## 3 Development of New Applications for DIC
### Inspection of the Automated Fiber Placement (AFP) Process
#### 3.2.2 Proof of Concept  
In order to use DIC to measure the three-dimensional shape of composite tows and consequently extract any gaps and/or overlaps between them, there should be enough random textures with contrast on the tows and the substrate layer. However, composite tows made of carbon fiber and epoxy are usually black, and they are laid on a black substrate as well (previous layer), which makes it difficult, if not impossible, for DIC to capture the features of interest (i.e., gaps and overlaps). Applying a random pattern using a marker or spray paint is not a viable option during inspection, as it takes time to apply the pattern and it might introduce unwanted materials (i.e., marker ink) between composite layers. In order to overcome this difficulty, a random pattern was projected by a digital projector to a set of tows laid on a composite substrate. A regular digital projector (i.e., a computer data projector) with full High-definition (1920 \(\times\) 1080 pixels) capability was placed at the correct distance from the sample under inspection and projected a pattern to the area of interest. Since only the shape measurement was of interest in this application, and not strain measurement, there was no need for a pattern to be actually applied on the surface, and only projection served this purpose. After a regular calibration procedure, stereo DIC was used to evaluate the possibility of detecting the tows' locations and the gap between them.  
Figure 21a shows a flat panel consisting of a substrate layer and two tows, in which a random pattern was projected on them. As it can be seen from Figure 21c, DIC was able to detect both tows' locations and the gap between the ends of the tows. However, the shape measurement results were relatively noisy for inspection purposes; the main reason for this was due to the quality of the projected pattern on the composite layer. Since a digital projector was used, not only were the white dots projected, but the black background of the pattern was also projected, which made the pattern look noisy on the composite layer. It was suggested that an optical projector should be used instead of a digital projector to avoid this issue.  
In order to evaluate the potential of DIC to measure gaps and overlaps, assumed to be of good quality and have a random pattern, a spray technique was used to generate white dots randomly on a gap and overlap features between two tows, as shown in Figure 22. For evaluation purposes, two tows were placed in such a way to create a continuously decreasing gap, and the other two tows were placed to simulate continuously decreasing overlap. It was found that, if a good random pattern quality is used, gaps and overlaps as small as about 0.4 mm could be detected.  
Figure 21: (**a**) Two composite tows on a composite substrate. (**b**) Random pattern projection. (**c**) DIC shape measurement results, z-axis represents thickness direction.  
Furthermore, other types of defects beside gaps and overlap that might occur during AFP can be detected and inspected by DIC. Two examples of such defects are twisted tow and damaged tow. During AFP and when the AFP head places composite tows on the substrate, the placed rows might get twisted, which is referred to as a twisted tow defect, or the fibers might be pulled out by the compaction roller creating a damaged tow defect. Figure 23 shows the capability of DIC to detect such defects.