Ultrasonic testing (UT) performed in immersion tanks or using squirters or bubblers is an established non-destructive testing technology for automated inspection of wide variety of steel, aluminum, and composite assets. Ultrasonic immersion testing lends itself to high volume and/or surface area inspection applications that required high degrees of location accuracy, precision and repeatability of ultrasonic testing results. Immersion testing using automation robotics may be fitted with conventional ultrasonic (UT) and phased array ultrasonic testing (PAUT) hardware. This article discusses some of the key aspects of immersion testing systems used across many non-destructive testing industries.
Figure 1: 6-axis ultrasonic testing immersion system.
Advantages of Immersion Ultrasonic Testing
Immersion UT is a multi-axis automated ultrasonic testing process that provides improved Probability of Detection (POD) of hard-to-find material defects in production environments. There are three main mechanisms that are used to couple ultrasound from the ultrasonic transducer into the test part using water: Immersion tanks, squirters and bubblers. The tested component and the ultrasonic transducer are both immersed in the tank in the former. Squirter and bubblers deploy similar mechanisms the enable ultrasound to be coupled from the transducer to the test part via a water jet or constant stream in the case of the bubbler. Immersion ultrasonic inspection is used mainly in production/fabrication or forming applications to accurately locate, classify and assess defects in large plates, bars and forged materials.
Advanced high-resolution images are created using a high-resolution mechanical scanner with conventional UT and PAUT equipment. High resolution A-scan, B-scan, and C-scan 2-dimensional images are created and can be analyzed post inspection to generate accurate documentation of flaw size and location.
Ultrasonic Immersion Testing – Scanner Specifications
An ultrasonic immersion scanner is a complex assembly of structural and automated components that are designed to manipulate the test piece and/or translate the ultrasonic transducer in 3-D to provide the required inspection coverage with the desired ultrasonic testing solution. Ultrasonic testing solutions can be designed using longitudinal and shear waves with both conventional and phased array platforms. The most basic scanning system comes equipped with X-, Y-, and Z-axes as shown in Figure 2. The X-axis is typically the long axis of the tank. The Y-axis is aligned with the tank width. The Z-axis control the water path depth and is aligned with the tank depth. Additional axes may be acquired adding to the overall system capability and cost. These axes include a roller system (R-axis) as shown below. This is commonly referred to as the R-axis and can provide rotation speeds up to 60 RPMs. Articulation around the X-, Y-, and Z- axes are also available for a total of 7-axes. Axis performance is generally defined by accuracy, precision, resolution and speed. X-, Y-, and Z- axis resolution is typically in the millimeter range (0.040”). X-, Y-, and Z- axis speed may be in the 10- 500 mm/s range depending on the application. Rotation axes specified in angular units for both resolution and speed.
Figure 2: Ultrasonic testing immersion system axes.
Ultrasonic Immersion Testing – Data Analysis
Non-destructive testing data analysis and reporting is an ambitious process as it relates to UT immersion testing. At sub-millimeter resolutions data files are very large and there are multilayers of ultrasonic NDT data to be analyzed. Custom ultrasonic NDT analysis software can be used and modified by project or off-the-shelf solutions like Olympus’ FocusPC can be used to acquire and manipulate ultrasonic testing results. Example UT data is shown below in Figure 3 using FocusPC software. The top right data shows a single ultrasonic A-scan over an approximately 1” range. The ultrasonic reflection is from a flat-bottomed hole (FBH) and multiple back wall reflections are observed. The red gate shown, almost over the entirety of the ultrasonic A-scan, dictates what is displayed on the supporting amplitude C-scan, distance C-scan, and cross-section B-scans shown. The time compensated gain (TCG) is shown directly above the A-scan. In this case, 3 TCG points were assigned.
An amplitude C-scan is shown on the bottom right. In this case, each pixel is assigned a color in accordance with the color map assigned to the C-scan. For this ultrasonic C-scan, lower amplitudes are assigned blue while higher amplitudes are assigned yellow and orange. The 9 individual panels are clearly observable in the amplitude C-scan. In addition, the 5 flat bottom holes (FBH) on the end of each plate are detected accurately by the ultrasonic C-scan.
Figure 3: Ultrasonic testing immersion example data.
The ultrasonic C-scan image on the top right is a position-based image meaning that position at which the ultrasonic reflection crosses the red gate is mapped in lieu of the amplitude. This provides a clearer image of the 9 different pockets and flat bottom holes detected by the ultrasonic test. C-scan results are again color coded by thickness with the light to dark blue the thinnest and the dark reds the thicker parts. The bottom graph displays and end view or D-scan which may by swapped out with an end view B-scan. The image viewed depends on the cursor position of the C-scan. The blue data cursor identifies which cross-sectional slice is viewed in the B-scan. Likewise, the red data cursor identifies which cross-sectional slice is viewed in the D-scan
A separate controls user interface is required to control automation and transfer important ultrasonic scan plan testing parameters between the programmable logic control (PLC), motors / encoders, and FocusPC or comparable software. The machine interface is used to communicate with the PLC and Olympus’ FOCUS CONTROL. FOCUS control is the software library that interfaces with FOCUS PC. A proven communication protocol is used to communicate with the required PLC. Data is saved in a database and scanned files of FocusPC (.fpd) are stored under the software subdirectory for future analysis.