Spray-on sensing technology for structural health monitoring

2 min read

A team of researchers from Hong Kong Polytechnic University (PolyU) has developed sensors which can be sprayed directly on to flat or curved surfaces. The sensors can be networked to extract rich real-time information on the health status of the structure being monitored. Due to its light weight and low fabrication cost, large quantities of sensors can be deployed in a sensor network for detecting hidden flaws of structures, which the researchers say will pave the way for a new era of ultrasonics-based structural health monitoring.

Professor Su Zhongqing, lead researcher in PolyU’s Department of Mechanical Engineering, said: “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.”

Compared to conventional ultrasound sensors which cost over $10 each and weigh a few grams, PolyU’s nanocomposite sensor costs .50 and weighs 0.04g for each. As such, more sensors can be adopted in one structure, generating more information for analysis, with less weight added to the structure.

The technology encompasses 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 regime 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.

Made from a hybrid of carbon black (CB), graphene, conductive nano-scale particles, and polyvinylidene fluoride (PVDF), the nanocomposite sensor can be easily and flexibly tailored to different sizes towards various engineering applications.

The sensor’s high sensitivity to structural change is due to the optimised nanostructure of the hybrid, which allows the sensor to identify changes in piezoresistivity of the nanocomposite. To this end, Prof Su and his team conducted numerous tests on the weight ratio of nanofillers in order to optimise the conductivity of the nanocomposite.

Each sensor is connected to a network via a wire printed on the structure. By analysing and comparing the electrical signals converted from the electric resistivity, the network can spot the defect in a structure, as well as translate the signals into 3D images.

Prof Su added: “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.”