Imaging techniques for non-destructive testing

Gilles Heymes, winter semester 2016/17

The interpretation of non-destructive testing (NDT) results is complicated and requires a lot of know-how in raw-data interpretation as well as for processing them for imaging. Commercially available, compact testing devices like ultrasonic and radar testing devices generate diverse images which have to be interpreted by the user in order to determine the structure of the material of interest. Several image enhancing methods have been developed to facilitate the interpretation of these images. In the following sections different data presentations from several imaging techniques for non-destructive testing will be described. Furthermore, a special method of ultrasonic application, two image enhancing methods as well as a short section about tomography image generation will follow.

Data presentation

The three most common formats for data presentation generated by NDT applications are A-scan, B-scan and C-scan. These presentations display the response of the material of interest to the stimulation from an NDT transmitter such as an ultrasonic probe.

A-scan: The received energy of the test specimen in response of a stimulation is displayed as a function of time in the A-scan presentation. From the time delay of the response to the stimulation the depth can be calculated under consideration of parameters of the specimen such as the permittivity for RADAR applications or the sound velocity for ultrasonic applications. The A-Scan is a 2D presentation with time and energy displayed on the axis. The main information of an A-Scan is the determination of the existence of discontinuities and their depth on a single point of the test specimen. On figure 1 an A-Scan for a specimen with a discontinuity is pictured. The ultrasonic waves are reflected from the discontinuity as well as from the back-wall (also called back-wall echo) of the specimen. These reflections are caused by the difference in impedance of the discontinuity, or the surrounding medium, and the specimen. The same reflection phenomenon occurs for RADAR testing due to differences in the permittivity.

B-scan: The B-scan presentation is a profile view of the test specimen. It can be understood as multiple A-scans strung together to form a 3D presentation with the depth and the one-axial position as axis and the third dimension, the energy amount, displayed as a color code presentation. (Figure 2)

C-scan presentation: The C-scan provides a plan-type view of the location and size of test specimen features. It can be seen as multiple B-scans strung together to form a 4D presentation with the depth and two one-axial positions as axis and the fourth dimension, the energy amount, displayed as a color code presentation. (Figure 3)^[1]

Ultrasonic applications

In the following sections the focus is on image enhancing methods for ultrasonic applications. Those enhancing methods are similar for other applications working with wave propagation. For an insight of ultrasonic transmission principles please refer to the following articles: [1][2]

A different technique for ultrasonic testing is the Time of flight Diffraction Technique (TODF) of which the working principle is explained in the following section.

Time of flight Diffraction Technique (TODF)

The time of flight diffraction technique is commonly used for weld-joint inspection. It uses a pair of ultrasonic transducers sitting on opposite sides of the weld-joint of interest. In a non-defective case the receiver detects two waves from the transmitter, one is the reflection of the back wall the other is the lateral wave along the surface from the transmitter to the receiver. In case of a discontinuity, as represented in figure 4, more reflection waves are received from the receiver caused by reflections of the emitted waves on the discontinuity. The depth of the discontinuity can by precisely calculated by the path difference of the waves deduced with trigonometrical laws. The main advantage of TOFD is the speed of detecting discontinuities with a reproducibility and accuracy proven <0,5mm.^[2][3][4]

Image enhancing techniques

In the following two sections image enhancing techniques are presented. The Synthetic Aperture Focusing Technique (SAFT) and the Full-Matrix Capture Technique. The main difference of these two techniques is the use of different ultrasonic transmitters for measurements.

Synthetic Aperture Focusing Technique (SAFT)

An improvement of the image resolution of ultrasonic images can be obtained with the Synthetic Aperture Focusing Technique (SAFT) without the additional use of traditional ultrasonic lenses. Discontinuities in materials such as a reinforcing steel bars in concrete are shown as crescents in ultrasonic B-scans. These crescents are due to the dispersion of the ultrasonic waves in form of a cone (by approximation). The width of the cone at a given range is called “aperture width”. Due to conical dispersion, waves are reflected on discontinuities even before the transmitter is positionedright above it. These reflected waves have a longer sound path to travel than the waves reflected with the transmitter right above the discontinuity, as pictured on figure 5. This path differences cause time of flight differences of the ultrasonic waves and are the reason for the crescent representation of discontinuities. The aim of the SAFT algorithm is to display discontinuities at their location without width spreading crescents to facilitate the interpretation of ultrasonic images.

In order to eliminate or at least minimize the effects of the transmitter aperture beam width over an B-scan or C-scan image, SAFT calculates, for each image line the position where the signal should be inside an adjacent line. In practice the process recognizes crescents in the image and shifts those to a straight line with a smaller extension by correlating the individual signals.(Figure 5)^[5]

Full-Matrix Capture

The Full Matrix Capture (FMC) technique is an application of the phased array technology. It consists of capturing and storing all possible time-domain signals (A-Scans) from every transmitter-receiver pair of elements in the array. A visualization algorithm, such as the Total Focusing Method (TFM), post-processes all raw information in order to enhance image resolution. This visualization algorithm works with similar principles as SAFT and correlates the different information in order to display clearly the location and size of discontinuities. The main difference between SAFT and FMC/TFM is, that FMC/TFM is used when phased array transmitters are used. Because of the larger dataset received with phased array transducers the image processing requires more computational capacities then the SAFT algorithm. A display of the data collection with an phased array transmitter is pictured on figure 6.^{[6] [7]}

Ultrasound computed tomography (USCT)

In Tomography application the main approach is to scan the object of interest with a receiver and transmitter placed opposite on the object. Both sensors turn around the object whereby they always stay in a straight line to one another. This is the case for the classical x-ray computed tomography and the 2D USCT. For 2D USCT the approach is to assume the ultrasonic pulse to propagate along a narrow straight line from the transmitter to the receiver. With this approach the same back-projection methods for image reconstruction as for x-ray CT can be used. Further details of different types of back projection algorithm can be found in the following article: [3]^[8]

The narrow straight line approach for ultrasonic waves has however several major drawbacks. First of all, the three dimensional dispersion of the waves is not taken into account. Furthermore, reflection and deviation of the waves through the specimen are not considered in this model. In order to correct these drawbacks, the 3D USCT was developed. The 3D USCT scans the entire body as a 3D object and not as for 2D USCT or most x-ray CT as numerous 2D scans strung together to form a 3D body. The main advantages of the 3D USCT are simultaneous recording of reflection, attenuation and speed of sound images, high image quality, and fast data acquisition. 3D USCT systems implement unfocused ultrasound emission and reception using spherical wave fronts for imaging. The received emissions are reconstructed by synthetic aperture postbeamforming to create USCT images.^[9]

Literature

1. NDT Recources center: Data Presentation
2. Hecht, A.: Time of Flight Diffraction Technique
3. TWI: Time-of-Flight Diffraction
4. Berke M.: Detection of discontinuities
5. A.W. Elbern: Synthetic Aperture Focusing Technique for Image Restauration
6. Tremblay P., Richard D.: Development and validation of a full matrix capture solution
7. Dao G. et al.: Full Matrix Capture with a Customizable Phased Array Instrument
8. Jiřík R. et al.:Sound-Speed Image Reconstruction in Sparseaperture 3-D Ultrasound Transmission Tomography. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency control, vol. 59, no. 2, p.254. February 2012
9. N.V. Ruiter et al.: First Results of a Clinical Study with 3D Ultrasound Computer Tomography. ISBN:978-1-4673-5686-2/13, IEEE6512013 Joint UFFC, EFTF and PFM Symposium