## 2 Conventional Digital Image Correlation (DIC) Applications
### Bending Behavior of Thick-Walled Composite Tubes
#### 2.3.3 DIC Measurements  
**Deformation results:** Axial and hoop deformations captured by the first camera pair located in front of the tube is shown in Figure 13 just before failure. Axial deformation results (Figure 13a) showed that in the upper part of the tube, due to compression, both sides were moved towards each other, while in the lower part, due to tension, they were moved away from each other. This indicates the smooth transition of the load to the test article. The tube deformation in the hoop direction (Figure 13b) revealed that maximum deflection happened at the mid length of the tube, closer to the neutral axis than the lower part. This is an interesting finding and can be explained due to ovalization of the tube cross section.  
Axial and out-of-plane deformations captured by the second pair of cameras looking to the top of the tube is shown in Figure 14 just before and after failure. Figure 14a shows that both sides of the tube deformed toward each other, and the axial deformation was zero at the middle of the tube. Maximum out-of-plane deflection before failure occurred at the mid length of the tube as expected (Figure 14c). After failure, the location of the maximum deformation moved below the failure zone (Figure 14d).  
Figure 12: Bending test setup including the DIC system and thick composite tube (after failure) [16].  
Figure 13: Deformation of the tube from the front view just before failure: (**a**) axial deformation \(u\) (mm), (**b**) hoop deformation \(v\) (mm) [16].  
**Strain results:** Figure 15 shows the contour plot of the axial strain just before and after failure measured by the first camera pair located in front of the tube. While it shows a negative axial strain in the upper part and a positive axial strain in the lower part, the absolute strain values at the top and the bottom side of the tube were nearly the same as expected in a typical pure bending case (Figure 15a). After failure (Figure 15b), the maximum axial strain was accumulated at the failure location on the upper right side of the tube.  
Axial strain measured by the second camera pair located above the tube, just before and after failure, is shown in Figure 15. Just before failure (Figure 15c), the maximum compressive strain was measured at the top position in the tube, and one could expect the failure location by observing the accumulation of the axial strain at the right side of the tube (purple color pattern). After failure (Figure 15d), the maximum compressive strain accumulated around the crack that propagated parallel to the fiber directions of the outer layer [16]. The failure mechanism can be explained by a kink band mechanism [17; 18]. Kink bands develop under compression stress due to plastic microbuckling induced by nonlinear matrix deformation. It is a complex phenomenon affected by many factors like fiber failure, matrix failure, fiber-matrix interface strength, etc. For the tube under bending (Figure 15c), delamination started under the outer layers (i.e., [25; 26; 27] layer) at the top right side of the tube accompanied with matrix cracking causing the delaminated layers to slide parallel to the fiber direction (i.e., 25 degrees) towards the neutral axis of the tube. This sliding motion removes the constraint on the fibers in the layer below (i.e., \(-25\) degrees) causing microbuckling [16].  
Figure 14: Axial deformation \(u\) (mm) just before (**a**) and after (**b**) failure. Out-of-plane deformation \(w\) (mm) just before (**c**) and after (**d**) failure [16].