## 3 LPBF Sensing Techniques
### Ultrasonic Techniques
Figure 33: Example provided by [187] showing the LU defect detection results of the LPBF Ni alloy sample (**a**,**c**) and the LPBF Ti alloy sample (**b**,**d**). **Resonant ultrasound spectroscopy (RUS)** is a measuring technique to quantify the resonance frequency of a material sample following a mechanical excitation [195]. RUS utilizes the excitation of the elastic stationary wave frequency inherent to a given material to determine the elastic tensor of the material and ultimately analyze part quality. Deviation from the natural frequency can indicate heterogeneities, such as part defects. The RUS technique can obtain the entire elastic tensor from a single crystal sample in a single rapid measurement [196], allowing for relatively fast, contact-free measurements. RUS local resonance behavior data depends on the specimen's local mechanical properties, often requiring assisting technologies to determine the exact locations of defects. [PERSON] et al. [197] studied the elastic mechanical properties of LPBF-fabricated SS316L samples using RUS. Spectra characteristics obtained from the RUS results showed that the first frequency peak splits and the separation behavior become consistent with the creation of undesired texture and inhomogeneous porosity distributions in part. [PERSON] et al. [198] investigated the utility of nonlinear RUS (NRUS) to predict fatigue life in cylindrical Ti6 Al4V LPBF-printed samples. NRUS was reported to be most sensitive to defects close to the displacement nodes and least sensitive to defects close to the strain nodes. [PERSON] et al. [195] investigated the feasibility of RUS for inspecting complex AM lattice structures with varying numbers of missing struts. Differences between pristine and defective lattices were distinctly identified using RUS. [PERSON] et al. [199] utilized RUS to analyze SB-CoNi-10C LPBF specimen part quality. EBSD texturing results were quantitatively in agreement with RUS results. [PERSON] et al. [200] investigated the viability of RUS in predicting porosities in the AlSi7 MgO.6 LPBF-printed part. The RUS results successfully indicate the presence of a lack of fusion porosities. **Phased array ultrasonic testing** (PAUT) is a technique that can transmit and receive ultrasonic wave information independently at different times using multi-element transducers. PAUT reportedly allows for reduced inspection times and can increase ultrasonic testing reliability and sensitivity [201]. According to the literature, bulk ultrasonic wave data from ultrasonic probes can be collected using the total focusing method (TFM), plane wave imaging (PWI), or their Fourier-domain counterparts. In the PAUT process of LPBF part defects, the extracted ultrasonic data can be converted to 2D images, which can be used to detect defects. PAUT has a fast inspection rate, an excellent repeatability factor, and is highly effective in corrosion detection, though it is considered one of the more difficult and complex ultrasonic testing equipment to operate. Additionally, PAUT is not advised for thin shell structures due to the presence of dead zones that can produce interface distortions of the image [202]. [PERSON] et al. [203] detected defects in aluminum LPBF components using a laser-induced phased array (LIPA) technique. The LIPA technique detected cylindrical defects (0.5 mm or less in diameter and as deep as 26 mm on the sample surface). [PERSON] et al. [184] used a high-frequency PAUT to evaluate the part quality of a cubic AlSi10 Mg sample fabricated using LPBF with intentional defects. XCT was used to validate the PAUT-produced data, and it showed excellent agreement, indicating that almost all artificial defects in the test sample were detected. [PERSON] et al. [202] utilized PAUT to detect defects in thick parts fabricated by LPBF, as shown in Figure 35. Practical defects, such as cylindrical lack of fusion defects, were generated by reducing the laser power at prespecified locations.