Updated: Sep 13
Introduction to Phased Array Ultrasonic Testing System Verification
Phased array ultrasonic testing (PAUT) equipment calibration can mean many things and is addressed via many standards including ASTM E2491, ASTM E317, ISO 22232-1:2020, ISO 22232-2:2020, and ISO 22232-2:2020 (Did I lose you yet?). Additionally, there are other calibrations related to the performance of the combined phased array pulser/receiver and probe combinations including velocity, wedge delay, sensitivity, and time compensated gain (TCG), or distance amplitude correction (DAC) calibrations. This article focuses on phased array calibration techniques outlined in ASTM E2491 Standard Guide for Evaluating Performance Characteristics of Phased-Array Ultrasonic Testing Instruments and Systems.
ASTM E2491, Standard Guide for Evaluating Performance Characteristics of Phased-Array Ultrasonic Testing Instruments and Systems presents a variety of calibration blocks including the phased array beam assessment block. This block is used for phased array beam characterization and system performance characterization. It may also be used as a baseline block to establish long-term instrument performance variations, DAC/TCG curve generation, linear/angular resolution, beam and focusing performance, including steering capability.
Assessment of PAUT Beam Steering Calibration Block
ASTM E2491 Annex A – Determination of Phased-Array Beam Steering Limits This annex describes procedures to determine practical limits for beam steering capabilities of a phased-array probe in the active plane. The PAUT beam steering block assessment consists of two sets of side drilled holes (SDHs) along a 25 mm and 50 mm radius separated by approximately 20 mm. All side drilled holes are 0.060” diameter. There are 19 total SDHs for each beam steering radius set. The first hole is located at 0 degrees. The last hole is located at 85 degrees close to the end of the useful beam steering range. An actual PAUT test block is shown in Figure 2. This PAUT Type B calibration block consists of 19 side drilled holes with the first two separated by 2.5 degrees.
Assessment of PAUT Beam Steering
Assessment of beam steering is based on comparison of signal to noise ratios (SNRs) at varying beam angles. Non-destructive testing phased array applications where focusing is necessary may perform better than unfocused applications. The beam steering capability will be theoretically limited by the element width and PAUT transducer frequency. Practically, the beam steering capability is defined by the 6 dB separation criterion. This can be studied by incrementally decreasing the spacing between targets and observing the minimum target separation that provides 6 dB separation . Similarly, evenly spaced angular reflectors may be used as the targets. For example, when a phased array probe is setup to sweep +35° on a Type B PAUT beam steering assessment block, the higher of the pair of the SDHs which achieves a 6-dB separation shall be considered the maximum steering capability of the probe, wedge and focal law configuration.
Figure 3: 2 inch radius focus with 5 MHz, 32-element PAUT transducer, with 0.60 mm element width.
Example data is shown above for a 5 MHz, 32-element PAUT transducer, with 0.60 mm element width. This phased array data was acquired using a focal depth of 1 inch and an angular resolution of 0.5 degrees. The objective of the test was to determine if the selected PAUT transducer, wedge, and focal law configuration could adequately steer longitudinal waves over the 0 to 30 degree range. The active focal law, or beam, is shown on the left and the S-scan on the right. The 2” radius sound path targets start at 0 degrees and extend out to 28 degrees. The first two holes are separated by 2.5 degrees. 6 dB separation is clearly observed between all the measured targets. For example, the 0 degree target peak amplitude is 84% full screen height (FSH). The target amplitude decreases to a minimum of 38.3% FSH between the 0 and 2.5 degree target, and therefore the 6 dB separation is achieved. After the second target moving clockwise towards an increasing angle, 6 dB separation is clearly observed up to the final target at 28 degrees. Based on these phased array testing parameters, the selected PAUT transducer, wedge, and focal law configuration are adequate for the inspection goals.
Similar data for the same 5 MHz, 32-element PAUT transducer at a 1” depth is shown below. In this case, the 0 degree target peak amplitude is 79%. The target amplitude decreases to a minimum of 44% FSH between the 0 and 2.5 degree target, and therefore the 6 dB separation is not achieved. The angular resolution is 0.5 degrees but decreasing the resolution to 0.25 may achieve the 6 dB separation. Alternatively, increasing the element quantity will narrow the beam profile to potentially achieve the desired results.
Figure 4: 1 inch radius focus with 5 MHz, 32-element PAUT transducer, with 0.60 mm element width.
The guidelines in ASTM E2491, Standard Guide for Evaluating Performance Characteristics of Phased-Array Ultrasonic Testing Instruments regarding the assessment of beam steering is a useful process to determine practical steering limits based on 6 dB separation. The Type B phased array ultrasonic testing (PAUT) beam steering assessment block should be used to confirm the performance of the transducer, wedge, and focal law configuration are compliant with inspection goals and relevant inspection standards.
1. ASTM E2491 – 08 Standard Guide for Evaluating Performance Characteristics of Phased-Array Ultrasonic Testing Instruments and Systems
2. ASTM E317 Practice for Evaluating Performance Characteristics of Ultrasonic Pulse-Echo Testing Instruments and Systems without the Use of Electronic Measurement Instruments
3. ISO 22232-1:2020 Non-destructive testing — Characterization and verification of ultrasonic test equipment — Part 1: Instruments
4. ISO 22232-2:2020 Non-destructive testing — Characterization and verification of ultrasonic test equipment — Part 2: Probes
5. ISO 22232-3:2020 Non-destructive testing — Characterization and verification of ultrasonic test equipment — Part 3: Combined equipment
6. E.A. Ginzel, D. Johnson, Phased-Array Resolution Assessment Techniques, Phased-Array Resolution Assessment Techniques (ndt.net)