NDT Testing

The work of engineers in the testing of aircraft components is paramount to the safety and durability of an aircraft both in the air and on the ground. One of the daily tasks of an aviation engineer includes testing of materials and components which can be done in two ways: destructive and nondestructive testing. While destructive testing is extremely useful in examining ductility1 , tensile strength2 and fracture toughness3 , as is suggested by the name this comes at the heavy cost of materials that could be used in aircraft. As such non-destructive testing (NDT) is an effective method of examination as it allows analysis without destroying the integrity of a component.

NDT takes place during production, maintenance and in the case of unexpected events. While the obvious and most significant benefit of NDT is safety and accident prevention there are many other valuable outcomes which include;

- Enhanced reliability,
- Cost reduction and;
- Greater knowledge and insight for improved future safety preparation.
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There are multiple methods of NDT which include liquid penetrating, electromagnetic testing, acoustic emission testing, ground penetrating radar, laser testing, leak testing, magnetic particle testing, magnetic flux leakage, radiographic and neutron radiographic testing, microwave testing, thermal testing, vibration analysis, visual inspection and ultrasonic testing. Two methods used most at Pacific Turbine Brisbane are liquid penetrating and magnetic particle testing.

Testing begins with determining whether the metal/material is ferrous or non-ferrous. This dictates whether it will need to undergo Fluorescent Particle Testing (FPT) or Magnetic Particle Testing (MPT). If the metal is ferrous, then it is magnetic and hence needs MPT. If it is non-ferrous, FPT is the method of choice.

Magnetic Particle Testing (MPT)

Magnetic particle testing looks at finding surface and sub-surface defects in ferromagnetic materials. The component is covered in a penetrant which contains fluorescent particles and is then placed in a machine which flows a specific amperage to create the necessary magnetic field over the defect.

The part is then examined under ultraviolet (UV) light which will highlight any of these defects. The part is then demagnestised. Nil defects found means the part is fit to return to service.

Fluorescent Penetrant Inspection (FPI)

Similarly, except this time for those components which are not ferromagnetic- FPI is used. When a part is in need of inspection it firstly needs to be warmed to allow any defects to expand which allows penetrant to enter surface defects.

The penetrant is applied for a specific dwell time depending on the level of sensitivity. The penetrant is removed, the object is dried and then the part is coated with a developer powder for a specific time.

The object is then placed under a UV light, which illuminates the fluorescent penetrant and allows for identification of any deficiencies.

  

As can be seen in the image, this axle has completely snapped due to the microscopic cracks in the material.

These testing methods typically follow visual and dimensional inspections. With such potentially deadly outcomes for any damaged or flawed component NDT is an essential part of an extensive testing checklist.

This work can only be undertaken by a specialist who has received training in this area of testing which includes specialty courses and number of scheduled hours.  

 

  1. Ductility- the ability to be hammered thin or stretched into a wire without breaking. If the substance is ductile it will be able to do so without breaking.
  2. Tensile strength- this is a measurement of a materials maximum stress point while being stretched or pulled before breaking.
  3. Fracture toughness- this examines the strength of a material with the presence of a pre-existing flaw.
  4. Discontinuity- is an interruption or imperfection in the physical properties of a material.