On-Site Health Monitoring of Composite Bolted Joint Using Built-In Distributed Eddy Current Sensor Network

Abstract

There is an urgent need to monitor the structural state of composite bolted joints while still remaining in service; however, there are many difficulties in analyzing their strength and failure modes. In this paper, a built-in distributed eddy current (EC) sensor network based on EC array sensing film is developed to monitor the hole-edge damages of composite bolted joints. The EC array sensing film is bonded onto the bolt and consists of one exciting coil and four separate sensing coils. Experiments are conducted on unidirectional composite specimens to validate the ability of the EC array sensing film to quantitatively track the damage that occurs at the hole edge and to investigate the performances of the EC array sensing films with different configurations of the exciting coil. Experimental results show that the induced voltage of sensing coil changes only if the damage appears on the laminate structure where that particular sensing coil is located, whereas the induced voltages of the other sensing coils on other laminate plates remain unchanged. Numerical simulation based on the finite element method is also carried out to investigate and explain the phenomena observed in the experiments and to analyze the distribution of the EC around the bolt hole. Both experimental and numerical simulation results demonstrate that the developed EC array sensing film can effectively identify not only whether there is damage at the hole edge but also the damage location within the thickness and quantitative size.

Mechanical Joining of Fibre Reinforced Polymer Composites to Metals—A Review. Part I: Bolted Joining

“…The monitoring of bolted joints in composites is of great importance, because they constitute weak spots in such structures and the failure of the whole structure is likely to initiate from the bolted joint. However, techniques used most commonly for non-destructive testing of fibre reinforced polymer composites, such as: ultrasonic C-scans—according to ASTM E2580: Standard practice for ultrasonic testing of flat panel composites and sandwich core materials used in aerospace applications, shearography—according to ASTM E2581: Standard practice for shearography of polymer matrix composites and sandwich core materials in aerospace applications, thermography—according to ASTM E2582: Standard practice for infrared flash thermography of composite panels and repair patches used in aerospace applications, acoustic emission—according to ASTM E2661: Standard practice for acoustic emission examination of plate-like and flat panel composite structures used in aerospace applications, radiography—according to ASTM E2662: Standard practice for radiographic examination of flat panel composites and sandwich core materials used in aerospace applications are dedicated for flat panels and are difficult to perform in the case of bolted joints due to the inaccessibility of the joint [ 131 ]. These techniques are not capable of real-time monitoring of damage evolution either [ 132 ].…”

“…Due to the electrical properties of CFRP, eddy current (EC) method is practical and easy to implement [ 141 ]. Liu et al developed eddy current array sensing film and used it for monitoring the damage around the hole-edge of composite bolted joint [ 131 , 141 ]. A rectangular flexible eddy current array sensing film printed on a flexible substrate was bonded around the bolt.…”

“…The whole structure of eddy current senor consists of an exciting coil and a receiver coil. The principle of measurement is shown in Figure 12 [ 131 ]. The EC sensor is manufactured on a flexible substrate with printed flexible circuit technology and bonded on the bolt in composite joint.…”

“…The EC sensor is manufactured on a flexible substrate with printed flexible circuit technology and bonded on the bolt in composite joint. To monitor bolt hole-edge damages in both radial and axial directions, the EC sensing film is made of an exciting coil and several sensing coils [ 131 ]. According to Faraday’s Law, when an alternating electric current is applied to the exciting coil, a primary alternating magnetic field is produced and then an EC is be generated in the conductive composite material [ 131 ].…”

“…The occurrence of damages disturbs the generated EC due to the change of flow paths and consequently affects the secondary magnetic field. Thus, the damages in the composite can be detected [ 131 ].…”