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Advances in Structural Integrity Evaluation and Residual Strenght Prediction of Pressure Vessels using Acoustic Emission Technique T. Chelladurai, A.S. Sankaranarayanan and K.K. Purushothaman Vikram Sarabhai Space Centre, Trivandrum - 695 022, INDIA Contact

ABSTRACT

Ever since the first application of Acoustic Emission (AE) technique to a rocket motor case almost four decades ago, acoustic emission technology has made rapid strides in the integrity evaluation of pressure vessels made of metallic as well as composites. With the technological revolution that is taking place in electronics and software, the day is not far off when acoustic emission can predict precisely the residual life of a pressure vessel. The authors experimented with a number of Ti-lined Kevlar-epoxy composite pressure vessels slated for aerospace application with optimal number of AE channels. On the basis of the AE data acquired during the auto-frettaging pressure cycle and the subsequent proof pressure cycle, it is found possible to estimate the residual life of the hardware with its attendant degradation represented by the 'felicity ratio' etc. Moreover, the emerging philosophy for the prediction of the left over life of a hardware at a test pressure which is much lower than the maximum expected operating pressure level is outlined in the paper. Besides, the possibility of residual life estimation for Ti-6Al-4V alloy pressure bottles of low margin is also discussed in the paper with reference to our in-house AE studies carried out on a number of such aerospace hardware. At the Vikram Sarabhai Space Centre, Trivandrum, the AE technique has also demonstrated to be an efficient tool for detecting and characterising the severity of defects like lack of fusion, which was, missed by conventional NDT techniques. By and large, the AE technique has made remarkable advances in the recent times to qualify itself as a unique mandatory tool for the acceptance of pressurised systems in whatever application calling for stringent quality requirements.

1.0 INTRODUCTION

Acoustic Emission (AE) technique is a fast maturing NDT tool ideally suited for the structural integrity evaluation of pressurised systems during a proof pressure test or in actual service, as the case may be. The unique capability of the technique, therefore, lies in its ability for the real time monitoring of active defects. Whether a material is a good or a bad emitter of AE is governed by the microstructure as well as the presence of inclusions and second phase particles in the case of metallic. And in the case of composites, the resin-fibre system form prolific emitters of AE. It has been our experience in Vikram Sarabhai Space Centre that Ti alloy pressure bottles have demonstrated to be moderate emitters of AE as compared to Fibre Reinforced Plastic hardware (FRP). The AE response from titanium alloy is expected to be from active micro cracks or from local dents that get corrected up possibly by micro yielding so long as the peak amplitude levels are nominal and the hit rates show a decline during 'pressure holds'. By and large, the micro cracks have a tendency to get blunt with increasing pressure. When any of the micro cracks go into intermittent or sustained growth stage, AE would provide intermittent or continuous emissions and the severity is indicated in terms of certain governing parameters such as peak amplitude, event duration, hit rate and so on in real time. Systematic specimen level studies have been carried out on similar Ti alloy material by several investigators in the past. Moreover, there have been significant advances in the area of AE instrumentation in the recent past where by better sensitivity, fast processing rate and real time assessment are achievable.

In the present study, the authors share with the NDT community, their experience derived from the AE response of :

1. Ti lined kevlar-epoxy pressure bottles designed and fabricated for aerospace application and
2. Ti alloy pressure bottles fabricated using EB weld process for aerospace application.

These hardware have been AE monitored during their hydrostatic pressure test in order to assess their structural integrity for their intended application. The residual life prediction potential of the AE technique in respect of this type of hardware is also discussed. In the case of the latter an attempt is made to verify whether the degradation is indicated by strain gauges bonded on the hardware. A correlation of AE with respect to radiographic assessment of defects is also included.

2.0 HARDWARE DETAILS:

The schematic of the Ti lined FRP pressure bottles studied in the paper is given in Fig. 1.
These bottles find their application in the propulsion systems of spacecraft and upper stages of launch vehicles. The combination of composites having higher specific strength and metal liner with its impermeability would yield tankages of higher specific strength. The pressurant tank under investigation employs annealed Titanium alloy - Ti 6 Al 4V - for the liner and Kevlar 49-epoxy composite for the overwrap. The design of the liner is such that it will be under compressive stress, less than its critical buckling stress, when the vessel is pressurised to proof pressure level and brought back to zero pressure. The first such pressurisation cycle is known as auto frettaging. The liner henceforth operates in an offset elastic strain range when pressurised within proof pressure range.

Fig 1: AE INSTRUMENTATION FOR TITANIUM LINED FRP BOTTLE

Fig 2: AE INSTRUMENTATION FOR TITANIUM PRESSURE BOTTLE

The Ti alloy pressure bottles (Fig. 2) studied in the paper have been realised from Ti alloy (Ti 6Al 4V) plates that were hot formed into hemispheres, machined and then Electron Beam welded. The bottles are in the annealed condition and carry optimum burst strength factor befitting aerospace requirement.

3.0 AE INSTRUMENTATION & LOADING SEQUENCE

The bottles - both made of Ti lined kevlar-epoxy and Titanium alloy - were instrumented with optimal no. of AE sensors (150 kHz resonant type) as shown in Figs. 1 and 2 respectively following zonal location philosophy giving cognizance to the requirement of fast processing rate and also circumventing the problem of data overflow. Appropriate preamplifier with 40 dB amplification and appropriate filter in the range 100 kHz to 1 MHz were used in the instrumentation chain. The state-of-the-art AE system MISTRAS-2001 was used for the AE data acquisition and processing. It was ensured that Hsu-Nielsen pentel pencil break anywhere on the bottle hardware was picked up satisfactorily by the AE sensors. While the Ti lined kevlar-epoxy pressure bottles were additionally subjected to an initial auto frettaging cycle upto 505 ksc with no appreciable pressure hold except that required for strain recording, all the pressure bottles - whether Ti lined FRP or Titanium alloy - were subjected to a proof pressure cycle wherein the bottles underwent pressurisation in steps of 50 ksc to the proof pressure of 495 ksc with a 'pressure hold' of maximum 1 min. at each of the steps barring the 'pressure holds' at 330 ksc and 495 ksc, where the'pressure hold' duration was for 15 minutes. However, wherever required, a repeat pressure cycle was resorted to upto 330 ksc, which is the Maximum Expected Operating Pressure Level (MEOP), so as to observe 'Kaiser Effect'.

4.0 AE PERFORMANCE AND INFERENCES

The AE data of the hardware studied in either case of the bottles brought out the following results and inferences.

a) Ti Lined Composite Pressurant Tanks:


• The auto frettaging pressure cycle upto 505 ksc performed on these bottles indicated resin crazing upto a pressure of 300 ksc. During this phase there has been substantial increase in hit rate along with encountering a large no.of high amplitude emissions most often exceeding 80 dB and long duration events exceeding 3 milli sec duration. Moreover, the resin-crazing phase very much reveals the inadequacies in the fabrication. The presence of spikes at various stages of resin crazing phase indicates the undue degradation taking place in the hardware leading to premature failure.
• The hardware which exhibited emissions with less than 80dB peak amplitude during the first pressure cycle demonstrated a burst pressure of about 1.6 times the maximum pressure reached during that cycle.
• The repeat pressure cycle upto proof pressure level normally exhibits very few emissions. The peak amplitude level rarely crosses 66 dB in the case of good hardwares. Good hardwares also exhibited a felicity ratio of more than 0.9.Moreover,the hardwares which indicated a felicity ratio of 0.9 in conjunction with absence of events above 66 dB peak amplitude level confirmed a burst pressure of about 1.4 times the maximum reached pressure during the previous cycle. It is also observed that the lesser the level of peak amplitude or grater the felicity ratio the hardware demonstrates a higher burst pressure.

From the above it is clear that the peak amplitude level during the first or subsequent pressure cycles and the felicity ratio during the repeat proof pressure cycles could provide a reliable means for estimating the life of the hardware.

b) Ti. Alloy Pressure Bottles :


• The local yielding that takes place on EB weldment is discernible from the very level of emission rate as well as the peak amplitude level. The massive local yielding taking place at lack of fusion regions is indicated by a quantum jump in the emission rate and is also characterized predominantly by the amplitude level and energy level which are low. However, there was a marked increase in the event duration level - majority of the emissions touched 1700 microsec duration.
• The critically active flaws relating to lack of fusion gave out alarm signals at as early as 30% of burst pressure!
• At the same time, it was interesting to note that strain gages mounted in the suspected critical regions could not provide any noticeable failure indications.
• Bottles which were branded defective due to lack of fusion by AE, in fact,had earlier been given a clean chit by radiography. A repeat radiograph could reveal the unfused zone even as much as 300 mm long and 2 to3 mm wide.

5. Conclusion/Recommendations :

• The AE studies performed on Ti. Line composite pressure bottles confirm the potential of AE technique for predicting the residual life of the hardware based on the AE performance registered during the auto frettaging cycle.
• Unlike the rubber lined composite bottles, the Ti. lined composite bottles are complex and a repeat cycle do not authentically represent the degradation level undergone by the composite at 50% MEOP or so simply because the load sharing by the liner and composite are not clearly known.
• The felicity ratio indicated during the repeat/ subsequent cycle, however, plays a key role in representing the level of degradation undergone by the hardware.
• The AE studies and investigation carried out on Ti alloy bottles confirm the potential of AE technique for the real time detection of active flaws.
• In the case of Ti bottles, the severity of the 'lack of fusion' induced flaws have been effectively cautioned by AE.
• A good correlation is found to exist between radiography and AE technique as to the presence of flaws. While Radiography is capable of providing the size and type of flaws, AE could indicate the severity of flaws.
• At the same time, it became difficult to establish any direct correlation between AE and strain values measured by strain gauges at critical regions of the hardware.
• It may be noted that premature degradation due to a flaw growth is quite different from the degradation exhibited due to lower thickness/reduced mechanical property. While the former is expected to provide adequate advance warning, the latter cannot be provide the same.
• A repeat cycle would bring out the real picture of the hardware in terms of active flaws or otherwise.

Acknowledgement:

Authors remain grateful to The Director, VSSC for kind permission to submit this paper to the WCNDT. Authors are indebted to Dr. A.R.Acharya, Deputy Director, VSSC and to Dr. G.V.Rao, Group Director, Structures, VSSC for their constant encouragement to develop this technique. Sincere thanks are due to Dr A.V.S.S.S.R Sharma for his valuable suggestions . Authors also remain thankful to Mr. V.A.Chacko and Mr.Alexander Mathew for their support in bringing out this paper.

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