# Measurement of Absolute Acoustic Nonlinearity Parameter Using Laser-Ultrasonic Detection
## 5 Conclusions  
In this study, the measurement of the absolute acoustic nonlinearity parameter (\(\beta\)) was conducted using the laser-ultrasonic detection method, where a laser interferometer based on TWM in the PRC was used as the receiver. This detection method provides (1) noncontact and NDE to measure the \(\beta\) parameter; (2) owing to the direct ultrasonic displacement detection, an inspection ability of different materials without complex calibration when compared to conventional piezoelectric detection, and (3) applicability for general milling machined surfaces owing to the use of photorefractive interferometer. First, the absolute ultrasonic displacement amplitudes measured in the general milling machined surfaces (Ra: 0.64 \(\mu\)m) for the copper and Al6061-T6 specimens using photorefractive interferometric detection indicated high correlation with those measured on an optically flat surface (Ra: 8 nm) using conventional Fabry-Perot interferometric detection. For the copper and Al6061-T6 specimens with general milling machined surfaces, the absolute \(\beta_{Cu}\) and \(\beta_{Al}\) values measured by the proposed method were 3.4 and 4.9, respectively, which matched well with those of the conventional piezoelectric detection method and the cited reference ranges. These results indicate that the proposed laser detection method is \begin{table}
\begin{tabular}{c c c c} \hline  & \multicolumn{3}{c}{**Measurement of \(\beta\) Results**} \\ \cline{2-4}  & **Laser Detection** & **Piezoelectric Detection** & **Reference Range** \\ \hline
**Copper** & 3.4 & 3.2 & 2.1–3.5 [35; 36] \\ \hline
**Al6061-T6** & 4.9 & 5.0 & 4.5–5.69 [9; 37] \\ \hline \end{tabular}
\end{table} 
Table 2: Measured absolute parameters (\(\beta_{Cu}\)) and (\(\beta_{Al}\)). applicable to components with different material properties and rough surfaces. A future study is warranted to apply the proposed laser detection method to different materials and assess material degradation such as fatigue and thermal aging. Conceptualization, S.-H.P. ; methodology, S.-H.P., J.K., D.-G.S. and K.-Y.J. ; validation, S.-H.P. and S.C.; formal analysis, S.-H.P. ; investigation, S.-H.P. ; resources, S.-H.P. and K.-Y.J. ; writing--original draft preparation, S.-H.P. ; writing--review and editing, S.-H.P. and K.-Y.J. ; supervision, S.C. and K.-Y.J. ; funding acquisition, K.-Y.J. All authors have read and agreed to the published version of the manuscript. This work was supported by a Korea Institute of Machinery & Materials grant funded by the Korea government (MSIT) (NK230). Not applicable. The authors declare no conflict of interest. * [PERSON] et al. (2020) [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] Applicability of nonlinear ultrasonic technique to evaluation of thermally aged CF8M cast stainless steel. _Nucl. Eng. Technol._**2020**, _52_, 621-625. [CrossRef]
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