The above-mentioned results were obtained by the quasi-
static experiments. Further work has also been performed on
fatigue loads affecting the MBN signal. Kettunen and Ruuskanen
[98] measured the MBN signals of two low-carbon structural
steels under tension–compression fatigue cycles. It was found
that the MBN signals increased in the beginning and then kept an
approximate constant value without large oscillations until the
final failure of the specimens. Karjalainen and Moilanen [99]
further tested the MBN signals of the mild steel under the
constant deflection bending fatigue. It was demonstrated that
the overall signal amplitude decreased during the fatigue test. In
some cases,there existed a sudden increase in the signal level