Measurement of acoustic nonlinearity with phase-sensitive nonlinear reverberation spectroscopy and noncontacting electromagnetic-acoustic transduction

The operating principle of all forms of nonlinear reverberation spectroscopy (NRS) involves measurements of time-dependent changes in resonant frequency and vibrational amplitude during free decay after acoustic excitation to extract information on nonlinearity and loss. An advantage of NRS over swept-frequency nonlinear resonance techniques is the short duration of resonant ringdown and associated minimal temperature drift during data acquisition. An innovation in NRS at NIST employs phase-sensitive reception and electromagnetic-acoustic transduction to provide additional advantages of simplified signal analysis, reduced noise, and elimination of damping and nonlinearity from contacting transduction. Phase-sensitive noncontacting NRS of axial-shear modes of an Al (0.2 at.% Zn) cylinder demonstrates a precision of measurements of fractional frequency shifts during resonant ringdown on the order of tens of parts per billion, exceeding the greatest reported precision achieved with nonlinear resonant ultrasound spectroscopy (NRUS). The technique has been employed to sense industrially relevant levels of porosity in additively manufactured (AM) aluminum cylinders, porosity in AM 17-4 stainless steel cylinders, and designed-in voids in AM CoCr specimens with complex geometries. The NRS measurements of AM aluminum, combined with Ritz modeling, X-ray CT, and SEM measurements, specifically provide evidence of nonlinearity from hysteretic friction between irregular surfaces of lack-of-fusion defects.