The Basics of Non-Destructive Material Testing – Part 3
Hailey Kupiec posted on June 29, 2016 |
Leak testing, guided wave testing and visual inspection.

To conclude the three-part series on non-destructive testing we discuss leak testing, guided wave testing and visual inspection.

7. LT – Leak Testing

Leak testing is used to detect faults in a variety of fluid systems so that corrective action may be taken. Leaks require a pressure difference to generate flow, which runs from higher to lower pressures.

A variety of methods are used to detect flow, including:

  • Pressure Decay: The part is filled with air to a set pressure (pressurized). It is then disconnected from the air supply. The air pressure in the part is monitored and the leak rate calculated based on the pressure change over time.
  • Mass Airflow: The part is pressurized and remains connected to the air supply. An equilibrium is reached such that the air flowing into the part from the supply is equal to that leaking out. At this point the internal pressure of the part is stable and the leak rate is directly measured as the incoming air flow rate.
  • Observation: The part or assembly is pressurized and submerged in a tank of water. Any bubbles that form reveal the location of a leak. If the part is too large or unwieldy to be submerged, the suspected leak area is coated with a soap solution to create bubbles.
  • Gas Tracer (Helium): The part or assembly is enclosed in a sealed chamber. The part and/or the chamber is evacuated and one is charged with helium. The gas lost to the other volume is measured with a mass spectrometer.

8. GWT – Guided Wave Testing

By Sprialboy (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

By Sprialboy (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons

Guided wave testing is used to inspect pipelines as well as train tracks, metal plate structures and rods. It can inspect up to 100m from a single location.

Though guided wave testing is also known as long-range ultrasonic testing, the two methods are fundamentally different. GWT uses a lower frequency (10-100kHz) of ultrasound. Axially symmetric waves propagate horizontally through the material in both directions from the test site.

A ring with multiple transducers is attached around the circumference of the object. These transducers are in pulse-echo mode, meaning that they both produce the waves and receive reflected signals. The reflected signals are caused by defects in the material as well as by cross sectional changes such as welds. The arrival time and predicted speed that a wave travels in the medium (often calculated using weld echoes) allows the user to determine the location of defects. 

9. VT – Visual Testing/Inspection

Visual inspection is the oldest and most basic method of non-destructive testing, but it’s also powerful. Visual testing is used for surface flaw identification and inspection of equipment and structures. All other forms of flaw detection must ultimately be confirmed visually. Magnetic particle inspection and dye penetrant inspection are methods used to enhance visual testing.

Visual inspection using a borescope. By U.S. Navy photo by Mass Communication Specialist 3rd Class Gary Prill [Public domain], via Wikimedia Commons
Visual inspection using a borescope. By U.S. Navy photo by Mass Communication Specialist 3rd Class Gary Prill [Public domain], via Wikimedia Commons

Proper surface preparation, such as etching and cleaning, vastly improves the accuracy of visual testing. Magnifiers, microscopes and other such devices are aides in visual testing. A well trained, experienced inspector can detect most signs of damage, making visual testing highly reliable.

In part 1, we discuss radiographic testing, ultrasonic testing and magnetic particle inspection. Part 2 covers dye penetrant inspection, eddy current testing and thermographic inspection.

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