This technique is used for the detection of internal and surface
(particularly distant surface) defects in sound conducting materials.
principle is in some respects similar to echo sounding. A short
pulse of ultrasound is generated by means of an electric charge
applied to a piezo electric crystal which vibrates for a very
short period at a frequency related to the thickness of the crystal.
In flaw detection this frequency is usually in the range of one
million to six million times per second (1 MHz to 6 MHz). Vibrations
or sound waves at this frequency have the ability to travel a
considerable distance in homogeneous elastic material, such as
many metals, with little attenuation. The velocity at which these
waves propagate is related to the 'Youngs Modulus' for that material
and is characteristic of that material. For example the velocity
in steel is 5900 metres per second, and in water 1400 metres per
energy is considerably attenuated in air, and a beam propagated
through a solid will, on reaching an interface (e.g. a defect,
or intended hole, or the backwall) between that material and air,
reflect a considerable amount of energy in the direction equal
to the angle of incidence.
contact testing the oscillating crystal is incorporated in a hand
held probe which is applied to the surface of the material to
be tested. To facilitate the transfer of energy across the small
air gap between the crystal and the test piece, a layer of liquid,
usually oil, water or grease, is applied to the surface.
mentioned previously, the crystal does not oscillate continuously
but in short pulses, between each of which it is quiescent. Piezo
electric materials not only convert electri-cal pulses to mechanical
oscillations, but will also transduce mechanical oscillations
into electrical pulses; thus we have not only a generator of sound
waves but also a detector of returned pulses. The crystal is in
a state to detect returned pulses when it is quiescent. The pulse
takes a finite time to travel through the material to the interface
and to be reflected back to the probe.
normal method of presenting information in ultrasonic testing
is by means of a cathode ray tube, in which horizontal movement
of the spot from left to right represents time elapsed. The rate
at which the spot moves is such that it gives the appearance of
a horizontal line on the screen. The system is synchronised electronically
so that at the instant the probe receives its electrical pulse
the spot begins to traverse the screen. An upward deflection (peak)
of the line on the left hand side of the screen is an indication
of this occurrence. This peak is usually termed the initial pulse.
the base line is perfectly level the crystal is quiescent. Any
peaks to the right of the initial pulse indicate that the crystal
has received an incoming pulse reflected from one or more interfaces
in the material. Since the spot moves at a very even speed across
the tube face, and the pulse of ultrasonic waves moves at a very
even velocity through the material, it is possible to calibrate
the horizontal line on the screen in terms of absolute measurement.
The use of a calibration block, which produces a reflection from
the back wall a known distance away from the crystal together
with variable controls on the flaw detector allows the screen
to be calibrated in units of distance, and therefore determination
of origins of returned pulses obtained from a test piece.
is therefore possible not only to discover a defect between the
surface and the back wall, but also to measure its distance below
the surface. It is important that the equipment is properly calibrated
and, since it is in itself not able to discriminate between intended
boundaries of the object under test and unintended discontinuities,
the operator must be able to identify the origin of each peak.
Further as the pulses form a beam it is also possible to determine
the plan position of a flow.
height of the peak (echo) is roughly proportional to the area
of the reflector, though there is on all instruments a control
which can reduce or increase the size of an indication - variable
sensitivity in fact. Not only is part of the beam reflected at
a material/air interface but also at any junction where there
is a velocity change, for example steel/slag in a weld.
all faces of a test piece not only discovers the three dimensional
defect and measures its depth, but can also determine its size.
Two dimensional (planar) defects can also be found but it is best
that the incident beam impinges on the defect as near to right
angles to the plane as possible. To achieve this some probes introduce
the beam at an angle to the surface. In this manner longitudinal
defects in tubes (inner or outer surface) are detected.
of the indications on the cathode ray tube requires a certain
amount of skill, particularly when testing with hand held probes.
The technique is, however, admirably suited to automatic testing
of regular shapes by means of a monitor - an electronic device
which fits into the main equipment to provide an electrical signal
when an echo occurs in a particular position on the trace. The
trigger level of this signal is variable and it can be made to
operate a variety of mechanical gates and flaw warnings.
the velocity of sound in any material is characteristic of that
material, it follows that some materials can be identified by
the determination of the velocity. This can be applied, for example
in S.G. cast irons to determine the percentage of graphite nodularity.
This process can also be automated and is now in use in many foundries.
A typical equipment is the 'Qualiron'.
the velocity is constant, as it is in a wide range of steels,
the time taken for the pulse to travel through the material is
proportional to its thickness. Therefore, with a properly calibrated
instrument, it is possible to measure thickness from one side
with an accuracy in thousandths of an inch. This technique is
now in very common use. A development of the standard flaw detector
is the digital wall thickness gauge. This operates on similar
principles but gives an indication, in LED or LCD numerics, of
thickness in absolute terms of millimetres. These equipments are
easy to use but require prudence in their application.
of Ultrasonic Flaw Detection:
and lengths up to 30 ft can be tested.
size and type of defect can be determined.
sensitive if required.
of being fully automated.
to only one side necessary.
of Ultrasonic Flaw Detection:
require interpretation (except for digital wall thickness gauges).
degree of skill necessary to obtain the fullest information
from the test.
thin sections can prove difficult.
NDT has manufactured a wide variety of Automatic Ultrasonic Inspection
Equipments, ranging from testing small powered metal valve guides
to testing steel mill caster rolls. Details of some equipments
are given on this website.
is taken from the Insight NDT technical paper entitled 'A Brief
Explanation of Non-Destructive Testing Methods'. A copy of the full
paper in Adobe Acrobat format is available by clicking Here.
of other Insight NDT technical papers relating to Ultrasonics please
refer to the 'Technical Papers' section of
this website, which is available from the main menu.