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Figure 4. Examples of AM for phononic crystals and acoustic metamaterials. a: Acoustic metamaterial that may enable acoustic cloaking, fabricated from titanium using a laser powder bed fusion (LPBF) technique known as direct metal laser sintering. Reproduced from Cushing (2022), with permission from the AIP. b: Acoustic metal foam with prescribed porosity enabled by metal LPBF. Reproduced from Konarski (2021), with permission from the AIP. c: AM acoustic “filter” that can be used to improve nondestructive evaluation measurements. Reproduced from Smith and Matlack (2021), with permission from the AIP.
structures have enabled researchers to explore materials that exhibit acoustic phenomena like acoustic cloaking (Cushing, 2022). In addition, small changes to complex geometric structures can allow researchers to finely tune the frequency range over which the acoustic metamate- rial operates (Arretche and Matlack, 2018). Recent work has even shown that AM can be used to create meta- materials with porosity as an additional design variable, as shown in Figure 4b (Konarski, 2021). Because AM makes it possible to fabricate complex geometries in many different length scales, acoustic metamaterials can now operate over a wide range of frequencies, from hertz up to megahertz frequencies.
AM has also opened the door to various applications of acoustic metamaterials, such as eliminating damaging vibrations from structures (Arretche and Matlack, 2018; Gerard et al., 2021). Recent work has shown how acous- tic metamaterials that are highly anisotropic, meaning that their mechanical properties are different along dif- ferent directions, can be used to guide acoustic waves that have large wavelengths compared with the size of the metamaterial (Yves and Alù, 2021). Such materials could be fabricated using AM techniques. Continued advancements in AM, including honing our ability to print multiple materials in the same structure or even print more advanced materials like piezoelectrics (Lewis, 2006), will certainly push what is possible in terms of acoustic wave control with acoustic metamaterials.
Applications in Ultrasonic Nondestructive Evaluation
The discrete layer-by-layer approach of AM means that the resulting materials, and thus their mechanical response, can be very different from their traditionally manu- factured counterparts. Furthermore, seemingly minor differences from one print to another, even using the same machine, can result in materials with different mechanical properties. One application of acoustics has been to use ultrasound as a nondestructive evaluation (NDE) tool to determine the mechanical and microstructural properties of AM parts. For AM to be successfully adopted by indus- tries that require highly precise part creation (e.g., nuclear, automotive, aircraft), reliable and fast NDE methods to qualify, characterize, and quantify damage in these new materials is crucial. The ability to accurately measure the properties of materials created using AM remains a critical challenge that, when properly addressed, will enable more widespread adoption of AM. Ultrasonic NDE methods are one promising approach to evaluate AM materials.
Various ultrasonic inspection techniques have been applied to AM structures, particularly metals. Ultrasonic parameters such as the wave velocity, attenuation, and nonlinearity coefficients have been shown to be capable of sensing AM-specific features such as texture or pores (Kim et al., 2021) and nanometer-sized defects (Bellotti, 2021). One way to assess these properties is to use a Rayleigh wave measurement setup as shown by Bellotti
Spring 2022 • Acoustics Today 53