VISUAL TESTING (VT)
Visual testing includes all non-destructive testing techniques that use electromagnetic radiation in the field of visible light, i.e. in the band of wavelengths between 400 and 700 nm approximately the geometry and power of which can highlight the defects sought. Optical instruments including magnifying glasses, endoscopes or remote visual systems can, when necessary, be more sensitive that the naked human eye or reach complex or restrictive geometrical areas. The operator can be given characterisation aids, in the form of laser lines, for example.
Visual testing is direct if there is no interruption in the optical path between the surface inspected and the operator’s eye. This category includes tests by the naked eye and those using magnifying glasses, mirrors, lenses, borescopes, fibre optics, etc.
Visual testing is indirect if the optical path is interrupted between the surface inspected and the operator’s eye. This category includes tests using photographs, cameras, video-endoscopes, etc.
Visual testing can detect any surface-breaking defect (cracks of all kinds, scratches, porosities, shrink holes, cold shut, lines, seams, laps, cold cracks, deposits, traces of corrosion, migrating bodies, lamination, etc.).
The method produces an image of the part and any indications detected. In most cases, this image can be recorded for test traceability purposes. Determining the location and dimensions of indications precisely can be complex when testing manually with optical instruments; this characterisation is simple to achieve and can be very precise in a mechanised test.
2. Test method
The test is performed by an inspector in conditions that allow the defect sought to be detected on the part inspected. The main parameters that influence inspection sensitivity are described below.
Lighting geometry must be adapted to the defect sought, to create the contrast required for detection:
- Raking light is preferred when looking for a surface accident (crack, scratch, sign of impact, etc.): the linear indications are detected even better when they are perpendicular to the raking light flux;
- Coaxial lighting is preferred for surface colouration (burns, corrosion, etc.).
The lighting must be powerful enough for the light receiver so that a strong enough signal can be collected. The standards stipulate minimum illuminance of 160 to 500 lux for direct testing, depending on the type of test. This value should be appropriate to the sensitivity of the image sensor in indirect testing.
Resolution (the ability of the system to see small defects) must suit the test configuration: the more restrictive it is, the smaller the size of the field inspected; optical instruments, cameras, mechanised positioning systems, etc. may then be necessary to ensure the sensitivity. In addition, the use of optical instruments that increase the resolution generally reduces the depth of the system field, which increases the implementation constraints even further.
Details in a width of more than a few tens of micrometres can be detected with the naked eye. Indications in the order of one micrometre wide can be visualised using optical instruments.
Colour (indirect testing)
The colour test provides additional information that is especially important when looking for corrosion or deposits. However, in indirect testing, obtaining colour images implies a considerable drop in sensitivity and the lateral resolution of the sensor as well as a significant rise in the volume of data: its use should be limited to cases of genuine need.
Visual testing is a method widely used in all sectors of industry, mainly for process testing or to supplement other NDT methods. It can detect and above all characterise all types of surface defects on a part, provided that the characteristics of the testing equipment used are compatible with the application (sensitivity, depth of focus and positioning accuracy in particular).
It can be used on all types of material in both manufacturing and maintenance.
Small variations in geometry, the grade of the material inspected, its mechanical characteristics, etc. have little effect on the sensitivity of the method: visual testing is therefore a general method for performing surface tests when no other method can be used.
Visual testing has its limits, however, in terms of:
- sensitivity on high gloss surfaces or when looking for very fine discontinuities in parts with complex geometries (given the shallow depth of focus correlated with this type of test);
- selectivity in looking for fine defects where the geometry is not significantly different from the nominal surface state of the part: in this case, its qualification can be highly complex as the classification of indications depends strongly on the human factor;
- testing time when looking for small defects on large parts due to the small field size associated with this type of test.
4. Advantage of the method
Visual testing requires a very limited initial investment. This increases with the sensitivity and performances required in terms of characterisations sought (remote visual inspection, mechanised inspection). It ensures the traceability of inspected data and can take place in a very hostile environment (temperature, corrosive environment, ionising radiation, etc.).
The method does not use effluents or strong magnetic fields and its health, safety and environment (HSE) impact is limited to the operator’s eye strain. This can be significant in tests lasting a long time and tests with inappropriate lighting intensity or on-screen analysis.
The fact that indications are detected on the image directly by the human brain, which is still the most efficient image processing system, gives the method a very broad spectrum of applications, as it can be adapted to all the materials to be inspected, or an extensive variety of surface defects to be sought. The downside to this is that the method only rarely has quantitative reporting criteria and indications cannot therefore be detected in automated fashion in most cases: the classification and characterisation of indications are therefore complex.
5. Related standards
Standards currently in force
NF EN 13301-10 Non-destructive testing - Terminology - Part 10: Terms used in visual testing
NF EN 13018/A1 Non-destructive testing - Visual testing - General principles
NF EN 13927 Non-destructive testing - Visual testing - Equipment
PrNF EN ISO 18490 Non-destructive testing — Evaluation of vision acuity of NDT personnel (publication planned in 2015).
Text prepared by COFREND in conjunction with Matthieu Taglione (AREVA NDE-Solutions INTERCONTROLE).