Nonlinear resonance ultrasound spectroscopy (NRUS) is a growing NDT technique that tracks amplitude-dependent changes in resonance frequency to assess the presence of damage, such as dislocations and micro-cracks . The amount of frequency shift with increasing amplitude is typically quantified with the nonclassical nonlinear parameter f . NRUS has been used in metals to study fatigue damage ,  , creep , stress corrosion cracking , thermal aging –, and porosity . Unlike linear techniques, nonlinear approaches are sensitive to the presence of micro-damage, which is of great interest to uncover early forms of damage, and help setting out adequate maintenance plans. Additionally, resonance approaches enable quick inspection of parts, irrespective of how complex the geometry is. A typical way of conducting NRUS tests is to bond a piezoelectric (PZT) disc to the sample. The PZT is used as a source and allows one to achieve large excitation amplitudes (nonlinear elastic regime with dynamic strain of order ~10-5). Unfortunately, the bond between the sample and the disc introduces additional elastic nonlinearity, and measurements must be repeated several times, with ungluing/regluing of the PZT in-between each test. Moreover, the intrinsic nonlinearity of the sample cannot be determined, and only relative differences in nonlinearity across a set of samples can be examined, that is, only relative measurements can be made. This makes consistent and repeatable measurements difficult to achieve . Being able to conduct NRUS tests in a non-contact fashion would be transformational, making the measurements much faster, more reproducible, and absolute. Recently, air-coupled transducers have been used as an excitation source for NRUS tests on small metallic prismatic specimens –, reaching dynamic strains of order ~2×10-6. A “candy can” cavity with multiple PZTs has also been used successfully as an excitation source to conduct nonlinear measurements ,  . The cavity consists of a focusing chamber with an aperture for sample excitation. Combined with a time reversal technique, the cavity achieves higher incident pressure and narrower focus than commercial focused air-coupled transducers , . If a high enough excitation can be achieved, it could likely be used as an excitation source for NRUS testing. Pulsed Nd:YAG lasers have also been used as a non-contact excitation source for resonance ultrasound spectroscopy (RUS) , . Such laser could be used for NRUS testing as well, in combination with signal processing developed for impact-based NRUS, when a single hammer impact excitation is used for NRUS testing –. The objective of this study is to compare the NRUS parameters of additively manufactured (AM) and wrought cylindrical 316 stainless steel samples with four different heat treatments. We compare the results of four different excitation methods: in-contact piezoelectric discs, air-coupled transducers, a “candy can” cavity, and laser excitation.
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