5 Methods of Non-Destructive Testing of Welds

Non-destructive testing of welds is an essential process to determining a welds structural integrity and ensuring a good weld has been achieved. If it is not carried out, poor welds and weld defects, such as a lack of sidewall fusion and crater cracks, won’t be identified and could lead to premature failure. Below we cover 5 methods of non-destructive testing of welds.

Visual Inspection of Welds

Visual inspection is the most used non-destructive method of testing welds. Once a weld has been completed, the first check undertaken is a visual check. A visual inspection consists of an inspection for surfaces discontinuities with the eye. Although simple, visual inspection can provide quick feedback on the quality of a weld.

Advantages of Visual Inspection of Welds

• – Cheap
• – No expensive equipment is required
• – Quick identification of surface defects

Disadvantages of Visual Inspection of Welds

• – Training is necessary
• – Cannot defect subsurface defects
• – Subjective – Open to opinion
• – Good eyesight required
• – Report must be completed

Magnetic Particle Inspection (MPI)

Following visual inspection, Magnetic Particle Inspection (MPI) is the next most common method of non-destructive testing of welds. Magnetic particle inspection is used in a range of industries, highlighting its popularity and reliability.

Unlike visual inspection, magnetic particle inspection can detect subsurface defects however, they must be near the surface. The use of contrast paint allows for an easy visual representation of any surface break defects.    

Advantages of MPI Testing Welds

• – Visually identify defects      
• – Inexpensive compared to other non-destructive testing methods. 
• – Can inspect welds quickly.    
• – Can detect surface and near-surface defects    
• – Surface preparation not required (or little)      

Disadvantages of MPI Testing Welds: 

• – Training required     
• – Difficult to test restricted access welds

Eddy Current Testing of Welds

Eddy current testing utilises the principle of electromagnetism to detect surface break and sub-surface flaws. A coil is energised by alternating currents creating a magnetic field around the coil, which is induced into the conductive material of the test piece. Due to the change in the magnetic field (from the alternating currents), local currents are induced into the test piece, known as eddy currents.

The eddy currents generate their magnetic field, which opposes and modifies the coils magnetic field. The phase and magnitude changes seen in the primary coil then determine the detection of any defects in the test piece.

Eddy current testing is not very popular for the testing of welds but is more popular for testing entire components.

Visualisation of Eddy Currents Induction. Credit: Stefan Trache. License: CC BY-SA 4.0

Advantages of Eddy Current Testing of Welds:

• – Can detect subsurface defects as well as surface-breaking defects
• – Testing can be undertaken on complex geometries
• – Provides immediate feedback
• – Equipment is small and lightweight

Disadvantages of Eddy Current Testing of Welds:

• – Very susceptible to magnetic permeability changes making it difficult to test welds in ferromagnetic materials
• – Unable to detect defects that are parallel to the test piece surface
• – Not a consistent penetration depth

Ultrasonic Testing of Welds

As the name suggests, ultrasonic testing utilises ultrasound to detect defects. A beam of ultrasonic sound waves is input into the test piece. As the beam travels through the material it only experiences an insignificant loss. The beam reflects off the surfaces and returns to the transducer in the probe. When the beam encounters a defect, it reflects the transducer. The change in electrical energy shows ds detectable defect and is highlighted by abnormal spikes in the graph output.

Advantages of Ultrasonic Testing Welds

• – Equipment is portable
• – Provides consistent results
• – Detects surface and subsurface weld defects

Disadvantages of Ultrasonic Testing Welds

• – High degree of operator skill is required, therefore must be qualified
• – No permanent inspection records
• – The reading is subjective as different operators may interpret the results differently.

Ultrasonic testing of a pipeline weld for defects

Liquid/Dye Penetrant Testing of Welds

Dye penetrant testing is also known as liquid penetrant testing and is a non-destructive testing method that is popularly used to detect surface-breaking defects.

The method works by applying penetrant (generally a bright colour) to the test surface and allowing sufficient time to allow the penetrant to soak into the defects. The time for this process is known as the dwell time. As defects in welds tend to be smaller, a greater dwell time is required.

Once the dwell time has passed, excess penetrant is removed by using a solvent. However, depending on the penetrant different solvents will be used. When the excess penetrant is removed a developer is applied to the weld to draw out any penetrant in defects, giving a visible indication.

To inspect the weld, the inspector will use either light with suitable intensity or ultraviolet light depending on if a visible fluorescent penetrant was used

Advantages of using liquid/dye penetrant to test welds

• – Test can be carried out quickly
• – The testing process is cheap
• – Complex geometries can be tested
• – Highly portable equipment

Disadvantages of using liquid/dye penetrant to test welds

• – Rough as-welded conditions make it difficult to remove excess penetrant, potentially introducing false indications
• – Can only detect surface breaking flaws
• – The penetrant can cause skin irritation
• – Pre-cleaning the weld is necessary as contaminants can mask defects.