The Main Methods of NDT (Non Destructive Testing)
At RTUTec we specialize in detecting technologies. The benefit of using such technologies for testing and quality control is that they do not cause damage to the item that is being tested or analyzed and yet new information is detected. Non destructive testing technologies are used in a variety of applications. This is a review of the main methods used today.
Radiographic Testing (RT)
Radiographic testing is a non destructive testing method based on the ability to use short wavelength electromagnetic (X-ray) or radioactive radiation in order to detect defects and flaws inside the tested object - without destroying it. The X-rays or gamma radiation from isotopes such as Ir-192, Se-75 and even Co-60, penetrate the tested object and hit the detector behind it. The image resulting exposes the defects.
Radiographic testing is used in applications as welding quality tests, authentication of artifacts, casting and production quality control, composite material testing, forensics research and more, detecting defects or discontinuities such as porosity, erosion, corrosion, fatigue cracks among others.
Radiographic testing protocols are ever changing to meet the innovative technologies, which are emerging in this field, from film X-ray via imaging plates and scanners and all the way to direct digital radiography. The testing equipment varies according to the size and type of tested object, as well as its location. High energy X-ray sources are used in laboratories, while other cases may be tested with a mobile X-ray cabinets located on site or even with portable digital X-ray systems in field conditions.
Ultrasonic Testing (UT)
Ultrasonic testing is a non destructive testing method based on the ability to use propagation of ultrasonic waves in order to detect defects inside tested object but also to characterize materials and measure their thickness, while keeping the inspected object intact. This method can be used to test many materials including steel, wood, concrete and composites.
When the reflection method (or pulse-echo mode) is used, the ultrasonic transducer will both send and receive the ultrasonic waves. Distance (or material thickness) is measured as a function of the arrival time of the reflection, while its amplitude and intensity represents flaws and changes in the material. In attenuation method (or through-transmission mode) the ultrasonic waves will be sent through the tested object from a transmitter to a receiver.
Ultrasonic waves are also used for diagnosis and treatment of humans and animals. A ultrasonic / radio frequency wave transducer can create effects of mechanical vibration, diathermy, cavitation and shock wave to relief muscular and bone conditions, among which are tendon pains, muscle strains and contractions, inflammation, calcification of muscle and bone, edemas and hematomas.
Thermographic testing is an non destructive testing method based on the ability to use infra-red radiation measurements as an indicator of temperature changes. According to the "Black Body Radiation Law", any object with a temperature above absolute zero emits infra-red radiation. Thermographic devices translate the infra-red radiation measurements into a colored image, indicating the temperature of various objects in the inspected environment.
With a thermographic device warm blooded animals and people, as well as heated objects become visible even at night when there is no adequate source of illumination. A damp spot (indicating fungus or water leaks) can be detected by its cooler temperature, while electric cables are seen as brighter hotter stripes. Inflammation can be observed due to its extra heat.
Thermographic devices are used in applications such as security and surveillance, for facility management inspections, product quality checks, forensic research, as well as physiotherapy analysis and even treatment.
Eddy Current Testing
Eddy current testing is a non destructive testing method based on the ability to make use of electromagnetic induction to characterize surface and skin depth defects in conductive non-ferromagnetic materials. Eddy currents are closed circular currents creating a magnetic field, which is opposed to the magnetic field that induced the Eddy currents. (In its most basic form, the inducing magnetic field is created by a probe containing a single coil of conductive wire, which is excited with an alternating electrical current) .
Discontinuities and flaws in a conductive material, as well as variations in the electrical conductivity and magnetic permeability are demonstrated in changes of phase and amplitude of the Eddy current. The changes recording in the testing device are analyzed to detect defects in the tested object. Eddy current testing is popular in the aerospace industry as well as in the oil and gas industry for the testing of tubes and pipes.
Magnetic Particle Testing
Magnetic particle testing is a non destructive method based on the ability to use magnetic particles as an indication for surface and sub-surface flaws in the tested objects, which are other wise not visible to the naked eye. This testing method can only be used on ferromagnetic materials such as nickel, iron, cobalt and some metal alloys.
Magnetic particle testing process starts by putting a magnetic field into the inspected object. When using direct magnetization method, the electric current is passed through the object and a magnetic field is created in the material. When using an indirect magnetization method the magnetic field is applied from an external source and no electric current is passed through the tested material.
Discontinuities and flaws on the surface create a magnetic flux leak, as the air is not able to hold the magnetic field (volume per unit) like the ferromagnetic material can. The next step in the test is to apply ferrous iron particles to the surface of the inspected object. the particles are attracted to the area of the magnetic flux leakage (if such an area exists) thus indicating the presence of a defect.
Liquid Penetrant Testing
Liquid penetrant testing is a non destructive method based on the ability to use liquids to detect defects in non-porous materials such as plastics, metals and ceramics. This is a low cost testing method that is widely used for the inspection of quality for casting and forging manufacturing processes, as well as defects in new welds. The method is also used for testing in-service parts for fatigue cracks and erosion from usage.
The liquid penetrant is applied to the tested object by spraying, dipping or brushing. After enough time has been allocated to penetration (which occurs based on the capillary cation principle), the excess of the liquid is removed and the developer is applied. If there are defects on the surface of the tested object, such as porosity, hairline cracks and leaks, these will be penetrated by the liquid and the developer makes this penetration clearly visible (under a suitable light, depending if fluorescent or non-fluorescent liquid was used).
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