ACOUSTIC EMISSION MONITORING AND EVALUATION OF NICKEL HYDROGEN BATTERY CELL CASES

Transcription

1 Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation NDE 2011, December 8-10, 2011 ACOUSTIC EMISSION MONITORING AND EVALUATION OF NICKEL HYDROGEN BATTERY CELL CASES K K Purushothaman 1, B Binu 1, Jeby Philip 1, S A Pillai 1 and S A Ilangovan 2 1 Structural Design & Engineering Group, 2 Energy Systems Group Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala, India ABSTRACT Acoustic Emission Technique (AET) is one of the most promising non destructive techniques for monitoring and evaluation of pressure vessels and other structural members. It is an effective way of detecting fracture and fatigue behaviors of materials. Being a volume technique AE can detect the presence of defects like cracks, delaminations, debonds, certain weld defects etc anywhere in the component, which are active, or that can grow under service loading conditions. In certain cases AE can also detect yielding behavior. This technique is being widely used as a reliable tool for structural integrity assessment during qualification and acceptance testing of large number of structural components in aerospace industry. Nickel-Hydrogen cells are used in satellites. The cell case is made of INCONEL-718 alloy and cylindrical in shape capped with tori-spherical end domes. The cell case is made by joining two parts together by means of cirseam TIG weld. The terminals are also welded to the case. During normal operating conditions the cell case will be under internal pressure of 64 Ksc. During service the cells are subjected to large number of charging and discharging cycles, which causes pressure variation in the case. So ensuring the structural integrity of the welds is very important. AE monitoring has been carried out on Nickel-Hydrogen assembled cells during acceptance pressure tests by using Helium gas in order to assess the structural integrity by ascertaining whether the hardware is free from any active defects especially in the TIG weld joints and also that no leakage exists anywhere especially in the cirseam and terminal joints during the proof pressure test. The quality of the hardware is also graded based on AE data. In this paper, the basic concepts, terminology and common equipment setup are discussed. The experiment conducted to capture the Acoustic Emission pertaining to the different failure modes, elongations in bottom dome and leak in the terminals etc in real time and the parameters correlated with the defect formation and failures are described. Keywords: Acoustic Emission, Amplitude, Duration, Energy, Kaiser effect, Roll over, NiH 2 Cell. INTRODUCTION Nickel Hydrogen batteries are used in satellites as power source. They are lighter in weight and provide higher ampere hour (AH). During transfer orbit, eclipse period in-orbit and peak power needs, spacecraft entirely depends on batteries for its electrical power. Battery cell delivers the electrical energy by conversion of chemical energy from hydrogen gas and nickel hydroxide material. In the cell, hydrogen gas under pressurized condition is stored. The Nickel Hydrogen cell case is made of Inconel-718 encloses with a stack consisting of electrodes and other hardware components which function in the high-pressure environment. The electrodes are assembled with the shell by a terminal weld. The cell case with cylindrical shell and tori-spherical end domes is hermetically sealed with stack inside through cirseam TIG welding. Inconel 718 (Nickel alloy) cell case is chosen based on the reasons such as strengthto-weight ratio, alkaline stability and resistance to fatigue crack growth etc. During charging, the pressure inside the cell may go up to 65Ksc and during discharge the pressure reduces to around 35 Ksc. Thus during the life cycle, the cell is subjected to a number of pressure cycles due to the charge and discharge processes. Also the pressure of hydrogen also has to be maintained without any leak. For reliable operation of the cells, it is essential that the cell cases are free from any defects and it should be leak proof. To ensure this they are subjected to proof and qualification pressure tests before acceptance. In order to assess the structural integrity of the assembled Nickel-Hydrogen cells, AE monitoring was carried out during the acceptance pressure tests using Helium gas, to verify whether the hardware is free from any active defects especially in the TIG welded joints. Acoustic Emission (AE) technique is a proven technique for the structural integrity evaluation of pressurized systems. The technique has been successfully employed for clearing and grading structural integrity of the Nickel Hydrogen assembled cells.

2 376 Purushothaman et.al : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation ACOUSTIC EMISSION TECHNIQUE The phenomenon of acoustic emission is the spontaneous release of transient elastic energy due to deformation, fracture or both. In metals the sources of AE are dislocation motionplastic-deformation, fracture, micro crack initiation, crack growth etc. The defects in the welds, which were very minute, may go undetected during their normal NDT. These microscopic defects may eventually grow during their proof pressure test. Acoustic Emission Technique (AET) is a non-destructive testing method with capabilities for online monitoring the structural integrity of hardwares, detecting leaks and incipient failures in structural components. It is a highly sensitive technique, which detects even active microscopic events in a material. AET which is an active technique differs from other NDT methods in the fact that the energy detected is released from within the test object rather than being supplied externally (like UT) and is capable of detecting the dynamic processes associated with the degradation of structural integrity. The advantage of AE is that, a whole structure can be monitored with a few sensors. Studies carried out on NiH 2 pressure vessels using Acoustic emission testing demonstrated the capability of the technique for identification of minute leaks, localized yielding, defects in TIG welds during acceptance pressure test. INSTRUMENTATION The schematic representation of the instrumented hardware is shown in each Fig.1. AE monitoring was done by using the DISP AE Workstation supplied by M/s Physical Acoustic Corporation. The cells were instrumented with 4 AE sensors of 150 KHz resonant frequency. AE CH-1 is placed near to the Positive and negative terminals on the top dome, CH-2 on the cylindrical region near to cirseam weld, CH-3 in the bottom dome where elongation was observed in some of the earlier batch cell cases during acceptance pressure test. It has been ensured that Hsu - Nielson pencil break given anywhere on the hardware could be picked up by any of the AE sensors, which indicate the adequacy of the coverage and any active defect occurring anywhere on the hardware would be indicated on-line. Zonal location monitoring scheme was followed for the test. LOADING CYCLE The Cells have been subjected to three pneumatic proof pressurization cycles in the following sequence ksc (Maximum Expected Operating Pressure) ksc (Proof Pressure) ksc with a pressure hold of 3 min at proof and 1 min at all other steps. AE ACCEPTANCE CRITERIA FOLLOWED Roll over of AE hits should be observed during any pressure hold. There should not be successive high amplitude AE hits above 70dB peak amplitude level, during any pressure hold. Build up of average AE hit rate during successive pressure holds indicates degradations resulting from crack growth in the weld, lack of fusion in the weld, and micro level yielding. If any abnormality in AE (like cluster of signals, intermittent high amplitude spikes etc) is observed during the first cycle the cell is to be repeat pressure tested - up to the previous max pressure to rule out the presence of active defects. AE GRADING / ASSESSMENT Based on AE performance during the pressure test, the cells were classified in to four categories as shown in Table 1. Table 1: Grading of cells Cell cases Grading Observations Type A Very good Cells which exhibit very less AE signatures and has not violated any AE criteria during the acceptance pressure test. Type B Good Cells which exibited relatively more AE signatures during pressurisation phases. But during repeat pressure cycle no significant AE is observed. Fig. 1 : AE instrumentation of cell case Type C Bottom dome Cells with more number of AE elongation signals during MEOP/PPT from (inferior than bottom dome region during other Very good /Proof pressure cycle. Good cells) Otherwiswe normal behavior is observed. Type D Not acceptable Cells which register more high (Cirseam weld amplitude AE signals from cir failure / seam weld region/ Top dome Terminal leaks) terminal regions during pressure hold periods during I st and repeat pressure cycles. Indication of active defect.

3 NDE 2011, December 8-10, AE performance of cell type A (very good cell case) AE performance of a Good cell case during acceptance pressure test is shown in Fig: 2. During the first MEOP cycle, the cell registered a few low amplitude AE hits during pressurization phase. Clear Roll Over phenomenon was observed during all pressure holds including MEOP hold. During the proof pressure clear Kaiser effect was observed up to MEOP. Beyond MEOP few AE hits were registered during pressurisation phase up to 96ksc. All the pressure holds including proof pressure hold was very clear. During repeat MEOP cycle few low amplitude low duration AE hits were observed up to 40ksc which is due initial noise in the assembled cell due to pneumatic gas. The cell did not register any AE hits beyond 40ksc including MEOP hold. Based on AE data the integrity of the cell is found to be very good. Fig. 2 : AE performance of the cell type A (Very good cell case) Fig. 3 : AE performance of the cell type B (Good cell case)

4 378 Purushothaman et.al : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation AE performance of cell type B (good cell case) The AE performance of a good cell case during acceptance pressure test is shown Fig: 3. During the first MEOP cycle, the cell registered intermittent high amplitude AE hits. Most of them are from terminal region during pressurization phase. Roll Over phenomenon was observed during all pressure holds including MEOP hold. During proof pressure cycle, few low amplitude-low duration AE hits were observed from the terminal region and the cell was subjected to repeat PPT instead of repeat MEOP. During repeat proof pressure cycle, low amplitude-low duration AE hits were observed which are mostly from terminal region. Repeat proof pressure hold was very clear. Based on AE data the structural integrity of the cell is assessed as good. AE performance of cell type C (Bottom dome elongation) The AE performance graph of an elongated cell is shown in Fig.4. During MEOP cycle the cell registered few AE hits during pressurization phase. Rollover was observed, All pressure hold including MEOP hold were clear. During PPT cycle, AE behavior of the cell was found normal up to MEOP. From 70ksc onwards the cell has registered cluster of low amplitude AE hits without observing Rollover phenomena. Most of the AE hits were registered from CH-3 deployed on the bottom dome. This indicates the occurrence of micro level yielding, leading to geometrical corrections and minor elongation in the bottom dome. During repeat proof pressure test also few low amplitude signals were obtained mostly from bottom dome region indicating further yield / geometrical corrections taking place. AE PERFORMANCE OF CELL TYPE D a. Cirseam weld failure The AE performance graph of cirseam weld failed cell data is shown in Fig.5. During MEOP cycle, the cell case has registered cluster of low amplitude AE signals with high duration corresponding to leak at lower pressure onwards from cir-seam weld region and the test was aborted. Leak through the cirseam weld region was confirmed by snoop solution test. b. Terminal leaks The AE performance of a cell with terminal weld failure, during acceptance pressure test is shown Fig: 6. During MEOP cycle, from lower pressurization phase onwards, the cell has registered more number of high amplitude AE hits. The AE hit rate and peak amplitudes during pressurisation phases upto 20 Ksc were high compared to cell A indicating some degradation in the hardware. During further pressurisation the hit rate and Event durations further increased and became saturated. Energy and peak amplitudes became significant. Roll over was not observed during 30 Ksc pressure hold. This could be due to the initiation of terminal defect and initiation of leak near the negative terminal region. A warning Fig. 4 : AE performance of the cell type C (Bottom dome elongation)

5 NDE 2011, December 8-10, Fig. 5 : AE performance of the cell type D (Cirseam weld failure) Fig: 6 : AE Performance of the cell type D ( Terminal leaks) was given at this stage about the possibility of leak in the hardware. Since the leak was very minor and not enough to effect the pressure gauge reading, the cell was further pressurized to 40 Ksc. The AE parameters became more significant during pressurisation and pressure hold at 40 Ksc. A small pressure drop was also noticed in the pressure gauge at this stage confirming leak. The cell was then depressurized to 10 Ksc and inspected with soap solution. A very minor leak in one of the two terminal circular weld caps was identified which confirmed the AE findings. CONCLUSION The potential of acoustic emission technique in real time monitoring and structural integrity evaluation of Inconel 718 Ni H2 cell cases have been demonstrated. Acoustic emission capability to detect the different modes of degradation like yielding, defect growth, leak etc and identification of the location is also illustrated. Based on the AE behavior cells could be graded also.

6 380 Purushothaman et.al : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation ACKNOWLEDGEMENTS The support provided by Shri.Praveen PS, EXMD, Shri.Shripati Ingale, EXMD/ VSSC and Smt. Sujatha S, NFED/ VSSC are highly appreciated. REFERENCES 1. R. V. Williams. Acoustic Emission. Adam Hilger Ltd., Bristol, UK, 1980; R. S. Williams, et al. Final Report Acoustic Emission Monitoring of TIG Welding. Babcock & Wilcox, Lynchburg, Virginia, July 1978; 20: Ronnie K. Miller. Acoustic Emission Testing (Non Destructive Testing Handbook, Volume 5). American Society for Nondestructive Testing, 1987; Acoustic Emission ASTM Special Technical Publication 505 A Symposium presented at the December committee week ASTM, Bal Harbor, Florida, 7 8 December 1971; C. P. Kotwal, A. Sharma, J. Paul, M.I. Junaidh, S. Tripathi, A.S. Sankaranarayanan, B. Pant and K.K. Purushothaman, Application of Acoustic Emission Technique for Material Characterization of Ti6Al4V Alloy, Proc. National Seminar on Non-Destructive Evaluation Dec. 7-9, 2006, Hyderabad. 6. Sharma.A, M.I. Junaidh, K.K. Purushothaman, C.P. Kotwal, J. Paul, Shalini Tripathi1, B. Pant1 and A.S. Sankaranarayanan, Online Monitoring of Electron Beam Welding of TI6AL4V Alloy Through Acoustic Emission, Proc. National Seminar on Non-Destructive Evaluation Dec. 7-9, 2006, Hyderabad.