Magnetic and Ultrasonic Testing – An Overview

Heavy industries such as Aerospace, Nuclear, Petrochemical, Power and Paper besides Pharmaceutical, Automotive and Pipeline all need to be inspected from time to time. Such inspections are necessary in order to prevent failures from occurring since failures may not only be costly to rectify but could also bring the manufacturing process to a grinding halt. These inspections have to be carried out in a way as not to disrupt the industry’s functions. This, therefore, necessitates the use of nondestructive testing (NDT) processes. The various types of NDT commonly used are Eddy Current Testing, Penetrant Testing, Radiographic Testing, Specialized Testing, Magnetic testing, Visual Testing, and Ultrasonic Testing.

Magnetic testing methods are generally used for the examination of pipelines, welds, turbine blades and various structures. The various services that are used could be the Magnetic Flux Leakage (MFL), the Alternating Current Field Measurement (ACFM) and the Portable Yoke. The MFL is generally used for detecting pitting and corrosion in steel structures. In this technique, the structure is initially magnetized and thereafter an analysis is carried out of the areas that do not show flux lines. There is an advanced MFL technique that uses a tool called the In-line Inspection Tool (ILI) that travels inside the pipeline being inspected. The collected data is analyzed to yield defects pertaining to corrosion and pitting. Further improvement to the resolution is achieved by using multiple sensors. The ACFM technique, on the other hand, can be used on either ferrous or nonferrous materials with equal ease. No direct contact being necessary, this technique is effective through coatings as well.

The various ultrasonic testing methods being used relate to the detection and monitoring of the oxide that’s built up and corrosion. Thickness of samples is also measured. Different services in use include Phased Array or PA, the Electromagnetic Acoustic Transmission or EMAT and Automated Ultrasonic Testing or AUT besides Time of Flight Diffraction or TOFD. The PA reveals structural defects that are hidden by the use of a number of probes. These probes create a high-resolution beam that can be swept in order to produce the object’s internal structure.