Ultrasonic testing

In ultrasonic testing (UT), very short ultrasonic pulse-waves with center frequencies ranging from 0.1-15 MHz and occasionally up to 50 MHz are transmitted into materials to detect internal flaws or to characterize materials. A common example is ultrasonic thickness measurement, which tests the thickness of the test object, for example, to monitor pipework corrosion.

Ultrasonic testing is often performed on steel and other metals and alloys, though it can also be used on concrete, wood and composites, albeit with less resolution. It is a form of non-destructive testing used in many industries including aerospace, automotive and other transportation sectors.

How it works

In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer is typically separated from the test object by a couplant (such as oil) or by water, as in immersion testing. However, when ultrasonic testing is conducted with an Electromagnetic Acoustic Transducer (EMAT) the use of couplant is not required.

There are two methods of receiving the ultrasound waveform: reflection and attenuation. In reflection (or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves as the “sound” is reflected back to the device. Reflected ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object. The diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance, representing the arrival time of the reflection. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after traveling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing their presence. Using the couplant increases the efficiency of the process by reducing the losses in the ultrasonic wave energy due to separation between the surfaces.

Advantages

  1. High penetrating power, which allows the detection of flaws deep in the part.
  2. High sensitivity, permitting the detection of extremely small flaws.
  3. Only one surface needs to be accessible.
  4. Greater accuracy than other nondestructive methods in determining the depth of internal flaws and the thickness of parts with parallel surfaces.
  5. Some capability of estimating the size, orientation, shape and nature of defects.
  6. Non hazardous to operations or to nearby personnel and has no effect on equipment and materials in the vicinity.
  7. Capable of portable or highly automated operation.

Disadvantages

  1. Manual operation requires careful attention by experienced technicians. The transducers alert to both normal structure of some materials, tolerable anomalies of other specimens (both termed “noise”) and to faults therein severe enough to compromise specimen integrity. These signals must be distinguished by a skilled technician, possibly, after follow up with other nondestructive testing methods.
  2. Extensive technical knowledge is required for the development of inspection procedures.
  3. Parts that are rough, irregular in shape, very small or thin, or not homogeneous are difficult to inspect.
  4. Surface must be prepared by cleaning and removing loose scale, paint, etc., although paint that is properly bonded to a surface need not be removed.
  5. Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being inspected unless a non-contact technique is used. Non-contact techniques include Laser and Electro Magnetic Acoustic Transducers (EMAT).
  6. Inspected items must be water resistant, when using water based couplants that do not contain rust inhibitors.

Standards

International Organization for Standardization (ISO)
  • ISO 7963, Non-destructive testing – Ultrasonic testing – Specification for calibration block No. 2
  • ISO/DIS 11666, Non-destructive testing of welds – Ultrasonic testing of welded joints – Acceptance levels
  • ISO/DIS 17640, Non-destructive testing of welds – Ultrasonic testing of welded joints
  • ISO 22825, Non-destructive testing of welds – Ultrasonic testing – Testing of welds in austenitic steels and nickel-based alloys
European Committee for Standardization (CEN)
  • EN 583, Non-destructive testing – Ultrasonic examination
  • EN 1330-4, Non destructive testing – Terminology – Part 4: Terms used in ultrasonic testing
  • EN 1712, Non-destructive testing of welds – Ultrasonic testing of welded joints – Acceptance levels
  • EN 1713, Non-destructive testing of welds – Ultrasonic testing – Characterization of indications in welds
  • EN 1714, Non-destructive testing of welds – Ultrasonic testing of welded joints
  • EN 12223, Non-destructive testing – Ultrasonic examination – Specification for calibration block No. 1 is replaced by the EN ISO 2400:2012 “Non-destructive testing – Ultrasonic testing – Specification for calibration block No. 1″
  • EN 12668-1, Non-destructive testing – Characterization and verification of ultrasonic examination equipment – Part 1: Instruments
  • EN 12668-2, Non-destructive testing – Characterization and verification of ultrasonic examination equipment – Part 2: Probes
  • EN 12668-3, Non-destructive testing – Characterization and verification of ultrasonic examination equipment – Part 3: Combined equipment
  • EN 12680, Founding – Ultrasonic examination
  • EN 14127, Non-destructive testing – Ultrasonic thickness measurement
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Nondestructive testing

Nondestructive testing or Non-destructive testing (NDT) is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage. The terms Nondestructive examination (NDE), Nondestructive inspection (NDI), and Nondestructive evaluation (NDE) are also commonly used to describe this technology. Because NDT does not permanently alter the article being inspected, it is a highly valuable technique that can save both money and time in product evaluation, troubleshooting, and research. Common NDT methods include ultrasonic, magnetic-particle, liquid penetrant, radiographic, remote visual inspection (RVI), eddy-current testing, and low coherence interferometry. NDT is commonly used in forensic engineering, mechanical engineering, electrical engineering, civil engineering, systems engineering, aeronautical engineering, medicine, and art.

Methods

NDT methods may rely upon use of electromagnetic radiation, sound, and inherent properties of materials to examine samples. This includes some kinds of microscopy to examine external surfaces in detail, although sample preparation techniques for metallography, optical microscopy and electron microscopy are generally destructive as the surfaces must be made smooth through polishing or the sample must be electron transparent in thickness. The inside of a sample can be examined with penetrating electromagnetic radiation, such as X-rays or 3D X-rays for volumetric inspection. Sound waves are utilized in the case of ultrasonic testing. Contrast between a defect and the bulk of the sample may be enhanced for visual examination by the unaided eye by using liquids to penetrate fatigue cracks. One method (liquid penetrant testing) involves using dyes, fluorescent or non-fluorescent, in fluids for non-magnetic materials, usually metals. Another commonly used method for magnetic materials involves using a liquid suspension of fine iron particles applied to a part while it is in an externally applied magnetic field (magnetic-particle testing). Thermoelectric effect (or use of the Seebeck effect) uses thermal properties of an alloy to quickly and easily characterize many alloys. The chemical test, or chemical spot test method, utilizes application of sensitive chemicals that can indicate the presence of individual alloying elements. Electrochemical methods, such as electrochemical fatigue crack sensors, utilize the tendency of metal structural material to oxidize readily in order to detect progressive damage.

 

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Magnetic particle inspection

Magnetic particle inspection (IPC ) is used for detection of surface and subsurface defects in objects of various shapes and dimensions, made of ferromagnetic materials or objects with a nonmagnetic coating.

With it can be revealed cracks of various origins , hairline , sunsets, lack of fusion welded joints and other imperfections opening width of a few micrometers.

Magnetic method has the following advantages:

high sensitivity ;
ease of control and the ability to test various shapes and sizes of parts on the same flaw ;
the ability to control parts which are in construction;
relatively high performance control.

Magnetic particle inspection process operations are conducted in the following sequence :

training component;
magnetization ;
applying to the surface of the indicator ( powder or slurry) ;
inspection of parts ;
transcript pattern and grading of the indicator ;
demagnetization and control demagnetized ;
removal of residues from the part indicator.

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