Performance Evaluation of a Carbon Nanotube Sensor for Fatigue Crack Monitoring of Metal Structures

Abstract

This article describes research that investigated the ability of a carbon nanotube (CNT) sensor to detect and monitor fatigue crack initiation and propagation in metal structures. The sensor consists of a nonwoven carrier fabric with a thin film of CNT that is bonded to the surface of a structure using an epoxy adhesive. The carrier fabric enables the sensor to be easily applied over large areas with complex geometries. Furthermore, the distributed nature of the sensor improves the probability of detecting crack initiation and enables monitoring of crack propagation over time. Piezoresistivity of the sensor enables strains to be monitored in real time and the sensor, which is designed to fragment as fatigue cracks propagate, directly measures crack growth through permanent changes in resistance. The following laboratory tests were conducted to evaluate the performance of the sensor: (1) continuous crack propagation monitoring, (2) potential false positive evaluation under near-threshold crack propagation conditions, and (3) crack re-initiation detection at a crack-stop hole, which is a commonly used technique to arrest fatigue cracks. Real-time sensor measurements and post-mortem fractography show that a distinguishable resistance change of the sensor occurs due to fatigue crack propagation that can be quantitatively related to crack length. The sensor does not show false positive responses when the crack does not propagate, which is a drawback of many other fatigue sensors. The sensor is also shown to be remarkably sensitive to detecting crack re-initiation.

The Experimental Verification of Direct-Write Silver Conductive Grid and ARIMA Time Series Analysis for Crack Propagation

“…With this sensor type, an equivalent crack propagation rate could be estimated in real time, due to the parallel conductive strands’ connection and the stepping sensor output characteristic. The sensors investigated in [ 18 , 19 , 20 ] are in the form of a continuous sheet, so the temperature and strain effects cannot be distinguished from the output signal without additional equipment. Furthermore, the use of carbon black or carbon nanotubes for the sheet sensor prevents visual inspection of the area under the sensor.…”

“…The arrangement of multiple side electrodes enabled the detection of several cracks within the applied film sensor. Furthermore, in [ 19 ] and [ 20 ], carbon nanotubes were investigated to develop electrically conductive patterns to detect fatigue cracks. The so-called “smart sensing skin” for structural health monitoring applications was investigated [ 20 ].…”

Effect of Vulcanization on the Electro-Mechanical Sensing Characteristics of Multi-Walled Carbon Nanotube/Silicone Rubber Composites

“…However, the cross-linking network between these conductive fillers and the polymer matrix is weak, the preparation method is complicated, the reversibility is poor, and it is difficult to match the application requirements. Since the emergence of carbon nanotubes (CNTs), their light weight, high conductivity, high strength, and high aspect ratio have made them suitable fillers for the damage sensing of polymer composites with good comprehensive performance [ 12 , 13 , 14 , 15 ].…”

Structural Health Monitoring of Fatigue Cracks for Steel Bridges with Wireless Large-Area Strain Sensors

“…Further development of the antenna sensor led to a wireless patch antenna sensor [ 25 ] for strain and crack sensing, which was validated through laboratory experiments. A carbon nanotube (CNT) sensor was studied for monitoring and detecting fatigue cracks in metal structures utilizing laboratory tests [ 26 ], and a wireless stretchable sensor network [ 27 ], including piezoelectric, strain gauge, and resistive temperature sensors, was integrated to obtain measurements over an aerospace composite wing. Of interest to this research, a soft elastomeric capacitive (SEC) sensor was proposed to monitor strain over a large area [ 20 ].…”