# A Review on the Deformation Behavior of Silver Nanowire Networks under Many Bending Cycles
## 4 Deformation Mechanism of Silver Nanowire Network under Various Deformations  
The interesting point about the silver nanowire network is the differences in the deformation mechanism according to the type of deformation. As Figure 9 indicates, compression stress was applied to a silver nanowire network when the network was positioned close to the center of the curvature (Figure 9a). By contrast, tensile stress was applied to the silver nanowire network when it was located away from the center of the curvature (Figure 9b). The amount of stress applied to the silver nanowire network was independent of the type of stress as far as the network was located from the same distance from the neutral plane.  
Figure 8: Resistance change of (**a**) copper thin films with different thicknesses and (**b**) silver nanowire networks with different density. (**c**) Low-magnification and high-magnification SEM images of copper thin films and silver nanowire network after the cyclic bending test. (**d**) Numerical calculation of change in the resistance of low-density and high-density silver nanowire networks, using the finite element method. Reproduced with permission from ref. [42]. Copyright 2019, Elsevier.  
The results of the cyclic bending test in Figure 10 show that the increase in resistance was dependent on the bending condition. Four hundred thousand bending cycles with a strain of 2.5% were imposed while measuring the in situ resistance. Even though the amount of stress was the same, the increase in the resistance of the silver nanowire network was higher for tensile bending than for compressive bending. In the case of the un-annealed silver nanowire network, the decrease in the resistance was higher when tensile bending was applied to those samples, indicating that the tensile strain causes a larger deformation than the compressive strain (Figure 10).  
This difference in the resistance change behavior depending on the stress type resulted from the differences in the deformation mechanism between the compressive force and tensile force. As Figure 11 shows, the silver nanowires extended along the direction of the tensile force, whereas buckling occurred for silver nanowires exposed to compressive strain. Owing to the buckling, stress was dispersed across the network and fewer failures occurred for silver nanowires under compressive strain. In addition, the buckling resulted in failure along the length of individual nanowires (Figure 12c,d), unlike the silver nanowire networks under tensile bending for which the failure occurred at the junction (Figure 12a,b). The different failure under compressive strain was due to the stress localization at the sub-grain boundaries formed by buckling as shown in Figure 13. Therefore, even if the sample was exposed to the same amount of strain, a smaller change in the resistance occurred for the compressive strain. This implies that silver nanowires show different deformation mechanisms under different types of strain; thus, different design factors should be applied depending on the status of stress imposed on the silver nanowire networks.  
Figure 10: Resistance change of (**a**) annealed and (**b**) un-annealed silver nanowire networks under cyclic bending. Reproduced with permission from ref. [43]. Copyright 2016, Elsevier.  
Figure 9: Schematic illustration of stress type depending on the position of silver nanowires on flexible substrates: (**a**) compressive strain and (**b**) tensile strain. Reproduced with permission from ref. [43]. Copyright 2016, Elsevier.  
Figure 11: SEM images of silver nanowire networks under tensile stress in (**a**) low and (**b**) high magnification, and compressive stress in (**c**) low and (**d**) high magnification. The strain value was 2.5% for both samples. Reproduced with permission from ref. [43]. Copyright 2016, Elsevier.  
Figure 12: SEM images of silver nanowire networks after 400,000 cycles of bending (**a,b**) under tensile strain and (**c,d**) under compressive strain. Reproduced with permission from ref. [43]. Copyright 2016, Elsevier.