring the load state of the specimen from the ultrasonic data is also presented. Because most structures are subjected to changing load states in service, it is anticipated that understanding the changes due to load state will be important for implementing effective guided wave structural health monitoring systems in the field.

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