The most common UT inspection techniques involve the use of bulk waves in which the boundaries of the structure are just reflectors that do not fundamentally change the mode of propagation. Bulk waves only have two modes; longitudinal and shear, and are typically used to inspect areas near the transducer.
Guided Wave (GW) testing on the other hand is a technique in which the ultrasonic waves propagate through the boundaries of a structure, and these boundaries actively affect the mode of propagation. Unlike bulk waves, there could be hundreds of guided wave modes with different velocities and frequencies on a given structure. The number of modes exponentially increases the complexity in the analysis, but also opens the door to countless new applications of UT.
Since the boundaries of the structure permit ultrasonic waves to travel long distances with little loss of energy, one of the most common applications of guided waves is to cover long expanses of material from a single point of inspection. These applications are commonly referred to as Long Range UT (LRUT) and Medium Range UT (MRUT), depending on the distance covered. Under this classification, Short Range UT is used exclusively for bulk wave techniques (normal beam, angled beam) since they are typically used to inspect the area immediately below or in close proximity of the transducer.
Long Range UT (LRUT) refers to the use of guided waves for inspection of hollow cylindrical structures, such as pipes and tubes, using a ring of transducers placed around the structure. The interaction of all the beams from the different transducers contained within the tube geometry create a unified wave front that fills up the tube geometry and permits traveling long distances. Depending on the tube material and interfacial boundary conditions (e.g. tube coatings, surrounding soil / cement, liquids or solids inside the pipe), LRUT rings can be used to inspect up to approximately 300 ft (100 m) in front and behind the inspection ring.
The two available types of commercial LRUT rings are piezoelectric and magnetostrictive EMAT. In case of piezoelectric systems, the transducers are pressure-coupled or glued onto the pipe. On magnetostrictive EMAT systems, a highly magnetostrictive thin strip made out of FeCo or a similar material is also pressure-coupled or taped / glued on the tube, and an EMAT coil is positioned on top to generate the ultrasound on the strip, which is subsequently coupled into the component.
Medium Range UT (MRUT) is a term introduced in the last decade to differentiate between bulk wave (short range) UT techniques and LRUT. MRUT uses Lamb or Shear Horizontal modes and can be used to inspect tubes and pipes, as well as plates / tanks and other non-round structures. While LRUT requires complex analysis of multimodal tubular modes (flexural, longitudinal, and torsional), MRUT uses single-sensor guided wave formulations which are much easier to calculate and interpret. The distance covered with MRUT varies with sensor aperture and tube diameter, but due to beam spread, maximum coverage is approximately 9 ft (2.7 m) in tubes, and 16 ft (5 m) in plates and tanks. In addition to easier interpretation, the main advantages of MRUT over LRUT are the smaller dead zone (typically less than 50 mm vs 1 to 2 m for LRUT), and higher resolution and defect detection capabilities, which can be up an order of magnitude greater than LRUT.
There is not a theoretical limit in terms of maximum thickness that can be inspected with guided waves, but for practical purposes, the limit is often set at around 1” (25 mm). Thickness up to 1.5” (37 mm) is also possible, but the presence of multiple modes can make interpretation difficult.
Ultrasound in EMAT guided waves can be generated using two methods:
As explained under the EMAT Technology section, for guided waves with Lorentz force, ultrasonic waves are induced into a test object with two interacting magnetic fields within the EMAT sensor. A relatively high frequency (RF) field generated by electrical coils interacts with a low frequency or static field generated by magnets to generate a Lorentz force in a manner similar to an electric motor.
This disturbance is transferred to the lattice of the material, producing an elastic wave. In a reciprocal process, the interaction of elastic waves in the presence of a magnetic field induces currents in the receiving EMAT coil circuit.
Techniques that use Lorentz forces (such as the MRUT-Lamb technique) require the sensors to be in close proximity to the part, but can handle some lift-off caused by coatings or contamination. Although it varies with frequency, MRUT-Lamb techniques can typically handle up to 3mm lift-off.
Innerspec’s patented guided waves using magnetostriction method to produce ultrasonic waves requires the adherence of a magnetostrictive strip (made of FeCo) on the inspected part. This strip can be pressure-coupled or taped / glued to the inspected part.
With the magnetostrictive sensor, the inspector will swipe the magnetostrictive strip with a permanent magnet (in the sensor’s direction of inspection). This action generates a biased magnetic field on the strip. The interaction between the biased magnetic field and the dynamic field induced by the tangential current flow within the sensor’s coil creates magnetostrictive strains on the strip. In turn, this strain induces ultrasonic waves in the magnetostrictive strip, which is then transferred to the lattice of the inspected material.
Magnetostrictive techniques (such as the MRUT with Shear Horizontal wave modes technique) require that the strip is applied to the base material. The material can have paint or epoxy as long as it is hard and strongly adhered to the part. Once the strip is properly coated to the pipe, the thickness of coating is not relevant.
The MRUT-Lamb technique is a guided waves technique that uses Lamb waves to generate ultrasonic scans. The Lamb waves are generated with EMAT Lorentz force. The MRUT-Lamb technique can be used in two different configurations:
Features of the MRUT-Lamb technique:
• Range | Up to approximately 5 ft. on each side of the sensor.
• Scan Speed | Up to 1” per second.
• Coatings | Inspection of coatings of up to 0.125” (3.0 mm).
The MRUT-SH technique is a guided waves technique that uses Shear Horizontal (SH) waves to generate ultrasonic scans. The SH waves are generated with magnetostrictive force, with the application of magnetostrictive strip (MS Strip). As one sensor is utilized for both transmitter and receiver (i.e. Pulse-Echo mode), the sensor will detect reflections from defects along the material.
The MRUT-SH technique provides an increase of 30 to 40 dBs in amplitude, as compared to the MRUT-Lamb technique.
Features of the MRUT-SH technique:
• Range | Up to approximately 3 m (~10 ft) on each side of the sensor.
• Scan Speed | Up to 1” per second.
• Coatings | Inspection of coatings of up to 0.030” (0.75 mm).
Check this paper to know more about inspection with MRUT techniques:
Innerspec’s Long Range UT (LRUT) options include a simplified version with just an circumferential ribbon for axis-symmetric transmission and reception, and a more sophisticated version that provides synthetic focusing using a scanner for reception. These wave modes are optimised for Long Range Pipe Inspection.
Unlike conventional rings, Innerspec’s LRUT tool does not create flexural modes during transmission due to the uniform loading around the circumference generated by the Ribbon.
During reception on the other hand, defects will naturally generate flexural modes since they are not distributed uniformly around the circumference. Innerspec’s LRUT Scanner is able to make discrete stops as per the maximum number of Torsional Flexural modes which can exist based on the pipe geometry and frequency. The complete data from “n” stops is used to resolve maximum possible flexural modes providing much better focusing and resolution.
As an example, a high-density traditional ring with 16 channels will be able to provide maximum resolution of 22.5º (45º for a conventional 8 channel ring), whereas on an 8” pipe, Innerspec’s LRUT Scanner will make 40 stops that equate to 9º of circumferential resolution. In addition to better focusing, the Axis-symmetric wave mode will also provide smaller dead zone and greater penetration. During experiments typical defects of 1.5% CSA and smaller can be detected in ideal conditions.
Yes, the wave can propagate beyond elbows. However, accuracy, resolution, and sensitivity are affected, dependent on the technique.
Yes, one of the most important features of VOLTA is the possibility to combine techniques:
By applying a delay in sequence between the pulses of two coils:
The MRUT-SH technique requires the application of magnetostrictive strip (MS Strip), in order to generate the required magnetostrictive force. The MS Strip can be adhered via the following method:
MS Strip Adhesion Options
Double-Sided Tape – Standard (P/N 027V0086)
Double-Sided Tape – Rough Material (P/N 027V0083)
Epoxy (P/N 116V0052)