## 2 Conventional Digital Image Correlation (DIC) Applications  
### DIC- and Gage-Measured Strain Comparisons  
Two experiments were designed with the main goal of evaluating the accuracy of DIC strain measurements in composite structures. In both experiments, strain gages were used as a reference to evaluate the strain measurements obtained by DIC.  
In the first experiment, buckling under an axial compression test on a composite cylinder was performed, and a stereo DIC system (from LaVision Company, Ypsilanti, MI, USA called \"StrainMaster\") was used to measure strains. The composite cylinder was made of graphite/epoxy prepreg using hand layup and autoclave processes. Preparation of the composite cylinder included applying some random speckles on the cylinder's surface using a spray paint technique (speckle size 3-5 pixels in the area of interest, density about 50%). The test setup is shown in Figure 1. Before the start of the test, calibration of the cameras was performed by moving a standard calibration panel in front of the cameras.  
The composite cylinder was loaded through the aluminum end plates at a rate of 1 mm/min, and buckling occurred after 2 min from the start. The axial strain measured by a strain gage installed at the middle of the cylinder and the DIC system before and after buckling occurred is plotted in Figure 2a,b, respectively. As can be seen from Figure 2, before buckling, the measured axial strains by strain gage and the DIC system were in good agreement. However, after buckling occurred (around T = 120 s), the  
Figure 1: Axial buckling test setup for evaluating DIC [15].  
results from two techniques (i.e., strain age vs. DIC) did not agree quantitatively, although they had a similar trend. This could be attributed to the fact that after buckling, due to large local deformation, debonding between the strain gage and composite surface may have occurred. Accuracy for a specific test setup depends on many factors including speckle pattern size and quality, resolution of the camera used, lighting, stereo angle, lens selection, etc. For the setup used in this experiment (5 Megapixel camera system from LaVision, Ypsilanti, MI, USA), the accuracy for measuring the local strain could be expected to be around 200 microstrains [15].  
In the second experiment, with the goal of comparing DIC strain measurements with strain gage measurements, an L-shape composite angle was tested under tension using a universal testing machine. The experiment involved large deformations of the composite angle under tension. A unidirectional strain gauge was installed on the outer surface of the composite angle, and a random pattern was created on the composite angle surface using permanent marker (Figure 3).  
A stereo DIC system (from Correlated Solutions Company, Irmo, SC, USA called \"VIC-3D\") was used to measure strains. The strains measured by both strain gage and DIC are compared in Figure 4. As can be seen, DIC results show an overall good agreement with strain gage results. The deviation between DIC and the strain gage measurements was between 50 to 200 s of the test, which may be due to the large out-of-plane deformation of the composite angle as it was flatt  
Figure 3: L-shape composite angle with a random pattern and installed strain gage [16].  
Figure 2: Comparison between strain gage and DIC (**a**) before and (**b**) after buckling [15].  
out-of-plane deformation, the motion of the points was perpendicular to the image plane, which may cause error in DIC results. However, after 200 s of the test, as the L-shape angle flattened, an excellent agreement between DIC results and strain gage results was observed [16].