## 4 Experiments and Discussion
### The Effect of Front-Surface Roughness on Notch Detection  
The main purpose of the experiment was to investigate the effect of rough and smooth surfaces on the ultrasonic detection capacity of back-surface micro-notches. The results were obtained first from rough front surfaces of testing samples while back surfaces were smooth, using the experimental equipment and parameters shown in Figure 10. Notches with depth 100 \(\upmu\)m, 150 \(\upmu\)m, and 250 \(\upmu\)m for the ten samples listed in Table 1 were measured in immersion coupled mode. For example, Figure 12 shows three average signals of notch 3 on samples with different \(R_{a}\). It can be seen that the mean \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Processing Method**} & \multirow{2}{*}{**Sample No.**} & \multirow{2}{*}{\(R_{a}\) (\(\upmu\)m)} & \multirow{2}{*}{\(R_{sm}\) (\(\upmu\)m)} & \multicolumn{2}{c}{**Notch Depth D (\(\upmu\)m)**} & \multirow{2}{*}{**Notch Width**} & \multirow{2}{*}{**Material**} \\ \cline{5-6}  & & & & **1** & **2** & **3** & & **W (\(\upmu\)m)** & \\ \hline \multirow{5}{*}{\
\begin{tabular}{c} CNC turning \\ (Rough surface) \\ \end{tabular} } & 1 & 3.2 & & & & & & \\  & 2 & 6.3 & & & & & \\  & 3 & 12.5 & 1320 & 250 & 150 & 100 & & \\  & 4 & 17.0 & & & & & \\  & 5 & 23.0 & & & & & 100 & Q23A Steel \\ \hline \multirow{5}{*}{
\begin{tabular}{c} Vertical CNC milling \\ (Rough surface) \\ \end{tabular} } & 6 & 3.2 & & & & & & 100 & Q23A Steel \\  & 7 & 6.3 & & & & & \\  & 8 & 12.5 & 900 & 250 & 150 & 100 & & \\  & 9 & 17.0 & & & & & \\  & 10 & 23.0 & & & & & \\ \hline \hline \multicolumn{2}{c}{Ginding (Smooth surface)} & All samples & 1.6 & 85 & 250 & 150 & 100 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of test samples with rough and smooth surfaces. CNC: Computerized Numerical Control. \(R_{a}\): the arithmetic average heights of rough surface. \(R_{sm}\): the horizontal parameter of rough surface. Figure 11: (**a**) The distribution of notches 1–3 on rough surface and smooth surface of test sample plate; (**b**) test samples 1–10; all steel plates are of the same size (240 mm \(\times\) 50 mm \(\times\) 90 mm). amplitudes of notch signals dropped rapidly and the average signal shapes changed with an increase in \(R_{a}\). In addition, the noise signals caused by the rough surface grew slightly with larger \(R_{a}\). There is good agreement with the simulation results. Figure 13a shows the relationship between the normalized mean amplitude of pulse-echo signal of notch and roughness \(R_{a}\) of front surface. The results indicated that the mean amplitude of notch signal was sensitive to notch depth. It can also be seen that larger \(R_{a}\) corresponds to weaker \(\overline{As_{max}}\) of notch signal; the mean amplitude of the deeper notch signal reduced rapidly with an increase in \(R_{a}\), and was the same when \(R_{a}\) was larger than 17 \(\upmu\)m. It indicated that the attenuation of energy intensity of pulse-echo increased with enhancement of multiple scattering caused by surface roughness \(R_{a}\). Considering the experimental uncertainties of the attenuation measurement, the agreement between simulation results and measured data was deemed satisfactory. Meanwhile, the relationship between signal amplitude ratio factor (SARF) and \(R_{a}\) is presented in Figure 13b. It can be seen that the SARF value decreased with \(R_{a}\) because of decrease in \(As_{max}\) of the notch signal and increase in noise signal. When the roughness \(R_{a}\) was lower than 12.5 \(\upmu\)m, the impact  
Figure 12: Ultrasonic time-domain mean signals of notch 3 on different experimental samples with front-surface roughness \(R_{a}\) = 3.2 \(\upmu\)m, 6.3 \(\upmu\)m, and 12.5 \(\upmu\)m.  
Figure 13: (**a**) Normalized mean signal amplitudes of notches reduce as front-surface roughness \(R_{a}\) increases; \(\Upsilon\)’ represents CNC turning, ‘M’ represents vertical CNC milling. (**b**) The signal amplitude ratio factor (SARF) reduces as front-surface roughness \(R_{a}\) increases.