recent developments in ndt and shm in the united states

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009

Recent developments in NDT and SHM in the United States

John S. POPOVICS 1

1 Department of Civil and Environmental Engineering University of Illinois, Champaign-Urbana, USA, [email protected]

Abstract Recent research work on non-destructive testing (NDT) and structural health monitoring

(SHM) for civil engineering materials and structures from the United States is described. Developments in three topic areas are discussed: non-linear analysis of ultrasound and vibration for concrete, internal imaging for concrete using two different data types and SHM and NDT for steel cables and tendons. In each case the technical background is described and the innovations are emphasized.

Résumé Des recherches récentes sur l'évaluation non destructive (NDT) et le suivi de santé

structurelle (SHM) des matériaux du génie civil et des structures aux Etats-Unis sont décrites. Les développements sur trois sujets sont discutés : l'analyse non-linéaire des ultrasons et des vibration pour le béton, l'imagerie interne du béton utilisant deux types de données différentes (électrique et rayon X) et le SHM et le NDT des câbles d'acier accessible et non accessible. Dans chaque cas le contexte technique est décrit et les avancées misent en relief.

Keywords Concrete, Imaging, Non-linear Ultrasound, Steel, Tomography, Vibration, X-ray

1 Introduction In this paper, I aim to recognize and bring attention to recent and interesting research

developments in the United States that may not be known to the NDT-CE technical community at large, either because the researchers are from a different technical community, or because the methods themselves are unusual. I hope to communicate these findings to a new community and thus to evoke interest, discussion and further development on these important topics.

2 Non-linear behavior of mechanical waves Research on the non-linear behavior of ultrasonic wave propagation and vibration

phenomena in metals has been underway for some time. In the classic wave propagation analysis, the higher (non-linear) orders of strain are retained, and are characterized by a series of non-linear coefficients (β, γ, etc.) associated with each subsequent increase in strain anharmonicity. Non-linear behavior is characterized by dispersion of the wave pulse, generation of multiple wave harmonics above the fundamental, generation of harmonic side bands when two wave frequencies are mixed, and apparent material softening (modulus reduction) with increasing wave amplitude [1]. Non-linear analysis of ultrasound provides enhanced sensitivity to micro-defects in the material, and had been used to characterize micro-porosity, the early stages (initiation) of fatigue cracking, and plasticity driven damage in metals [2-5]. Researchers at Sandia National Lab in the United States extended this study to ceramic-like materials (rock) in a careful and extensive set of studies with ultrasound, vibration resonance, and combinations of the two [6-8]. More recently, other researchers in


the United States have extended this work to concrete [9-11]. In particular, Jacobs and Kurtis at Georgia Tech University have leveraged their experience on non-linear ultrasound for metal alloys to develop a unique procedure to examine cement-based materials. They utilize a hybrid ultrasound-vibration approach to characterize early stages of alkali-silica reaction damage in concrete samples [12]. They correlate the sideband energy produced by the frequency mixing, illustrated in Figure 1, to increased ASR damage level in concrete samples. With this method they are able to detect the development of ASR damage in concrete after only two days of exposure to aggressive (ASR producing) environment. In another study, Popovics and Payá have shown enhanced sensitivity to low levels of mechanical damage (cracking) in concrete using non-linear vibration resonance.

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Figure 1. Non-linear acoustic approach for concrete: measurement approach (left) and

illustration of sideband production (right) (Images courtesy of Prof. Lawrence Jacobs)

3 Internal Imaging

Images can effectively transmit important NDT data to practitioners. Great advances in imaging technology have been realized recently in science and engineering, for the example in the medical field. However, imaging development for civil engineering lags behind. One effective method for image construction from a data set is tomography. Tomography describes a process of reconstructing a cross-sectional image using ray projection measurement data through the section. Resolution of these images is generally limited by the ray coverage density and the wavelength of the energy used in measurements [13]. However, the phenomenological base of the data from which the tomographs are constructed vary: a broad range of data types can be used. Here work on tomographic reconstruction for concrete using two different types of signal data, electrical impedance and X-ray absorption/scattering respectively, are described.

3.1 Electrical impedance tomography Researchers at the University of Michigan are exploring the self-sensing capability of fiber

reinforced cementitious composites (FRCC) as a “smart” material capable of distributed sensing for crack detection. FRCCs are an emerging high-performance civil engineering material that exhibits extremely high strength and ductility. FRCCs contain high volumes of distributed steel fibers, and thus the material has measureable conductivity. Lynch has utilized electrical impedance tomography to recreate the spatial distribution of conductivity within FRCC materials based only on boundary measurements, as illustrated in Figure 2 [14, 15]. The image reconstruction process is semi-analytical, requiring iterative matching of


analytical and experimental data to arrive at the final impedance image. Micro- and macro-cracks can be imaged in fine detail since cracks can be considered non-conducting compared to the FRCC material. This approach represents a distributed sensing scheme that can directly sense and locate cracks, unlike other current health monitoring approaches that use point sensors that indirectly detect damage through physics-based modeling.

Figure 2. Approach to produce impedance tomographs: measurement approach (left) and

inverse solution scheme (right) (Images courtesy of Prof. Jerome Lynch)

3.2 X-ray Microtomography Researchers at the University of Maine have led an effort to apply X-ray microtomography

to cement-based composites. X-ray microtomography is similar in practice to medical CAT-scans. To achieve high spatial resolution within the image, Landis uses an extremely bright synchrotron X-ray source combined with appropriate optics and scintillation applied to small samples. The high flux and monochromatic X-ray source allows one to distinguish very subtle differences in X-ray absorption and scattering, and therefore material phase differences, within the material. In the much of the work, 6 micron voxels defines the spatial resolution, although resolution approaching 1 micron has been achieved [16]. The image in Figure 3 (left) shows a mortar shard where sand grains are distinguishable from cement paste as dense and porous regions. The nondestructive nature of the X-ray scans opens up the possibility of quantifying pore structure and pore connectivity without any special specimen preparation [17].

Using a specially developed in situ loading frame, specimens were imaged while subjected to progressively higher and higher compressive load. Through micro-tomographic reconstruction, internal damage and fracture can be quantified and measured without any need to resort to simple idealizations of crack geometry [18, 19]. An example of a fractured cylinder loaded in axial compression is shown in Figure 3 (right). Current work is aimed at developing discrete element models for which there is a direct correspondence between the material’s physical structure and the corresponding computational model [20].

4 NDT and SHM for metal tendons and cables Steel cables and tendons are very important elements in civil engineering structures,

providing critical load bearing roles in bridge stays or embedded within concrete in pre-stressed and post-tensioned concrete elements. Despite their importance, they tend to be insufficiently monitored only because they are very difficult to inspect.

Initial Conductivity Distribution

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Figure 3. X-ray micro-tomographs of mortar samples showing detain of microstructure and pore network (left) and micro-cracking after loading (right) (Images courtesy of Prof.

Eric N. Landis)

4.1 Ultrasonic guided waves Researchers at Imperial College in London carried our pioneering work on the use of

ultrasonic guided waves for evaluation of embedded steel multi-strand tendons and rock bolts [21]. Ultrasonic guided waves propagate within a long wave-guide, for example a steel bar, and have multiple velocity and attenuation characteristics depending on the wave frequency, waveguide geometry and wave excitation details. The strength of the guided wave approach is that these multiple modes can be specifically tuned and selected to optimize detection characteristics. The ultrasonic guided wave approach has been extended to multi-strand tendons by researchers at the University of California at San Diego. There, DiScalea uses linear guided wave propagation for monitoring stress level within multi-wire strands for post-tensioned concrete structures and for defect detection within the strands. To monitor stress levels, the inter-wire energy leakage between strands in the multi-strand tendon is monitored and related to the applied tensile stress level, as illustrated in Figure 4 [22]. A semi-analytical finite element modeling approach (SAFE) is used to simulate the guided wave propagation within the complicated multi-strand axis-symmetric waveguides; the SAFE simulations help in the selection of the optimal wave modes and in the interpretation of the data and diagnosis of the tendon [23]. Defect detection within multi-wire tendons is achieved using ultrasonic guided waves; the reflected and forward scattered pulses of specific guided wave modes within the tendon are interpreted [24]. The guided wave modes are selected based on performance criteria such as spatial resolution, attenuation, excitability and sensitivity to specific defects.

4.2 Magneto-elastic sensing

Researchers at Northeastern University have developed novel magneto-electric sensors for nondestructively monitoring in situ stress in steel pre-stressed tendons and bridge cables. Magneto-elastic stress sensors utilize the dependence of the magnetic properties (e.g. the magnetic permeability) of structural steels on the state of stress of the material [25, 26]. The sensors are comprised of a principal and secondary set of cylindrical wire coils that are wound around the test object. Magnetic fields are generated when electric current is passed through the principal coil, and the interaction field is detected by the secondary coil. The


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Figure 4. Study of inter-strand wave leakage to monitor tensile stress in tendon (Images

courtesy of Prof. Francisco L. Di Scalea)

magneto-elastic sensors function in a fully contactless manner, and do not touch or alter the inspected material in any way except by magnetization.

This technology more directly measures the internal stress in steel tendons and cables, as compared to other NDT methods, since the magnetic permeability in ferromagnetic materials is a function of magnetic history and applied field. Furthermore, only a relatively small length of sample cable (less than 2 m) is needed for laboratory calibration of the material used in structure. The sensors allow for easy installation in new structures and in-situ installation adaptability for existing structures. Other attractive features include compact sensor size, ease of operation, high accuracy of stress determination (within ±3%), and a theoretically unlimited service lifetime.

The sensors have been applied to cable-stayed bridges [27, 28], arch-suspension bridges, post-tensioning concrete box girder bridges [29], and a large domed space structure that contains high tension steel bracing cables, as shown in Figure 5.

5 Summary

Work on a range of interesting NDT and SHM technologies are currently being carried out in the United States. These efforts promise to bring improvement to the state of technology in this important field, and to have impact on the international engineering and science communities.


EM Sensors

Figure 5. Application of EM sensors in Kumagaya Dome in Japan. Map of bracing cable layout (left) and photo of sensors in situ (right) (Images courtesy of Prof. Ming L. Wang)

References 1. Blackstock, D.T. (2000) Fundamentals of Physical Acoustics, Wiley Inter-Science Inc.,

New York. 2. Buck, O., Morris, W.L. and Richardson, J.M. (1978) “Acoustic harmonic generation at

unbounded interfaces and fatigue cracks,” Applied Physics Letters, Vol. 33, No. 5, pp. 371-373.

3. Herrmann, J., Kim, J.Y., Jacobs, L.J., Qu, J., Littles, J.W., and Savage, M.F. (2006), “Assessment of material damage in a nickel-base superalloy using nonlinear Rayleigh surface waves,” Journal of Applied Physics, Vol. 99, No. 12, p. 124913.

4. Kim, J.Y., Jacobs, L.J., Qu, J., and Littles, J.W. (2006) “Experimental characterization of fatigue damage in a nickel-base superalloy using nonlinear ultrasonic waves,” Journal of the Acoustical Society of America, Vol. 120, No. 3, pp. 1266-1273.

5. Pruell, C., Kim, J.Y., Qu, J., and Jacobs, L.J. (2007) “Evaluation of plasticity driven material damage using Lamb waves,” Applied Physics Letters, Vol. 91, p. 231911.

6. Johnson, P.A., Zinszner, B. and Rasolofosaon, N.J. (1996) “Resonance and elastic nonlinear phenomena in rock,” Journal of Geophysical Research, Vol. 101, No. B5. pp. 11553-11565.

7. Van Den Abeele, K.E.A, Johnson, P.A. and Sutin, A. (2000). “Nonlinear elastic wave spectroscopy (NEWS) techniques to discern material damage: Part I nonlinear wave modulation spectroscopy (NLWS),” Research in Nondestructive Evaluation, Vol. 12, pp. 17-20.

8. Van Den Abeele, K.E.A, Carmeliet, J., Ten Cate, J.A. and Johnson, P.A. (2000). “Nonlinear elastic wave spectroscopy (NEWS) techniques to discern material damage: Part II single-mode nonlinear resonance acoustic specroscopy,” Research in Nondestructive Evaluation, Vol. 12, pp. 31-42.

9. Chen, X.J., Kim, J.-Y., Kurtis, K.E., Qu, Wu, S.C. and Jacobs, L.J. (2008) “Characterization of progressive microcracking in Portland cement mortar using nonlinear ultrasonics,” NDT&E International, Vol. 41, pp.112-118.

10. Warnemeunde, K. and Wu, H-C. (2004) “Actively modulated acoustic non-destructive evaluation of concrete,” Cement and Concrete Research, Vol. 34, pp. 563-570.


11. Xiang, H., Newtson, C.M. and Woodward, C. (2008) “Optimization of NLUT results to determine dynamic properties of concrete,” ASCE Journal of Materials in Civil Engineering, Vol. 20.

12. Chen, J., Jayapalan, A., Kurtis, K.E., Kim, Jin-Yeon, and Jacobs, L.J. (2009) “Nonlinear wave modulation spectroscopy method for ultra-accelerated assessment of alkali-silica reaction in Portland cement mortar,” ACI Materials Journal, accepted for publication.

13. Jackson, M. J., and Tweeton, D. R. (1996) “3Dtom: Three-dimensional geophysical tomography,” Rep. of Investigations 9617, U.S. Bureau of Mines.

14. Hou, T. C. and Lynch, J. P. (2009). “Electrical Impedance Tomographic Methods for Sensing Strain Fields and Crack Damage in Cementitious Structures,” to appear in Journal of Intelligent Material Systems and Structures.

15. Hou, T. C. and Lynch, J. P. (2005) “Monitoring Strain in Engineered Cementitious Composites using Wireless Sensors,” Proceedings of the International Conference on Fracture (ICF XI), Turin, Italy.

16. Diamond, S., and Landis, E. N. (2007). “Microstructural Features of a Mortar as Seen by Computed Tomography.” Materials and Structures, Vol. 40, pp. 989-993.

17. Lu, S., Landis, E. N., and Keane, D. T. (2006). “X-Ray Microtomographic Studies of Pore Structure and Permeability in Portland Cement Concrete.” Materials and Structures, Vol. 39, pp. 609-618.

18. Landis, E. N. (2006). “Damage Variables Based on Three Dimensional Measurements of Crack Geometry.” Strength, Fracture & Complexity, Vol. 3, pp. 163-173.

19. Landis, E. N., Zhang, T., Nagy, E. N., Nagy, G., and Franklin, W. R. (2007). “Cracking, Damage and Fracture in Four Dimensions.” Materials and Structures, Vol. 40, pp. 357-364.

20. Landis, E. N., and Bolender, J. E. (2009). “Explicit Representation of Physical Processes in Concrete Fracture.” Journal of Physics D: Applied Physics, (submitted).

21. Beard, M.D., Lowe, M.J.S. and Cawley, P. (2003) “Ultrasonic guided waves for the inspection of grouted tendons and bolts,” ASCE Journal of Materials in Civil Engineering, Vol. 15, pp. 212-218.

22. Bartoli, I., Salamone, S., Phillips, R., Lanza di Scalea, F., Coccia, S., and Sikorsky, C. (2008) “Monitoring Prestress Level in Seven-wire Prestressing Tendons by Inter-wire Ultrasonic Wave Propagation,” Journal of Advances in Science and Technology - Embodying Intelligence in Structures and Integrated Systems, Vol. 56, pp. 200-205.

23. Marzani, A., Viola, E., Bartoli, I., Lanza di Scalea, F., and Rizzo, P. (2008) “A Semi-analytical Finite Element Formulation for Modeling Stress Wave Propagation in Axisymmetric Damped Waveguides,” Journal of Sound and Vibration, Vol. 318, pp. 488-505.

24. Rizzo, P., Sorrivi, E., Lanza di Scalea, F., and Viola, E. (2007) “Wavelet-based Outlier Analysis for Guided Wave Structural Monitoring: Application to Multi-wire Strands,” Journal of Sound and Vibration, Vol. 30, pp. 52-68, 2007.

25. Wang, G. and Wang, M. (2003) “Stress monitoring of multi-strand cable through the measurement of magnetic permeability” KSCE Journal of Civil Engineering, Vol. 7, pp. 667-673.

26. Wang, G. and Wang M. (2004) “The utilities of U-shape EM sensor in stress monitoring,” International Journal of Structural Engineering and Mechanics, Vol. 17, pp. 291-302.

27. Varsha S. and Wang, M. (2005) “Measurement of corrosion using electro-magnetic sensors,” Journal of Smart Materials and Structures, Vol. 14, pp. S24-S31.


28. Wang G., M. Wang, M. and Zhao, Y. (2006) “Application of EM stress sensors in large steel cables,” Journal of Smart Structures and Systems, Vol. 2.

29. Wang, M.L. (2008) “Long term health monitoring of post-tensioning box girder bridges, International Journal of Smart Structures and Systems, Vol. 4, pp. 711-726.

recent developments in ndt and shm in the united states

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