Using Non-Destructive Testing for Weld Inspection

In assemblies and structures, welds work to keep structural integrity strong enough to last through the decades. However, over time, weld connections suffer fatigue and encounter loads which can lead to weld failure if they’re not found in time.


Many might assume that a weld is “good quality” if it looks secure. However, weld inspections involve far more than simple visual inspections to determine the quality of the weld connections.

However, cutting into the weld to see if it’s strong enough would be counterproductive. Destroying the weld is not an option, as it would destroy the structure or assembly that the weld secures. Instead, technicians use nondestructive testing (NDT) to determine the quality of a weld.

While standards of weld quality differ between industries and even between specific projects, proper examination techniques can ensure quality and safety. Technicians should inspect welds throughout the duration of the project. However, weld also ought to be inspected periodically after the project is finished, to determine safety of the structure for years afterward.

Technicians use the following techniques in weld inspections:

Visual Testing (VT)

This is the most efficient and cost-effective method in weld inspection. It is also the first method that should be used, because submitting an obviously poor weld to more sophisticated techniques would waste time and money.

Visual testing should occur before, during, and after the welding process during constructions. In order to visually inspect a weld, a technician needs good light, a pocket rule, a magnifying glass, a hand-held mirror, a weld size gauge, and a straight edge.

Technicians should inspect the equipment before striking the first arc weld. They check that the parts meet specifications for:

  • Quality
  • Clean from grease, oil, etc.
  • Type
  • Size
  • Freedom from defects

They should also check that the pieces to be welded meet acceptable standards of flatness, dimensional accuracy, and straightness.

During fabrication, the welder should inspect the weld bead and end crater to find possible problems. If a weld includes cracks, gas or slag inclusions, inadequate penetration, undercut, or surface porosity, the weld will have to be performed again. By discovering these problems early in the welding process, technicians can prevent expensive repairs.

Visual inspection is limited, because it can only determine imperfections in the surface of the weld.

Liquid Penetrant Testing (LT)

To find surface pinholes or cracks that are not visible to the naked eye, technicians can perform liquid penetrant testing. They use two types of liquids, called “penetrants”: fluorescent and visible dye.

With fluorescent testing, a technician applies the liquid to the surface of the weld. Capillary action pulls the liquid through the openings. Then the technician uses a “developer” to draw the penetrant to the surface, where it can be viewed with an ultraviolet light.

With a dye penetrant, the technician uses brightly colored dyes and ordinary light to see penetrations.

Ultrasonic Testing (UT)

This method uses high-frequency sound beams to detect interruptions in the continuity of the weld’s material. When the sound beam strikes a defect, part of the sound is reflected back. An instrument collects measurements and projects them on a video screen for the technician.

This method allows technicians to measure defects in the surface and subsurface of the weld, even flaws that are too small to be detected by some other methods.

Magnetic Particle Testing (MT)

This method is only suitable for magnetic materials. It can detect defects that occur on the surface of the weld and those that lie slightly below the surface. It is especially useful in detecting surface cracks, incomplete fusion, inadequate penetration, undercut, and defects on the base metal. Technicians may use this method to inspect the edges of materials before welding, or it can be used to inspect the finished weld.

To perform the test, technicians place probes on each side of the area. A high amperage passes through the material between the probes. This causes a magnetic flux that falls at right angles to the current’s flow. Then the technician dusts a magnetic powder on the material’s surface. The dust clings to areas with leakage problems more than elsewhere.

Radiographic Testing (RT)

Also known as x-ray testing, radiography is one of the most widely used method in nondestructive testing. Technicians use x-rays to detect the internal soundness of welds. They use x-rays and gamma rays that pass through metal more opaquely than through other materials. When the rays pass through the segment of welded metal, not all the radiation passes through the metal.

Different materials produce different wavelengths, which then get translated to a photo-sensitive film. Any section that includes slag, porosity, or other material will cause less absorption of the x-ray. These areas appear to be darker on the radiograph.

Based on these x-rays, technicians can detect defects in the interior of the weld, which are invisible to other methods of nondestructive testing.

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