Visual Testing

Visual Testing (VT) is the most fundamental and most widely used non-destructive testing method, in which discontinuities, deformation, corrosion and geometric deviations on the surface of a material or welded joint are assessed through the inspector's sense of sight — either with the naked eye or with auxiliary optical equipment. Although the physical principle appears simple, the reliability of the method is closely tied to measurable parameters such as light, viewing angle, observation distance and the visual perception of the inspector. Because the human eye can distinguish surface defects only under adequate illumination and within a limited angle, VT is not a casual glance but a systematic examination process carried out under conditions defined by standards. The method is generally the first step of all other NDT techniques, since any clearly visible surface defect must be detected before moving on to more advanced (and more costly) examinations.

The application begins with the preparation of the surface to be examined. Any contamination that could mask defects — slag, spatter, oil, rust or paint — is removed from the surface, after which adequate light of the correct orientation is directed onto the examination area. The relevant standards require an illumination intensity of at least 500 lux on the examination surface, a viewing angle typically greater than 30 degrees, and a distance between the eye and the surface not exceeding approximately 600 mm. The examination may be direct (where the inspector can reach the surface) or indirect (with mirrors, endoscopes, borescopes or camera systems where access is restricted). In indirect examination the optical system used is expected to offer a resolution capable of distinguishing detail equivalent to that of direct examination.

The equipment used is selected according to the condition of the surface and the size of the defect sought. Basic hardware includes torches and lamps providing adequate illumination, low-magnification magnifiers of 2x–10x, mirrors, weld gauges for measuring the profile and dimensions of the weld seam, and measuring rules and feeler gauge sets for comparing crack and gap widths. A calibrated lux meter is used to verify the illumination intensity; traceable calibration of the measuring instruments is mandatory for the validity of the results. Video endoscope and borescope systems come into play for internal surfaces, pipe bores and confined spaces that are difficult to access.

Welded fabrication is the field in which visual testing is applied most intensively. Here surface cracks, porosity, lack of penetration, undercut, excess weld metal, slag residue, spatter outside the seam, incorrect seam profile and deviations in size and regularity are assessed. In addition, surface defects in castings, signs of corrosion and erosion in pressure vessels and pipelines, deformation in bolted connections and the integrity of coatings and paint are also examined within the scope of visual testing. The examination is often planned in three stages: before welding (weld preparation), during welding (between passes) and after welding.

Evaluation of the examination result is made by comparing the detected indication with the acceptance criteria defined in the relevant fabrication or acceptance standard. Not every indication is recorded; discontinuities exceeding the size and type thresholds set by the standard are classified as "acceptable" or "not acceptable". The report clearly documents the area examined, the equipment and lighting conditions used, the standard and acceptance level applied, the location and dimensions of the indications found, and the final evaluation. The reliability of the method depends directly on the qualification of the personnel carrying out the examination; the personnel must therefore be certified at the relevant level in accordance with TS EN ISO 9712 and have passed periodic vision tests. TÜV AUSTRIA SILA KALİTE performs visual testing as an inspection body accredited to TS EN ISO/IEC 17020, with traceably calibrated equipment and certified personnel.