Keywords

Introduction

Acoustic NDT is widely applied at hazardous production facilities in the gas, oil, and petrochemical industries, as well as in mechanical and nuclear engineering, utilities, and other industries due to the need for safety analysis of the feasibility of operating metal structures (main pipelines, NPP units, pressure vessels, supporting building structures, etc.) [1,2,3,4,5,6,7,8,9,10,11,12,13].

The probability of plastic destruction in critical components of structures and equipment can be evaluated using devices implementing the time-of-flight method of the acoustic NDT and providing the measurement of the current mechanical stresses arising from technological influences.

In order to ensure the reliability of results obtained in measuring mechanical stresses, such devices should have a traceability to the standard of the corresponding physical quantity (Fig. 1). However, as shown by the analysis of the register of approved patterns of measuring instruments, the measured value in these devices is represented by the transmission time of various type (P and S) elastic waves excited in the material of the controlled object, i.e., traceability is ensured to a time, not a mechanical stress, standard. The analysis of the verification procedures confirmed that the standards used for verification (oscilloscope, pulse generator, and frequency meter) do not transfer a unit of mechanical stress to the devices under verification.

Fig. 1
A flowchart to ensure the reliability of results obtained in measuring mechanical stresses. It starts with object that comprises standard, characteristics, quantity to measurement technique. Calibration comprises of measuring principle, measurement method, measuring instrument and measurement uncertainty that connects to measurement result.

Ensuring traceability of measurements to standards reproducing SI units

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In order to control SSS, standardized measurement techniques are used [14,15,16,17,18], which provide for the transfer of the dimension of a mechanical stress unit by constructing a calibration characteristic using a sample made of a material similar to that of the structure or equipment under study. At the same time, the fact that the properties of materials in the structure (equipment) under study were evaluated using samples of a different shape and dimensions is completely ignored, the contribution from the heterogeneity of the initial material in mechanical properties is not taken into account, and no uncertainty is estimated for the calibration characteristic obtained by the method of regression analysis. Thus, the basic principle of ensuring traceability in SSS measurements is violated: the result can be correlated with the reference system through a documented continuous chain of calibrations, each of which contributes to the measurement uncertainty [19].

Ensuring the traceability of mechanical stress measurements has two alternative perspectives.

The authors of [20] believe that, during the calibration of instruments implementing the time-of-flight method of the acoustic NDT, traceability to mass and length units is ensured, therefore, CRMs with an assigned value of the second kind modulus of elasticity (Young's modulus) can be applied for calibration.

The studies [21, 22] propose apply only CRMs for ensuring traceability to the mechanical stress unit, since obtaining a response from the stresses arising in the material during loading depends on external factors, i.e., the stresses are method-dependent and the traceability to the primary reference procedures of the National Metrology Institute is required.

Both methodological approaches recognize the use of CRMs as a necessary condition for ensuring the traceability of mechanical stresses.

Therefore, for mechanical stresses, the concept of traceability can be represented by Fig. 2, where the reference system consists of a material, sufficiently homogeneous and stable with respect to a certain property established in order to be used in the measurement process, i.e., a CRM [23].

Fig. 2
A flowchart to ensure the reliability of results obtained in measuring mechanical stresses. It starts with object reference material that comprises certified, stress-strain state and mechanical stress to measurement technique. Calibration comprises of measuring principle, measurement method and so on, connects to measurement result.

Concept of traceability for mechanical stress

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The present study was aimed at the development and certification of special shape CRMs traceable to SI units of force and length and intended for transferring a unit of mechanical stress to measuring instruments, where a time-of-flight method of acoustic NDT is realized.

Materials and Methods

In order to ensure the representativeness of mechanical stress measurements in objects manufactured using various technological processing techniques, the CRM material was represented by:

  • rolled sheet made of structural steel;

  • forging made of high-quality structural high-alloy steel.

The homogeneity study of the CRM in the form of a sheet or forging was a complex methodological task. Firstly, for calibration of measuring instruments implementing the time-of-flight method of the acoustic NDT, a CRM should have a shape and dimensions (hereinafter referred to as the form factor) different from proportional CRMs used to determine the mechanical properties under static tension [24]. Secondly, the main feature of measuring the mechanical properties of materials involves the fact that measurements cannot be repeated on the same sample due to its failure under loading.

The CRM inhomogeneity represents a source of a significant error (uncertainty) component; therefore, at the first stage, the acoustic anisotropy was estimated by an indirect method [18] in order to select an area, sufficient for the manufacture of a CRM on a sheet or forging according to the form factor and homogeneous in acoustic parameters. The results of evaluating the acoustic anisotropy are graphically represented in Fig. 3.

Fig. 3
A computer 3D simulation of acoustic anisotropy. The ranges in the figure are 0.20 to 0.24, 0.16 to 0.20 and so on. In the left simulation, the range 0.12 to 0.16 covers the maximum area of the graph. In the right simulation, the range 0.16 to 0.20 covers the maximum area of the graph.

Results of evaluating the acoustic anisotropy

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At the second stage, using proportional CRMs, the spatial homogeneity in the distribution of mechanical property was estimated in accordance with the algorithm provided in [25] by the method of static tension [24].

The CRMs were recognized as homogeneous. The obtained estimates of the error (uncertainty) from heterogeneity cannot be neglected; therefore, these values should be taken into account in estimating the error (uncertainty) of the CRM certified values.

The establishment of the CRM certified values, as well as the estimation of their errors, was carried out using the 1st echelon state standard of the force unit in accordance with the algorithms provided in [25].

The shelf life of the CRMs was established on the basis of the data about the shelf life for similar CRMs of the steel mechanical properties (GSO 10957–2017). The CRM shelf life comprises 10 years for all certified characteristics.

The developed CRM set was included in the State Register of approved patterns of certified reference materials as GSO 11544–2020/11545–2020.

The values of the GSO 11544–2020/11545–2020 metrological characteristics are presented in Tables 1 and 2.

Table 1 Metrological characteristics of GSO 11544–2020
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Table 2 Metrological characteristics of GSO 11545–2020
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The traceability of certified values to SI units was realized by means of direct measurements using the 1st echelon State standard of force unit in accordance with the State verification schedule for force measuring instruments and State verification schedule for length measuring instruments in the ranges from 1·10–9 to 100 m and 0.2 to 50 µm (wavelengths).

CRMs are designed for pattern approval testing of instruments measuring mechanical stress using a time-of-flight method of acoustic non-destructive testing based on the phenomenon of acoustoelasticity, verification and calibration of instruments measuring mechanical stress using a time-of-flight method of acoustic non-destructive testing, and the accuracy control in mechanical stress measurements.

Discussion and Conclusions

In the course of the study, unique CRMs of steel mechanical properties for ensuring the uniformity of mechanical stress measurements were developed and approved.

A distinctive feature of the developed CRMs involves the certification of the “proportional limit” and “modulus of elasticity E” characteristics. The certified values of these characteristics promoted for unambiguously setting the area of elastic deformation during verification and (or) calibration of the instruments measuring mechanical stress using the acoustoelasticity phenomenon, as well as for establishing the coefficients of elastic-acoustic coupling [16].

It should be noted that the certified values of the “modulus of elasticity E” characteristic differ from the reference data. The certified values of the characteristic for GSO 11544–2020, made of structural steel, and GSO 11545–2020, made of high quality structural high alloy steel, comprise 95 and 195 GPa, respectively, while their reference values are equal to 200 and 215 GPa. This difference is due to a number of reasons, each of which contributes to the reliability of the reference data. First, the available reference data contain no information on the uncertainty of the values obtained. Secondly, for reference data, no information is provided on the method of material processing for obtaining the reference data. For example, the CRM, made from rolled sheet, has surface hardening affecting the values of material properties obtained during testing. Therefore, the key task of ensuring the reliability of the results of mechanical property measurements involves the availability of the CRMs for calibration of non-destructive testing instruments associated with the technology of manufacturing controlled materials.

It should be noted that not only standards, but also reference systems in the form of GSO (State certified reference materials) and primary reference procedures should be used in verification schedules, representing the urgent task of legal metrology in the Russian Federation. The local verification schedule, based on the principles of Fig. 2, ensures using the GSO for pattern approval tests for instruments measuring mechanical stresses and implementing various methods of the acoustic NDT.