Burst pressure estimation of reworked nozzle weld on spherical domes

Transcription

1 Indian Journal of Engineering & Materials Science Vol. 21, February 2014, pp Burst pressure estimation of reworked nozzle weld on spherical domes G Jegan Lal a, Jayesh P a & K Thyagarajan b a Cryo Structures and Tanks Division, Liquid Propulsion Systems Centre, Indian Space Research Organisation, Thiruvananthapuram , India b Department of Mechanical Engineering, Pon Jesly College of Engineering, Nagercoli , India Received 19 December 2012; accepted 16 August 2013 Propellant tanks of rockets or launch vehicles have nozzles welded to their domes as interfaces for valves or pressurisation/vent lines. These nozzle welds are critical since more often the failure happens here when the tank is pressurised. The considerable difference between the yield and ultimate strength of weld as compared to the parent material is the reason for the low burst pressure of the nozzle welds. To eliminate unacceptable weld defects like porosities, cracks etc., the defective portions are rewelded and are called weld reworks. Residual stress present because of rework, further reduces the burst pressure. In this study, burst pressure estimation of a typical nozzle welded to the dome is analysed by finite element method. The effect of geometric non-linearity of the structure in the prediction of burst pressure is clearly brought out. Also, the burst pressure reduction when weld reworks are carried out in a portion of the weld length and reduction in burst pressure when this rework length increases is demonstrated using finite element analysis Keywords: Nozzle, Weld strength, Burst pressure The liquid stages of satellite launch vehicles have propellant tanks that store propellants under pressure. The tanks have spherical end domes that support nozzles with openings through which the tanks are pressurised or vented. Often, the burst pressure of the tanks is the pressure at which this nozzle weld fails. This is because of the considerable difference in the yield and ultimate strength between the weld and parent material and due to the effect of geometrical nonlinearity. The reduction in weld strength due to weld reworks and the consequent further reduction in property also influences the burst pressure. Finite element analyses to estimate the burst pressure of pressure vessels have been carried out by many researchers 1-4. The accuracy of the burst pressure estimation depends on the accuracy of determination of the strength of the material and weld and knowing the true stress-strain plots, and carrying out finite element analyses with geometric and material non-linearities with small load steps and fine mesh at the critical locations. Material stress-strain graphs are generated from actual testing of parent material and weld specimen. An elasto-plastic finite element analysis invoking geometric non-linearity is carried out and burst pressure of nozzle estimated. *Corresponding author ( jeganlal_isro@yahoo.com) Materials and Methods Material property For the launch vehicle liquid propellant tank, AA2219 an aluminium-copper alloy is a widely used material because of its high specific strength, high fracture toughness, ductility and weldability. The Young's modulus of the material at room temperature (300 K) is N/mm 2 and Poisson ration Six specimen of parent material in T87 condition (i.e. solution heat treated, cold worked and aged) and six welded specimen are tested. Tested stress-strain curve of the specimen are shown in Figs 1 and 2. Tensile test specimen is shown in Fig. 3. Chemical composition of the alloy is given in Table 1. The yield strength and ultimate strength values for the Fig. 1 Stress-strain curve for AA2219-T87 (parent material)

2 LAL et al.: BURST PRESSURE ESTIMATION OF REWORKED NOZZLE WELD ON SPHERICAL DOMES 89 Fig. 5 Axisymmteric FE model Fig. 2 Stress-strain curve for AA2219 weld Alloying element Fig. 3 Tensile test specimen Fig. 4 Configuration of nozzle Table 1 Chemical composition of AA2219 % composition Copper 6.3 Manganese 0.3 Titanium 0.06 Vanadium 0.1 Zirconium 0.18 Aluminium balance Table 2 Yield and ultimate strength values of AA2219 T87 at room temperature, N/mm 2 Material AA2219 T87 Yield strength Ultimate strength Parent metal Weld parent material and weld material at room temperature are given in Table 2. von Mises failure criterion When the material is subjected to multi directional stresses, the failure is determined by the von Mises stress which is given by σ von = 1 2 Fig. 6 FE model with 3D shell elements (( σ σ ) + ( σ σ ) + ( σ σ ) ) where σ 1, σ 2, σ 3 are principal stresses. The material yields when the σ von is equal to the yield strength of the material and fails when the σ von is equal to the ultimate strength of the material as obtained from the uniaxial tensile test of the material. The criteria is applicable to ductile materials especially metals and is validated for AA 2219 by experiments 2,5. Configuration Configuration of the portion of the tank end dome with nozzle is shown in Fig. 4, along with salient dimensions. The dome is 3.4 mm thick and is a sphere of radius 2265 mm. the nozzle weld diameter is 254 mm and it has an opening of 100 mm diameter. FE Model Two finite element models, an axisymmetric model and a half shell model, are made. Axisymmteric model is made to predict the burst pressure of a nominal weld without any defect and also to validate the 3-D shell model which is used to predict the burst pressure of welds with defects. Both the models are shown in Figs 5 and 6. Multi-linear stress-strain

3 90 INDIAN J ENG MATER SCI., FEBRUARY 2014 specimen are input at a small weld length and non-linear analysis carried out. Analysis repeated increasing the rework length to find its effect on burst pressure. Fig. 7 von Mises stress at weld versus pressure Fig. 8 von Mises stress distribution over the dome Fig. 9 Variation of plastic strain versus pressure curves as shown in Figs 1 and 2 are input for the analysis. Analysis Fixed boundary condition is given at the periphery of the dome. Symmetry boundary condition is imposed at the line of symmetry in the shell model. A linear elasto-plastic analysis and a non-linear (geometric) elasto-plastic analysis carried out with nominal weld properties using ANSYS (ver.11) finite element analysis software. Also, rework and resultant weld strength reduction observed due to residual stresses in actual test Results and Discussion Burst experiments are carried out by many researchers 1-4,7-9. Finite element analyses with geometric and material non-linearities are already carried out by the authors simulating the failures validating the finite element techniques. von Mises as the failure criteria for biaxial stress state for the material AA2219 is experimentally proved 2,5. Effect of geometrical non-linearity The von Mises stress at the weld is plotted with respect to pressure, both for linear and non-linear axi-symmetric analysis (Fig. 7). The burst pressure prediction based on linear analysis is 0.88 MPa, whereas it is 1.78 MPa from the non-linear analysis. This is because of the decrease in the radius of the dome with pressure resulting in increase in stiffness. This reduces further deflection, stresses and strains. The von Mises stress distribution in the entire dome is shown in Fig. 8. The equivalent plastic stain versus pressure is shown in Fig. 9. It indicates yielding at 0.4 MPa pressure and the equivalent plastic strain of 11.2% at the burst pressure. Effect of reworks at the weld Often weld reworks carried out to eliminate unacceptable weld defects like cracks lead to higher residual stresses and reduced strength at the reworked region. The reduction in weld strength for two reworks is 20 N/mm 2 found from present experimental results and matches with the values given in the letrature 6. If the weld rework is carried out on the entire weld length, then the strength of the weld is reduced by 20 MPa and the burst pressure reduces correspondingly. If weld rework is not carried out on the entire weld length and done only on a portion then what will be burst pressure is being attempted to answer by analytical means with experimental support. The percent reduction in burst pressure versus percent rework length for different nozzle weld diameters is plotted in Fig. 10. Burst pressure reduces with increase in weld rework length. But beyond 25% rework on weld

4 LAL et al.: BURST PRESSURE ESTIMATION OF REWORKED NOZZLE WELD ON SPHERICAL DOMES 91 Fig. 10 Variation of burst pressure versus rework length Fig. 12 Comparison of hoop strain FE prediction versus test Fig. 11 von Mises stress distribution with 6% weld rework length length further reduction in burst pressure is insignificant. The reworked region yields before the yielding of non reworked weld region due to lower yield strength and stresses redistributes if the percentage of reworked region is less. The von Mises stress distribution for 6% rework length given in Fig. 11 clearly shows the higher stress in the nearby no reworked weld due to stress redistribution. This stress redistribution to the nearby high stiffness and strength weld region gradually reduces with increase in percentage of weld rework since the total load that has to be resisted by the circular weld section is constant. Beyond 25% weld length rework the weld behaves as if the strength reduction of 20 MPa is almost for the entire length. Apart from weld reworks, for other defects which reduces the strength of the weld joint like porosities, local thickness reduction due to grinding and weld collapse also, the same logic and analysis is applicable leading to the inference that for this type of dome nozzle configuration, any reduction in weld strength will not cause proportional reduction in burst pressure for defect lengths less than 25% of total weld length. Fig. 13 Comparison of meridional strain FE prediction versus test Experimental validation To validate the FE analyses, strain gauging was done near the nozzle weld for the same nozzle configuration welded in a propellant tank. Pressure test to an internal pressure of 0.37 MPa (abs) carried out. The predicted strain from FE analysis and the test data were compared and good match observed validating the FE model and analysis procedure. The comparisons of strains in the hoop and meridional directions are shown in Figs 12 and 13. Conclusions The following conclusion can be drawn from this study: i. Finite element analysis with geometric and material non-linearity gives accurate burst pressure prediction. ii. For cir-seam nozzle welds on tank domes, weld defects like porosities, thickness reduction or weld rework over small lengths will not cause proportional reduction in burst pressure. iii. The optimum ratio of defective weld length to total weld length is 0.25, above which the burst pressure is proportional to the reduced weld strength. If the

5 92 INDIAN J ENG MATER SCI., FEBRUARY 2014 ratio is less than 0.25, then the burst pressure is higher than the strength reduction. References 1 Sang Z F, Xue L P, Widera G E O & Lin Y J, Trans ASME, J Pressure Vessel Technol, 127 (2005) Jones D P & Holliday J E, Trans ASME, J Pressure Vessel Technol, 122 (2000) Kisioglu Y, Brevick J R & Kinzel G L, J Pressure Vessel Technol, 127 (2005) Rajan K M, Deshpande P U & Narasimhan K, J Mater Process Technol, (2002) NASA CR-65412, "Biaxial strength characteristics of selected alloys in a cryogenic environment". 6 Welding and forming qualification for sizing of ARIANE-5 EPC tank bulk-heads, Proc. Sympos on ARIANE-5 Structures and Technologies, Majzoobi G H, Farrahi G H, Pipelzadeh M K & Akbari K, Int J Pressure Vessels Piping, 81 (2004) Kaptan A & Kisioglu Y, Int J Pressure Vessels Piping, 84 (2007) Wang H F, Xue L P, Widera G E O & Sang Z F, Trans ASME, J Pressure Vessel Technol, 131 (2009)