# Review on the Electrical Resistance/Conductivity of Carbon Fiber Reinforced Polymer
## 4 CFRP Conductivity Measurements and Techniques for Non-Destructive Testing Applications
### Delamination Measurements Based on Electrical Conductivity
#### 4.2.2 Electrical Resistance Change Method  
Most of the reported studies on CFRP testing focus on the electrical resistance change method (ERCM) for monitoring the bulk resistance of the CFRPs [58; 63; 139; 140]. The relationships between the recorded electrical resistance change and other conditions (loading conditions and/or mechanical degradation of materials) were obtained. Experimental findings were verified with finite element (FE) models. Electrical methods, such as ERCM, can sense the evolution and development of damages in CFRPs [140; 141; 142]. Electrical conductivity-based detection methods were extensively investigated. ERCM had been used to detect the internal damage of CFRP laminates. The through-thickness resistance is responsiveto delamination. Since CFRP was adopted as sensors for damage detection in the method, ERCM did not decrease fatigue strength or static strength. ERCM is suitable to existing structures. ERCM does not increase the structural weight and shows good monitoring performance in stiffness reduction caused by fatigue loads. The damage accumulation process can be observed [142; 143; 144; 145; 146; 147; 148; 149; 150; 151]. [PERSON] studied the electrical response of a conductive laminate with a delamination, experimentally confirmed the relationship between orthotropic electric conductance and fiber volume fraction, and explored the effects of measured orthotropic electric conductance on delamination monitoring through FEM analyses [32]. [PERSON] developed an electrical meso-model based on the consideration of transverse cracks with local delamination for in-plane loading and then increased the consideration of the out-of-plane loading [121]. The model indicates that the through-thickness and transverse conductivities of the ply can be changed by the transverse cracking with local delamination. The model's capability and accuracy were proven [121]. The fiber volume fraction has significant effects on the through-thickness and transverse electric conductance, and the through-thickness electric conductance significantly affects the delamination detection results with ERCM [143]. The influence of transverse cracks along with delamination was explored by the response surface method [144; 145; 146]. The method successfully identified the delamination size and location. The effects of matrix cracking on electric resistance changes between electrodes were explored by FEM analyses [144]. The data set of electric resistance changes can be obtained for calculating response surfaces through the simple calculation with a straight delamination crack model. The data normalization method can significantly improve the estimation performance [142]. Half of the applied electric current flowed in the surface layer along the fiber direction, whereas the other half tended to flow in the bottom surface layer along the fiber direction [146]. The difference caused the electric current flow along the thickness direction as well as a large electrical resistance change in front of a delamination crack. The same research group monitored the location and dimension of a delamination crack in the specimen through multiple electrodes mounted on the single surface [148]. The relationship between delamination locations/dimensions and measured electrical resistances were experimentally explored by response surface method and the least square errors method. The ERCM involves the application of two or four electrodes for sending current and different numbers of electrodes for measuring voltage [145; 150]. The differences between two-electrode ERCM and four-electrode ERCM are listed as follows [34; 151]:  
(1) Four electrodes (Figure 4): in the four-probe technique for piezoresistance analysis of CFRP composites, both the inner measurement electrodes and outer current introduction electrodes are mounted on the external laminate surface and subjected to the mechanical strain. Compared to the two-electrode technique, the four-electrode technique is suggested for monitoring the changes in resistance.