NDT Methods
Non-Destructive Testing
Methods


Your are either au fait with non-destructive testing (NDT) and are interested in furthering your training, or maybe you have heard of NDT and would like to know more about it and how to progress into the field of NDT.

Firstly, what is destructive testing? Engineers design components and structures and these parts on completion will be subjected to physical tests until the component or structure breaks. The design engineers and the fracture mechanics engineers can now decide how these units can be improved and their working life can be extended. They could for example build an aircraft and once everything is satisfactory, put it into service.

Once the aircraft or component is put into service it is monitored to see how it is holding up to its working conditions and to assure that the material or structure is not deteriorating significantly - here non-destructive testing is used. In other words, the material, component or unit is inspected in such a way that its structural integrity is not at risk. Non-destructive testing is used to determine the condition of the material and to find any flaws which may have developed in-service before they reach the point where the failure occurs, which could have catastrophic results.

The methods used in non-destructive testing range through twelve different principles. Even though there are so many methods, there is no single method that will supersede another. They are often used in combination as complimentary to each other because each has its own specific applications.

At S.A.N.D.E. we offer training in the six main methods, namely - Radiographic Testing (RT), Ultrasonic Testing (UT), Magnetic Particle Testing (MT), Liquid Penetrant Testing (PT), Eddy Current Testing (ET) and Visual Inspection (Testing) (VT). The other methods are Neutron Radiography, Thermography, Acoustic Emission, Leak Testing, Laser Testing and vibration Analysis.

Radiographic Testing

Short wave radiation energy, namely x-rays or gamma rays are passed through the material under inspection. The energy which passes through the material is captured on a receptor (x-ray film or viewing screen). After due processing the shadow picture on the film is viewed and evaluated.

Ultrasonic Testing

Mechanical sound energy pulses are passed into the material and if there are any flaws in the material they act as reflectors. The energy then bounces off these reflectors and is returned to the instrument as an echo. Electronic images are then displayed on a monitor.

Eddy Current Testing

Circulating electrical currents are induced in conductive materials. Any flaws in the material will cause a disruption in the flow of these eddy currents which may be sensed by a coil and displayed on a monitoring device.

Magnetic Particle Testing

Magnetic fields are introduced into a ferromagnetic material. If there are any surface or sub-surface flaws in the material, magnetic flux leakage fields are generated by these flaws. The flux leakage fields may be detected with the use of ferromagnetic particles when attracted to the flux leakage site. Flux leakage fields may also be found by the use of special (Hall effect) sensors.

Liquid Penetrant Testing

This method relies on its ability to detect surface breaking flaws. The surface of the component/unit is prepared carefully and thoroughly, then a strongly coloured liquid is applied to the surface after which the excess is removed from the surface and a developer, which acts as a blotter, is applied and draws trapped penetrant back onto the material surface. Flaws are then revealed as red or fluorescent stains on a contrasting background.

Visual Inspection

Visual inspection is a common method of NDT and quality control, data acquisition, and data analysis. Visual Inspection can used in inspection of raw materials, maintenance of facilities and in service equipment using raw human senses such as vision, touch and/or any non-specialized and specialized inspection equipment under the appropriate lighting conditions.