What are the common non-destructive testing methods?

1. Ultrasonic Testing (UT)
Technical Principle
Ultrasonic testing uses sound waves with a frequency of more than 20 kHz to move through materials to find flaws including cracks, pores, and inclusions. This causes reflection, refraction, and diffraction. You may find out where the faults are, how big they are, and what they are like by looking at the variations in the amplitude, position, and shape of the reflected waves. Ultrasonic probes change electrical impulses into sound waves, which travel through the material and back to the probe. The sound waves are then changed back into electrical signals so they can be shown and analyzed.
Main strengths
Strong penetrating ability: can find metals, non-metals, and composite materials that are a few millimeters to a few meters thick.
It is possible to find faults with an accuracy of ± 0.1 mm by measuring the time it takes for sound waves to go through them.
Very sensitive: can find flaws as small as 0.1mm in diameter.
Wide range of uses: Works well with welds, castings, forgings, composite materials, and more.
Uses in Business
Aerospace: Finding cracks and delamination problems inside turbine blades and wing coverings.
Petrochemical industry: Check the quality of the welds and the corrosion resistance of pressure vessels and pipes.
Detecting casting flaws in engine cylinder blocks and gearbox housings is part of making cars.
Power industry: keeping an eye on the thinning of boiler tube walls and fatigue cracks in nuclear power station equipment.
Typical Scenario
Ultrasonic tests showed that the internal porosity of the Boeing 787 Dreamliner's engine blades was higher than normal after they were made using SLM (Selective Laser Melting) technology. After being treated with hot isostatic pressing (HIP), the porosity went down from 5% to 0.1%, and ultrasonic testing showed that the tolerance range went down from ± 0.03mm to ± 0.005mm.
2. Radiographic Testing (RT)
Technical Principle
When X-rays or gamma rays go through materials, the difference in density between damaged and undamaged portions causes varying amounts of radiation to be absorbed. This creates images on film or digital detectors that show where the flaws are, how big they are, and what shape they are.
Main strengths
Visual imaging: showing the shape of a defect immediately using film or digital photographs.
High resolution: can find small flaws that are 0.1mm long and 0.01mm wide.
Strong applicability: Can be used to find defects inside metal, non-metal, and composite materials.
Uses in Business
Aerospace: Finding cracks and other problems inside engine turbine discs and combustion chambers.
Automotive manufacturing: Checking the quality of the fusion in welded connections (such spot welding and arc welding).
In the nuclear power business, keep an eye on weld flaws and the corrosion of reactor pressure vessels.
Electronic packaging: finding BGA solder junctions for virtual soldering and cracks inside chips.
Normal Case
To find burrs and air holes at cross holes in the making of hybrid transmission valve bodies, Toyota conducts X-ray inspection. Real-time imaging technology cuts the time it takes to find something from 30 minutes with traditional film methods to 5 minutes. It also keeps the tolerance variation within ± 0.008mm.
3. Testing with Magnetic Particles (MT)
Technical Principle
Magnetic particle testing uses magnetized ferromagnetic materials like carbon steel and low-alloy steel to make a leakage magnetic field at the site of the defect. This field attracts the magnetic powder that is put on the surface, creating magnetic traces that show where the defect is and what shape it is.
Main strengths
Very sensitive: can find cracks in surfaces that are 0.1 μm wide.
Simple to use: The gadget is light and easy to use for testing on site.
Low cost: The cost of testing is only one-fifth that of radiographic testing.
Uses in Business
Railway industry: finding cracks and damage to bolt holes on rail treads.
Petrochemical industry: Check for stress cracks and surface corrosion on pipelines and pressure vessels.
Shipbuilding: Checking for lack of fusion and slag in ship hull welds.
Power industry: Keep an eye on the surface flaws of generator rotor guard rings.
Normal Case
China's high-speed railway uses magnetic particle testing technology to check the wheel rim surface 100% of the time. This technology can find micro cracks that are 0.05mm deep, which prevents driving accidents caused by fatigue fracture and doubles the wheels' service life.
4. Testing for Liquid Penetration (PT)
Technical Principle
Penetration testing uses the way liquids move through small openings to get fluorescent or colorful dyes into the surface opening flaws of materials. After imaging agents work, visible markers are created to show where and what shape flaws exist.
Main strengths
Wide applicability: can find practically any non-porous materials, like metals, ceramics, polymers, and more.
Flexible operation: No need for big machines; can be used in the field or at high altitudes.
Low cost: The cost of testing is only one-third that of ultrasonic testing.
Uses in Business
Aerospace: Finding cracks in the surface of turbine blades and landing gear that are caused by fatigue.
Automotive manufacturing: Check the casting porosity of engine cylinder blocks and gearbox housings.
Nuclear power equipment: finding tiny cracks on the surface of stainless steel welds.
Construction industry: Look at the surface flaws in steel structure welds.
A Typical Case
Fluorescent penetrant testing technology is used to find surface flaws on the titanium alloy skin of Airbus A350 aircraft wings. Ultraviolet light makes fissures that are 0.02 mm wide very easy to see. The detection rate is ten times better than regular eye inspection, and the tolerance pass rate has gone up to 99.5%.
5. Eddy Current Testing (ET)
Technical Principle
The principle of electromagnetic induction is used in eddy current testing. When a test coil with alternating current gets close to a conductive substance, it causes eddy currents to flow through the material. The material's qualities (conductivity, magnetic permeability) and flaws determine the size, phase, and flow shape of the eddy currents. Detecting changes in coil impedance tells us if there are any problems.
Main strengths
Non-contact detection: No need for coupling agents, works well on high-speed automated production lines.
Quick detection speed: can find pipes or wires that are several meters long in a minute.
Detectable thin layer: works with conductive materials that are 0.1mm thick or greater.
Uses in Business
Aerospace: Finding cracks in the blades of aircraft engines that are caused by fatigue and corrosion of rivet holes in the skin of the fuselage.
Energy industry: keeping an eye on how the inside wall of heat exchanger pipes is corroding and how the wall thickness of boiler pipes is getting thinner in nuclear power plants.
In the manufacturing business, check to see if the heat treatment and surface flaws of copper wire for aluminum alloy wheels for cars are the same.
Rail transit: finding cracks in the tread of high-speed rail wheels and damage to the holes for rail bolts.
Normal Case
Tesla employs eddy current testing equipment to check the surface of copper foil online while making battery electrodes. Multi-frequency eddy current probes can find scratches that are only 0.01mm deep. This cuts the chance of electrode breakage by 80% and extends the battery's cycle life to more than 2000 times.