The extraordinary sensitivity of nonlinear ultrasonic waves to the early stages of material
degradation makes them excellent candidates for nondestructive material characterization.
However, distinguishing weak material nonlinearity from instrumentation nonlinearity
remains problematic for second harmonic generation approaches. A solution to this problem
is to mix waves having different frequencies and to let their mutual interaction generate sum
and difference harmonics at frequencies far from those of the instrumentation. Mixing of bulk …