Nonlinear Acoustic Shear Wave Detection of Zero Gap Kissing Bonds in Encapsulated Packs
Nonlinear acoustic shear wave harmonic detection resolves zero gap kissing bonds in encapsulated battery packs by measuring contact acoustic nonlinearity.
Second Harmonic Generation
Meaning
Ultrasonic inspection methods analyze the frequency shifts that occur when a sound wave propagates through a material with micro-structural defects. The phenomenon of second harmonic generation occurs when a monochromatic acoustic wave passes through an elastic medium containing micro-cracks, generating a signal at twice the excitation frequency. This frequency doubling is driven by the nonlinear elastic response of the material under dynamic strain.
In quality control, measuring the amplitude of this second harmonic provides a direct measure of the micro-scale damage, such as fatigue or micro-voids, before macro-defects appear. The technique is highly sensitive to early-stage material degradation.
Elastic Origin
Acoustic wave propagation through a material with closed cracks experiences a non-symmetric stress-strain relationship. Compressive stresses are transmitted across the crack faces, while tensile stresses are not, causing the wave to distort. This distortion introduces even-order harmonics into the acoustic spectrum, with the second harmonic being the most prominent.
The amplitude of this harmonic increases with the density of the micro-defects.
Experimental Procedure
Receiver sensors are tuned to the second harmonic frequency while filters suppress the primary excitation frequency to ensure accurate measurement. Transmitters must be driven at a low distortion level to avoid introducing system-level harmonics into the material.
Quality Assessment
Manufacturing audits use this metric to evaluate the quality of laser welds in battery packs. Low second harmonic amplitudes confirm the mechanical joint is solid and free of micro-cracks.