A team of researchers from Hong Kong Polytechnic University (PolyU) has developed sensors which can be sprayed directly onto flat or curved surfaces. The sensors, made from a hybrid of carbon black (CB), graphene, other conductive nano-scale particles, and polyvinylidene fluoride (PVDF), can be networked to extract rich real-time information on the health status of the structure being monitored.
The technology includes a sensor network with a number of the sprayed nanocomposite sensors and an ultrasound actuator to actively detect the health condition of the structure to which they are fixed. When the ultrasound actuator emits guided ultrasonic waves (GUWs), the sensors will receive and measure the waves. If damage is detected, such as a crack in the structure, propagation of GUWs will be interfered by the damage, leading to the wave scattering phenomena to be captured by the sensor network. The damage can then be characterised quantitatively and accurately.
The sensor can measure an ultrasound signal from static to up to 900kHz with ultralow magnitude. The acquisition of wave scattering in an ultrasonic scheme allows detection of cracks as small as 1 to 2mm. This response frequency is claimed to be over 400 times more than the highest frequency achievable by current nanocomposite sensors.
While conventional ultrasound sensors can measure a wider range of ultrasound waves when compared to those developed by the team, the high cost and weight of the conventional sensors make a large quantity application infeasible, limiting the quantity of data to be acquired. Thanks to the new sensor's light weight and low fabrication cost, large quantities of sensors can reportedly be deployed in a sensor network for detecting hidden flaws of structures, which the researchers say may pave the way for a new era of ultrasonics-based structural health monitoring.
The team stated that “This nanocomposite sensor has blazed a trail for implementing in-situ sensing for vibration, or ultrasonic wave-based structural health monitoring, by striking a balance between the cost of sensors, and the quantity of data acquired by the sensors”. The researchers also added that “Due to its light weight, the novel nanocomposite sensors can be applied to moving structures like trains and aeroplanes. That will help to pave the way for real-time monitoring of these structures in future, enhancing safety of the engineering assets and retrofit the traditional system maintenance philosophy.”