Visual testing is frequently dismissed as inferior to other mainstream non-destructive test methods due to the misnomer of being crude and basic.
This paper is aimed at dispelling such a myth by revealing the complex issues confronted when engaged in visual interrogation of test items.
Currently there are only a few Internationally recognized codes and standards that address visual testing. They vary in degrees of content ranging from brief guidelines for general scrutiny or to extended considerations when addressing finite analysis of many product forms.
One new standard, which has just completed the formal vote stage prior to issue, is the General Principles Document prEN13018 that addresses requirements for the application of visual testing. This document details many factors required to fulfill a successful visual test and is one of the more comprehensive general documents to date. Other globally recognized documents specific to visual testing are ASME boiler and pressure vessel code sections V and XI, EN 970 and AWS codes for welding, and Euronorms EN763, EN1370 and EN12454 (plastic piping and steel castings respectively).
ASME V for many years has included visual testing as a recognized nondestructive method requiring training and certification of personnel to the recommendations of SNT-TC-1A.
ASME XI relies upon visual testing as a main method of pre-service and in-service inspection mandating training and certification of personnel in accordance with CP189.
Product standards also address the necessity for visual examination of materials and components endeavoring to ensure quality control and /or condition monitoring of in-service components. Typical examples include, pressure vessels and boilers, power generation equipment, chemical process plant, structural engineering including steels and concrete.
Fig 1: Hydraulic stabilizer and rigid mounting examined by direct visual testing|
Standards however are not aimed as tutorial documents therefore this has to be addressed as a separate issue.
Although visual testing is thought of as an elementary inspection, test results are frequently subjective and cause for debate. Unlike most other nondestructive test methods visual testing rarely uses instrumentation or mediums of enhancement to highlight flaws and discontinuities. Observance of large areas therefore requires comprehensive analysis of visual characteristics requiring evaluation prior to focussing on features of interest. Even in the event of image enhancement equipment being utilised frequently you have to question "Do we all see the same Image?"
Fig 2: Carbon steel extracted from an Amine plant|
An obvious requirement is of visual acuity, but what methods of verification are available and are the regular series of opthalmic checks suitable for specific visual testing applications? Most importantly though, do we all perceive the same image? Even when using remote imaging systems, interpretation is conducted manually by personnel who use the human brain to process the data. The most advanced automated camera system cannot replace human judgement therefore emphasizing an essential requirement for personnel training.
Assuming we all perceive the same image are we correct? Visual testing is frequently confronted by optical illusions. Prescription of correct visual techniques including lighting can often eliminate confusion founded from oblique visual angles or monocular observation.
Direct Visual Testing
Simplified analysis of the direct visual test sequence allows us to consider individual stages. This process of consideration is vital in procurement of successful test procedures.
A prime consideration is to that of light. Surface finish, profile, color, contrast and nature of inspection dictate the necessary illumination for adequate reflected energy to reach the eye. Codes and standards cannot adequately address all combinations of surface reflectivity or contrast and they only offer basic guidance with rudimentary mandates on minimum values.
Visual acuity and adaptation control the ability to receive quality images on the retina whilst mental preparation, past experience and educated knowledge facilitate perception.Physical fatigue contributes to misperception, therefore careful consideration of work cycles should not be excluded when planning visual tests. Concise written instructions or criteria can be used to aid the application of a technique improving probability of detection and assisting reliable interpretation.
Remote Visual Testing
Remote visual testing combines many facets of direct visual testing with simple or complex imaging devices. The same features that dominate direct visual tests affect light reflected from the test surface in the form of luminous flux. However during remote visual testing the eye is separated from receiving unimpeded visual information by a series of optical or imaging devices.
Lenses are used to focus luminous energy onto optical or imaging devices within the test system. In turn an imaging device or camera transfers data to a processor before formatting it into a visual display. Only then do we return to the full sequence of direct visual testing thus compounding sources of image transfer error prior to reliance of human perception and interpretation.
Consideration has also to be given to the variety of applications the visual tester is frequently expected to perform. Such tests are not only in the form of nondestructive observations, but often require scrutiny of destructive test coupons and analysis of fracture surfaces from component failures.
Product forms encountered may be in the form of raw materials, minerals, metals, ceramics or composites and polymers, all of which the visual tester is expected to be proficiently competent in evaluating.
Visual testing is not only employed for various material applications but is then utilised to examine components and structures at various stages during the products life cycle. Typically this may include intermediate stages of manufacturing inspection, pre-service inspection, in-service inspection and post service inspection. Personnel would therefore encounter different features requiring observation emanating from characteristic faults occurring during the product life cycle.
Fig 3: Chemical attack on carbon steel pipework|
Frequently the visual test personnel are allowed to select equipment for specific applications. This choice may be well founded, but seldom proven by performance demonstration trials. Indeed the question arises who is eligible to prescribe equipment pertaining to specific applications? Assuming the correct choice of apparatus has been selected the next hurdle is to ensure its correct use. Equipment manufacturers are eager to demonstrate equipment capabilities but are rarely willing to spend time assisting with applications. In many cases you question the ability of some sales personnel when presented with the applied use of visual test instruments.
Applying the test instrument correctly relies upon optimizing variable parameters. This not only pertains to the principle apparatus but also encompasses ancillary equipment to enhance the visual image. Data may then be viewed and recorded for reporting and archival purposes. Again selection of the most appropriate media to ensure acceptable resolution has to be made by someone and therefore who is competent?
In order to understand the complex issues surrounding equipment selection the following factors require consideration:
Accessibility: will the equipment reach, focus, fit, or be chemically compatible with the test environment and are necessary ancillary services available?
Resolution: Is the system capable of resolving the desired features, remembering that resolution is a higher order of sensitivity.
Image format: Will the image data recording media be transferable to other archival systems and will the format reproduce the desired resolution?
2D or 3D: Two-dimensional recorded images often lack perspective required to evaluate changes in profile or contour. Monocular optical aids also suffer from a restricted perspective typified when viewing objectives through an endoscope.
Equivalent acuity: Can the optical aid or imaging system achieve an equivalent acuity as expected of the human eye?
Depth of field: Over what range is the depth of field achieved by the test arrangement?
Magnification: is the image magnified or distorted?
Illumination: What is the required level of illumination to operate the test system?
Fig 4: Undesirable glare when viewing toward a source of illumination.|
Having thought about the preceding points and selected a test arrangement the next stage is to assess its performance. Unlike most other mainstream nondestructive methods there are few codes and standards that stipulate criteria to be achieved. Draft European standard pr EN 13927 is one useful text offering standardization and guidance when selecting and proving visual test equipment or systems of visual imaging.
Most systems are assessed in a process of performance demonstration on either mock test pieces or components fitted with targets.
Equipment checks must assess magnification, focal distances, spherical and chromatic aberration, resolution both displayed and recorded, environmental and thermal stability, and evidence of damage or wear. Pre test equipment checks should be recorded as part of the visual test operation in the same manner as expected by other nondestructive test methods.
Pre inspection assessments may be required to justify equipment selection and preempt any test restrictions or limitations. Restricted access should be considered including the equipment portability, access through hatches and whether the necessary supply services are available. Once accessibility has been dealt with the achievable angle of view must be suitable to gain the desired perspective. Indeed this consideration has to include the field of view, depth of field and achievable focal distance.
Naturally the concept of illumination has to be assessed, but how? Will you require photometry at the objective to verify suitable levels of illumination and what means of illumination are at your disposal? Selection of a specific type of light source will bias the spectral distribution of luminous energy received by the imaging system and that may prejudice correct interpretation of the subject. Therefore filament types and light transfer mediums are just as important as achieving a set level of illumination.
Aside from all the previous thoughts one additional problem requiring a solution is once you obtain your image how do you assess it for dimensional features? Magnified images are renown for disproportionate perspectives therefore assessment of a feature is most desirable against a means of comparison at the objective surface. Knowledge of means by which features can be quantified is another facet to the skill of visual test personnel, which is not to be confused with the science of metrology.
Why rely on Visual Testing?
You are probably thinking why on earth do people rely on visual testing, as this certainly is far more complex than originally understood? Well this test method has many attributes offering a broad potential for many applications. Visual testing when controlled correctly will perform as efficiently as any other method. Images obtained are frequently non-intrusive requiring minimal disruption to plant operation. Remote applications allow safe working distances to be maintained in hazardous environments extending working hours due to reduces exposure times.
Fig 5: Submersible Remotely Operated Vehicle|
Submersibles allow penetration to depths untenable by divers whilst transmitting real-time data to the topside operators. Complex inspection plans can be programmed into robotic systems minimizing lack of detection due to erratic scan patterns. To summarize, visual testing can be as simple or complex as the job demands whilst retaining cleanliness and so minimizing contamination, yet allowing images to be recorded by a range of media to offer permanent records for inspection history.
Need for Training
Predictably the subject of training returns as the key to successful application of visual testing. Training offers the potential to develop knowledge and skill in both prescribing and applying the visual testing method. As delivered in all other non-destructive test methods, the three tiered grading of syllabi offer degrees of expertise suitable for most visual applications. Training courses may be generalized to address unified certification programs or tailored specifically to individual requirements. Specific courses not only allow adaptation to product forms but also enable advanced systems of visual imaging to be utilised. Remote visual systems in the form of robotics or remotely operated vehicles require skilled personnel to acquire quality images, where as personnel viewing images for interpretation purposes require different skills. This concept is not unlike that experienced by the industrial radiographer, who operates the equipment to obtain an image which may then be interpreted be a separately skilled person.
Basic training caters for individuals typically involved in obtaining information by direct visual means. Frequently referred to as a level 1, this grade used in certification signifies a level of competence to extract relevant information from a test item whilst working to detailed written instructions. Advanced training can follow to promote a degree of competence expected of persons performing direct visual or remote visual applications requiring concise evaluation or complex manipulation of equipment. Level 2 certification is assigned for this practical grade to signify the advanced capability.
Levels of experience and knowledge needed for prescription of techniques and formulation of test procedures are typified by the prerequisites laid down in current level 3 certification programs. Level 3 grading is the top tier of certification designed to encompass specific method skills whilst embodied with a broad understanding of most non-destructive test methods. Training of level 3 students involves comprehensive tutorials followed by wide ranging assessments over a planned program of learning to ensure coverage of the broad subject matter. Only at this stage is it generally expected that a visual test person would encounter all product forms and sample the majority of imaging options. However it is unreasonable to expect even the level 3 to be an expert with all visual test applications and they therefore tend to specialize in respective fields.
All training courses build on foundations of standardized terminology, thus reducing confusion when formulating or working with procedures and techniques. Details written in procedural documents must to be concise, precisely controlling all variables that will be encountered during a test. The key to consistent results is in the preparation of test instructions, including accurate recording thresholds for relevant visual features. Conflicting interpretation of the application codes and standards may also result in further test inconsistencies. Therefore training courses are also offered by organizations specifically addressing interpretation of codes.
Is Certification Necessary?
I think that you will agree training would be beneficial for visual test applications, but is certification necessary? Certain codes mandate certification this is exemplified by the application of ASME XI. Certification is required in accordance with CP189 to categories segregated as VT1, VT2 and VT3. Each category specifies visual applications and is available at levels 1 or 2 with specific criteria pertaining to eligibility of trainers and prospective level 3 persons. Not all code or standards require certification but how else do you assess the competence of test personnel? An age-old phrase of justification is that of "continued satisfactory performance" but what does this mean? Is this a quantifiable value or a misnomer of accuracy? Surely by virtue of demonstrating continuous performance by an internal or external means of assessment, automatically achieves traceability and quantification. But this approach is little short of the certification process advocated by many as costly and unnecessary. Certification schemes are available to operate as company internal quality measures with internal or external moderation. Quality control documents typified by written practices based around SNT-TC-1A, CP189, NAS410 or prEN4179 are used globally for internal certification being moderated by internal or independent external examiners. Certification in accordance with EN473 and ISO9712 is differs from those previous by classification as central certification in a similar vein to college examination bodies. Although both routes to certification have attributes and detriments they do ensure adequate coverage of all industrial needs with flexible options to address specific requirements.
Should you require any further guidance on any of the points mentioned in this paper please contact Lavender International NDT Ltd. via email at Stephen@lavender.demon.co.uk or phone 44 (0) 1226 765769 addressing enquiry's to Tim Armitt.
- EN 473: General Principles for Qualification and Certification of NDT Personnel.
- EN 763: Plastics Piping and Ducting Systems. Injection Molded Thermoplastic Fittings. Test Method for Visually Assessing Effects of Heating.
- EN 970: Non-Destructive Examination of Fusion Welds. Visual Examination.
- EN 1330-1 Non-Destructive Testing. Terminology. List of General Terms.
- EN 1330-2 Non-Destructive Testing. Terminology. List of Common Terms.
- pr EN1330-10 Non-Destructive Testing. Terminology. Terms used in Visual Testing.
- EN 1370: Founding. Surface Roughness. Inspection by Visual Tactile Comparitors.
- pr EN 4179: Qualification and Approval of Personnel for Non-Destructive Testing.
- EN 12454: Founding. Visual Examination of Surface Discontinuities. Steel Sand Castings.
- pr EN13018: Non-Destructive Testing. General Principles. Visual Testing.
- pr EN 13927: Non-Destructive Testing. Visual Test Equipment.
- ISO9712: Non-Destructive Testing - Qualification and Certification of Personnel.
- SNT-TC-1A Recommended Practice: Personnel Qualification and Certification in Non-Destructive Testing
- ANSI/ASNT CP-189-1995: ASNT Standard for Qualification and Certification of Nondestructive Testing Personnel
- NAS410 NAS Certification and Qualification of Non-Destructive Test Personnel
- ASME Boiler & Pressure Vessel Code Section V: Non-Destructive Examination.
- ASME Boiler & Pressure Vessel Code Section XI: Rules for In-service Inspection of Nuclear Power Plant Components. (Section IWA2000)
- Useful reading:
ASNT Handbook Volume 8: Visual & Optical Testing