Fatigue and other cracks, found either visually
or with the aid of such techniques as magnetic particle inspection,
in machinery and plant invariably pose the problem that whilst the
position and length are easily ascertained, the depth to which the
crack penetrates, is not immediately obvious. However, it is usually
the depth of the crack, which determines the fate of the defective
component; whether or not the item can continue in service under
observation with planned remedial action by repair or replacement,
or whether the item has to be immediately scrapped.
As an aid to gaining full information about the
crack we have developed the QUADENT crack depth meter. With this
portable instrument the depth of a surface breaking crack can be
quickly and easily measured. Little skill is necessary and the information
is presented in absolute terms of depth. The propagation rate can
be determined and in longer cracks the profile can be plotted.
The QUADENT can be used on steel, iron, stainless
steel or any metals of relatively low conductivity.
principle of AC potential drop
range 0 to 100mm
±10% of reading or 1mm whichever is the greater
with Off/Charge, Metric and Inches settings
Metric to 0.1mm or Inches to 0.01 Inch
with an internal charger
Up to 1000
measurements from a fully charged battery
four hardened steel, spring loaded, electrodes
in a housing incorporating
an automatic zero button
Optional calibration block for calibration verification
Optional spare sensor probes and contact pins
The measurement of crack depth uses the principle of electrical
AC potential drop. In this method the increase in potential drop
in a current passing between two electrodes in contact with a sound
surface adjacent to the crack, is compared to the voltage drop when
the crack is situated between the two electrodes. Since the current
traveling around the crack effectively increases the distance between
the measuring electrodes the voltage drop will be greater. The increase
is proportional to the crack depth.
Current passed between two outer electrodes A & D
Voltage measured between electroded B & C
The AC current between electrodes A & D is approximately 0.5
amp at a frequency of 1500Hz. At this frequency the current flows
preferentially close to the surface (skin effect) and therefore
closely follows the contours of the crack. Even in a very tight
crack with fracture faces in contact, the path of least resistance
is still around the bottom of the crack.
The application of the electrodes to the material surface is by
means of a hand held probe. This houses four spring-loaded electrodes
so that they will conform to curved surfaces.
The mode of application is to apply the probe to the surface of
the material adjacent to, but not affected by, the crack and press
the zero button integral in the probe. This action compensates for
different conductivity characteristics of the material so that in
effect the increase in voltage drop is related directly to the unaffected
surface. The display will indicate 0.0. Following this procedure
the electrode probe is moved so two middle electrodes straddle the
crack - one on each side. The path length being effectively extended
by the presence of the crack the display will indicate the measured
The QUADENT will not only measure crack depth but has the ability
to discriminate between surface marks, scores etc., which have the
appearance of cracks, and genuine cracks. If during the measurement
process the LED displays 0.0 then there is not a crack.
||0 - 100mm
(0 - 4)
of indicated depth or 1mm (0.040) whichever is the greater
||3½ digit red LED
or Inches, switch selectable
powered with integral charger. Battery Low indicator.
to operate from 110V or 230V.
to 1000 measurements from fully charged battery
three-position switch. Off/Charge, Metric & Inches
ABS plastic with handle 275mm (10.75") by 250mm (9.75")
by 75mm (3.0")
hardened steel, spring-loaded, electrodes in SRBP housing incorporating
automatic zero button. Including connecting cable
steel relative permeability 240 with graduated slot 0-10mm depth
is supplied, with comprehensive instructions, in a rigid carrying
case with foam cut outs to accommodate the various items.
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