TWI Knowledge Summary
Ultrasonic testing
- A-Scan Thickness Survey
- A-Scan Weld Inspection
- Pulse Echo Imaging
- Time Of Flight Diffraction (TOFD)
- Surface Ultrasonics
- Long Range Guided Ultrasonics
A-Scan Thickness Survey
How does it work?
Thickness measurements are performed using a conventional flaw detector and a compression wave probe, which sends longitudinal waves into the component at normal incidence to the surface. Signals are displayed on the flaw detector screen in the form of an A-scan, in which the horizontal axis represents distance and the vertical axis represents signal amplitude. Since a 0° compression probe is being used, the horizontal axis is equivalent to depth from the scanning surface. When the probe is placed on the surface of the component, a reflection appears at a range corresponding to the thickness of the component at that point. The use of an A-scan display allows the operator to distinguish more easily between signals originating from embedded plate flaws and the nominal back wall response. Also, the dynamics of the back wall echo can be observed on the A-scan display to detect the presence of pitting.
Conventional twin-crystal 0° compression probes are generally used to detect hidden corrosion. However, where pitted surfaces are being assessed for remaining thickness, pencil probes are used. These have a pointed tip which is designed to fit into the pits, so that the remaining thickness can be measured where the external pitting is at its most severe.
What will it find?
Internal corrosion pitting and general erosion in most metals. A-Scan thickness surveys are also used for the inspection of parent material for inclusions and laminations.
Where is it used?
Generally used for thickness surveys on pressure vessels, pipelines, storage tanks and ship hulls.
A-Scan Weld Inspection
How does it work?
A strong specular reflection is required to resolve a flaw response from the background noise level with pulse echo ultrasonics. For planar flaws (cracks, lack of fusion, etc.) a specular reflection will only result if the ultrasonic beam is normal (or near normal) to the plane of the flaw. Angled beam shear wave probes are commonly used for the manual ultrasonic inspection of welds in ferritic steels, as these provide the only way of directing ultrasound into the weld body when the cap reinforcement is still present. Where a weld cap restricts probe movement, the sound can be reflected off the bottom surface and directed into the weld body under the cap.
Where sound is angled directly at the area of interest, this is referred to as "half skip testing". "Full skip" testing occurs when the bottom surface is used to reflect the sound before it enters the weld.
For a typical girth weld, a 45° probe is used for inspecting the root region, and 60°/70° probes for the sidewall fusion faces and weld body. The behaviour of the echo-dynamic pattern and shape of the flaw response (with respect to probe movement) can be used to identify the type of flaw, estimate the length and, in some cases, the through-wall height of the flaw.
Vertically orientated planar flaws can be a particular problem for detection using an angle probe in pulse-echo mode. However, a variation of angled shear wave ultrasonics is the Tandem technique, which is normally used for the detection of vertical flaws in thick section components. Two 45° shear wave probes are positioned in a jig, one behind the other facing the area of interest. The rear probe is used to transmit ultrasound into the joint area and the front probe receives sound reflected from flaws within the insonified region. By moving the probes relative to each other, it is possible to obtain full-through thickness coverage.
The type of material to be inspected affects the choice of angle probe. Shear wave probes are commonly used for examining welds in fine grained materials such as ferritic steels and aluminium. Welds in coarse grained materials such as stainless steels, duplex stainless, copper and composites have a severe attenuating effect on shear waves and can cause beam skewing effects at fusion faces. For welds in these types of materials, angled compression waves are used. However, these have a longer wavelength than shear waves, so there is a reduction in their resolving power. A-Scan weld inspection using angled compression wave probes can be very difficult due to the presence of spurious mode converted signals on the flaw detector display. Consequently, such probes are restricted to half skip testing and are preferably used in conjunction with an imaging system.
What will it find?
Most manufacturing flaws (lack of sidewall fusion, lack of root fusion, lack of root penetration, porosity, solidification cracking, etc.) and in-service flaws (fatigue cracking, stress corrosion cracking, etc.).
Where is it used?
Inspection of welds made in both ferritic and non-ferritic metals in pressure vessels, pipework, storage tanks, bridge structures etc.
Pulse Echo Imaging
How does it work?
By using the standard ultrasonic probes used for pulse-echo thickness measurement and weld inspection with a position encoded scanner device and appropriate software, images of flaws can be generated and electronically saved. Imaging systems provide a highly repeatable inspection and are capable of showing the extent of scan coverage. Due to their ability to store data, they can be used to "fingerprint" a component for comparison with any repeat inspections.
Programmed parameters such as material velocity and beam angles are used in conjunction with positional data to automatically plot flaw responses in the correct position on top, side and end view images showing the volume of material inspected. Many different systems are commercially available, offering two or three dimensional images. Each image view is known by a particular name as follows:
C-Scan image
This is another name for a top (or plan) view image. C-Scans can be obtained from immersion testing systems (where a 0° compression wave probe is scanned across an area through a water path, i.e. non-contact scanning) or from direct 0° contact scans.
Depending on the mode of operation selected, the colour coding levels on the image may represent signal amplitude or range. The latter case is used for automated corrosion mapping where on-screen cursors can be used to show the thickness at any point and sectional thickness plots.
B-Scan and D-Scan images
These images are usually through-thickness side and end view slices which are produced by scanning a probe beam in a linear fashion across an area of interest. The B scan is normally used as a transverse section through a weld and is taken in the scanning direction whereas the D scan is a longitudinal view and is taken in the index direction which is orthogonal to the scanning direction. The B-Scan image is normally acquired through a flaw where it has its greatest through-wall extent and provides an estimate of both remaining ligament and height. As the probe is moved, the A-Scan signals are recorded and plotted according to probe position, range and probe angle. Owing to the beam divergence, the response from a point reflector (e.g. pore) will be plotted along the beam axis even when it does not lie on it, causing arc shaped indications on the B-Scan image. These characteristic arcs vary in shape and size according to the width of the ultrasonic beam at different depths within the material.
Some of the more advanced imaging systems are also capable of generating an amplitude colour coded side view image and of storing all of the raw A-Scan data acquired. For automated scans, individual probes can either be "raster" scanned in the conventional manner, focused at a fixed stand-off (thereby targeting a particular depth zone) or part of a Phased Array.
What will it find?
Manufacturing flaws (lack of sidewall fusion, lack of root penetration, lack of root fusion, porosity, etc.), in-service flaws (fatigue cracking, stress corrosion cracking, corrosion, erosion, etc.) and parent material flaws (inclusions and laminations).
Where is it used?
Thickness surveys and parent plate or weld inspection on ferritic and non-ferritic pressure vessels, pipework, storage tanks, bridge structures, etc.
Time Of Flight Diffraction (TOFD)
How does it work?
Most ultrasonic techniques rely on receiving specular reflections from defects, even if only from particular facets. Time of flight diffraction (TOFD) detects flaws using the signals diffracted from the flaw's extremities. Two angled compression wave probes are used in transmit-receive mode, one each side of the weld. The beam divergence is such that the majority of the thickness is inspected, although, for thicker components, more than one probe separation may be required. When the sound strikes the tip of a crack, this acts as a secondary emitter which scatters sound out in all directions, some in the direction of the receiving probe. A 'lateral wave' travelling at the same velocity as the compression waves, travels directly from the transmitter to the receiver. The time difference between the lateral wave and the diffracted signal from the flaw provides a measure of its distance from the scanned surface. If the flaw is large enough in the through wall dimension, it may be possible to resolve the tip diffracted signals from its top and bottom, thereby allowing the through wall height of the flaw to be measured.
Due to the low amplitude of the diffracted signals, TOFD is usually carried out using a preamplifier and hardware designed to improve signal-to-noise performance. As the probes are scanned along the weld, the RF A-Scan signals are digitised and displayed in the form of a grey-scale image showing flaws as alternating white and black fringes.
Depending on which direction the probes are moved over the component surface, it is possible to construct 'end-view'; (B-scan TOFD) or 'side-view' (D-scan TOFD) cross-sectional slices. TOFD can also utilise Synthetic Aperture focusing or beam modelling software to minimise the effects of beam divergence, thereby providing more accurate location and sizing information.
TOFD is generally recognised as the most accurate ultrasonic technique for measuring the through-wall height of planar flaws that lie perpendicular to the surface and as a method for detecting and quantifying crevice corrosion at the weld root. At present, national standards for the application of TOFD exist, however, no acceptance criteria have been agreed upon.
What will it find?
The TOFD technique is suited for the detection and sizing of all types of embedded flaws, especially those planar in nature. However, the detection of small near the scan surface flaws can be more difficult due to the presence of the lateral wave response which often occupies several millimetres of the depth axis on images. The TOFD technique becomes less reliable when:
- there is a high density of defects;
- the material contains scattered inclusions; and
- with coarser grained material.
Where is it used?
TOFD is most widely used in the Petrochemical and Nuclear industries, for the inspection of butt welds in pressure vessels, process pipework, etc. TOFD is often used to provide critical flaw sizing data for input to Engineering Critical Assessments (ECA).
Surface Ultrasonics
How does it work?
As the name suggests, surface waves (or Rayleigh waves) travel along the surface of components, penetrating to a depth in the order of one ultrasonic wavelength. When a surface wave probe is used in pulse-echo mode, it is suitable for the detection of surface breaking flaws since, provided that the beam direction is normal to the plane of the flaw, sound will be reflected back to the transducer. Surface wave probes can also be used in transmit-receive mode, such that when the signal detected by the receiver is weakened or totally disappears, it signifies that a surface breaking flaw lies between them. Surface waves are sensitive to surface condition and will be attenuated by excess couplant left on the surface. Since energy is concentrated in the surface region, small blemishes on the surface can give rise to spurious indications.
Creeping waves are high angle compression waves, which propagate immediately beneath the inspection surface. They are used for a number of applications where surface-breaking or very near-surface planar flaws need to be detected. As creeping waves propagate, they interact with the inspection surface causing secondary shear waves to be emitted. This continuous transfer of energy, from one wave mode to another, means that creeping waves are attenuated rapidly and inspection is only effective over a relatively short range (40 - 50mm / 1.6 - 2.0"). For this reason they are normally used to inspect specific areas such as the toe of welds, where the probe can be placed in close contact with the area of interest. Unlike true surface waves, creeping waves are relatively insensitive to the condition of the inspection surface and do not require excess ultrasonic couplant or dirt to be removed.
What will it find?
Planar flaws that break the surface and lie at 90° to the scanned surface e.g. fatigue cracks in weld toes and stress corrosion cracking.
Where is it used?
Detection of environmental cracking in petrochemical plant and fatigue cracks in civil engineering structures such as bridges.
Long Range Guided Ultrasonics
How does it work?
In recent years much research and development work has gone into the development of techniques for the rapid screening of pipework for corrosion/erosion. This work has resulted in systems such as Teletest ® and Wavemaker TM. These systems make use of low frequency guided waves to detect corrosion/erosion in pipework. In the Teletest ® Multi-Mode system a tool comprising five rings of piezoelectric transducers is clamped around the pipe and ultrasound is sent in both directions along the pipe. Three of the rings excite longitudinal waves in the pipe and two excite torsional waves. Using this system, the wave mode can be optimised for any given pipe. Data from one particular direction of propagation is interpreted in a single test. The signal obtained is similar to a conventional ultrasonic A-scan, where the horizontal axis represents distance along the pipe measured in metres and the vertical axis represents signal amplitude, which is indicative of the severity of the corrosion. Unlike conventional A-scans, the signals are displayed from three different wave modes, namely symmetrical, horizontal flexural and vertical flexural. The relative intensities and characteristics of these three signals are important in identifying different distributions of corrosion. The recent development of the Teletest Focus ® system allow the transducers to be fired as a phased array and the ultrasound to be focused at a specific spot both along the pipe and around its circumference. These allow the circumferential extent and position of corrosion locating to be determined and improve the signal to noise ratio. Although propagation distances vary according to pipe geometry, contents, coating/insulation and general condition, it is not unusual that a range of up to 30m (100') in either direction from the transducer can be inspected. The technique is equally sensitive to internal and external corrosion, but cannot distinguish between them.
The principal advantage of this technique is that it provides 100% initial screening coverage, and only requires local access to the pipe surface (i.e. removal of a small amount of insulation) at those positions where the transducer array is to be attached. It is suitable for use on pipe diameters above 50mm (2.0") and on wall thicknesses up to 40mm (1.6").
Further information on TWI's long range guided wave ultrasonics activities, and information about the Teletest ® system and services provided using it.
What will it find?
Detection of both internal and external corrosion/erosion in thermally insulated, coated and buried pipework, corrosion under pipe supports and hidden welded joints, irregularities in girth weld shape.
Where is it used?
Petrochemical process pipework, oil and gas transmission lines, gas manifolds, offshore risers, jetty lines and power station boiler tubes.
Further information
TWI offers training courses on ultrasonic testing.
TWI Industrial Members and JoinIT North East users have access to the NDT Selector Toolkit
Additional information about ultrasonic testing can be found in the items detailed below:
FAQ: What are the main properties of a sound wave relevant to ultrasonic testing?
FAQ: What are the principles of ultrasonic examination in Non-Destructive Examination (NDE)?
FAQ: What are the advantages of using twin crystal ultrasonic probes?
Ultrasonic inspection - technology file
You can use the Weldasearch literature database to supplement what you find in JoinIT.
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