There are 3 main NDT methods for the inspection of welding surface crack detection, Liquid Penetrant Testing (PT) , Magnetic Particle Testing (MT) and Eddy Current Testing (ET). We shall consider how they are used, and what types of welding discontinuities they can be expected to find. We shall examine their advantages over other inspection methods and their limitations.
Liquid Penetrant Testing (PT)
This is probably the most common method of surface crack detection used. We shall consider the two most common methods of this testing method, Color Contrast and Fluorescent Dye. Both these methods use the same fundamental procedure. First, the application of a penetrating liquid to the surface of the weld to be tested, followed by a predetermined soak period to allow for adequate penetration of the liquid penetrant into any surface breaking discontinuities. Second, the careful removal of any excess penetrant, usually with a solvent or sometimes with a water wash, dependent on the method used. Third, the application of a developer to withdraw penetrant left behind within any discontinuity. These three steps are followed by the interpretation and evaluation of the test results. This will involve the detection of penetrant bleed-out from within any surface discontinuity. The method of detection is different for the color contrast and the fluorescent dye. The color contrast method is dependant on the bright contrast between the red penetrant dye and the white developer background covering the surface of the weld being tested, and the evaluation of the test is conducted in ordinary light. The fluorescent dye method is assisted by the use of an ultraviolet light (black light) that is used to illuminate the fluorescent dye and assist in the interpretation of the test.
This type of testing is limited to the detection of surface breaking discontinuities, that is, discontinuities which are open to the surface to which the penetrant has been applied. It cannot detect discontinuities that are sealed within the body of the weld such as internal porosity, or fusion defects. It is not usually suitable for testing rough or porous materials because interpretation of the test results can be hindered by false indications.
When compared to unassisted visual inspection, this type of inspection can provide a more sensitive inspection method that is more likely to detect smaller and finer surface breaking discontinuities, such as hair line cracks and micro surface porosity. This type of inspection may be suitable for both ferrous and nonferrous materials.
Magnetic Particle Testing (MT)
This is an NDT method used for detecting cracks, porosity, seams, inclusions, lack of fusion, and other discontinuities in ferromagnetic materials. Surface discontinuities and shallow subsurface discontinuities can be detected by using this method. This testing method consists of establishing a magnetic field in the part to be tested, applying magnetic particles to the surface of the part, and examining the surface for accumulations of particles that indicate discontinuities. A magnet will attract magnetic particles to its ends or poles, as they are called. Magnetic lines of force or flux flow between the poles of a magnet. Magnets will attract magnetic materials only where the lines of force enter and leave the magnet at the poles. If a magnet is bent and the two poles are joined so as to form a closed loop, no external poles will exist and consequently it will have no attraction for magnetic material.. This is the basic principle of magnetic particle testing. As long as the part has no cracks or other discontinuities, magnetic particles will not be attracted. When a crack or other discontinuity is present in the part being tested, north and south magnetic poles are set up at the edge of the discontinuity.
Only ferromagnetic materials can be tested by this method. Ferromagnetic parts that have been magnetized during testing may retain a certain amount of residual magnetism. Certain parts may require demagnetization if they are to function properly in service.
When using surface crack detection, whether liquid penetrant testing or magnetic particle, you should always consult the relevant specification involved for levels of acceptability and qualifications for equipment and operators. These methods of inspection are specialized and should be carried out by suitably trained and qualified inspection personnel.
Eddy Current Testing (ET)
Eddy current tester can be used for a variety of applications such as the of crack detection (discontinuities), thickness measurement of metal , detection of metal thinning due to corrosion and erosion, determination of coating thickness, and electrical conductivity measurement and magnetic permeability. Eddy current testing is an excellent method for detecting surface and near surface defects when the probable defect location and orientation is well known.
Defects such as cracks are detected when they disrupt the path of eddy currents and weaken their strength. The images to the right show an eddy current surface probe on the surface of a conductive component. The strength of the eddy currents under the coil of the probe ins indicated by color. In the lower image, there is a flaw under the right side of the coil and it can be see that the eddy currents are weaker in this area.
Of course, factors such as the type of material, surface finish and condition of the material, the design of the probe, and many other factors can affect the sensitivity of the inspection. Successful detection of surface breaking and near surface cracks requires:
- A knowledge of probable defect type, position, and orientation.
- Selection of the proper probe. The probe should fit the geometry of the part and the coil must produce eddy currents that will be disrupted by the flaw.
- Selection of a reasonable probe drive frequency. For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity. For subsurface flaws, lower frequencies are necessary to get the required depth of penetration and this results in less sensitivity. Ferromagnetic or highly conductive materials require the use of an even lower frequency to arrive at some level of penetration.
- Setup or reference specimens of similar material to the component being inspected and with features that are representative of the defect or condition being inspected for.
The basic steps in performing an inspection with a surface probe are the following:
- Select and setup the instrument and probe.
- Select a frequency to produce the desired depth of penetration.
- Adjust the instrument to obtain an easily recognizable defect response using a calibration standard or setup specimen.
- Place the inspection probe (coil) on the component surface and null the instrument.
- Scan the probe over part of the surface in a pattern that will provide complete coverage of the area being inspected. Care must be taken to maintain the same probe-to-surface orientation as probe wobble can affect interpretation of the signal. In some cases, fixtures to help maintain orientation or automated scanners may be required.
- Monitor the signal for a local change in impedance that will occur as the probe moves over a discontinuity.