Glass fibre reinforced polymer (GFRP) composites have various applications, but due to the brittle nature of the epoxy matrix and reinforcing fibres, efficient damage evaluation are greatly needed for the inspection of composite materials. NDI methods such as the ultrasonic C-scan, distributed piezoelectric transducers or lamb waves have been employed, but still with some weaknesses. Recent studies show electrical resistance measurement, more sensitive than the traditional NDI method, has been utilized in GF/EP laminate composite.

Researchers with the University of Sydney, Institute of Process Engineering (IPE), Beijing Aeronautical Science & Technology Research Institute developed a new method of electrical resistivity tomography to assess low-velocity transverse impact damage in GF/EP composite laminates, and made a comparation with C-scan images.

Besides, GF/EP composite laminates with an epoxy matrix modified by CB and copper chloride (CC) were manufactured by the vacuum assisted resin infusion (VARI) technique in their work. The effects of CB nanoparticles and CC on improvement in Modes I and II interlaminar fracture toughness and impact damage resistance were investigated. Delamination growth was calibrated by in situ electrical resistance changes during interlaminar fracture tests. A damage index based on the change in electrical resistance was introduced.

The GF/EP laminates were prepared using a VARI technique in combination with hot pressing. A GF/EP laminate with a high fibre volume fraction of 61% was prepared. The interlaminar fracture toughness of the GF/EP laminates was characterized using the double cantilever beam (DCB) test (Mode I) and the end notched flexure (ENF) test (Mode II) on a universal material testing machine. The experimental results showed that the change in electrical resistance varied consistently with Mode I delamination growth. So they concluded change in electrical resistance could be used to complement conventional methods in defining the onset of delamination for determining Modes I and II interlaminar fracture toughness.

The drop-weight impact test was performed using an impact device. GF/EP laminate samples were impacted by 50%, 100%, 150% and or repeated impact 25% of standard energy, respectively, and the impact damage was measured on a matrix of conductive points in both in-plane and through-thickness directions.The image of the in-plane electrical resistivity tomography estimated the intensity of the presence of damage based on a network of electrical resistance measurements. The result of through-thickness electrical resistivity tomography was more closely associated with the actual damage states. Therefore, an enlightenment in this study comes: combination of the two methods may achieve practical and precise damage evaluation and may be a practical in situ damage monitoring technique.

The paper was published in Composites Part A : Applied Science and Manufacturing.