Combining X-Ray Tomography and Acoustic Emission Analysis for the imaging of fracture formation processes

Sascha Rommel, winter semester 2020/2021

The combination of X-Ray tomography and acoustic emission analysis yield different advantages for detecting cracks and the imaging of crack growth. There are two different methods: One is to detect and localize crack growth during loading of a specimen with the acoustic emission analysis and confirm the data afterwards with a X-Ray tomography image. The other is a combination of the two methods in situ during the experiment. Therefore, both methods are used during experiments. X-Ray tomography and the acoustic emission analysis and a combination of those two techniques can be used in mechanical, building and civil engineering.

1 Basics

1.1 X-Ray Tomography

X-Ray tomography is a technology based on X-Rays, i.e. electromagnetic waves with a wavelength between 0.1 to 10 nanometers. Digital computed tomography (CT) scanners are used today. [1] For the history of X-Rays and the fundamental physical principles see the article about X-Ray tomography. For new developments, especially micro-computed tomography (micro-CT) see the article about Micro-CT. With CT the type, the location and the geometry of a defect can be determined.

1.2 Acoustic Emission Technique

Acoustic emission technique (AET) is widely used to detect elastic waves (such as p- and s-waves) emitted by a growing fracture. The AET is classified as a “passive” testing method, because there is no source emitting a signal, as the signal arises within the material itself. If there would be an active external source emitting a signal, then the method would be classified as “active”. [2b] (See Figure 1)

The passive AET can be divided into two basic techniques: The parameter-based AET and the signal based AET. The parameter-based technique usually measures the count of the acoustic emission signal. Whereas the signal-based technique (receiver arrays) localizes the source of the signal and therefore, the signal can be evaluated regarding the frequency spectrum. [3] The localization methods are based on the time difference of the signal’s first arrival at different sensors. A 3D-localization requires at least four sensors. If there are more then four sensors in use, which is favorable, an error can be calculated in additionally. [2b, 3] For the sensor types and more information on the fundamentals see the article about acoustic emission testing. For more details on localization methods and acoustic emission testing on fiber-reinforced materials see the article about Acoustic Emission Analysis of fiber-reinforced high performance materials.

2 Combining Acoustic Emission Testing and X-Ray Tomography

2.1 Motivation

Compared to other non-destructive testing methods, the acoustic emission technique brings the advantage that the entire history of a certain part can be monitored without interrupting the testing or monitoring cycle. [2a] For instance testing with CT – which is a scanning method – requires to stop the loading or testing process of the specimen. [1] The localization of defects is vital for non-destructive testing methods. Therefore, there are a few established methods in acoustic emission testing in order to localize the source of the acoustic emission event. These techniques work very good with homogenous isotropic materials. However, if the material is heterogeneous (e.g. concrete or fiber reinforced plastics) these methods might not always be absolutely accurate and a certain error in localizing the effect must be taken into account. [2b] However, a considerable disadvantage of AET is its impossibility to determine whether the source is a new crack or an already existing crack which is propagating. [2a] The aim of combining AET with CT or micro-CT is to overcome the difficulties related to the acoustic emission technique. With CT/micro-CT it is possible to determine the exact position of a crack or damage and also whether the crack is new or propagating from an existing one and therefore compensates this disadvantage of the AET. Due to the fact that a rough localization of the damage can be made with AET, there is no need to do a CT-scan of the whole part or specimen but only the region where the damage is located. This not only allows shorten the time taken for the CT-scan, but also to cut additional expenses caused by a time-consuming procedure [6]. Nevertheless, one big disadvantage related to this method is the interruption of the experiment or monitoring while carrying out the CT-scan.

In addition, the two methods can be used in succession. Therefore, the AET is used during the experiment and later the results can be verified through a CT-scan. In these cases, the error of the AET can be determined more precisely than just using the AET. [4]

2.2 Challenges

The biggest challenge in combining the two methods is, that the loading of the specimen must be interrupted to carry out the CT scan. In [5] it is stated that a 5 to 10 minute relaxation time is needed for fiber reinforced plastics in order to obtain a clear CT-image. The loading cylinder is also held in a constant position. Due to relaxation effects the force on the specimen is decreasing during that time.

Today the combination of these two methods is only possible in laboratories, due to the complex experimental setup. It may be used for material characterization such as fiber reinforced plastics, other composite structures and reinforced concrete. It is practical to use the acoustic emission analysis as a monitoring technique for measurements on parts in the field. If the specimen (partly) fails, then a X-Ray tomography can be used to validate the measurements of the AET afterwards.

3 Examples

3.1 Fiber reinforced plastics

Damage on Fiber reinforced plastics can be categorized into three types of failures: Matrix cracking, fiber breakage and interface delamination. In order to detect these damages during an experiment the acoustic emission technique can be used. In [5] the acoustic emission analysis is combined with a in situ X-Ray tomography in order to detect fiber breakage, matrix cracking, and interface delamination. For the experiment a three-point bending test was conducted on a glass fiber reinforced molding compound based on an unsaturated polyester-polyurethane hybrid. During the test both, an acoustic emission analysis and at certain load steps an in situ X-Ray tomography, were made. For the test setup see Figure 2.

A peak frequency analysis of the acoustic emission signals showed three clusters at certain frequency bands. With the help of the in situ X-Ray analysis two of these three clusters can be assigned to matrix cracking and fiber breakage. The third cluster could not be assigned to a defect as the resolution of the CT-images was not efficient in this example.

3.2 Ceramic Matrix Composites

In [6] a combination of acoustic emission analysis and in situ synchrotron X-ray microtomography was used to characterize the damage evolution in ceramic matrix composites, which were recently introduced in the high-pressure turbine shrouds of commercial jet engines. Here four-point bending and tensile tests were conducted. As in the example of fiber reinforced plastics the loading cylinder is held at a constant position after certain load steps do the X-Ray microtomography. In this example matrix cracking and fiber damage could be clearly distinguished with the acoustic emission analysis and the images of the X-Ray microtomography revealed an insight into the crack formation process.

3.3 Reinforced Concrete and Corrosion

A big challenge for reinforced concrete are corrosion processes of reinforcement steels. Because of that, corrosion processes and cracking were investigated in [4]. Here AET was used to detect corrosion cracks during the accelerated corrosion in the laboratory. At certain intervals a micro-CT was performed with the specimens to validate the AET signals. The experiments validated that with AET it is possible to quantify the corrosion damage in the specimens and the moment of cracking can be clearly distinguished from the change in acoustic emission energy. It also became quite clear that in these experiments the localization of the AE events was quite accurate. Thus, the specimens were made of reinforced mortar no rocks have been used as filling material and only pores were present. Hence the specimens were not quite as heterogenous as normal concrete. It was nevertheless concluded that the acoustic emission technique can be adequately used for monitoring structures in terms of corrosion cracking.

Summary and future application

The combination of the acoustic emission technique with CT or micro-CT can be appropriately combined. The CT can be used to validate the results of the acoustic emission technique and to determine in which part of the specimen the cracking occurred. Especially in composite structures such as fiber reinforced plastics with CT images fiber cracking, matrix cracking, and delamination can be distinguished and assigned to certain frequency clusters at the AET.

Future applications of this combination will mostly be carried out in laboratories, as CT or micro-CT are mainly stationary and very expensive. It will be used for material characterization, the research on damage mechanisms, and the verification of the acoustic emission technique on different damage and crack propagation mechanisms. The latter is quite important as the AET is highly promising for monitoring structures and should be used more frequently in the future on a broader variety of structures and parts. A better understanding of crack mechanisms ensures the safety of structures in the field.

References

[1] Thompson, A., Leach, R., Carmignato, S. (Ed.), Dewulf, W. (Ed.): Introduction to Industrial X-Ray Computed Tomography in Industrial X-Ray Computed Tomography. Springer International Publishing, Cham (2018), pp. 1-10.

[2a] Grosse, C.U., Ohtsu, M. (Ed.): Acoustic Emission Testing. Springer Verlag, Heidelberg (2008), pp. 3-9.

[2b] Kurz, J.H., Köppel, S., Linzer, L. M., Schechinger, B., Grosse, C.U., Ohtsu, M. (Ed.): Acoustic Emission Testing: Chapter 6 – Source Localization. Springer Verlag, Heidelberg (2008), pp. 101-143.

[3] Große, C.U.,Schallemissionsanalyse. In: Grundlagen der Zerstörungsfreien Prüfung. Arbeitsblätter im Rahmen der Vorlesung, Lehrstuhl für Zerstörungsfreie Prüfung der Technischen Universität München. München (2020), pp. 182-204.

[4] Van Steen, C., Pahlavan, L., Wevers, M., Verstrynge, E.: Localisation and characterization of corrosion damage in reinforced concrete by means of acoustic emission and X-ray computed tomography. Construction and Building Materials (2019) 197, pp. 21-29.

[5] Bartkowiak, M., Schoettl, L., Elsner, P., Weidenmann, A. K.: Combined In Situ X-Ray Computed Tomography and Acoustic Emission Analysis for Composite Caracterization – A Feasibility Study. Key Engineering Materials (2019) 809, pp. 604-609.

[6] Maillet, E., Singhal, A., Hilmas, A., Gao, Y., Zhou, Y., Henson, G., Wilson, G.: Combining in-situ synchrotron x-ray microtomography and acoustic emission to characterize damage evolution in ceramic matrix composites. Journal of the European Ceramic Scociety (2019) 39, pp. 3546-3556.