Non-Destructive Testing - how far should you go?

Every day millions of components are produced around the world to help build everything from a simple toy to fighter jets. For every manufacturer involved in the process, a decision must be made on how to test the component to ensure it meets the design criteria. For the vast majority, batch inspection and testing is sufficient; however, where high-precision engineering is involved the testing must be exhaustive and comprehensive. Components for aerospace, motorsport, medical devices and other ultra critical areas are required to be tested to a very high standard.

As a manufacturer of precision gear components to a wide range of industries including aerospace and motorsport, with the additional ability to conduct exhaustive NDT in-house, Micro Precision provides an illustration of how and why this is such a crucial process.

The ability to inspect and test a component without compromising the item itself is a vital technique which can save time and money. Parts are valuable and often used in life-critical applications, so both accuracy and lack of damage is prescient for the process. Hence, each test must be conducted by suitably qualified personnel, using properly calibrated equipment and documented in such a way so as to satisfy the closest scrutiny. It is important that the organisation performing the testing is accredited to the relevant bodies for the industries involved.

The National Aerospace and Defence Contractors Accreditation Program (NADCAP), for example, is a universal standard administered by the Performance Review Institute (PRI) to ensure manufactured aircraft and aero-engines are of the highest quality, and is now specified by the majority of prime manufacturers in these sectors. The accreditation allows aerospace equipment makers to be confident of receiving the highest standard of service, conforming to strict industry guidelines.

There are a wide range of methods and techniques involved in NDT, and therefore selecting the correct method for each application is very important to ensure that the results meet the client's requirements and expectations. In this case we are looking at the more common methods of Magnetic Particle inspection (MPI), Liquid Penetrant Inspection (LPI) and Nital Etching.

MPI: Magnetic Particle Inspection

This process is used for detecting surface and slightly subsurface discontinuities in ferroelectric materials such as iron, nickel, cobalt, and some of their alloys. The process applies a magnetic field to the part and the presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak. Ferrous iron particles are then applied to the part, either dry or in a wet suspension. If an area of flux leakage is present the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication. This procedure can be applied to both new components and items while in service to show any signs of cracking due to fatigue.

LPI: Liquid Penetrant Inspection

Liquid, or dye, penetrant inspection is an extension of visual inspection and is used for detecting surface-breaking flaws, such as cracks, laps and folds, on any non-absorbent material's surface.

Firstly, the surface to be inspected is cleaned thoroughly to remove all traces of dirt and grease. A brightly coloured or fluorescent liquid is then applied liberally to the component surface and allowed to penetrate any surface-breaking cracks or cavities. The time the liquid is allowed to soak into the material's surface is normally about 20 minutes, but can be specified otherwise. After soaking, the excess liquid penetrant is wiped from the surface and a developer applied to make any indications visible.

Despite being one of the popular NDT methods, liquid penetrant testing is often misused. Test surfaces may not be cleaned adequately, the contact time between the penetrant and the test surface may be too short, or the excess penetrant is removed carelessly (i.e. from flaws as well as from the test surface). For these reasons, it is important that the personnel carrying out LPI are properly trained and qualified, for example, in accordance with the British Institute of Non Destructive Testing's PCN certification scheme or equivalent schemes such as those operated by the Certification Scheme for Welding Inspection Personnel (CSWIP) and American Society for Non-destructive Testing (ASNT). NAS (National Aerospace Standard) 410 outlines the minimum qualification requirements for all personnel that perform NDT. Following a specific procedures manual is required to ensure the process is completed fully and correctly. This procedure will lay down the exact timings, cleaning regimes and temperatures to be used throughout the testing procedure.

Nital Etching

The Nital etch process, with fully trained personnel, provides a means of detecting surface defects of all kinds, but with particular emphasis on the detection of grinding abuse. Where components have been subjected to surface-hardening treatments such as carburising and hardening or nitride hardening, the surface is vulnerable to damage caused by various grinding techniques, which may not be visible unless Nital etched and inspected by trained personnel with specialised equipment.

Micro Precision is fully approved and equipped to carry out the Nital etch inspection process and have NADCAP accreditation for chemical processes as well as NDT itself. This accreditation ensures that the results are properly recorded as well as details of the chemicals used and their manufacturers, personnel involved in the testing, their accreditations and training records, calibration records for the equipment used, maintenance and inspection reports are all properly maintained.

Micro Precision also holds AS9100, which is the quality management standard specifically written for the aerospace industry. It had long been considered by some entities, such as the Federal Aviation Administration (FAA), that the ISO 9000 series of standards were inadequate in terms of ensuring quality and safety in the "high-risk" aerospace industry.

Terry Grubb, Managing Director of Micro Precision, says: "We have been in business for over 30 years, supplying some of the highest-profile commercial engineering operations in the world, from the latest generation of aircraft to Formula 1 cars. We have worked hard with clients such as Rolls Royce, Boeing, Goodrich, GE, Moog and many others to ensure that we maintain their approvals. Our continual investment in the very latest high accuracy machines, the best people and the most exacting approvals such as NADCAP, we believe we are in the right position to provide the type of precision engineering today's growth sectors demand."

To find out more about non-destructive testing, please go to www.microprecision.co.uk.

Micro-Precision Gear Technology Ltd

Duxons Turn
Hemel Hempstead
HP2 4SB
UNITED KINGDOM

+44 (0)1442 241027

enquires@microprecision.co.uk

www.microprecision.co.uk

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