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What Are the Different Methods of NDT Testing?

Non-Destructive Testing or NDT Testing is a collection of different material tests that can be carried out to assess the mechanical, physical and chemical properties of different materials. NDT Testing is a popular choice for many industries, as it does not cause damage to the material.  

There are a wide variety of methods of NDT testing, each with their own benefits and advantages depending on what information needs to be obtained. Here are some of the most popular methods of NDT testing. 

Eddy Current Testing (ECT)

Eddy Current Testing (ECT) is used to detect defects within conductive materials. 

The process involves using a probe with two coils; a primary coil made of conductive wire, which is excited with an alternating electric current to create a magnetic field, and a secondary coil that is used to measure the amplitude of the resulting eddy currents.

Changes in the metal thickness or defects within the near-surface material will interrupt the amplitude and pattern of the eddy currents. These variations help to identify the presence and location of defects, such as shallow surface fatigue cracks, stress corrosion cracks, and corrosion pitting. Conductivity measurements can also help to identify the material and its composition.  

Eddy Current Testing is adaptable for many industries, but it is especially crucial for sectors that require highly sensitive detection, such as aviation and aerospace. In these fields, even the smallest crack or defect could have catastrophic consequences.  

Often used for: 

  • Pipes and tubes 
  • Aircraft fuselage and wings 
  • Heat exchangers 
  • Welds 

Advantages: 

  • Sensitivity to small cracks and defects 
  • Suitable for testing high-volume production of components.  
  • Test objects need minimal preparation 
Ensure precision and reliability with Eddy Current Testing

Dye Penetrant Inspection (DPI)

Dye Penetrant Inspection, also known as Liquid Penetrant Inspection (LPI) or Penetrant Testing (PT), is widely used for surface-breaking defects in non-porous materials, and non-ferrous materials.  

This method is particularly effective in detecting welding surface defects in castings and forgings, such as hairline cracks, surface porosity, leaks, and fatigue cracks.  

Before testing can begin, the surface area needs to be cleaned and prepared to remove any dirt, debris, or contaminants that could interfere with the application of the penetrant.  

Once the surface is prepared, a brightly coloured or fluorescent dye is applied to the material and left to soak into any flaws for a period ranging from 5 to 30 minutes, with smaller flaws requiring a longer processing time. After the dye has soaked in, the excess dye is removed using a solvent remover.  

A white developer is then applied to the sample, which helps to draw the penetrant out of defects and onto the surface, making them visible for inspection. This is known as “bleed out”.   

An inspection of the flaws then takes place, using appropriate lighting to examine the flaw; colour contrast penetrant needs good white light, while fluorescents need to be used in darkened conditions with ultraviolet light or black light.  

Often used for: 

  • Castings 
  • Welds 
  • Shafts 

Advantages:  

  • Highly sensitive to surface cracks and defects 
  • Portable dye kit means tests can be carried out on-site 
  • Instant results  
Enhance your quality assurance with Dye Penetrant Inspection

Magnetic Particle Inspection (MPI)

MPI or magnetic particle testing (MT) or crack testing, is an inspection method used to detect defects on the surface of ferromagnetic materials, such as steel, iron, nickel, and cobalt and their alloys.  

Before testing can begin, the material must be cleaned to remove any dirt, oil or debris that may interfere with the test.  

A strong magnetic field is then applied to the material using a magnetic yoke on the surface. While the magnetic field is present, magnetic particles are applied to the surface. These particles will migrate toward any surface or near-surface defects, which will interrupt the magnetic field.  The inspector examines the surface of the material to look for these disruptions.  

If the material is free of defects, the magnetic field will remain uninterrupted, with the magnetic flux predominantly inside the material. However, if there are surface-breaking flaws, the magnetic field will be interrupted, causing a local magnetic flux leakage around the defect on the surface.  

The inspector interprets these indications to determine the severity of the defect, considering its size, shape, and location. After the analysis, the material is cleaned again to remove the magnetic particles.  

MPI is widely used across various industries to help determine if a part, component or piece of equipment is fit for use.  

Often used for: 

  • Castings  
  • Internal tank inspections 
  • Engine and suspension components  

Advantages: 

  • Results are immediately visible on the surface 
  • Very sensitive at detecting hairline cracks  
  • Can be used to inspect irregular shaped components  
Uncover hidden flaws with Magnetic Particle Inspection

Positive Material Identification

Positive Material Identification (PMI) is used to confirm the chemical composition of metals. This technique provides insight into the percentages of certain elements within the material, ensuring that products or components are made of the appropriate materials.  

There are two key methods for Positive Material Identification; X-ray Fluorescence (XRF) analysis and Portable Optical Emission Spectroscopy (OES), also known as Spark Testing.  

XRF analysis uses a hand-held analyser or gun to scan the metal and identify key elements within the material. However, it cannot detect carbon or lighter elements, and is not suitable for pure carbon steels.  

Portable Optical Emission Spectroscopy (OES) uses a spectrometer to generate an electrical spark between the electrode and metal sample in an argon atmosphere. This causes the atoms to vaporise and emit light, which varies in colour and intensity depending on the material. OES  is more suitable for distinguishing levels of carbon, although it requires need surface preparation before testing and can leave slight burn marks on the material after testing.   

PMI is crucial in many industries as a quality control process to prevent material mix-ups, which can happen at any stage of a process and potentially have disastrous consequences. Industries like petrochemicals, oil and gas use PMI testing as an additional safety and quality measure. 

Often used for: 

  • Pre-purchase verification of steel 
  • Post-purchase material certification Identification of suspected rogue material  

Advantages: 

  • Ensures manufactured products are made of the right alloy 
  • Supports compliance with quality and safety standards 
  • Identifies potential material mix-ups  
Gain certainty with Positive Material Identification

Radiography

Radiography uses X-rays or gamma rays to detect signs of imperfections within the internal structure of materials.  

Using a radiation source, such as X-rays, Iridium192 or Selenium75, the radiation passes through the material and is absorbed at different rates depending on the material’s density and thickness. Defects and flaws will absorb radiation differently compared to the surrounding material. A detector on the opposite side of the material captures the transmitted radiation, creating an image the reveals the internal features of the material. This image is then analysed to identify potential defects or irregularities.  

Radiography is a valuable tool for many industries, as it can provide in-depth information on a material’s internal structure without damaging the component. However, there are more risks associated with Radiography compared to other non-destructive methods due to the use of radiation. Therefore, strict safety protocols and risk assessments are required to protect workers and the environment.  

Often used for: 

  • Pressure vessels 
  • Welds, Castings and small components 
  • Storage tanks  

Advantages: 

  • Able to detect small defects 
  • Ideal for testing large circumference items (e.g. butt welds) 
  • Permanent record in form of a radiograph image 
Gain insights into your material’s structure with Radiography

Ultrasonic Testing

Ultrasonic Testing uses high-frequency sound waves to detect internal flaws within a variety of materials, such as metals, plastics, composites, and ceramics.  

The process begins with cleaning and preparing, the material to remove any debris or contaminants that may interfere with the test. A gel or liquid is then applied to allow sound wave transmission via a transducer.  

When the sound waves encounter a boundary within the material – such as a flaw or the back of the surface – they are reflected back to the transducer. The transducer captures these reflected waves, and the data is displayed on a screen, allowing for interpretation of the material’s internal structure and identifies the location and severity of any flaws.  

Phased Array Ultrasonic Testing (PAUT) is an advanced form of UT that involves multiple ultrasonic tests to generate a more detailed, cross-sectional image of a material. This enables more precise inspections, especially for complex or larger structures, by providing a comprehensive view of the internal structure.  

Many industries use Ultrasonic Testing, especially where internal integrity of the material is crucial for safety and performance. It is also useful for industries where different materials are used for parts and components.  

Often used for: 

  • Weld inspections 
  • Thickness measurements
  • Composite materials  

Advantages: 

  • Detects flaws deep within the material
  • Highly sensitive for detects small defects  
  • Immediate results allow for quick decision-making  
Understand your material’s internal structure with Ultrasonic Testing

Visual Inspection

Visual Inspection involves observing the test material for any visible damage, flaws or irregularities. It is usually the first step in assessing the material, but additional NDT methods are often used afterwards.  

The material is first prepared and cleaned to remove any dirt or debris, and the inspector examines the material area either direct observation or using magnifying tools such as microscopes or borescopes to assess the material’s condition. Other tools, including lighting equipment, cameras, and endoscopes can provide an in-depth assessment of both external and internal surfaces.  

Visual Inspection is widely used in weld quality inspections, as many welding flaws are surface-level and can be easily identified by the naked eye or using magnifying devices. This method is a crucial initial step in ensuring the integrity and safety of welded structures before using more advanced NDT techniques.  

Often used for: 

  • Weld inspections 
  • Surface defects  
  • Regular maintenance and inspections  

Advantages: 

  • Easy to perform with minimal equipment 
  • Low-cost requiring basic tools 
  • Can be performed without the need for complex set up  
Ensure surface integrity with Visual Inspection

Things to consider when selecting an NDT method

With many different NDT methods available, it is important to consider various factors that may influence your decision. Factors such as the shape and type of material, as well as its surface condition, play a crucial role in the decision-making process.  

The location and environment of the material that needs to be test are also critical considerations. If the material is located in a difficult or remote location, this may limit the type of NDT tests that can be performed.  

Explore our Guide to Choosing the Right NDT Method

How BES Group can help you with NDT Testing

Understanding the different NDT Methods and their suitability means that you can make an informed decision about the type of test you need. Our team of expert inspectors and technicians can support you with all your NDT testing needs, whether you have a small sample that requires analysis or large-scale projects that need complex testing.  

Contact BES Group today to learn more

Related services

Eddy Current Testing

Find out more

Liquid (Dye) Penetrant Inspection

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Magnetic Particle Inspection

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Positive Material Identification

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Radiography

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Ultrasonic Testing

Find out more

Phased Array Ultrasonic Testing

Find out more

Visual Inspection

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