Assessing each process, its tools, advantages, and disadvantages
June 13, 2006
Five types of nondestructive testing are common for tube and pipe weld inspection, and each has advantages and disadvantages that may make one more suitable than another for your inspections.
Nondestructive testing is one quality control function and complements other, long-established methods.
By definition, nondestructive testing is the testing of materials for surface or internal flaws or metallurgical condition without interfering in any way with the integrity of the material or its suitability for service.
The technique can be applied on a sampling basis for individual investigation or may be used for 100 percent checking of material in a production quality control system.
Five nondestructive testing methods are most common, and each has advantages and disadvantages that will determine whether it is suitable for your particular testing application. These techniques are:
Basics. Radiographic testing can detect internal defects in ferrous and nonferrous metals. X-rays, generated electrically, and gamma rays emitted from radioactive isotopes penetrate radiation that is absorbed by the material they pass through. The greater the material thickness, the greater the ray absorption. These rays help form a latent image that can be developed and fixed in a similar way to normal photographic film.
Tools. Various radiographic and photographic accessories are necessary, including radiation monitors, film markers, image quality indicators, and darkroom equipment. Radiographic film and processing chemicals also are required.
Advantages. In radiographic testing, information is presented pictorially. A permanent record is provided, which can be viewed at a time and place distant from the test. This type of testing is useful for thin sections and is suitable for any material. Sensitivity is declared on each film.
Disadvantages. Radiography is not suitable for several types of testing situations. For example, radiography is inappropriate for surface defects and for automation, unless the system incorporates fluoroscopy with an image intensifier or other electronic aids. Radiography generally can't cope with thick sections, and the testing itself can pose a possible health hazard. Film processing and viewing facilities are necessary, as is an exposure compound. With this method, the beam needs to be directed accurately for 2-D defects. Also, radiographic testing does not indicate the depth of a defect below the surface.
Basics. Magnetic particle inspection can detect surface and near-surface discontinuities in magnetic material, mainly ferritic steel and iron. The principle is to generate magnetic flux in the article to be examined, with the flux lines running along the surface at right angles to the suspected defect. Where the flux lines approach a discontinuity, they will stray out into the air at the mouth of the crack. The crack edge becomes magnetic attractive poles, north and south. These have the power to attract finely divided particles of magnetic material, such as iron fillings. Usually these particles are an iron oxide 20 to 30 microns in size. They are suspended in a liquid that provides mobility for them on the surface of the test piece, assisting their migration to the crack edges. However, in some instances they can be applied in a dry powder form.
Tools. Basically, magnetic crack detection equipment takes two forms. First, for test pieces that are part of a large structure, or for pipes and heavy castings, for example, that can't be moved easily, the equipment takes the form of just a power pack to generate a high current. For factory applications on smaller, more manageable test pieces, bench-type equipment — with a power pack, an indicating ink system that recirculates the fluid, and facilities to grip the workpiece and apply the current flow or magnetic flux flow in a methodical, controlled manner — generally is preferred.
Advantages. Magnetic particle inspection generally is simple to operate and apply. This testing is quantitative, and it can be automated, apart from viewing. However, modern developments in automatic defect recognition can be used in parts with simple geometries, such as billets and bars. In this case, a special camera captures the defect indication image and processes it for further display and action.
Disadvantages. This type of nondestructive testing is restricted to ferromagnetic materials, as well as to surface or near-surface flaws. Magnetic particle inspection is not fail-safe; lack of indication can mean that no defects exist, or that the process wasn't carried out properly.
Basics. Dye penetrant testing is used frequently to detect surface-breaking flaws in nonferromagnetic materials. The part to be tested must be cleaned chemically, usually by vapor phase, to remove all traces of foreign material, grease, dirt, and other contaminants from the surface, generally, but also from within the cracks. Next, the penetrant, which is a fine, thin oil usually dyed bright red or ultraviolet fluorescent, is applied and allowed to remain in contact with the surface for about 15 minutes. Capillary action draws the penetrant into the crack during this period. The surplus penetrant on the surface then is removed completely, and a thin coating of powdered chalk is applied. After the appropriate development time, the chalk draws the dye out of the crack to form a visual indication, magnified in width, in contrast to the background.
Tools. Various substances can be used and may be applied in many ways, from simple application with aerosol spray cans to more sophisticated means, such as dipping in large tanks on an automatic basis. More sophisticated methods require tanks, spraying, and drying equipment.
Advantages. A quantitative analysis, dye penetrant testing is simple to do and is a good way to detect surface-breaking cracks in nonferrous metals. It's suitable for automatic testing, but with the same limitations that apply to automatic defect recognition in magnetic particle inspection.
Disadvantages. Dye penetrant testing is restricted to surface-breaking defects only. It is less sensitive than some other methods and uses a considerable amount of consumables.
Basics. This technique detects internal and surface (particularly distant-surface)defects in sound-conducting materials. A short pulse of ultrasound is generated by means of an electric charge applied to a piezoelectric crystal, which vibrates for a very short period at a frequency related to the thickness of the crystal. This pulse takes a finite time to travel through the material to the interface and to be reflected back to the probe. Probing all faces of a test piece reveals the 3-D defect, measures its depth, and determines its size.
Tools. Modern ultrasonic flaw detectors are fully solid-state, can be battery-powered, and generally are built to withstand work site conditions. The process can be automated and now is used in many foundries.
Advantages. Ultrasonic flaw detection can be used to test thickness and length up to 30 feet. This type of testing can determine defect position, size, and type. It's a portable type of testing that offers extreme sensitivity when required and can be fully automated. Access to only one side is necessary for testing, and no consumables are used.
Disadvantages. No permanent record is available unless one of the more sophisticated test results and data collection systems is used. The operator can decide whether or not the test piece is defective while the test is in progress. Test indications require interpretation, except for digital wall thickness gauges. A considerable degree of skill is necessary to get the most information from the test. Finally, very thin sections can be difficult to test with this method.
Basics. The eddy current technique can detect surface or subsurface flaws and measure conductivity and coating thickness. This testing is sensitive to a test piece's material conductivity, permeability, and dimensions. For surface testing for cracks in single or complex-shaped components, coils with a single ferrite-cored winding normally are used. The probe is placed on the component and "balanced" by use of the electronic unit controls. As the probe is scanned across the surface of the component, cracks are detected.
Tools. Most eddy current electronics have a phase display that allows the operator to identify defect conditions. Some units can inspect a product simultaneously at two or more different test frequencies. These units allow specific, unwanted effects to be electronically canceled to give improved defect detection. Most automated systems are for components with simple geometries.
Advantages. Suitable for automation, eddy current testing can determine a range of conditions of the conducting material, such as defects, composition, hardness, conductivity, and permeability. Information can be provided in simple terms, often go or no-go. Phase display electronic units can be used to obtain greater product information. Compact, portable testing units are available, and this type of testing doesn't require consumables, except for probes, which sometimes can be repaired. This technique is flexible because of the many probes and test frequencies that can be used for different applications.
Disadvantages. Many parameters can affect the eddy current responses. This means that the signal from a desired material characteristic (for example, a crack) may be masked by an unwanted parameter, such as hardness change. Careful probe and electronics selection is necessary in some applications. Also, tests generally are restricted to surface-breaking conditions and slightly subsurface flaws.
Mark Willcox, managing director, and George Downes can be contacted at Insight NDT Equipment Ltd., Kings Thorn, Herefordshire HRZ 8AW, England, 44-0-1981-541122, fax 44-0-1981-541133, www.insightndt.com.