# Measurement of Absolute Acoustic Nonlinearity Parameter Using Laser-Ultrasonic Detection  
[PERSON]  
1 School of Mechanical Engineering, Hanyang University, Seoul 04763, Korea; [EMAIL_ADDRESS] (S-H-P); [EMAIL_ADDRESS] (D.-G.S) 1  
[PERSON]  
2 Korea Atomic Energy Research Institute, Daejeon 34057, Korea; [EMAIL_ADDRESS] 2  
[PERSON]  
1 School of Mechanical Engineering, Hanyang University, Seoul 04763, Korea; [EMAIL_ADDRESS] (S-H-P); [EMAIL_ADDRESS] (D.-G.S) 1  
[PERSON]  
3 Department of Flexible and Printable Electronics, and LANL-IBNU Engineering Institute-Korea, Jeonbuk National University, Jeonju 54896, Korea; [EMAIL_ADDRESS] (Correspondence: [EMAIL_ADDRESS] ([PERSON]@hanyang. Currently, a piezoelectric detection method is the most widely used method to measure \(\beta\), owing to its advantages of high sensitivity for the measurement of sub-nanometer-scale displacements and convenience because it utilizes general NDE equipment [18]. [PERSON] et al. [19] proposed this method for the first time where they first measured \(\beta^{\prime}\) using the through-transmission method and calibrated \(\beta^{\prime}\) to \(\beta\) based on the transfer function using a pulse-echo method. They successfully obtained the \(\beta\) values for 6061 aluminum alloys (Al6061-T6) and fused silica. [PERSON] et al. [20] used the piezoelectric detection method to characterize the material microstructures and successfully investigated the characteristics of second-phase precipitation in heat-treated Al6061-T6 [16] and thermally aged CF8M cast stainless steel [21]. They also evaluated the yield strength by ultrasonic reconstruction of a nonlinear stress-strain curve [22] based on the \(\beta\) measurements and successfully estimated the yield strength of heat-treated Al6061-T6. However, the piezoelectric detection method uses a contact-type piezoelectric transducer, which requires a couplant and special shoe for effective application [23]. Moreover, it requires a complicated calibration to compensate for the frequency-dependent sensitivity of the transducer and material-dependent-coupling coefficient between the transducer and tested material, which requires repeating the calibration whenever either the incident wave frequency or test material is changed [24]. To address these issues, studies have reported the use of noncontact detection methods such as capacitive [25] and laser detection methods [26] to measure \(\beta\). [PERSON] et al. [25] used a capacitive receiver, which measures the change in capacitance between one side of a specimen and the receiver to measure \(\beta\) for the aluminum alloy specimens. [PERSON] et al. [26] used a Michelson-type laser interferometer to measure \(\beta\), which is a well-known method that provides a direct means of absolute displacement amplitudes. These methods can be applied without couplant and special shoes. They allow the user to neglect the effect of coupling at the contact interface compared to piezoelectric detection owing to their broad frequency range up to several megahertz [27]. However, they required a test specimen with an optically flat surface with a roughness level of approximately several nanometers, which makes it difficult to apply to the general NDE industry. In this study, a laser-ultrasonic detection method is used to measure \(\beta\), where a photorefractive interferometer based on two-wave mixing (TWM) in the photorefractive crystal (PRC) is used as the receiver. This method provides (1) noncontact and nondestructive measurement of \(\beta\); (2) the inspection ability of different materials without complex calibration owing to the direct ultrasonic displacement measurement, and (3) applicability for the \(\beta\) measurement even on the general milling machined surfaces of components because of the use of a scattered light beam. These advantages provide the potential for the quantitative characterization of hidden damage and material microstructures.