- 1 Methods and Definition
- 2 Non-Destructive Testing Methods
- 2.1 Acoustic Emission Testing (AE)
- 2.2 Electromagnetic Testing (ET)
- 2.3 Ground Penetrating Radar (GPR)
- 2.4 Laser Testing Methods (LM)
- 2.5 Leak Testing (LT)
- 2.6 Magnetic Flux Leakage (MFL)
- 2.7 Microwave Testing
- 2.8 Liquid Penetrant Testing (PT)
- 2.9 Magnetic Particle Testing (MT)
- 2.10 Neutron Radiographic Testing (NR)
- 2.11 Radiographic Testing (RT)
- 2.12 Thermal/Infrared Testing (IRT)
- 2.13 Ultrasonic Testing (UT)
- 3 What is the Difference Between Destructive and Non-Destructive Testing?
- 4 What are the Advantages of using NDT?
Methods and Definition
Non-destructive testing (NDT) is also known as non-destructive examination (NDE), non-destructive evaluation (NDE), and non-destructive (NDI). This process involves a large variety of techniques used by various industries to evaluate the properties of a part, material, product, system, or weld without impacting the original piece.
Non-Destructive Testing Methods
The following are NDT test methods:
Acoustic Emission Testing (AE)
AE is an NDT technique that detects and monitors the release of ultrasonic waves that are given off when material fractures under a lot of stress. This test monitors for damage during mechanical testing. When a material is about to crack the emissions are generally too low in amplitude or too high in frequency to be detected by a person’s ear. AE allows a person to detect higher frequencies and lower intensities in highly stressed areas to detect cracks before they even form. This is often used during proof tests of a pressure vessel, leaks, and active corrosion.
Electromagnetic Testing (ET)
ET is a testing method that uses an electric current and/or magnetic field inside an object that a person desires to test to observe the electromagnetic response. This test is used to detect cracks, measure material thickness, measure coating thickness, and measure conductivity. It is often used by the airline industry to inspect the exterior of an aircraft.
There are 3 types of electromagnetic testing:
- Eddy current testing. This test uses a coil to produce an electromagnetic field in a conductive material.
- Alternating current field measurement. This test uses an alternating current into the surface of an object to detect surface-breakage.
- Remote field testing (RFT). This probes a bunch of transmitter coils to detect internal and external defects. This is used to find defects in steel pipes and tubes.
Ground Penetrating Radar (GPR)
GPR is a geophysical locating method that works by sending radar pulses through the surface of an object to see images below the surface of an object. When the electromagnetic impulse hits an object, the density of the object sends back a signal alerting the user that there is something under the ground or in a material. A common example of a GPR is a metal detector that people use for fun to find buried treasure in the ground.
Laser Testing Methods (LM)
There are 3 types of Laser Testing Methods:
- Holographic testing. This uses a laser to detect fluctuations in the surface of a material that has been put under stress in the form of vibration, heat, or pressure. From there, the results are compared to the same type of material that is undamaged in order to show defects.
- Laser profilometry. This is a procedure that uses a fast-spinning laser light source to detect pitting, corrosion, cracks, and other forms of material degradation. It works by detecting fluctuations in the surface of an object.
- Laser stereography. This uses light from a laser to produce an image prior to surface stress. The stressed image and the original image are compared to determine if there are any defects.
Leak Testing (LT)
There are 4 types of Leak Testing:
- Bubble leak testing. This uses a tank of liquid solution to detect gas leading from the test piece. If the test piece is leaking bubbles will appear. For larger parts, a soap solution is used to test for bubbles.
- Pressure change testing. This is only used on closed systems because it uses either pressure or a vacuum to detect defects. A loss of pressure over a time period indicates that there is a leak.
- Halogen diode testing. This uses pressure to detect leaks through mixing air and halogen-based tracer gas together. Then, a halogen diode detection unit is used to find leaks.
- Mass spectrometer testing. This involves helium or helium mixed with air inside a test chamber with a halogen diode detection unit to determine if there are any variations in the air sample. If so, there is a leak. Sometimes, a vacuum is used to detect leaks. The vacuum works by using a mass spectrometer to sample the vacuum chamber to check for ionized helium. If ionized helium is detected, there is a leak.
Magnetic Flux Leakage (MFL)
MFL works by using a magnet to generate magnetic fields to magnetize steel structures like storage tanks and pipelines. If there is corrosion, pitting or wall loss in the steel structures the magnetic field will leak indicating a reduction in material and this sensor will determine the location and severity of the defect.
This technique transmits microwave frequencies which are received by a test probe. The test probe determines if there are any changes in the dielectric properties such as pores, shrinkage, cavities, foreign materials, or any cracks. This information is then displayed in the form of B or C scans. Microwave testing can only be used with dielectric materials which are any material that is a poor conductor of electricity.
Liquid Penetrant Testing (PT)
This technique is economical, versatile, and there is little need for training compared to other nondestructive methods. PT checks for material flaws, such as cracks or porosity, that are open to the surface by flowing a liquid into the flaw. This method creates a visible indication that there is a flaw in the material because liquid will seep out if there is a crack or flaw. Welds are the most common item inspected, but pipes, castings, bars, and plates can also be inspected using PT.
Magnetic Particle Testing (MT)
MT is also referred to as Magnetic Particle Inspection. MT uses magnetic fields to find cracks or leaks near the surface of ferromagnetic materials. Ferromagnetic material are materials that can be magnetized such as nickel, iron, steel, cobalt, and some of their alloys. When a crack or leak is present, the magnetic flux (magnetic field) leaks out of the metal. This magnetic flux collects ferromagnetic particles, such as iron powder, and makes the size and shape of the crack easy to see.
Neutron Radiographic Testing (NR)
NR is an imaging technique that is similar to x-rays. Unlike an x-ray, this method uses neutron radiation (neutron beams) to see inside a material. This technique only works to detect flaws in organic materials such as carbon and hydrogen which enable structural and internal components to be seen to find flaws. For example, lead and steel have weak interactions with neurons enabling NR to see the organic material that is preserved inside a metal object.
Radiographic Testing (RT)
RT works by enabling either x-ray or gamma radiation to go through an object to find any flaws that are present. This method enables the user to see the inside of their workpiece to detect internal defects such as corrosion or erosion and check for casting flaws or foreign objects. Depending on the density of the material either x-ray (used for thin materials) or gamma rays (used for thick materials) are used.
Thermal/Infrared Testing (IRT)
IRT is also referred to as infrared thermography which uses a thermographic camera to detect radiation (heat) coming off of a workpiece. The thermal imager converts it to a temperature and displays it as an image, showing the temperature distribution. It then produces thermograms which are images of the radiation. This information enables a person to see heat-producing objects that would otherwise be invisible. IRT is used in predictive maintenance and to determine the condition of a workpiece.
There are two types of IRT that are used in inspections:
- Passive thermography. This technique directly measures the surface temperature for evaluations. It uses sensors to measure the wavelength of the emitted radiation. If the emissivity can be measured, then the temperature is calculated and shown as a digital reading or colored image. Passive thermography is useful to detect overheating bearings, motor or electrical components, and to monitor heat loss from buildings.
- Active thermography. The technique measures the surface temperature for evaluation after applying an external energy source to produce a thermal contrast between the object and the background. Areas affected by the heat flow will show surface temperature variations signifying a defect and can analyze the condition of an object. This is used to determine bonding defects in objects or near-surface delamination (fractures).
Ultrasonic Testing (UT)
This method enables the transmission of high-frequency sound waves to examine and measure the thickness or internal structure of an object. This method works because high-frequency sound waves typically travel through a medium until they hit a boundary with another medium such as air. At this point, they reflect back to their source. This can be used to determine if there are any cracks or flaws within a material.
2 methods for receiving the ultrasound waves:
- Pulse-Echo Inspection. This method pushes a sound beam (pulsed waves) through the material to be tested. The sound waves will travel through the part and waves that are reflected back indicate that they reached the end of the piece or they hit an imperfection. The machine will display the results representing the arrival time of the reflection (intensity of the reflection and the distance).
- Through Transmission Testing. This method pushes ultrasounds through one surface and uses a separate receiver on another surface to detect the amount that reached it after traveling through the object. If there are any imperfections, the amount of sound transmitted will be reduced, indicating a flaw. This method does not measure thickness.
Various forms of nondestructive ultrasonic testing:
- Time of Flight Diffraction (ToFD). This method uses a pair of ultrasonic probes placed on opposite sides of a weld or item. One probe works as a transmitter, pushing out ultrasonic pulses and the other receives those pulses. If flaws are present, there will be diffraction of ultrasound pulses from the tips of the crack. This method can determine the depth of a crack and is used when true reflection cannot be obtained. ToFD is also used to inspect the rear wall of an object to determine if there is any corrosion.
- Immersion Ultrasonic Testing. This is the advanced form of ultrasonic testing because all frequencies can be used because there is no physical contact between the transducer and the material being tested. This method is ideal for smaller objects and determines how well an object will hold up when submerged in an immersion ultrasonic tank in various liquids, such as water. This enables a person to find the smallest defects by precisely reporting the size and location of sub-surface flaws.
- Air Coupled Testing. This ultrasonic testing method is a type of non-contact inspection technique that is used when materials cannot handle being placed in water. This is a lower frequency inspection that uses air with ultrasound testing. This is a water-free, sensitive inspection that checks for voids and cracks in water-compatible materials.
- Electromagnetic Acoustic Transducer (EMAT) Testing. This ultrasonic test is another type of non-contact inspection that pushes ultrasonic waves into an object with two interacting magnetic fields. This method can be used in extremely hot or cold temperatures. It is commonly used to inspect piping, vessels, tubes, and determine wall thickness.
- Guild Wave Testing (GW). This method is sometimes referred to as long-range ultrasonic testing (LRUT). GW places a ring of transducers around a pipe and uses ultrasonic sound waves to travel down both directions of the pipe. If they come in contact with corrosion or other flaws in a pipes wall the sound waves will be reflected back towards the transducers. The results from this method are sent to a computer to be analyzed. This test is commonly used to test pipes over long distances.
Advanced ultrasonic methods:
- Automated Inspection. This is a completely automated technique that uses an ultrasonic imaging system and encoded robotic scanners to examine an object.
- Phased Array Ultrasonic Testing (PAUT). This method uses advanced probes that are made up of many small elements that can be pulsed individually. A computer is used to calculate the phasing sequence (timing) to control when each element is fired. The phasing sequences enable wave focusing, scanning, and steering. This technique is used to inspect complex geometries for flaws.
- Full Matrix Capture (FMC). FMC is an advancement from the PAUT method a data. This acquisition strategy enables the capture of all possible ultrasonic data from a phased array probe. There is no need to focus or steer the beam because the entire region that is being tested is in focus. This makes FMC very advantageous. Also, it is simple to set up and use as well as being tolerant of misalignment and structural noise. The biggest disadvantage is that it is slower than PAUT and the file sizes are big.
- Virtual Source Aperture (VSA). This is a type of FMC that has better image quality. It also has smaller file sizes and acquisition speeds that make it better than PAUT.
- Vibration Analysis (VA). This surface mapping technique uses sensors to determine the amount of vibration from rotating machinery to determine if there are flaws in an object. Some sensors used are velocity sensors, accelerometers, and displacement sensors.
- Visual Testing (VT). This method is also known as visual testing examination or visual inspection. VT involves looking at a part to determine if there are any issues with the surface of an object. VT is often used with magnifying glasses and mirrors to enhance a person’s viewing. This method is commonly used in NDT because it is simple, low cost, and requires little to no equipment. It is frequently used to detect corrosion, damage, misalignment, and cracks in welds, storage tanks, boilers, pressure vessels, and piping.
What is the Difference Between Destructive and Non-Destructive Testing?
The biggest difference between destructive and non-destructive testing is that destructive testing destroys or alters an object making the part unable to be used and removed from service. Destructive testing is more expensive and wasteful. Whereas, NDT does not damage or alter an object enabling it to pass the test and be used. Also, there is no interruption beyond normal maintenance making NDT more cost-effective.
What are the Advantages of using NDT?
- Parts being tested are undamaged. This permits parts to be repaired instead of replaced if there are any flaws.
- Useful for testing on welds and ensuring weld procedures were done correctly.
- The majority of tests are safe for operators.
- Accurate form of inspecting parts because the test can be performed a number of times and the results of each test can be correlated together to ensure precision.
- Test techniques are cost-effective because the test does not destroy the part.
- Tests can be performed on the entire part to examine multiple critical sections.