Acoustic emission monitoring during proof pressure testing of a solid rocket motors

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1 More info about this article: Acoustic emission monitoring during proof pressure testing of a solid rocket motors Introduction: Nilesh Mane, Sandeep Jain, K R Mohanananthanarayanan, A K Mishra Vikram Sarabhai Space Centre, Trivandrum In Indian Space Research Organization(ISRO) solid rocket motors have been used in PSLV and GSLV MkIIand GSLV MkIII. Solid rocket motors are used as a core stage &as booster stage. High strength high alloy (HSHA) steel which is having excellent UTS and fracture toughness is being used in realizing these solid rocket motors using Tungsten Inert Gas welding. The material is very sensitive and crack prone & design is based on fracture toughness and hence crack detection & elimination is very important. Hence meticulous NDT is being done after welding of the segment to eliminate any kind of defects such as lack of fusion, cracks, undercut, low thickness, which may lead to failure of the segment well below the specified UTS. After machining, the hardware has to be qualified for flight by conducting proof pressure test (PPT).During PPT, subcritical cracks in weld seam may grow & may become critical which will ultimately lead to failure of the hardware which needs to be avoided by identifying dynamic crack growth during different stages of PPT. Acoustic Emission (AE) is the technique employed for this purpose. On line acoustic emission monitoring is used to identify the presence of not only any active defects but also dynamic crack growth, its location and to a limited extent, the severity. AE monitoring has been carried out on HSHA steel, during PPT (with oil medium) in order to confirm that hardware is sound enough for its intended application by the confirming following: 1. Ascertain whether the hardware is free from any growing defects especially in the weld areas like circumferential and longitudinal welds, T joints and repaired areas if any. 2. Whether any undue degradation occurs in the hardware during the PPT. 3. Attempt to identify theoretically active zones if any in the hardware, on the basis of linear location analysis. 311 Non-Destructive Evaluation 2016

2 Why acoustic emissiontechnique? Since conventional nondestructive testing methodssuch as Radiography Testing, Ultrasonic Testing, and PenetrantTesting etc.are not capable of detecting active defects hence AE is being employed. Acoustic emission is nothing but an elastic wave generated by local material change/micro-fracture (called as AE burst) due to stress. Acoustic emission being dynamic method used to identify growing defects if any and has following advantages: 1. Large areas can be covered by only few sensors 2. Defects which are growing during the test can be detected 3. Location of the defects can be identified easily 4. Inaccessible areas can be inspected e.g. insulated equipment can be inspected with only minor disturbance of the insulation Disadvantages: 1. Not all defects emit detectable stress waves 2. Confidence must be gained regarding type of loading necessary to promote detectable crack growth 3. Geometry, vessel contents, material properties and background noise can obscure signal detection 4. Data interpretation is not standardized for metals 5. Size of defect cannot be determined Acoustic emission in solid rocket motors PPT: Vikram Sarabhai Space Center which is realizing solid rocket motorsof 2.8 and 3.2 meter diameter which undergoes proof pressure test using oil medium before being used in flight. After welding and machining of the segments, firstly, the designer will analyze and identify critical locations which need to be monitored in addition to longitudinal welds such as repairs, low thickness, high mismatch etc.which are critical by design point of view and need to be monitoredduring testing. The pressurization sequence for solid rocket motors of 2.8 and 3.2 dia. hardware is as following: 312 Non-Destructive Evaluation 2016

3 The hardwaresubjected to internal pressure using oil medium. The normal sequence is: ksc- leak cycle, with a pressure hold of 2 min max.at 10 ksc. 10 ksc to 20 ksc, with 1 minute hold at 20 ksc. Back to 10 ksc for strain gauge repeatability check with min 1 minute hold at 10 ksc ksc (proof pressure), with 1 minute hold at every step (max. of 2 minute hold allowed below proof pressure) and 2minute(for 2.8 m dia.) and2& half minute(for 3.2 m dia.)hold at proof pressure. Depressurization within 5 seconds below threshold pressureand back to 10 ksc. And 1 min hold at 10 ksc. Back to near zero. AE sensors will be mounted on hardware welds using magnetic clamps. Its output will be connected to preamplifiers and subsequently to acoustic emission system. Refer fig.1 for typical arrangement. 313 Non-Destructive Evaluation 2016

4 Fig 1 typical testing assembly Fig.2 Typical sensor mounting on weld/repair location 1. Sensors-R15D type(150khz resonant) dB range 2. Preamplifiers -20/40/60 gains with single and differential inputs and power cable 3. DispLoc AE system with AE win software Refer the fig.1 for typical test arrangements. Firstly, paint will be removed from thelocation;the surface of the hardware will cleaned with acetone. The sensors will mounted on the hardware using magnetic clamps. High vacuum silicon grease will be used for bonding and to eliminate any air entrapment between sensor and hardware surface otherwise it may give erroneous results. After connecting sensors, preamplifiers and AE station in line; in AEwinsoftware, a program file will be made comprising various groups having intended channels, filter setting for parameters such counts, energy, rise time,peak Definition Time(PDT), Hit Definition Time(HDT), Hit Lockout Time(HLT)and max duration values,pressure transmitter inputs. Refer followingscreenshots. 314 Non-Destructive Evaluation 2016

5 Also, to monitor AE parameters such as amplitude, duration following graph will be set for each of group. In addition, each individual channel will monitored separately for its proper functioning and high amplitude emission in case signals are coming outside the group. For HSHA steel,after acquiring AE data on numerous tensile test specimens tests in lab using various sensors, preamplifiers; following parameters and criteria has been finalized for real time analysis during PPT of solid rocket hardware: 315 Non-Destructive Evaluation 2016

6 For each of the active source during pressure hold period, High amplitude emissions above 70 db at source shall not be more than 3 during one minute of hold period at source. Long duration emissions above 8 milliseconds shall not be more than 3 during one minute of hold period at source. Event rate shall not be more than one per second per sensor. Event roll-off shall be observed. Before the start of the test, each sensor will be checked for sensitivity. Generally, a sensors giving response of 99 ±1dB is acceptable. Any sensors not giving proper response will be either re-bonded OR replaced by another good one. Typical waveform observed while sensitivity check at AE sensors using Hsu-Neilson pencil breakage source with important parameters such amplitude, duration, initiation frequency and rise time etc. After calibration of all the sensors and confirmation of strain, hydraulic systems, safety etc. test will be started in sequence as mentioned earlier. 316 Non-Destructive Evaluation 2016

7 Case study 1- Use of AE to monitor location of high strains in solid rocket motors during PPT:- During PPT, AE can be helpful in decision making in cases where strain values observed very high against the predicted. By observing the AE acceptance criteria for the material, testing can be continued even strain values found beyond acceptable limit. In one of the 3.2 dia. Solid rocket motor PPT, Up to 50 ksc hold there was no issues observed in AE as well as strain gauging. However, at 60 KSC, 9096 micro strains were observed against prediction of 5638 at repair location carried out after static test. Hence the pressure was brought down to 10 KSC. During second pressurization cycle from 10 KSC to 66 KSC, up to 40 KSC Kaiser Effect was observed and at 50 and 60 KSC, some low amplitude events were observed, however performance of hardware was found normal &Violation of AE criteria was not observed. 317 Non-Destructive Evaluation 2016

8 At 66 ksc hold,some events up to 90 db were seen at one channel at repair location at the same time strain value crossed approx. more than 11000micro strain. Meticulous UT with higher gain was carried out; however no defect was observed in the joint. Above data shows how AE helped to monitor health of the hardware in real time and assisted in decision making for critical areas. 318 Non-Destructive Evaluation 2016

9 Case study 2 Leak detection during testing of solid rockets motors AE can be used to detect leaks from cracks or joints at early stages which helps to avoid catastrophic failures by aborting test. 319 Non-Destructive Evaluation 2016

10 AE signaturecaptured from Oil drops to find leak during test 320 Non-Destructive Evaluation 2016

11 Fig.3 typical AE waveform for oil drops Fig. various AE parameters for oil drop test 321 Non-Destructive Evaluation 2016

12 The data shows that for minute oil leak, the signals are having very low energy and amplitude. In one of the PPT, leak was observed at 30 ksc. The oil from flange joint started falling on dome just below it where the AE sensor was mounted on the hardware weld. Duringany repeatability cycle, Kaiser Effect(absence of AE signals till previous load cycle) must be observed to affirm absence of growing defects. However in this case felicity effect (occurrence of AE below the previous load) was observed. Also, some of the sensors were stopped working. Hence, the test was aborted and oil leakage was observed at flange joint. Refer below pics. 322 Non-Destructive Evaluation 2016

13 323 Non-Destructive Evaluation 2016

14 Fig. 4 channel data shows continuous low energy AE bursts due to oil leakage Summary: 1. During the manufacturing of motor cases, it has to undergo lot of processes periodically which induce defects/irregularities which may not be possible to get detected due to limitation in other NDT techniques. However, after complete manufacturing, during pressure test of motor cases, such defects may grow and AE can be used to identify such growing defects. 2. During testing, AE can be used to avoid catastrophic failures arising due to growing of dynamic defects to alarming levels 3. Thus AE can be used as a qualification tool to provide assurance such that hardware is sound enough for its intended application 324 Non-Destructive Evaluation 2016