·Table of Contents
·Workshop - Visual testing
European Standard for Visual Testing-General Principles PaperMacro C H Rolls-Royce Plc. Naval Marine - United Kingdom
As the first general visual testing standard for Europe it is felt to be very important that the general principles paper is received well and seen to be a simple working structure for evolution further at the next issue.
The new European Standards that are being created by working group 8 of the technical committee TC138 of CEN are proposed as the way of updating and harmonising the Visual testing work across Europe. They comprise of the following parts:-
The equipment standard has been very hard to finalise due to the great variation of equipment available for what is the oldest means of testing and is the subject of a separate paper, Ref 4.
The terminology standard forms part of a suite of terminology standards for non-destructive testing which cover all methods as well as a standard for general terms, Ref 5.
The general principles standard is the overall standard for visual testing generally and is not specific to any product. This will be discussed in more detail in this paper to outline the structure and approach taken. Product standards also exist such as ref 6.
It is structured to allow product standards to call it up as a parent document enabling specific detail to be added without the need to start from basics. It is hoped that more product and specific application standards will now be able to be generated in a modular way as has happened in other NDT Methods.
The current status of the visual testing standards is as follows:-
|General Principles||The standard has gone through a number of reviews and is currently acceptable to the standards panel allowing it to go out to final vote.|
|Equipment||This standard is currently in the enquiry phase for comment.|
|Terminology||This standard is currently in the enquiry phase for comment.|
This section defines the scope as specifying the general principles required for visual testing both directly and remotely when it is used to determine the compliance of a product with a specified requirement (e.g. surface condition of the part, alignment of mating surfaces, shape of part etc.). This includes the readings from measuring equipment for visual testing. Measuring equipment used for other methods is specifically excluded. In addition the standard does not apply to viewing activities linked to the use of any other destructive or non-destructive test method.
5.2 Normative References
The normative references included are those listed as references 3, 7 and 8 which cover the qualification of personnel and visual acuity testing.
Because of the diversity of definitions across Europe in the way visual testing is defined it was very difficult to set up a classification which could be understood by all to explain the main methods used in visual testing. To simplify this two definitions were set up to distinguish between direct and remote visual testing as this approach made the overall structure of the standard easier. These are
Direct Visual Testing:
visual testing where there is an uninterrupted optical path from the observer's eye to the test area. This is either unaided or aided via e.g. mirror, lens, endoscope or fibre optic.
Remote Visual Testing:
visual testing where there is an interrupted optical path from the observer's eye to the test area. Remote visual testing may cover the use of photography, video systems and robots.
Effectively these allow the standard and equipment standard (ref. 2) to be group specific requirements into common blocks since direct visual testing involves direct interaction with a view of a test area and remote visual testing modifies the view which changes the operator requirements.
5.4 Pre-test Documentation
This section calls for the creation of a written instruction, which is mandatory. In addition when required by the product standard or contract a written procedure shall be used. The requirements for the instruction and written procedure are prescribed as a minimum in the standard and these will be outlined further below.
The written procedure applies when the work is fairly involved or when a lot of components are involved. In addition, for particular production or inspection applications, generic procedures may be applicable with instructions only being specific to components. Thus the emphasis has been put on the minimum quality paperwork required to support the test.
The concept of working to a written instruction is common in NDT and some industries - but not all, and has helped to improve the quality and method of working. This section establishes the minimum test requirements in a simple form to be completed for each inspection or component and is a mandatory requirement.
The written instruction/procedure should consider the following for inclusion as a minimum: -
The list is not exhaustive but highlights those sections that should be included as a minimum. The procedure should also be demonstrated using a test-piece as close as possible to the real situation so that the true effectiveness of the procedure can be determined. This is particularly effective in the application of remote techniques but can be equally applicable with tight acceptance criteria in direct testing. Written instructions do not require a demonstration test-piece to be used as it is considered that these should be applied in simpler test arrangements and requirements.
Changes in equipment and details of the test arrangement should be assessed for their impact on the sensitivity levels before deciding if the procedure requires reproving.
The demonstration test-piece may be replaced by examples of the component to be tested or an approved system of reference which is physical evidence of typical indications in a form which can be referenced from i.e. pictures, example sections of components, for the operator to utilise during the test.
The written instruction can provide a pre-prepared form which allows the tester to quickly identify the extent of work whilst performing the test, and to also have somewhere to note and record the work that has been performed as well as any comments or results. It is intended to be as simple as possible and to allow a quality test to be performed. Its format is very dependent on the test being performed but where possible should be in a tick box format to speed the testing process not hinder it
5.5 Direct Visual Testing
The Standard then looks at the elements of visual testing and firstly reviews direct visual testing. This section is based on the traditional definition of visual testing by direct methods i.e. unaided using the eye at 600mm max and at an angle of not less than 30 degrees to the surface. The majority of this information is a repeat of existing working methods. This definition also covers aided direct visual testing which includes the use of mirrors and lenses as well as fibre optics.
The use of wording like x10 magnifying lenses has been avoided, as this is too specific for a general standard to be quoting and is open to abuse, although it has been used in standards in the past to their detriment. For example, a particular operator may use a set of x10 binoculars to inspect a pipe, not for getting further surface detail but because he could not get close enough as quoted in the procedure for general examination, thus not improving the inspection by their use but just achieving it.
The concept of general visual testing is also discussed to cover inspection at distances greater than 600mm as this is performed widely but never covered adequately in definitions. Examples of this are general checks for structural integrity such as clearances, loose or missing components, etc. Relaxed lighting conditions are applied to general visual testing, as the detail inspection requirements are not applicable. This makes the standard more versatile to those inspections actually being applied.
A minimum light level of 160Lx for general visual testing and 500Lx for fine work is set in the standard and this is a change from the figures traditionally seen in the UK. The change to these figures will set a more uniform lighting level across all the general and specific testing standards.
Care is also taken to draw the user's attention to points such as the choice of colour temperature and direction of lighting as well as considering the effects of surface reflectivity, which is often ignored.
5.6 Remote Visual Testing
This section highlights that the use of remote visual testing may be considered where applicable i.e. the use of endoscopes and fibre optics coupled to cameras or other suitable instruments. Where it is decided that this style of test should be utilised then its suitability needs to be proven so that its applicability can be evaluated. It is too easy to say that remote equipment is required and then not actually check that that remote equipment can perform the test required.
Remote visual testing is not just fibrescopes etc. it can also be much more complex devices such as a Submersible Remotely Operated Vehicle or a large manipulator with vision systems attached. The standard has been written generally enough to still be easily applied in proving the tests capability to perform to the acceptance criteria.
The requirements are that personnel working to this standard shall be proven to be familiar with relevant standards. This could be by the use of training records or incorporating the information in the procedure. In addition, the personnel should be informed about the relevant manufacturing procedure.
Vision tests shall have been performed to EN473 and adjusted to include a far vision test when performing general visual testing using the standard optotype in accordance with EN ISO 8596 (ref. 8) to visual acuity grade 0.63
This is a change because far vision has been included to ensure that the operator can perform the test, as there are more instances now of tests being performed at a greater test distance than 300mm as used for the Jaeger test. In addition some remote testing equipment focuses at greater distances than this for practical reasons and the comfort of the operator.
All visual tests are to be evaluated in terms of the acceptance criteria specified in the appropriate product standard i.e. a test is to be performed to prove that the results and acceptance criteria can be tested for on the actual component. The results of this test then need to be recorded for future reference. The written instruction that is always applied can give the space for this evaluation to be recorded. The evaluation may take the form of a simple resolution check under the test conditions or the pre inspection of set demonstration test-pieces as required.
5.9 Post-Test Documentation
This section covers the written test report that shall be provided when the work or contract requires it. The report is basically a summary of the test and the conditions under which it was performed. i.e.
This could also include the data from any evaluation tests that have been performed to check the procedure or equipment. Referencing the written instruction/procedure can complete a lot of the items above.
The standard calls for records to be maintained as agreed by the product standard or contract. This could take the form of paper records or sketches as well as tapes of the inspection (both audio and video) as necessary.
It is possibly worth considering, at the outset of a contract, which party should keep any records and for how long since storage is expensive. The recording quality of any video or digitally stored image should be clearly defined as it is difficult to achieve an image quality equal to the original and this may affect the usefulness of making or keeping a record.
Written instructions are intended to be the main method of working for simple tests which gives an easy structure to the tests. More complex and result critical tests should be performed using procedures and in these cases some demonstration of test capability of the procedure is called for. The procedure test demonstration is designed to be as simple as possible and can be adapted to suit the test criteria (see 5.8).
Changes will have to be made by those who do visual tests without any supporting documentation or outlined acceptance criteria, this will be of benefit to the tester who will then have clearer guidance on the requirements placed upon them.
The Mandatory use of written Instructions is new but is based on current practices and is the minimum documentation that is expected to be required. This can be as simple, or as complex a document as the test requires but certain minimum requirements are specified by the standard.
Procedures are called for where required and then it is required to show that they can achieve the acceptance criteria for the test specified.
The visual acuity tests for the operator have been updated to include a simple far vision test in addition to the existing near vision and colour vision test.
The implementation of this standard will be a major step forward in bringing all the NDT methods into line by having a series of standards to cover general principles, equipment and terminology.
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