Non-Destructive Examination (NDE) or Non-Destructive Testing (NDT) is use of non-invasive testing techniques to determine the integrity of a component or structure. This enables device component condition to be assessed, often in-situ, without detriment to its service life.
Whilst the majority of thorough examination is performed using Visual Inspection, when appropriate LTC uses it’s 40 years experience in NDE to select the most suitable NDT technique for each component, based on its design, material, size and location. All LTC engineers hold at least Level 2 PCN in Visual and Magnetic Particle Inspection, in addition to the techniques with some of the NDT techniques listed below.
Click below for further information on individual inspection techniques.
Visual inspection is the first tool in the inspection of any structure or component. Prior to any further NDE, a thorough visual inspection will be carried out. This inspection will reveal corrosion, damage and wear much more readily than other methods and can indicate where other investigative methods should be targeted. Over 90% of defects are found by the naked eye.
LTC also employ indirect visual aids such as video borescopes to enable remote visual inspection of internal cavities such as inside coaster chassis, GRP car/seat units, winch drums, themeing and ride booms/structures, etc. These techniques enable our inspection engineers to fully inspect such components in-situ without the time intensive process of stripping assemblies into component parts. The images from such devises are viewed live on a screen which can be recorded, uploaded to a computer for inclusion in reports or distribution between engineers and clients.
Magnetic Particle Inspection (MPI) is a NDE technique which uses the nature of magnetic fields to identify potential defects. In magnetised materials, the magnetic field tends to travel within the solid material. When an applied field meets a crack or other defect, it will tend to ‘jump’ across the crack. Inspection engineers use magnetic ink to pick up this ‘jumping’ mechanism and hence identify a flaw.
Finely divided iron or magnetic iron oxide particles, held in suspension in a suitable liquid (often kerosene) are used for the inspection; this magnetic fluid is referred to as the carrier. The particles are often coloured and can be coated with fluorescent dyes that are made visible with a hand-held ultraviolet (UV) light. This tends to improve the contrast of the inspection surface, and therefore easier to spot defects.
The suspension is sprayed or painted over the magnetised specimen during magnetisation which is provided by a direct current or with an electromagnet, to localise areas where the magnetic field has protruded from the surface. The magnetic particles are attracted by the surface field in the area of the defect and hold on to the edges of the defect to reveal it as a build-up of particles.
Ultrasonic testing in an NDE method which utilises an ultrasound transducer connected to a diagnostic machine that is systematically passed over the object being inspected. The transducer is coupled to the test item by a fluid, usually grease or glycerine to enable the sound waves to be transmitted into the specimen.
There are two methods of receiving the ultrasound waveform – via reflection or attenuation. In reflection (or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves as the “sound” is reflected back to the device. Reflected ultrasound comes from an interface, such as the back wall of the object or an imperfection within the object. The diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance, representing the arrival time of the reflection.
In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after travelling through the medium.
Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing their presence.
Specialist LTC inspection engineers are equipped digital UT equipment. They have several advantages over older, analogue technology, including:
- A greater repeatability of results
- Quicker and more accurate calibration
- A capacity to store data – results, trace patterns, etc.
- Compatibility with computer software, permitting uploading of results for direct inclusion in reports
- Reduced weight makes the device easier to transport, whilst better materials make the devices much more robust.
Ultrasonic methods are also used to measure material thicknesses to detect or monitor corrosion. The principles used are the same as for defect location but utilising echoes from the entire thickness of the material rather than defects within it.
The instruments employed by LTC are able to measure material thickness as well as the thickness of any coating. It can also save “profiles” of material under inspection, giving a representation of any corrosion present. This allows us to retain a database of thickness reading for a particular project, which can be uploaded to a computer for reporting or analysis.
Eddy current testing utilises the principles of electromagnetic induction to detect flaws in conductive materials. Eddy current testing can detect very small cracks in or near the surface of the material, the surfaces need minimal preparation, and physically complex geometries can be investigated. It is also useful for making electrical conductivity and coating thickness measurements.
Eddy current testing relies on the attraction of magnetic particles to the flux leakage when an eddy current is passed through the material. This is an indication of the flaws existence, this flux leakage is caused by the flaw in the ferromagnetic material for which is being tested. A sample eddy current trace is shown below.
Eddy current inspection has recently been employed to inspect for cracking on welded items without the need to remove paint. The instrument we use is able to store defect details and upload them to computer. The testing devices are portable, provide immediate feedback, and do not need to contact the item in question. All our digital NDE equipment is from the same “family”, thus enabling common software to combine results from differing methods in reports.
Penetrant may be applied to both ferrous (magnetic) and non-ferrous materials, but for inspection of ferrous components magnetic particle inspection is preferred for its sub-surface detection capability. Dye Penetrant Inspection can be used to detect casting and forging defects, cracks, and leaks in new products, in addition to fatigue cracks on in-service components.
Either an X-ray machine or a radioactive source can be used as a source of photons. Neutron radiographic testing (NR) is a variant of radiographic testing which uses neutrons instead of photons to penetrate materials. This can see different things from X-rays, as neutrons have the ability to pass through lead and steel with ease but are stopped by plastics, water and oils.
Since the amount of radiation emerging from the opposite side of the material can be detected and measured, variations in this amount (or intensity) of radiation are used to determine thickness or composition of material. Penetrating radiations are those restricted to that part of the electromagnetic spectrum of wavelength less than about 10 nanometres.
Welding inspection should be performed both directly after manufacturing and during periodic in-service inspection. LTC inspection engineers are fully qualified to inspect welded structures and other fabrications and have significant experience in identifying weld defects. Many amusement devices are welded in structurally critical areas, so it is vital to use trained welding inspectors to ensure a thorough, competent examination.
Thermal imaging enables problems to be identified and rectified prior to failure avoiding costly, unscheduled downtime. Another benefit is the ability to remotely inspect systems without the need to interrupt ride operation, strip or have direct access to the area in question. It can also be used regularly to monitor frictional forces within ride systems, enabling a predictive maintenance regime to be employed. Examples of an overheated electrical installation and defective gearbox is shown below.
