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Non Destructive Testing – How To Choose The Right Method for the Job

Non Destructive Testing (NDT) covers a huge array of techniques used for detecting and evaluating flaws in materials and objects without damaging the material that is being tested.  It’s used across a variety of industries and applications, notably where equipment or component failure could impact negatively on costs, safety and performance.

Each method has advantages and disadvantages and some will be ideal for certain applications but of little or no value in another situation – so it is important to choose the NDT method that is right for the job.

The following table provides a brief summary of the most commonly used techniques, their main uses and the advantages and disadvantages of each method – and technicians and engineers may find this information helpful when deciding which method is most suitable for a particular application.

Visual Testing In-service inspections


Minimal training required

Immediate results


Limited scope of testing

Restricted to observing and identifying obvious visible surface discontinuities, which are generally larger in size

Requires adequate illumination of the test surface

Liquid Penetrant testing (PT) Used to locate cracks, surface defects, porosity over large surface areas that may not otherwise be visible. Portable

Minimal equipment investment

Simple and accurate

Rapid testing

Immediate results

Can test large surface areas/volumes  and complex parts

Needs cleaning afterwards

Chemical handling required

Only locates surface defects

Relies on visual detection

Only works on smooth surfaces


Eddy Current testing Detects surface and near-surface flaws such as cracks in conductive materials

Also used to differentiate materials based on their conductivity and magnetic permeability

Measures thickness of steels

Detects corrosion

Used on vessels, storage tanks, piping systems and columns etc and for tube and wire testing



Minimum preparation

Immediate results

Highly sensitive

Can be used for a number of industry applications, not just flaw detection eg the rapid sorting of small components for flaws, size variations or material variation


Equipment relatively expensive

Requires more skills and training than other methods

Rough surfaces may interfere with test

Only works on conductive materials

Limited penetration depth


Ultrasonic testing


Measures changes in material thickness and changes in material properties

Locates surface and sub-surface flaws

Superior depth penetration

Tests more than just flaws



Fast results

Suitable for wide range of materials/thicknesses including metals, plastics and wood

Superior depth of penetration

Only requires single-side access

Requires highly skilled technician

Surfaces have to be accessible

Surface finish can affect inspection

May miss linear defects which are parallel to the sound beam

Magnetic Particle Testing Detects surface and near-surface flaws in ferromagnetic materials Relative low cost of equipment

Limited skills required

Enables rapid inspection of large surface areas and/or complex parts

Sensitive to small discontinuities

Less surface preparation required than for liquid penetrant testing

Highly visible indications


Can only be used on ferromagnetic materials (ie those that can be magnetised)

Some site preparation required and parts must be cleaned and demagnetised afterwards

Relatively smooth surface required

Surfaces must be accessible

Results can be affected by non magnetic coverings, eg paint


Radiographic testing (X-ray) Used to detect  flaws in virtually any material (surface, sub-surface and internal)

Also used to measure material thickness

Minimal preparation required

Suitable for just about any material

Can locate and measure hidden, internal features or parts

No disassembly required when inspecting multi-layered and complex structures

Relatively  high financial investment

Requires high degree of operator training, skill and experience

Radiation hazard

Usually requires access to both sides of the structure


Isotope Radiography Detecting surface and internal/hidden flaws Portable

Cheaper than X-ray equipment

Suitable for inspecting hidden areas

Minimal site preparation


Safety hazard

Requires highly skilled operator

Sensitive to flaw orientation so operator experience is vital for correct interpretation of results

This table is not intended to be prescriptive nor exhaustive and given that NDT plays such a crucial role in ensuring that structural and mechanical components perform their function in a safe, reliable, and cost-effective way, it’s imperative to make the right choice of testing method for a particular application.

If you would like advice from an experienced professional about which NDT method will best meet your needs, you should give Nexxis a call.  They are a leading Australian supplier of technical equipment and have an extensive range of NDT tools for purchase or rent, plus they can customise a solution that will fulfil your requirements.  Call them on or visit their website, for more information.


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