# Detection of Impact Damage on PVA-ECC Beam Using Infrared Thermography
## 4 Results  
### Warm-up Period Analysis  
The proposed procedure to detect impact damage has been applied to a PVA-ECC beam specimen. It should be noted that a micro-crack began to occur on the specimen's front side surface after the 6 th impact. Then, as the impact tests continued, the micro-crack expanded and grew upwards. The thermal contrast images from the 1 st to the 5 th were shown in Figure 3. These figures represent the thermal signatures before the micro-crack appeared. As shown in Figure 3, the temperature differences of these first five impacts were similar. This is consistent with the result that no crack appears on the front side surface of the specimen. In the details, the temperature differences increased and concentrated gradually near the impact point of the specimen, especially after the 5 th impact. It can be observed that there is a high temperature abnormal zone on the right side area of the image after the 5 th impact, as the arrow indicates in Figure 3. After the 5 th impact, the temperature contrast increased and the maximum value of the legend reached to 4.5 \({}^{\circ}\)C. A large area of high thermal contrast occurred on the right side area of the images. The thermal contrast in right side area was higher than that in the left side area. It means that the thermal energy was concentrated continuously on the right side area of the specimen after the 5 th impact. Although we cannot find any micro-cracks on the surface until the 6 th impact, we could speculate that there may be buried or subsurface cracks inside the specimen.  
Figure 3: Thermal contrast of first five impacts in the warm-up period (The arrow indicates the high temperature abnormal zone).  
After the 6 th impact, a vertical and narrow micro-crack appeared on the surface (Figure 4). From this point, the thermal contrast images were totally different from the first five, as shown in Figure 5. All the four thermal contrasts reached 4.5\({}^{\circ}\)C and the temperature distribution was completely different, compared with the ones in Figure 3. It seems that the original high temperature zone on the right side area of the specimen suddenly cooled. The high thermal contrast zone was replaced by low thermal contrast zone. That may be interpreted by the explanation that there was a sudden energy release when the crack appeared on the surface. As a composite material, the PVA-ECC beam is comprised of materials with disparate thermal expansion coefficients. It will develop residual thermal stresses when subjected to a uniform thermal excitation [58]. If the beam develops cracks, the residual and mechanical stresses will change to accommodate the fracture surfaces. The redistribution of residual and mechanical stresses releases strain energy that can drive crack propagation--the energy release for crack growth [59; 60]. As the rectangular box indicates in Figure 5, an elongated abnormal zone corresponding to the cracks occurred. As the impact continued, the crack expanded upwards and the abnormal temperature zone became longer and wider, which is easier to observe. In general, the wider the crack width, the quicker the maximum temperature difference is reached. The temperature difference increases when the area of crack increases.  
Figure 4: Photograph of the specimen’s surface before and after impacts.