NON DESTRUCTIVE TESTING

NON DESTRUCTIVE TESTING

Over time various NDT methods were developed, each one having advantages and limitations making it more or less appropriate for a given application. According to recent market studies, modern NDT methods have become more quantitative and less obtrusive, which in many scenarios, translates into savings over time. As a result, these advanced NDT methods have the potential that could lead to significantly lower repair rates while maintaining existing safety standards as long as adequate criteria and procedures are adapted. As indicated, the industry is weary of replacing well-established and conventional procedures with advanced technology without a time consuming validation effort. However, several international projects have tackled these issues over time, some of which produced some tangible changes. For example, digital radiography is now accepted by ASME and has been used to produce acceptable images on material up to 100 mm thick using Ir-192. Furthermore, under the direction of the International Pipe Line and Offshore Contractors Association (IPLOCA), criteria have been developed for the use of a combination of automated ultrasonic testing and time of flight diffraction on the testing of weld discontinuities on pipeline welds. With the variety of NDT methods available, it is important to select the method that will provide reliable results. A combination of different NDT tests may be applied to provide assurance that the material or component is fit for use.

Radiographic Inspection

The equipment required to perform radiographic inspection can be either an X-ray machine, which requires some electrical input, or a radioactive isotope that produces gamma radiation. The isotope offers increased portability as no electrical power supply is required. Radiation detectors used are image intensifiers in fluoroscopic and real time imaging systems. Electronic imaging panels and phosphorescent imaging screens are used to produce digital images for computed and digital radiography.

Ultrasonic Testing

The primary benefit of UT is that it is considered to be a truly volumetric test. That is, it is capable of determining not only the approximate dimensions and location of a defect, but it will also provide the testing technician with information as to the type of defect. Another major advantage of UT is that it only requires access to one side of the material being tested and it will best detect those more critical planar discontinuities such as crack’s and incomplete fusion which may not be possible with radiographic testing. Because a variety of beam angles can be used, UT can detect defect which may not be detectable by radiography. Portable UT equipment is lightweight and often battery-powered. UT requires highly skilled technicians because interpretation of indications can be difficult. Reference standards are required for calibration and setting up of the equipment. Test scans can be recorded by most equipment providing automated scanning. This test method is generally limited to the inspection of butt welds in materials that are thicker than 6 mm.

Magnetic particle Inspection

Is a non-destructive testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current, which may be either alternating current (AC) or some form of direct current (DC) (rectified AC).

The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals. Ferrous particles are then applied to the part. The particles may be dry or in a wet suspension. If an area of flux leakage is present, the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication. The indication can then be evaluated to determine what it is, what may have caused it, and what action should be taken, if any.

Dye Penetrant Inspection 

Also called liquid penetrant inspection (LPI) or penetrant testing (PT), is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). The penetrant may be applied to all non-ferrous materials and ferrous materials , although for ferrous components magnetic-particle inspection is often used instead for its subsurface detection capability. LPI is used to detect casting, forging and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.

Visual Inspection 

Is a common method of quality control, data acquisition, and data analysis. Visual Inspection, used in maintenance of facilities, mean inspection of equipment and structures using either or all of raw human senses such as vision, hearing, touch and smell and/or any non-specialized inspection equipment. 

Inspections requiring Ultrasonic, X-Ray equipment, Infra-red, etc. are not typically regarded as Visual Inspection as these Inspection methodologies require specialized equipment, training and certification.