Laboratory capability accreditation for industrial CT testing

Original Ni Peijun, Zhang Weiguo and other non-destructive testing NDT 2021-12-30 19:33

Industrial CT (computed tomography) technology is an advanced non-destructive testing technology developed on the basis of medical CT in the 1970s and has been applied in China since the 1990s. Industrial CT inspection is not limited by the type of material, shape structure, and surface condition of the specimen. It can obtain two-dimensional or three-dimensional images corresponding to the geometric structure, material composition, and density characteristics of the specimen. It has been widely used in aerospace, aviation, and weapons. , automobile manufacturing, petroleum, electronics, machinery, new energy, archaeology and other fields. Since 1999, a number of national, military and industry standards for industrial CT testing have been formulated in China.

In recent years, with the development of industrial CT technology and CNAS (China National Accreditation Service for Conformity Assessment) laboratory accreditation, more and more laboratories have applied for industrial CT testing capability certification, which has raised new standards for the review and confirmation of industrial CT technology capabilities. Require. Since the existing standard CNAS-CL01-A006:2018 "Application Description of Testing and Calibration Laboratory Capabilities in the Field of Non-Destructive Testing" does not formulate corresponding review regulations for new technologies such as industrial CT, and there is no ready-made experience abroad to learn from, so in In the actual review, there are problems such as lack of pertinence in the review process, deviations in technical ability confirmation results, and inconsistent grasp of review standards.

Therefore, there is an urgent need to study the key aspects of industrial CT testing technology capability accreditation, put forward policies and suggestions for industrial CT testing technology laboratory accreditation, and improve the scientificity and effectiveness of accreditation work.

1. Basic situation of industrial CT laboratory accreditation

According to rough estimates, there are currently more than 400 domestic units with industrial CT testing equipment, and the total number of various types of industrial CT testing equipment in service is more than 600.

There are currently more than 40 standards related to industrial CT testing, including 4 international standards (ISO), 8 American Society for Testing and Materials standards (ASTM), 20 national standards (GB), and 3 national military standards (GJB). 12 industry standards (HB, QJ, WJ, etc.). Types of standards include technical guidelines, testing methods, equipment performance testing methods, etc.

Currently, only 11 laboratories in China have passed the CNAS industrial CT technical capability accreditation. The largest number of laboratories have applied for accreditation based on the standard GJB 5312-2004 "Industrial Computerized Tomography (CT) Testing", with a total of 9 laboratories; followed by GB/T 29070-2012 "Nondestructive Testing Industrial Computerized Tomography (CT) Testing General Requirements", there are 3 companies in total. Others such as ASTM E1570-19 "Standard Procedure for Fan Beam Computed Tomography (CT) Inspection", GJB 5311-2004 "Industrial CT System Performance Test Method", HB 20118-2012 "Industrial Ray Layer of Electron Beam Welded Joints for Aeronautical Engines" Some laboratories have also passed the accreditation of standards such as Analytical Imaging (CT) Detection Methods.

The 11th category of the currently used standard CNAS-AL06:2015 "Classification of Laboratory Accreditation Fields" is non-destructive testing, and there is no special classification for industrial CT technology. Although CNAS has many professional technical reviewers in the field of non-destructive testing, the proportion of reviewers who are familiar with industrial CT technology is relatively small, and it is difficult to identify them in the reviewer field code.

2. Capacity requirements for industrial CT inspection personnel

Personnel qualification requirements

In accordance with the requirements of section 6.2.2 of the standard CNAS-CL01:2018 "Criteria for Accreditation of Competency of Testing and Calibration Laboratories", the laboratory should document the functional requirements that affect the results of laboratory activities, including education, qualifications, training, technology Knowledge, Skills and Experience Requirements. Non-destructive testing personnel are usually required to undergo qualification certification before they can engage in testing work corresponding to the level of qualification they have obtained.

The standard GB/T 9445-2015 (ISO 9712:2012) "Qualification and Certification of Non-destructive Testing Personnel" does not cover industrial CT technology. In recent years, with the application of industrial CT technology, international industrial CT testing standards have put forward requirements for personnel qualifications, such as the standard ISO 15708-3:2017 "Nondestructive testing of industrial radiation computed tomography testing Part 3: Operation and interpretation" Section 4.5.2 stipulates that CT image analysis should be completed by qualified personnel.

Currently, the ISO 9712-2012 personnel qualification certification standard is being revised. It is understood that the draft standard ISO/CD 9712-2012 has divided radiography into film photography, computerized tomography (CT), digital radiography (DR) and Four majors, including computed radiography (CR), indicate that the CT testing major will also be included in the scope of personnel qualification certification in the near future.

China's national standards for industrial CT testing, military standards and industry standards all stipulate the requirements for industrial CT testing personnel to obtain relevant qualification certificates, such as the standard GB/T 36232-2018 "Non-destructive testing of welds by electron beam welding joints industrial computer tomography ( CT) detection method", GJB 5312-2004, etc.

Personnel qualification certification status

In November 2002, the Commission of Science, Technology and Industry for National Defense issued the standard GJB 9712-2002 "Qualification and Certification of Non-destructive Testing Personnel", which for the first time included computerized tomography testing into the scope of personnel qualification certification, and established a training base in Ningbo.

Standard GJB 9712A-2008 "Qualification and Certification of Non-destructive Testing Personnel" further improves the qualification certification requirements for industrial CT testing personnel.

From 2003 to 2015, the National Defense Nondestructive Testing and Accreditation Committee held a total of 13 industrial CT training courses for the defense industry, and issued more than 400 CT professional level I, II, and III certificates.

In 2016, due to the reform of government agencies and the transformation of functions, the National Defense Non-destructive Testing and Accreditation Committee no longer carries out the qualification certification of non-destructive testing personnel. In order to meet the needs of training and qualification certification of industrial CT non-destructive testing personnel, since 2017, the aviation, aerospace, military and other industries In accordance with the "unified training, separate certification" model, the Non-destructive Testing Society and the Non-destructive Testing Society jointly organized 5 industrial CT non-destructive testing personnel training courses in Ningbo, and issued a total of more than 350 Level I, II and III certificates.

In addition, since 2008, Chongqing True Testing Company has also carried out training for industrial CT professional level I and II personnel, and has issued a total of about 170 non-destructive testing branch certificates of the Chinese Mechanical Engineering Society.

In terms of specific training and certification, each industry has its own characteristics. For example, aviation, aerospace, and the Non-destructive Testing Branch of the Chinese Mechanical Engineering Society classify the industrial CT profession into the radiographic testing category, and conduct training and certification in accordance with radiographic testing RT (CT); military industry The industry still implements the GJB 9712A-2008 standard, and industrial CT is certified separately; the non-destructive testing branch divides industrial CT technology categories into castings and welds, while aviation, aerospace, military and other industries have no technical categories.

Industrial CT testing equipment requirements

Basic equipment configuration

Industrial CT detection equipment mainly consists of radiation sources, detector systems, mechanical scanning systems, reconstruction and visualization systems, etc. Different from medical CT, most industrial CT is customized according to user needs, and the structure, configuration, functional characteristics, technical indicators, software, etc. are the same between devices. Different ray sources and detectors can form different detection systems, such as single source and single detector (such as microfocus ray source + linear array detector), dual source single detector (such as conventional ray source and accelerator ray source + linear array detector) , dual sources and dual detection (microfocus ray source and conventional ray source + linear array and area array detectors), etc. The combination of ray source-mechanical scanning system-detector system plays a decisive role in the performance of the industrial CT system. It determines the scope of industrial CT inspection specimens and the quality of information that may be obtained. The reconstruction and visualization system is an important part of the industrial CT system. Determines how to reconstruct high-quality CT images in a short time.

Equipment performance indicators

The overall performance indicators of industrial CT equipment mainly include inspection specimen range, inspection time, image quality, defect detection capability, etc. Inspection specimen range indicators include specimen material, size, weight, etc.; detection time indicators include scanning and reconstruction time; image quality indicators include spatial resolution, density resolution, artifacts, etc.; defect detection capability refers to the discovery of pores, cracks, etc. Deficient abilities.

The overall performance index of industrial CT equipment is determined by the configuration and performance index of each subsystem. Different configurations have different functional characteristics and performance indexes. The indicators of the ray source include ray energy, ray intensity, focus size, dose stability, etc.; the detector indicators include detector size, number of channels, crosstalk, etc.; the indicators of the data acquisition and transmission system include signal-to-noise ratio, stability, dynamic range, Acquisition speed, consistency, etc.; mechanical system indicators include specimen range (weight, size, etc.), scanning method (second generation, third generation or cone beam), positioning accuracy of scanning axis, repeatability, etc.; reconstruction and visualization system indicators include reconstruction Algorithms, CPU (Central Processing Unit) running speed, memory size, image processing functions, etc. In addition, the quality of industrial CT imaging is affected by various factors such as beam hardening, scattering, scanning and reconstruction parameters. Calibration, correction and process parameter selection also have a great impact on the final image quality.

The overall performance indicators of industrial CT equipment usually restrict each other. For example, high-energy linear accelerator CT has high ray penetration capabilities and can detect high-density and large-size specimens. However, due to its generally large focus size, the final spatial resolution and defect detection capabilities are relatively poor. Micro-nano focus CT systems usually have high spatial resolution and can detect small defects, but they can only detect small samples of a few millimeters.

Equipment calibration and verification requirements

CNAS-CL01:2018 accreditation standards put forward a series of requirements for equipment calibration and verification. For example, before equipment is put into use or put back into use, the laboratory should verify that it meets the specified requirements; the equipment used for measurement should be able to achieve the required measurement accuracy. degree and/or measurement uncertainty to provide valid results.

Standard ASTM E1570-19 and ISO 15708-4:2017 "Nondestructive Testing-Industrial Radiographic Computed Tomography Testing Part 4: Verification" stipulates that equipment used for industrial CT testing should be verified. In relevant domestic and foreign testing method standards , also put forward specific requirements for the performance verification of industrial CT equipment.

The requirements for equipment performance verification according to different industrial CT testing method standards are shown in Table 1:

Table 1 Performance verification requirements for industrial CT equipment of different standards

As can be seen from Table 1, spatial resolution, density resolution, defect detection capabilities, etc. are all key indicators for verification. In contrast, existing domestic standards lack detailed provisions on verification requirements for overall equipment performance, component performance, artifacts, slice thickness, and dimensional measurement.

Equipment performance calibration and verification methods

Since there is currently no unified industrial CT equipment calibration specification at home and abroad, in actual work, verification methods are basically used to verify system performance.

The CT equipment performance verification methods specified in different standards are shown in Table 2:

Table 2 Industrial CT equipment performance verification methods stipulated in different standards

The verification methods of spatial resolution and density (contrast) resolution at home and abroad are generally the same. The spatial resolution verification methods include the disk method, the silk-type line-on-line method, the round hole (square hole) line-on-line method, etc., and the density ( Contrast) resolution verification methods include disk method, density difference method, etc., but there are differences in the measurement results of different test methods, and their equivalence issues need to be further studied.

Defect detection capabilities and dimensional measurement capabilities usually require the use of standard (comparison) test pieces and reference standard (object) comparison methods for verification. ISO 15708-4:2017 and ASTM E1570-19 standards specifically stipulate that standard test pieces in use ( When used to verify dimensional measurement accuracy, the standard test piece (or reference standard) should be metrologically traceable.

Standard ISO 15708-4:2017 provides a series of recommended verification methods for the performance of CT equipment components, and GB/T 25758.1~5-2010 provides a method for measuring X-ray focus size. Monitor the stability of the CT system and the consistency of test results by regularly checking changes in the performance of CT system components.

To sum up, there are relevant test method standards at home and abroad for the verification of key indicators such as spatial resolution, density resolution, defect detection capability, and dimensional measurement accuracy of industrial CT. The verification principles are generally the same, and the standard samples used are various. Distinctive; the new ISO 15708-4:2017 standard also provides some reference methods for component performance verification. Existing domestic standards have relatively few provisions on these contents.

Equipment performance calibration and verification recommendations

Industrial CT equipment has a complex structure and many performance parameters. The overall performance is affected by the performance of the components. Over time, the performance indicators will change. For this reason, equipment performance needs to be calibrated and verified to meet the requirements of the detection method.

When accepting industrial CT equipment, manufacturers generally need to conduct comprehensive performance testing and provide certification documents that the equipment performance meets the requirements. As a user, you mainly focus on application-related performance indicators, such as detecting equivalent steel thickness, spatial resolution, density resolution, defect detection capabilities, etc.

The author recommends scientific and reasonable calibration and verification of equipment performance based on the detection target characteristics. Standard test pieces (reference standards) that directly affect quantitative testing results or accuracy verification should be calibrated regularly, such as spatial resolution test cards, density resolution test cards, image size reference standards (such as dumbbell balls), etc. Calibration intervals It can be 1 year; the overall performance, component performance, etc. should be verified. The verification project parameters are determined by the laboratory based on the characteristics of the respective equipment and equipment usage, etc., and should include at least the following:

① Maximum equivalent steel thickness of inspectable products;

② Best spatial resolution;

③ Best density resolution;

④ The smallest detectable defect;

⑤ Dimensional measurement accuracy.

4. Key points of inspection process control

In addition to being closely related to personnel, equipment and other resource conditions, industrial CT inspection results are also critical to inspection process control, which usually includes inspection preparation, equipment calibration, scanning process parameter selection, image reconstruction and processing, result interpretation and evaluation, etc. These are all elements that need to be focused on during the on-site review.

No. 1 Test Preparation

Before testing, you need to understand the type, structure, size and detailed testing requirements of the material being tested, write a process procedure or process card, and verify whether it can meet the testing requirements, such as whether it can meet the minimum defect detection, whether it can achieve dimensional measurement accuracy, etc. . The verification of process procedures or process cards can be carried out using standard (comparison) test pieces, whose materials, shape, size, defect characteristics, etc. should be as close as possible to the samples being inspected.

No. 2 Equipment Calibration

Before detection, the equipment needs to be calibrated (calibrated), which usually includes calibration of the detector's dark field and bright field, centerline, and pixel size. These corrections can effectively reduce noise, ring artifacts, etc. in the final CT image, and improve defect detection capabilities and dimensional measurement accuracy.

No. 3 Scanning process parameter selection

Select appropriate scanning process parameters, including ray energy, focus size, slice position, slice thickness, scanning field diameter, filtering method, scattered ray correction, scanning time, etc.

No. 4 Image reconstruction and processing

Select appropriate reconstruction parameters and image processing methods to obtain high-quality CT images. Select image display methods such as grayscale, pseudo-color, magnification, or two-dimensional or three-dimensional, and make the image easy to observe by adjusting global or local contrast and brightness.

No. 5 Interpretation and evaluation of results

Observe and analyze images to identify structural images, defect images, artifacts, etc. According to the pixel value, shape, size and other information of the detailed features on the image, relevant analysis software is used to analyze and interpret the detailed features of the image target; according to the detection requirements and relevant standards, the CT image target features are positioned, qualitative, quantitative and consistent. Waiting for evaluation.

5. Measurement uncertainty assessment

Industrial CT is widely used in defect detection, size measurement, density characterization, reverse reconstruction and porosity analysis. In these applications, it is sometimes necessary to provide quantitative detection such as defect size, sample characteristic size, area area, density, porosity, etc. result. According to the requirements of the accreditation criteria, these laboratories that need to provide quantitative testing data should be able to evaluate the uncertainty of the measurement results and ensure the reliability of the measurement results under a certain confidence probability.

Measurement uncertainty assessment methods mainly include GUM method, Monte Carlo method, Globe method, etc. At present, the GUM method is the most widely used in the field of non-destructive testing. The GUM method is divided into two categories: direct assessment method and comprehensive assessment method.

A feasible method in the practical application of industrial CT is the comprehensive assessment method. The basic idea is to treat the CT imaging process as a "black box". During the assessment, only the final image is focused. Under normal detection conditions, the inspected parts are subjected to multiple repeated assessments. Inspection, comprehensively consider the uncertainty components introduced by repeatability factors (including personnel, photon statistical noise, mechanical system accuracy, data collection, reconstruction algorithms, image display, etc.), and then consider the uncertainty components of the standard specimen on this basis. and measurement rulers and other uncertain components, etc., and finally make a reasonable evaluation. Regarding the uncertainty assessment of dimensional measurement by industrial CT, relevant research work has been carried out in China.

It is worth noting that there will be systematic errors in the measurement results of industrial CT equipment. For this reason, the equipment needs to be calibrated using a standard specimen that is the same or similar to the material of the tested specimen before testing, and the system error must be corrected after testing; in addition , As the equipment running time increases, the performance of industrial CT equipment will also change, and the uncertainty of measurement results will also change. The laboratory should be able to identify the uncertainty of measurement results caused by stability factors.

6. Ensure the validity of results and others

In order to ensure the validity of test results, Article 7.7 of the CNAS-CL01:2018 accreditation guidelines stipulates various quality control methods, most of which can be applied to industrial CT testing. In terms of external quality control, there are no proficiency testing program providers (PTP) for industrial CT testing projects recognized by the standard ISO/IEC 17043:2010 "Criteria for Accreditation of Proficiency Testing Providers" at home and abroad. External quality control is mainly through laboratories. Comparison between implementations.

Due to the large differences in the configuration and performance of industrial CT equipment, planning needs to be done when comparing different laboratories, such as comparing equipment models, selecting samples and methods, determining result evaluation methods, etc. Usually, we look for those with similar energy and similar performance. Industrial CT equipment for comparison. In terms of internal quality control, spatial resolution test cards, density resolution test cards, etc. can be used to test performance indicators, period verification, sample retention and retesting, report result review, in-laboratory comparison, and blind sample testing.

In other aspects, it is also necessary to pay attention to the requirements of the CNAS-CL01:2018 accreditation criteria for facility and environmental condition control. For example, institutions engaged in industrial CT testing should obtain a "Radiation Safety License" issued by the government's environmental protection department, and radiation protection should meet GBZ 117- In accordance with the requirements of the 2015 "Radiation Protection Requirements for Industrial X-ray Inspection" and GB 18871-2002 "Basic Standards for Ionizing Radiation Protection and Radiation Source Safety", the industrial CT scanning room is equipped with safety interlocks, monitoring and alarm devices, etc. With the application of mobile industrial CT, industrial CT testing implemented in non-fixed locations should also meet the requirements of laws, regulations and testing methods.

Conclusion

As an advanced non-destructive testing technology, industrial CT technology has complex equipment structure, large differences in performance parameters, high requirements for testing personnel, and testing results are affected by many factors. It is difficult to recognize the laboratory's technical capabilities. The accreditation work has been proposed. new requirements. Judging from the current status of accreditation and accreditation research, the review work needs to focus on personnel qualifications, equipment calibration and verification, measurement uncertainty assessment, etc.

(1) In addition to meeting the testing methods and specific industry requirements, industrial CT inspection personnel should hold a CT professional qualification certificate. The authorized signatory recommends holding radiographic testing level III and CT-II level or holding a CT-III level certificate.

(2) The performance of industrial CT testing equipment needs to be calibrated and verified to meet testing requirements. Users should focus on application-related performance indicators, such as equivalent steel thickness detection, spatial resolution, density (contrast) resolution, defect detection capability, dimensional measurement accuracy, etc. To this end, the spatial resolution test card, density ( Contrast) resolution test cards, image size reference standards and other equipment should be calibrated, and the overall performance, component performance, etc. should be effectively verified.

(3) During the review, attention needs to be paid to testing preparation, equipment calibration, process parameter selection, result interpretation and evaluation, etc. Laboratories that provide quantitative data such as defect quantification and dimensional measurement should be able to evaluate the uncertainty of measurement results.

Author: Ni Peijun1 , Zhang Weiguo1, Guo Miao2, Fu Kang1, Pan Feng2

Work unit: 1. Ningbo Branch of China Academy of Ordnance Sciences

2. China National Accreditation Center for Conformity Assessment

First author: Ni Peijun, researcher, whose main research directions are industrial CT, digital radiography detection and ultrasonic detection.

Source: "Nondestructive Testing" Issue 9, 2021