Standard Test Blocks for Ultrasonic Nondestructive Testing: Regulation of Technical Requirements

The paper analyzes the existing regulatory documents specifying the characteristics of calibration (standard test) blocks in the form of reference materials RM-2 and RM-1 used in ultrasonic nondestructive testing. The regulated metrological characteristics of these test blocks are considered. It is shown that the defined metrological characteristics do not ensure the required reliability of nondestructive testing. The authors note differences in the regulated characteristics of single-type standard test blocks produced by different organizations and shortcomings in the regulation of acoustic characteristics. When using single-type standard test blocks, these shortcomings lead to the variation of instrument parameters, as well as a significant error in the evaluation of test results. The study shows the need to regulate ultrasonic velocity for the types of ultrasonic waves actually used in standard test blocks and to reduce the permissible error in measuring their velocity to ±(0.2–0.5)%. It is noted that ultrasonic wave attenuation in standard test blocks, defined only for the longitudinal ultrasonic wave at one frequency of 2.5 MHz, fails to provide the stated error of ±1 dB when adjusting the sensitivity of ultrasonic testing devices and assessing the size of the detected defect. Thus, the ultrasonic wave attenuation is proposed to be determined by comparing the amplitudes of ultrasonic echoes in these test blocks with that of the ultrasonic echo in the reference block. In this case, the reference block should have a simple geometric form, which would enable its certification according to the absolute value of the attenuation coefficient. It is shown that the current State Hierarchy Scheme for instruments measuring the propagation velocities and attenuation coefficient of ultrasonic waves in solid media requires revision, as it does not provide for the unit transfer of the attenuation coefficient for shear waves and does not fully reflect the actual state of things regarding the measuring instruments currently used in practice. In order to ensure the full comparability and reliability of measurement results obtained using standard test blocks, it is proposed to develop a single regulatory document specifying general requirements for their regulated metrological characteristics and measurement methods, as well as a local hierarchy scheme for the unit transfer of ultrasonic wave velocity and attenuation.