Methods and equipment for NDT are widely used for the quality testing of different objects in compliance with approved documents on them.
The International standard ISO 8402:94 "Quality. Vocabulary" (1) the term "inspection" is defined as "activity such as measuring, examining, testing or gauging one or more characteristics of an entity and comparing the results with specified requirements in order to establish whether conformity is achieved for each characteristic".
NDT of quality is realized by the way of measurement of different parameters and characteristics of products and industrial equipment by the devices of non-destructive testing and procedures of measurement that comply with definite requirements (reliability of testing, error of measurement, etc.). Assurance of reliability and reproducibility of the results of measurement in NDT of quality is of paramount importance. The link of the error of measurement with the characteristics of the reliability of testing is clearly seen.
In states of CIS a set of normative documents foresees attestation of the procedures of measurements. As far as the procedures of measurements are the integral part of testing of quality and the result of measurement presents information about quantitative properties of objects being tested and its quality, the assurance of the reliability of testing results must be confirmed by attestation of the procedures of testing.
In compliance with (1) the procedure is a "specified way to perform the activity".
The technological process of the quality testing for different objects embraces the following operations being realized in sequence:
- choice of methods and equipment for NDT with accuracy characteristics taken into account;
- preparation of items for testing;
- evaluation of operational capability and adjustment of NDT equipment;
- search, detection of defect, measurement of its size;
- evaluation of the defect allowance and quality of the object being tested;
- registration of the results of testing.
All these operations are presented as a rule in one normative document named "procedure of testing".
The procedure of testing is a normative document that stipulates a method (or a collection of methods) of non-destructive testing, nomenclature of tested parameters and norms of testing accuracy that ensure the required reliability of testing for definite object.
Validation of the procedure of testing shall be performed for confirm the correct choice of NDT method (methods) and equipment and completeness of requirements included in the procedure of testing. The fulfillment of these requirements permits to use this procedure in compliance with its scope and ensures that reliable results of testing with required accuracy would be received.
In European standard ISO 8402 the procedure of the procedure validation is defined by the term "validation - confirmation by examination and provision of objective evidence that the particular requirements for a specific intended use are fulfilled" and "objective evidence - information which can be proved true, based on facts obtained through observation, measurement, test or other means".
The authors think that it would be in order to realize development and validation of the procedure of testing similar to the development and validation of the procedures of measurement but taking into account the specific character of NDT.
In application to the NDT the procedure of testing shall cover the following clauses:
- purpose of the procedure;
- evaluation of the object for testing;
- choice of the method (methods) of testing;
- choice of NDT device and standard blocks, taking into account their accuracy characteristics and required reliability of testing;
- evaluation of operational capability and adjustment of NDT device, availability of their metrological assurance;
- requirements to the personnel qualification;
- conditions of testing realization;
- preparation for testing;
- realization of testing;
- processing of the results of testing and evaluation of their reliability;
- evaluation of the quality of the object having been tested and registration of the results of testing;
- rules of safety and labour protection.
For validation of the procedures of testing the following information shall be presented:
- basic requirements for the development of the procedure of testing;
- draft of the procedure of testing;
- programme and results of experimental definition of the parameters of testing reliability.
It is advisable to realize validation of the procedure of testing in 2 stages.
The first stage - expert appraisal of the procedure draft with the aim to analyze and evaluate confirmation of the methods chosen and NDT devices to the specified requirements of reliability and conditions of the testing realization. At this stage the confirmation of NDT equipment to the requirements of their metrological assurance shall be determined among others.
The second stage - experimental research of the procedure of testing that is carrying out in compliance with the programme of experimental definition of the reliability of testing. The programme regulates composition and content of this work.
In realizing testing of the quality of different objects the specified NDT methods and equipment are used. Moreover the NDT devices have standardized metrological parameters. But in carrying out testing of quality the error of testing can include as its component the error of testing method, the error submitted by NDT specialist. The conditions of the testing realization would influence on the final result of testing as well. It is clear that for receive valid evaluation of the procedure of testing it is necessary to estimate the error of testing, the indices of the reliability.
In estimating the procedure of testing all components of the error of testing must be taken into account and confirmed by the results of experimental research.
It is obvious that experimental researches in validating the procedures of testing must be realized either on real objects for testing or on physical simulators of the objects of testing that sufficiently tally with the real object of testing. In our case these may be the specimens with artificial defects of known form and size. The research shall be performed taking into account the requirements of the final result of testing, its reliability, with registration of specified norms for evaluate the quality of the objects of testing.
On the basis of positive results of the procedure validation the statement shall be issued about the possibility to use the procedure in practice and it shall be approved in specified order. The approval of the procedure legalizes it, i.e. gives to it definite status (for example, state standard, guidelines).
For assure reliability and objectivity of quality testing of different objects and industrial installation the necessity is pending to develop an approve a normative document for validation of the procedures of testing.
References
- ISO 8402:1994 Quality. Vocabulary
1317-86 Methodical guide. State System of Measurements. Results and Characteristics of the Error of Measurement. Forms of Presentation. Ways to Use in Testing the Samples of Products and Examination of their Parameters.
- EAL G-15 European Cooperation for Accreditation of Laboratories. Accreditation for Non-Destructive Testing Laboratories.
