# Multimode Guided Wave Detection for Various Composite Damage Types
## 3 Detection of Various Damage Types Using ABT-ABT on a 2-mm Quasi-Isotropic Composite Plate
### Composite Specim
#### 3.1.2 Impact Damage Due to Low-Velocity Impact  
A small coupon was cut from the 2-mm quasi-isotropic composite plate and was used to conduct the impact testing. A drop-weight impact tower was utilized to induce an impact on a fixed composite coupon. In this study, the coupon dimensions were changed from 152 mm \(\times\) 102 mm (ASTM D7136 standard dimensions) to 304 mm \(\times\) 204 mm. Note that the in-plane dimensions of the coupon were twice the size of the ASTM D7136 standard in order to make the panel size more realistic for guided wave propagation. In order to accommodate the larger plate, two rectangular frames were designed and manufactured. They were clamped to the ASTM D7136 fixture to conduct impact tests on the same drop-weight impact tower. The modified experimental setup is shown in Figure 5. The controlled impact event was recorded by a piezoelectric load cell, which could accurately record the data of the applied impact force (energy absorbed by the specimen during the impact event).  
Figure 4: Ultrasonic nondestructive testing (NDT) inspection results showing the delamination simulated by the Teflon insert: (**a**) B-scan image; (**b**) C-scan image.  
Figure 5: Dynatup 8200 drop-weight impact testing machine with modified fixtures.  
The details of the impact test are given in Table 1, which indicates that the coupon was impacted using a 3.06-kg impactor with a potential energy of 16.69 J. This energy was chosen using engineering judgment to try to obtain a 25-mm-impact damage size in the 2-mm-thick quasi-isotropic composite coupon.  
The force-time history and absorbed energy of the impact event are given in Figure 6. The load curve shows peaks at a certain maximum load and is parabolic in shape. When there are irregularities in the parabolic shape of the load curve, this indicates that the specimen has undergone extensive internal damage [30]. The maximum-recorded contact force of this impact event was 3.57 kN. The energy-time plot given in Figure 6 was taken from the force-time curve through integration of the data. The energy-time plot described the peak energy experienced and energy absorbed by the specimen. Here, the energy absorbed was the difference between the value at the end of the plot and the initial value. The difference between the peak energy observed and the energy absorbed could be used to determine the efficiency of energy absorption of the material. Furthermore, the energy-time history was able to demonstrate the percentage of impact energy that was absorbed by the specimen to create the irreversible process of damage. It was found that over 65% of the impact energy was absorbed by the specimen where the 25-mm-impact damage was formed.  
After the impact testing, an ultrasonic NDT inspection was conducted using a 10-MHz, 25.4-mm focused transducer. After an initial scan with the focus on the top surface of the specimen, the transducer was refocused at the depth where the impact damage size was the maximum for a clearer C-scan image. The NDT inspection results are shown in Figure 7. Multiple impact-induced delaminations across the thickness were observed in the B-scan results. It was found that the size of the impact damage obtained was around 25 mm (C-scan results).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mass (kg)** & **Drop Height (cm)** & **Velocity (m/s)** & **Energy (J)** & **\% of Absorbed Energy** \\ \hline
3.06 & 55.61 & 3.3 & 16.69 & 65 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Impact testing conducted on a 2-mm quasi-isotropic composite coupon.  
Figure 6: Force–time history and absorbed energy of the 2-mm [\(-\)45/90/45/0]\({}_{\text{2S}}\) quasi-isotropic composite coupon: (**a**) force history; (**b**) absorbed energy.