Did you ever think what could happen if a small piece of an aircraft engine breaks? How about creation of a tiny pinhole in a gas pipeline? as one can imagine, the consequence of such events can be a catastrophe. In reality there are many other scenarios that can cause a material failure which sometimes leads to a disaster. To prevent the occurrence of these incidences, in addition to a safe design, it is a need for inspection methods that can detect any possible defect without causing any damage to the current working system. Such an idea was the driving force for developing a new field known as non-destructive testing (NDT). Using NDT methods one can detect metal losses, cracks, corrosion sites, pinholes and this information is a great help for scientists to preform systematic risk assessments. However there are many different NDT methods, the most common methods can be listed as follow:
- Liquid penetration testing
- Eddy current testing (EC)
- Ultrasonic testing (UT)
- Magnetic flux leakage (MFL)
In liquid penetration testing a penetrant liquid is exposed to the specimen under investigation and after cleaning the extra liquid, a developer exposes to the surface that brings out the trapped penetrant from holes or cracks and reveals defect sites. In EC testing an AC magnetic field applies to the sample. The AC magnetic field produces an eddy current inside the sample and the eddy current itself generates a secondary magnetic field which its phase and amplitude can be detected by a receiving coil. The received signal contains information regard the cracks or pinholes. In UT by using a transducer and a filling agent between transducer and sample an ultrasound wave is sent to the sample and then the reflected wave is detected by a receiver that reveals many information about the specimen condition. The MFL is based on detecting the magnetic field disturbance in the area where there is a metal loss. In radiography by emitting X-ray from one side of workpiece and putting a photographic film on the other side of it, cracks or defects are imaged.