mobile magnetic resonance: nondestructive testing of ... · pipeline technology 2006 conference 1...

Pipeline Technology 2006 Conference

1

Mobile Magnetic Resonance: Nondestructive Testing of Polymer Pipes Alina Adams Buda; Bernhard Blümich RWTH Aachen University Germany

Abstract

Magnetic resonance is well known in medicine as a non-invasive diagnostic tool. The patient is positioned in a magnet tube and investigated with radio waves to obtain slice-selective images. Contrary to x-ray tomography, the soft matter contrast is excellent in MR imaging. The same technique would be of great use for non-destructive testing of products from polymers and similar materials, if the object were not to be transported to the measurement device, but instead the device be moved to the object, and if the object could be positioned on the device instead of cut to size to fit the magnet hole. Exactly these problems were solved at RWTH Aachen University with the invention of the NMR-MOUSE (Nuclear Magnetic Resonance MObile Universal Surface Explorer). Depending on the particular design, two-dimensional images can be measured with it similar to looking through a magnifying glass at arbitrarily large objects but beneath their surface, and depth profiles can be measured with high precision from depth up to 2 cm. The device can be transported as hand luggage by one person and is suited for state assessment of rubber products and plastic pipes. It has also been used for investigations of old master paintings and to characterize the wound healing of skin in vivo.

1. Introduction

Nuclear Magnetic Resonance (NMR) is much more known to the large public as a diagnostic tool in medicine [1]. The patient is positioned inside a magnet tube and then imaged with the help of radio-frequency waves. In addition to its use in medicine, NMR is powerful technique in many other fields such as material science, chemical engineering, and analytical chemistry for the investigation of molecular structure of liquid and solid samples, and in oil-well logging [2]. The principle of the method is based on an interrogation of the atomic nuclei of the sample, preferably protons (1H), in a permanent magnetic field with the help of radio-frequency irradiation [1].

With the exception of well logging, the objects of study are brought to the laboratory and investigated in the homogeneous field of supra-conducting magnets. For this purpose, the object has to be cut into pieces small enough to fit inside the magnet. But in the case of well logging, the approach is changed, and the NMR device is brought to the place or the object of study. The large object is now brought into contact with the small sensor from one side. In this context the notions of �mobile NMR� and �inside-outside NMR� are used. The NMR-MOUSE® is a further development of the �inside-outside� idea, but stronger magnetic fields and smaller sensors are used. The whole device comprising the NMR sensor as well as the electronics (the NMR spectrometer) is small enough to be carried as hand luggage by one person to the site of the investigation. The price paid for the gain in mobility and small size is the admission of inhomogeneous magnetic and radio-frequency fields. Compared to conventional NMR, measurements with inhomogeneous fields request the acquisition of multiple echoes, instead of the impulse response, and the decay of the envelope of an NMR


2

echo train is recorded. Nevertheless, the NMR-MOUSE data lead to results similar to those of conventional NMR for a broad range of materials [5, 6]. Moreover, the NMR-MOUSE can be used to record site-selective information at different depths of an object. Depending on the sensors characteristics, depths up to 2 cm can readily be achieved so far. The depth resolution can be as small as a few micrometers, so that the sensor can be employed to measure depth profiles through thin laminates. This feature is important, for example, in the study of old paintings and wound healing of skin but also for liquid ingress into various materials. But mobile NMR is also a suitable tool for the state assessment of polyethylene pipes by its ability to discriminate between rigid, mostly crystalline, and soft, usually amorphous material [5].

Polyethylene (PE) is used more and more for the production of gas and water pipes due to its low cost, light weight, simple handling, and good corrosion resistance. The present standard is PE100, which identifies pipes made from such materials to withstand a stress of 10 MPa for 50 years at room temperature [7]. A life-time prediction of such a long-lived material cannot reasonably be carried out at room temperature (RT). For example, pressure tests with and without point loads and with and without surfactants are performed on pressurised pipe sections at temperatures ranging from 60 to 100 °C [7, 8], and the results are extrapolated to room temperature using the Arrhenius law [7]. It is clear that such test can at best approximate the most probably aging conditions encountered in real use, and the that the transposition of the results obtained under idealized testing conditions to real-life conditions is not at all straightforward. Therefore, the life time prediction of pipes under ground is still under discussion [9, 10]. The accelerated thermal aging process mainly concerns physical aging or fatigue, while pipes in contact with soil and chemically treated water suffer from additional chemical aging or corrosion, and both processes are shortening the life time of the material. The physical aging describes the evolution of the polymer morphology established during the manufacturing process towards thermodynamic equilibrium. It is often manifested in an increase in crystallinity with time which may go hand in hand with an embrittlement of the material. Unless the temperature is close the glass transition temperature Tg, this process may take years. However, the physical aging is reversible: if a physically aged material is heated above Tg and then cooled down again, the original material properties established during manufacturing can be restored. On the other hand, chemical aging changes the molecular structure irreversibly, for example, by oxidation, chain scission, and the formation of cross links.

A proper evaluation of the aging effects would have to asses the variation of the chemical structure and morphology at different depths inside the material from surface to the bulk and at different lateral positions, as technical products from semi-crystalline polymers are known to have a crystallinity gradient from inside to outside due to the cooling process, but also lateral crystallinity variations due to statistical nucleation and associated shrinkage of the material in the crystalline domains. Conventional mechanical tests such as stress-strain measurements do not discriminate between surface and bulk effects. Even more, the tests at elevated temperature will eliminate change the polymer morphology by annealing, and the changes of the mechanical properties invoked at room temperature by fatigue and corrosion may be masked.

Therefore, to arrive at more conclusive data than those provided by the currently accepted testing conditions of accelerated laboratory aging which are defined in different norms and regulations, investigations with new methods more sensitive to the different aspects of the polymer morphology are required. Recently it was shown that NMR relaxometry in combination with differential scanning calorimetry (DSC) is a powerful combination of methods for studying the effects of pressure and temperature on the morphology and the lifetime of poly(propylene) pipes [6]. In this study the measurements were performed in a destructive manner by using samples taken from the pipes. But the


3

NMR-MOUSE explored in this work is a new and powerful NMR tool to conduct morphological investigations in a non-destructive fashion. The first use of the NMR-MOUSE for non-destructive morphological characterization of PE pipes has recently been reported [5]. PE-100 pipes were investigated in the new state, after squeezing with a commercial compression device, and after annealing at 80o C, well below the melting temperature. Also, the changes in morphology induced by a pressure load from the inside and a point load from the outside were investigated as a function of depth.

The aim of the present study was to further investigate the effect of annealing and natural aging on the polyethylene morphology by non-destructive NMR testing. We have investigated the morphological characteristics before and after annealing of a section from a PE100 pipe which has been used in a pressure test with a point load a year ago, as well as sections from different LDPE coatings of steel pipes that had been in the ground for twenty to thirty years. Finally, the non-destructive NMR methods has been applied to inspect a welding line of a PE pipe.

2. Experimental

The measurements were executed with different NMR-MOUSE sensors, which differ in their field profiles and, therefore, in their measurement characteristics. Nevertheless, for the investigated semi-crystalline polymers, the measured echo envelope could well be approximated by a bi-exponential function

y = Ashort exp(-t/T2effshort) + Along exp(-t/T2efflong) (1) in all cases, where the relative amplitude ratio Ashort/(Ashort + Along) is a measure of the NMR crystallinity while Along/(Ashort + Along) is a measure of the amount of the amorphous phase. The two morphological phases are distinguished by the differences in the values of the corresponding NMR relaxation times T2eff which are a measure of the molecular mobility and are independent of the amount of material. The crystalline phase (or generally rigid phase) is characterized by a short value of the relaxation time (here denoted by T2effshort) while the amorphous phase (or generally the mobile phase) is characterized by a longer value of the relaxation times (here denoted by T2efflong). Due to the fact that especially at temperatures close to room temperature the signal from the crystalline phase may include also signal from the rigid amorphous domains the NMR crystallinity is usually different from that obtained by X-ray measurements. Moreover, the relaxation rates 1/T2effshort and 1/T2efflong are measures of the chain packing densities in the crystalline and the amorphous phases, can be employed for a more detailed chartacterization of the polymer morphology. Therefore, any change in the phase composition or chain packing is associated with a change in the relaxation times. In order to follow these morphological changes in an easy and and fast manner, the following parameter w, defined as the ratio between sums of echoes, is extracted from our measured experimental data,

,100*,

,2

np

m

SS

w = (2)

where m < p <n. The parameter w is an indicator of the chain rigidity or the crystallinity as it grows with increasing crystallinity.


4

3. Results

Prior to this work, we have investigated the morphological changes induced by a pressure test at 5° C with intentation according to Kiesselbach on two PE100 water pipes with outer diameters of 10 cm and a wall thickness of 7 mm [5]. Three indentations of 0%, 1%, and 6% were applied simultaneously to the unpressurized pipes with a 10 mm diameter, round pin. The pipes were subsequently pressurized with water for 24 hours; one pipe to 15 bar which is 1.5 times the maximum pressure allowed water pipes, and th eother to 6 bar which is 1.5 times the maximum pressure allowed for the gas pipes. Following the pressure load, the depressurized pipes were mapped within a few days of the pressure test with the NMR-MOUSE at 1 mm depth from the inside. The points of indentation showed a distinctly higher value of w, indicating strained chain segments. Moreover, a shear band was discovered by NMR imaging in the region of the 6% deformation in the first pipe. Fig. 1. Fotos of the investigated pipe region studied region which had been deformed a year ago by 6% in a pressure test. The positions of some of the measured rings (left) as well as those of the points 1 and 5 along the circumference (right) are indicated. The middle of the deformed region corresponds to the point 5 on ring 5.

One year later, the second pipe was systematically scanned by the NMR-MOUSE from the inside in a region surrounding around the 6% deformation (Fig. 1). The experimental set-up is depcited in Fig. 2. A step motor rotated the pipe between NMR measurements to acquire NMR data at points along the inner circumference in 1 cm intervals, leading to 28 measurement positions in one ring. These data were automatically acquired and analyzed in about 1 hour and 20 minutes. The positions of different rings were adjusted manually, also in steps of 1 cm. Fig. 2. Experimental set-up used for the automatic NMR mapping measurements of pipes from the inside. The NMR-MOUSE designed for these measurements on curved surfaces (left) can be positioned semiautomatically (right) to the different points inside the pipe.

A 9 cm long section around the 6% deformation region was analyzed by the NMR-MOUSE on a 1 cm2 grid before and after annealing at 80° C for 36 hours. The resultant maps


5

of the NMR parameter w are depicted in Fig. 3. As already seen in our previous investigations, the pipe material is laterally inhomogeneous which means that the polymer morphology fluctuates from point to point along the pipe. In the present study, the existence of inhomogeneities is reflected by the differences in the values of the parameter w. A slightly higher value of w is identified by the green color near the coordinate (5/5) where the 6% deformation point is, indicating slightly higher chain order in the amorphous regions. However, both, the NMR crystallinity and the value of T2efflong appear smaller than shortly after the intentation test and are comparable to those in some other points. This suggests, that the effect of the deformation started to heal after removal of the pressure within one year�s time at room temperature. The same healing effect was observed for the shear band detected in the 6% deformation region of pipe 1. In this case the sear band disappeared completely after a few of months. Both of our results are in agreement with observations by others [12]. Fig. 3. Morphology maps of PE pipe section. Left: Before annealing. Right: After annealing at 80° C for 36 hours. The color code represents the NMR parameter w.

Fig. 4. Distributions of NMR parameter w (chain rigidity) before and after annealing at 80° C for 36 hours.

After annealing, the values of w increase all over the pipe section as indicated by the brighter colors in Fig. 3 (left vs. right). A quantitative data analysis by fitting eqn. (1) reveals an increase in the amount of crystallinity and the relaxation rate of the amorphous domains. This observation can be explained by melting of the smaller crystallites at the elevated annealing temperature and the larger ones grow and heal out defects. For the 6% deformation point at coordinate (5/5), the value of w is now in the normal range of values, and no indication of the deformation can be detected any longer. A new morphology has been


6

established by annealing at 80° C. Also, the deformation which could still be felt by touching the inner pipe wall had disappeard, so that the indentation defect was healed. A statistical analysis in terms of a distribution of w of the pipe sections before and after annealing summarized the comparison of the data in Fig. 4: in addition to the shift of the mean value to higher values after annealing a change in the width of the distribution is detected.

To assess the significance of the morphology fluctuations with regards to the mechanical stability of the pipe, the wall thickness was measured at both ends of the pipe and plotted in Cartesian and polar coordinates (Fig. 5). The thickness of the pipe wall varies along the circumference by about 0.3 mm. The highest thickness values are found between positions 1 to 4 and 25 to 28 and the lowest values between positions 5 and 24, which correspond to the top of the pipe during extrusion. Given the fluctuations in polymer morphology along the pipe wall and the variation of the pipe wall diameter, it is difficult to assess at this point, which of the two effects eventually dominates in determining the position of a fracture from pressure overload. Certainly, the NMR-MOUSE is a suitable tool to assess the morphological fluctuations and their variations with processing, mechanical load, and annealing. Fig. 5. Variation of the wall thickness with position along the circumference. The two data sets corresponds to the two ends of the pipe. Left: Cartesian representation. Right: polar representation.

While the investigations above concerned changes in the polymer morphology due to thermal and mechanical effects (fatigue), chemical change (corrosion) also leads to changes in the polymer morphology and can likewise be detected by the NMR-MOUSE. To this end, a section of a low density polyethylene (LDPE) steel-pipe coating in use for twenty to thirty years has been investigated. The polymer morphology was mapped at 1 mm depth over the whole available surface before and after annealing at 60° C for 24 hours and statistically analysed (Fig. 6). In contrast to the statistics from physical aging (Fig. 4), where the mean value of the w distribution shifted to higher values upon annealing, a much larger shift in the opposite direction was observed for the LDPE coatings. But the width of the distribution also increased upon annealing. This drastic change of the mean correlates with the elongation at break (Fig. 7). A dramatic increase of the elongation to break is observed after annealing. Before annealing the sample is brittle as a result of chemical aging. After annealing, the sample appears like new. Our hypothesis is as follows: before annealing, the macromolecular chains have been modified chemically by corrosion, but not given the opportunity to adjust their packing. As a result the material became brittle. By annealing the chain packing can equilibrate, and the sample gained elasticity, although the chemistry is now different from that of the original LDPE. Eventually, chemical gradient from the surface to the material below will be assessed as well by measuring depth profiles with the NMR-MOUSE. These


7

observations are in agreement with the data by Krietenbrink [9], who observed, that for some PE pipes recovered after 20 to 30 years of use, the elongation at break had dramatically deteriorated at room temperature, while the pressure test at 80° C predicted a further lifetime of 100 years. Clearly, the elevated temperature of the test has annealed the chemically aged material, and a material different from that of the aged pipe had been tested for life-time prediction. Fig. 6. Distribution of the morphological inhomogeneities before and after annealing at 60° C for 24 hours of a twenty to thirty year old LDPE steel coatings.

Fig. 7. Elongation at break for a twenty to thirty year old LDPE steel coatings. Left: Before annealing. Right: After annealing at 60° C for 24 hours.

Another aspect of the present study concerned the quality of welding lines. The life

time of a pipe line is determined not only by the quality of the material but also by the quality of the welding line. Measurements were performed with a newly designed NMR-MOUSE with a flat and narrow sensitive volume (Fig. 8). The lateral profile across the welding line was measured at a depth of 1 mm in two different positions. Due to heating and heat dissipation, the polymer morphology changes in the welded region compared to that of the normal material. An increase signal amplitude was detected at both positions. Yet there are differences in the width and the position of the NMR profiles across the welding line for both positions. We hope to exploit these differences for an assessment of the quality of welding lines by non-destructive testing with mobile NMR.


8

Fig. 8. 1D images across the welding line of PE plates scanned in a pointwise fashion by lateral displacement of an NMR-MOUSE. The images were recorded in two different regions along the welding line.

Summary and Conclusions

The NMR-MOUSE has been shown to be suitable for non-destructive characterization of morphological changes by physical and by chemical aging in semi-crystalline polymers. A section of a PE100 water pipe exposed a year before to a pressure test with indentation before was imaged in a point-wise fashion from the inside with the help of a dedicated NMR-MOUSE. The data reveal that annealing induces morphological changes in all parts of the pipe, which widen the distribution of morphological heterogeneities and shift its mean so that the material becomes more brittle. Yet at this point it is not clear, if the NMR map of heterogeneities in the pipe is sufficient for predicting mechanically weak spots of the pipe, as the wall thickness fluctuates as well. In addition to physical changes in PE100, chemical changes were studied in old steel-pipe coatings from LDPE. Annealing rejuvenates the highly brittle material and dramatically improves the elongation at break. This is in agreement with a shift of the NMR parameter distribution in a direction opposite to that observed for annealing the pipe from the pressure test. Furthermore, it has been shown, that the NMR-MOUSE promises to be of use for testing welding lines nondestructively. The investigations conducted so far illustrate the potential of the mobile NMR for non-destructive testing of the PE pipes and support the idea of applying the NMR-MOUSE to refine lifetime predictions. References

[1] B. Blümich, Essential NMR for Scientists and Engineers, Springer, Berlin, 2005. [2] B. Blümich, NMR Imaging of Materials, Clarendon Press, Oxford, 2000. [3] G. R. Coates, L Xiao, M. G. Prammer, NMR Logging Principles and Applications,

Halliburton Energy Services, Houston, 1999. [4] G. Eidmann, R. Savelsberg, P. Blümler, and B. Blümich, The NMR MOUSE: A

Mobile Universal Surface Explorer, J. Magn. Reson. A 122 (1996) 104 - 109. [5] B. Blümich, F. Casanova, A. Buda, K. Kremer, T. Wegener, Anwendungen der

mobilen NMR zur Zustandsbewertung von Bauteilen aus Polyethylen, 3R International 44 (2005) 349 - 354.


9

[6] V. M. Litvinov, M. Soliman, The effect of storage of poly(propylene) pipes under hydrostatic pressure and elevated temperature on the morphology, molecular mobility, and failure behavior, Polymer 46 (2005) 3077 - 3089.

[7] H. Brömstrup, Hrsg., Rohrsysteme aus PE 100, Vulkan-Verlag, Essen, 2004. [8] H.H. Kausch, Polymer Fracture, Springer, Berlin, 1987. [9] H. Krietenbrink, R. Kloth, 3R International 43, 576 (2004). [10] R. Grosse-Boes, R. Kloth, 3R International 43, 233 (2004). [11] Kiesselbach, G.: Sicherheit und Nutzungsdauer erdverlegter PE-Druckrohrleitungen,

gwf-Wasser/Abwasser 145/1, (2004) S.45 [12] Strobl, G.: The Physics of Polymers, Springer, Berlin, 1996.

mobile magnetic resonance: nondestructive testing of ... · pipeline technology 2006 conference 1...

Documents