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Description  |
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The invention concerns a technique for determining the depth of defects
near the surface of a workpiece. The technique utilizes the fact that a
defect prevents the propagation from an angle probe of ultrasonic waves
scattered in the structure.
There exist a number of methods for determining the depth of a defect by
ultrasonic waves. However, none of these methods produce sufficient
accuracy when measuring from the surface of a workpiece the depth of a
defect located in the vicinity of the surface and, for example, oriented
perpendicular to it. This is usually the case in materials testing, since
defects frequently occur near the surface or originate from there.
Although it is possible to demonstrate such defects with suitable probes
by the ultrasonic pulse-echo method, such as using transversal or
longitudinal waves which are propagated at a small angle relative to the
surface, or also surface waves, the signals indicated, such as the echo
height or stationary, do not provide any accurate information on the depth
of the defect. In the case of machined surfaces, only defects of a depth
in the order of up to 5 mm can be ascertained from the echo height. If the
defect is deeper, no proportionality can be expected.
The object of the present invention is therefore to find a method which
permits measurements proportional to the depth of defect to be made for
defects which run even deeper, e.g. down to 30-40 mm. This was achieved by
an angle probe producing an obliquely incident longitudinal wave beam
creating scatter waves in the structure which are received by a second
angle probe designed as a directional receiver for ultrasonic waves, to
provide a maximum signal on a defect situated between both angle probes at
a position depending on the depth of the defect.
Instead of two angle probes of variable distance relative to each other,
this arrangement may be modified by using two angle probes with a bank of
several oscillators (usually piezo-ceramic elements) which are
electrically switched as desired to reflect a position change.
A further possible arrangement involves the use of specially-focusing (e.g.
line or dot focus) angle probes or oscillator devices to obtain a more
accurate resolution for determining the depth of a defect.
Another feature of the invention is the use of angle probes which emit and
receive longitudinal waves to avoid disturbing surface waves.
A variation in the arrangement of the method is the placing of an angle
probe on both sides of a defect on the surface where a defect has been
established by other means. One of the angle probes operates as
transmitter, the other as receiver. The height of the first signal (a
longitudinal wave, if surface waves have been eliminated) following the
transmitting pulse is picked up by the receiver and recorded. The beam
angles of both angle probes are equal, as is the distance of either probe
to the defect. The angle probes are moved in a manner maintaining an
equidistant position of each angle probe to the defect throughout every
phase of the movement. During the movement of the angle probes, the probe
acting as receiver records the height of the signal which consists of
longitudinal waves scattered by the structure. Since a defect, if deep
enough, shades the ultrasonic beam of the emitting probe where the probes
are arranged at short distances from the defect, the receiving probe
cannot receive any signals at a greater height from the structure. Only
when the maximum-intensity line of the sound beam of the emitting probe
reaches the lower limit of the defect, will there be a marked rise in the
height of the signal received from the structure. Thus the pattern of the
signal height above the distance of the probes from the defect identifies
the depth.
In yet another arrangement, the movement of the angle probes may be
replaced by several smaller probes arranged at an increasing distance from
the defect, or by several piezo-electric oscillating elements on a plastic
support, where the transmitter and the receiver combinations are switched
on successively to simulate the movement of the angle probes.
A further variation of the method is the arrangement of two probes fixed
equidistant from the crack and on both sides of it. By changing the angle
of the oscillators in the probe, longitudinal waves of differing angle of
incidence are produced in the workpiece to scan the depth of the defect.
Only when the lowermost limit of the defect is reached will the receiving
probe indicate noise signals from the structure.
Another possible arrangement is one in which a probe is fixed on one side
of the defect, and this may be a compressional wave probe for receiving
longitudinal waves scattered perpendicular to the surface, while the
second probe on the other side of the defect is an angle probe for
obliquely incident longitudinal waves. The second probe is either changed
with regard to its angle of incidence or its position to the defect, or
replaced by several oscillators at different distances to the defect.
The particular advantage of the invention lies in the fact that, due to
scattering at the structure, the depth of a defect can be determined from
the surface of a workpiece without serious disturbances caused by coupling
variations and other parameters which affect the absolute echo height in
normal ultrasonic pulse-echo methods and which tend to vary greatly. This
applies regardless of whether the defect runs to the surface or not.
The invention is explained in detail in the following drawings showing
different arrangements.
In FIG. 1, two angle probes are arranged on both sides of the defect to
produce and receive longitudinal wave beams at the same oblique angle of
incidence relative to the normal at the surface.
FIG. 2 shows the same arrangement as FIG. 1, but with the waves of both
probes incident at a different angle.
FIG. 3 shows two probes in which the incident and reflected angle of the
wave beam can be adjusted.
FIG. 4 shows an arrangement in which a probe for longitudinal waves
operates as transmitter and a compressional wave probe as receiver.
FIG. 5 involves an arrangement in which a probe provided with several
oscillators is positioned on either side of the defect.
FIG. 6 gives a typical oscillogram for an arrangement according to FIG. 1
or FIG. 2 if the probes in FIG. 1 or FIG. 2 are moved away from the defect
in uniform movement.
Details concerning the features of the invention are described in the
illustrations.
The two angle probes with the transmitting oscillator 3 and the receiving
oscillator 5 are shown in FIG. 1. The longitudinal wave beam 7 produced by
probe 4 excites a limited volume and creates scatter waves in it at the
structural inhomogeneities 12 which are received by the directionally
receiving probe 6 with the imaginary wave beam 8 from the limited volume
outlined. The structure of the workpiece 1 with the surface 2 has to be
taken into account insofar as relatively clear signals may initially be
expected where the grain is coarse, but which become fainter again if a
grain even coarser than the former is encountered, since the receiving
wave 8 also scatters at the structure. By selecting the working frequency
of the ultrasonic pulse (e.g. 2 or 4MHz) generated by oscillator 3, it is
possible to adjust the frequency to the workpiece and its structure as the
individual case may require. For example, the workpiece may consist of a
combination of ferritic base metal and an austenitic cladding applied to
the surface area. With regard to frequency, the same applies to the
arrangements shown in FIG. 2, 3, 4 and 5.
By selecting suitably shaped oscillators, the sound field of the
transmitting probe (beam 7 in FIG. 1) and the receiving probe (beam 8 in
FIG. 1) can be made to form a line focus or dot focus in the area where
the two wave beams intersect. If the defect 11 is situated in the centre
of the intersection area, probe 6 will only receive very weak scatter
signals. The intensity of the scatter signals received by probe 6 will
increase only when, by equidistant separation of the probes from the
defect, the distances 9 and 10 between the probes and the defect
projection to the surface have increased to the point where both sound
beams intersect below the defect.
The oscillogram 13 of an ultrasonic pulse-echo device shows the resulting
echo envelope curve 18 indicated as a dotted line in FIG. 6, in addition
to the transmission pulse 14, the received longitudinal wave pulse 15 of
the scatter signals, parasitic surface waves and transverse waves 16 and
the zero line 17 of the trace on the cathode-ray tube. In suitable test
blocks, the location of the steep rise of envelope curve 18 can be found
as a function of the depth of artificial defects. With the aid of a
calibration curve found in this manner, the depth of defects in workpieces
can be very closely determined.
FIG. 2 shows an arrangement similar to that in FIG. 1, but with different
angles of the sound beams at the transmitting and receiving probe. This
arrangement permits the method to be applied even where the existing
geometry is unfavourable if, for example, the space available on one side
of the defect location is insufficient to allow the necessary movement of
the probe.
FIG. 3, probes 4 and 6 have no fixed oscillators, but are arranged to be
turned so as to permit the angle of incidence and reflection of the sound
beam to be changed. As a result, the point of intersection of beams 7 and
8 may move along the entire extension of the defect until probe 6 receives
a clear scatter signal from the bottom end of the defect. By automatic
recording of the relationship between the scatter signal received and the
angle is it possible to use the artificial defects for exact determination
of the depth following calibration similar to that in the arrangement of
FIG. 1. In FIG. 4, a compressional wave probe is used which is capable of
receiving a longitudinal wave beam propagated normal to the surface,
instead of the receiving angle probe 6. In the case shown in FIG. 4, crack
11 completely shades the exciting wave beam 7, so that the compressional
wave probe 6 will hardly receive any scatter signals.
In FIG. 5, the necessary movement of both probes 4 and 6 in FIG. 1 is
replaced by fixed or stationary probes 4 and 6 which carry several
oscillators on a suitably shaped plastic support. The transmitting
oscillators 3, 3A, 3B, 3C and the receiving oscillators 5, 5A, 5B, 5C are
cemented in position at an angle to ensure that longitudinal wave beams
are emitted and received (7 and 8, respectively). The angles of incidence
can be adjusted to suit the geometrical configuration of the application
involved. In actual operation, the oscillators 3 and 5, 3A/5A, 3C/5C of
this probe are successively switched on with an ultrasonic pulse-echo
device, working as transmitters and receivers. With each individual
oscillator suitably focussed onto the intersection area at defect 11,
structural scatter signals of any intensity are only received when the
intersection point is at the bottom end of the defect (in the case shown,
located by 3C and 5C). This arrangement may also be adjusted by means of
artificial or simulated defects in test blocks. The following procedure
provides a further simple means for adjustment: in the case of the probe
positioned on the defect-free surface of a workpiece, all oscillator
combinations 3/5, 3A/5A, 3B/5B, 3C/5C are successively switched on and the
scatter signal set at the same height for each combination, which may be
done by varying the transmitting power or the sensitivity of the receiving
probe. If there is a clear change in the height of the scatter signal when
switching from oscillators 3B/5B to 3C/5C, this indicates that the
lowermost tip of the defect is located between the points of intersection
of the wave beams of combination 3B/5B and 3C/5C.
The arrangements shown in FIGS. 1-5 may be varied in a number of ways. For
example, the oscillators may be of different shape (narrow rectangles as
oscillators with cylindrical lenses placed in front to achieve a suitable
focus area, and the like), but all these possible variations have the
objective of utilizing the structural scatter in order to obtain
information as to whether the point of intersection of two imaginary wave
beams is in the area to which the tip of the defect extends or below it.
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