TEXO Asset Integrity - NDT Inspection

The Purpose of Non-Destructive Inspection (NDT)

The purpose of NDT is to perform an examination, test, or evaluation of components without changing or altering that object in any way - determining the absence or presence of discontinuities, reducing the risk of damage to equipment or infrastructure. Non-destructive methods are used in manufacturing, fabrication, and in-service inspection to ensure product reliability and integrity, by maintaining uniform/reliable quality levels and reducing manufacturing and production costs. ?

Eddy Current Testing (EC)

Principles: Eddy current testing is based on the principles of electricity and magnetism and is suitable for the inspection of surfaces and just beneath the surface of conductive materials, volumetric inspection of thin conductive materials, and to determine the thickness of nonconductive materials. Due to its sensitivity EC is used for other applications such as material conductivity and permeability variations, spacing between conductive layers, and detecting discontinuities. ??

Application: Localised electrical fields are induced into the conductive test material by electromagnetic induction. The primary alternating current is passed through a conductor (copper coil) inside the probe developing a magnetic field around the coil. The coil generates a changing magnetic field using an alternating current, generating eddy currents as it interacts with the component. The variation in phase and magnitude of the currents are monitored by a second coil. If a flaw is present, this will cause a change in the eddy currents, which then will change the phase and magnitude of the eddy current field.?

Figure 1. Eddy Current – weld toe defect signal (Array Training, 2022)

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Advantages:

• Test results are instantaneous
• Can detect surface and near-surface cracks as small as 0.5mm
• Can test complex geometries

Limitations:

• The material under test must be electrically conductive
• Variable depth of penetration
• Unable to detect defects parallel to the test surface

Magnetic Particle Inspection (MPI)

Principle: MPI is a method for detecting discontinuities that are primary linear and located near or at the surface of the material under test – components and structures. MPI is restricted to materials that can support magnetic flux lines and are classified as – ferromagnetic, paramagnetic, and diamagnetic. However, only metals classified as ferromagnetic can effectively be inspected by this method, as seen below.?

• Ferromagnetic metals: strongly attracted – iron, nickel, and cobalt.
• Paramagnetic metals: very weakly attracted – austenitic stainless
• Diamagnetic metals: very slightly repelled cannot be magnetised – gold

Application: Electromagnets using either alternating or direct currents, or a combination of both. Detection media used is either dry powder or as a liquid carrier containing magnetic particles. The most common colours used are black and white which provide a contrast with the surface under test, however fluorescent particles are used for maximum sensitivity. Prior to magnetising the component, the contrasting background paint is applied to the surface and allowed to dry. The component is then magnetised, and the magnetic particles detection media is applied. If any defects are present on or near the surface a flux leakage field is created by due to the attraction of the particles forming visible indication.

Figure 2. Electromagnet Yoke ?(asnt.org, 2022)

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Advantages:

• Can be applied without AC power supply, by using battery powered yokes.
• Highly sensitive
• Indications are instantly visible and easy to interpret. ?

Limitations:

• Only effective on ferromagnetic materials
• Limited to near surface discontinuities
• Demagnetisation may be required before and after inspection

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Penetrant Testing (PT)

Principles: Penetrant testing is a method is used to detect surface discontinuities by bleeding the applied penetrant dye from the flaw. This technique is based on capillary action process where the dye/liquid enters the flaw and then is drawn back out to the surface by applying a thin developer application. Penetrant testing is primarily used for non-porous materials, as porous materials can potentially hide evidence of defects. ?

Application: The penetrant can be applied to the test surface by spraying, brushing or immersion. Once the component is coated in penetrant it is allowed to dwell (usually 20 minutes or longer dependant on component being tested) given the penetrant enough time to enter any defect that is present. After dwell time the excess penetrant is removed from the part, without removing the penetrant entrapped in the defect(s). The developer is then applied (dwell times normally between 15 and 60 minutes) drawing out the penetrant from the defect onto the surface, forming a visible indication, known as bleed-out. Following the developer dwell time, the component is then visually inspected under illuminated light of 3 500 lux and UV-A Intensity irradiance of 3 1000mW/cm2 (10 W/m2) for fluorescent penetrants.

Figure 3. Penetrant application, penetrant removal, and developer application (nde-ed.org, 2022)?

Advantages:

• High sensitivity for small discontinuities
• Components with complexed geometry can be tested
• Suitable for various materials – metallic, non-metallic, conductive, and non-conductive, and magnetic and non-magnetic.

Limitations:

• Pre-cleaning is critical to the inspection as contaminants can find potential defects
• Surface condition can affect sensitivity of inspection
• Only surface breaking defects can be detected

Ultrasonic Testing (UT)

Principles: Ultrasonic testing is a volumetric NDT method that uses high frequency sound waves that can find flaws inside material. The sound is introduced into the component and if it encounters a material of a different acoustic impedance the sound will reflect back sending a signal to a visual display screen. The distance of any reflector or back wall is determined by the speed of the sound or acoustic velocity through the material. The two common types of sound waves are compression (longitudinal) and shear (transverse). Compression waves cause atoms to vibrate back and forth whist transverse sound waves cause atoms to vibrate perpendicularly to the direction of sound. ???

Application: A couplant (liquid or gel) is applied to the surface of the test piece. This is used to aid the transmission of the ultrasonic energy between the transducer and the test piece. This is required due to the acoustic impedance mismatch between air and the material. The Compression transducer/probe is then placed on the surface of the test piece and the longitudinal wave transmits through the material. If the sound hits a reflector the sound will reflect back to the transducer – the sound will return to the transducer faster than it would if it was to return from the back wall. An example of this can be seen in figure 3, where the results are shown on the screen display. Shear wave inspection uses the same transducer/probe, but it is mounted on an angled wedge - the sound beam is transmitted at an angle commonly at angles of 45?, 60? and 70?. The probe is manipulated back and towards allowing the sound to pass through the volume of the weld under test. The reflectors are aligned perpendicular to the beam (as like compression beam) and are reflected back to the transducer and displayed on the screen.

Figure 4. Compression beam (nde-ed.org, 2022)?Figure 5. Angle beam (nde-ed.org, 2022)

Advantages:

• High sensitivity
• Gives immediate data and results
• Provides shape, size, orientation, and nature of defects?

Limitations:

• Rough, thin, or not homogenous surfaces are difficult to inspect
• Surface must be accessible to transmit ultrasound

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At TEXO Asset Integrity we fully understand the challenges our clients face and our multi-disciplined teams can mobilise at short notice and minimise any operational downtime on their assets. Contact TEXO today for all your Non-Destructive Testing requirements: [email protected] | +44 (0)1224 44 66 00

https://texo.co.uk/services/asset-integrity/

References

Figure 1 Array Training (2022), ‘Texo Asset Integrity – Eddy Current Inspection procedure.

Figure 2 asnt.org (2022), ‘Magnetic Particle Inspection Techniques [Online] Available at: https://www.asnt.org/MajorSiteSections/About/Introduction_to_Nondestructive_Testing.aspx

Figure 3, Nde-Ed.org (2022), ‘Penetrant and developer application [Online] Available at: https://www.nde-ed.org/NDETechniques/PenetrantTest/MethodsTech/popup4_7.xhtml

Figure 4,5, asnt.org (2022), Ultrasonic Inspection: UT techniques straight and angle beams [Online] Available at: https://www.asnt.org/MajorSiteSections/About/Introduction_to_Nondestructive_Testing.aspx ?