05 Methods for Testing the Quality of Steel Structures

A weld that looks perfect on the outside – but contains internal defects such as cracks, porosity, or slag inclusions – may withstand normal conditions well, but will become a critical point of failure when subjected to dynamic loads or extreme temperatures.

This is why NDT (non-destructive testing) has become a non-mandatory requirement in all important steel structure projects – especially for high-tech plants, where a structural failure can cause hundreds of millions of dollars in losses. So what is NDT? What methods are there? This article will provide some information about NDT.

What is NDT and why can't we ignore it?

NDT (Non-Destructive Testing) is a set of techniques for inspecting materials and welds without damaging or destroying the object being inspected. Unlike destructive testing (cutting samples, tensile testing to measure strength), NDT allows for 100% inspection of products before shipment or during use.

In industrial plant steel structures, NDT (Neural Design) is mandatory according to the following:

AWS D1.1 (American standard for structural steel welding – Most common for FDI projects)
EN ISO 5817 (European standard)
TCVN 6700 (Vietnamese Standard)

Method 1: VT – Visual Testing

This is the first and mandatory step – it cannot be skipped even if other methods are used. Welds that do not meet the VT standard will be required to be ground and re-welded immediately – UT or MT will not be performed until the weld meets the standard.

Visual inspection (or with a magnifying glass) allows for the detection of:

Cracks on the display surface
Uniformity of weld seams
Surface pores, exposed teeth.
Geometric dimensional deviations (weld flange height, width)
Undercut – the area of the base metal that is recessed due to arc cutting.

Evaluation criteria: AWS D1.1 Table 6.1 specifies each criterion in detail.

Limit: Only surface defects can be detected. Subcutaneous defects cannot be detected with the naked eye.

Method 2: PT – Penetrant Testing

Principle: Spray a red penetrant onto the surface to be inspected. The liquid penetrates into micro-cracks and gaps. After cleaning the surface and spraying a white developer, surface defects become clearly visible as raised red spots on a white background.

This method helps detect:

Surface cracks
Pinholes
Surface defects with opening size ≥ 0.005mm

Advantage: Low cost, easy to implement, and requires no expensive equipment.

Limit: Only surface defects are detected. Hidden defects are not detected.

When to apply: Inspect welds and steel cross-sections after machining, and check for cracks in structural components currently in use.

Method 3: MT – Magnetic Testing

Principle: Create a magnetic field within the steel component using electromagnets or coils. Sprinkle magnetic particles (in powder or suspension form) onto the surface. At the location of the defect, magnetic flux leaks out and attracts the particles, causing them to gather and form a "cloud" of particles that reveals the shape of the defect.

This method helps detect:

Surface and near-surface cracks (up to 3-5mm deep)
More effective than PT in detecting transverse cracks.

Advantage: Fast, sensitive, and effective for cracking perpendicular to the magnetic field.

Limit: Only applicable to magnetic materials (carbon steel, low-alloy steel). Not suitable for stainless steel or aluminum.

When to apply: MT is applied to fillet welds on beam-column connections where crack inspection at the weld root is required – a location not accessible by UT due to angle limitations.

Method 4: UT – Ultrasonic Testing

This is the most important and common method in industrial steel structures.

Principle: High-frequency ultrasound waves (2-10MHz) are emitted into the material through a transducer. The sound waves propagate inside and reflect back upon encountering continuous (defects, material boundaries) waves. The time and amplitude of the reflected signal are analyzed to determine the location and size of the defect.

This method helps detect:

Internal cracks
Internal air pores (porosity)
Slag inclusion
Lamination in steel plates

Outstanding advantages:

Detect defects at any depth within the material thickness.
Accurately determine the location and size of the defect.
No contact with chemicals is required, making it safe for the person performing the procedure.

Applicable standards: AWS D1.1 Clause 6.13, ASTM E164, ISO 11666

Method 5: RT – Radiographic Testing (X-ray Examination)

Principle: X-rays or gamma rays are shone through the material and recorded as images on film or a digital detector. Internal defects appear as darker areas (porosity, slag inclusions) or brighter areas (denseer regions) compared to the background.

Discovered:

Porosity — very noticeable
Slag inclusion
Cracks parallel to the beam (limited)

Advantage: For visual representation, easy interpretation, and record-keeping using film.

Limit:

Radiation safety — requires isolation zones and strict safety control procedures.
No blind cracks were detected parallel to the beam.
It takes more time than UT.
Depth of defect not specified

When to apply: RT is often used when photographic documentation is needed as legal evidence, or at the specific request of the client for certain critical welds.

A CJP weld on the main column of a $2 billion factory cannot be left to “hopefully be fine.” NDT inspection costs account for approximately 0.5–1.5% of the total steel structure cost — but guarantee the entire structure’s reliability for 30–50 years.

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