A continuous sensor for damage detection in bars

The concept of a continuous sensor for detecting vibration and stress waves in bars is investigated in this paper. This type of sensor is a long tape with a number of sensing nodes that are electrically connected together to form a single sensor with one channel of data output. The spacing of the sensor nodes can be designed such that the sensor is capable of detecting acoustic emissions (AEs) occurring at any point along the length of the sensor. An active fiber composite material or piezoceramic wafers can be used as the active element of the sensor. The characteristics of this new type of sensor were investigated by a simplified simulation and through experiments. The scope of the simulation is primarily to determine the characteristics of the new type of sensor that is proposed and the simulation is not intended to model the complex dispersion characteristics of guided waves in bars. The sensor was modeled as being integrated within a uniform cantilever bar to measure longitudinal stress wave propagation. Damping was included in the model to attenuate the wave as it travels. The sensor was connected to an electrical tuning circuit to examine the capability to filter out undesirable noise due to ambient vibration that would occur in practice. The elastic response of the bar was computed in closed form, and the coupled piezoelectric constitutive equations and electric circuit equations were solved using the Newmark-Beta numerical integration method. Strain, vibration, and wave propagation responses were simulated and results indicate that damage to the bar can be detected by recognizable changes in the sensor output as the wave propagates along the bar and passes over each sensor node. Experiments were performed to verify the concept of the continuous sensor on a composite bar. The testing showed the continuous sensor can replace four or more individual sensors to detect AEs and thus may be a practical method for structural health monitoring.