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Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general.
Posted by: Yosi Bar-Cohen , E-mail: Yoseph.Bar-Cohen@jpl.nasa.gov, on September 19, 1996 at 09:40:40:In Reply to: Re: Smallest detectable flaw size, especially for low frequency test (f= 0.5 MHz) and in general. posted by : Rolf D. on September 12, 1996 at 10:24:22:
Ultrasonic methods of detecting flaws are using the perturbation
effect of flaws on the propagating ultrasonic waves. The
delectability of the wave depends on the flaw characteristics,
i.e., size and the material content.
Material content - This characteristics is related to inclusion
flaws. A simplistic rule suggests that the larger the acoustic
impedance mismatch with the host material the greater the flaw
detectability.
Flaw size - Flaws will scatter ultrasounds that are impinging on
them. A flaw that is larger than the wavelength will form a
backscattering directivety, i.e., reflection. Flaw size equal
or greater than the wavelength will cause a reflected amplitude
that is equal to the impinging wave amplitude. When the size of
the flaw is smaller than the wavelength, the scattered wave
amplitude drops exponentially as a function of the
flaw-size/wavelength ratio. As a general rule: the minimum
detectable flaw size is considered the value of half the
wavelength.
As an example, for a steel part that is tested at 0.5 MHz, the
velocity is about 6x10^6 mm/sec and the wavelength is
12 mm. Therefore, the smallest detectable flaw is 6 mm.
Overall this is only a guiding criteria. The actual detectable
flaw depends on numerous factors that include instrumentation
capability and inspector attentiveness & qualification. This
factors are used to determine the probability of detection (POD)
of the flaw.
Yosi