Mechanical Failure Prediction of Composite Rocket Cases

Transcription

1 APPROVED FOR PUBLIC RELEASE Mechanical Failure Prediction of Composite Rocket Cases DrRoger Li Weapons Propulsion, WCSD, DST 1

2 Contents Introduction Why composite rocket cases? Why and how to predict failure Composite coupon tests and FEA models Results and discussions Burst by hoop stress Shear out between case and aft boss Fibre tow splitting by in-plane shears Conclusions 2

3 Key is smaller openings Metal polar bosses heavier than the composite case Joints are made of metals Required join strength is proportional to opening diameter. 3D woven composite can integrate case, nozzle throat and extension Judged by the white Teflon tow guiders, they are more likely weaving towpregs. Towpreg winding produces high and constant fibre volume fraction, is a better technology than conventional dry fibre/resin bath winding. 3 From news.cctv.com on 5 th June 2017

4 Mechanical failures of composite rocket cases Leakage at dome areas Tow splitting caused by in-plane shear failure of resin. Leakage at the case/boss joints Due to high out-plane shear stress Case bursts Slow burst following the leakages above or fast burst at cylinder. 4

5 Solution to leakage at dome areas Quasi-isotropic layup is the answer At least quadriaxial fibre orientation angles, eg. 0/90/-45/+45 High in-plane shear modulus and strength. Hart-Smith s "Ten-Percent Rule" 45 or 90 ply has only 10% of the axial stiffness and strength of a 0 ply. 0 or 90 ply has only 10% of the shear stiffness and strength of a 45 ply. Importance of a third winding angle for composite cases In-plane shear strength of a cross-plied laminate depends greatly on whether there are fibre tows aligning with maximum shear direction (normally ±45 ). In-plane shear strength of a fibre wound bottle depends greatly on whether there is helical fibre tows (normally ±35 ) aligning between hoop tow (normally ±89 ) and polar tows (normally ±10 ). 5

6 How to capture composite properties? ASTM D2290 split disk ring tensile test 1-inwideringscut from filament wound 5 ¾ inchdiametercomposite cylinder Different from flat coupon, out-plane shear in particular. Similar fibre orientation angles Orientation angles Hoop tow Helical tow Polar tow D2290 ring ±89 ±37 ±9 10-in case ±88 ±35 (50%±25 and 50%±45 ) ±18 6

7 ASTM D2290 test results Having subtracted the system compliance from the loading displacement Fibre type Maximum tensile Tensile extension at maximum stress/mpa load/mm T

8 ASTM D2290 simulation MPa 8

9 Composite properties derived from simulation In LS/DYNA, the test results can be reproduced by using Types 022 Composite_Damageor 040 Nonlinear_Orthotropicmaterial model formats. 9

10 10 Composite rocket case FEA model Unrealistic 3mm thickness without tow build-up at domes 10

11 Burst predicted at 17.5 MPa pressure Failed by hoop stress at cylinder with 1 st principal stress of MPa MPa 11

12 Failure could be much earlier at openings If there were no tow build-up and alignment around the openings MPa 12

13 APPROVED FOR PUBLIC RELEASE, 13th PARARI, Nov 2017 Significant case deformations ~7mm longitudinally ~2mm radically 13

14 Significant shear-out between case and bosses A highly elastic shear ply is required to seal the gap MPa 14

15 Modest in-plane shear stress only half of normal 10 ksi(69 MPa) shear strength of resin, very likely smoothen down by the quasi-isotropic layup. MPa 15

16 Conclusions APPROVED FOR PUBLIC RELEASE, 13 th PARARI, Nov 2017 LS/DYNA predicts the sample 10-indiametercomposite case would fail at a burst pressure of 17.5 MPa by high hoop stress in the cylinder area. A highly elastic shear ply between the composite case and aft boss is required, or there will be a shear-out failure before the predicted 17.5 MPa pressure. The maximum in-plane shear stress is only half of normal 10 ksi(69 MPa) shear strength of matrix resin, demonstrating the advantage of a quasi-isotropic layup for preventing the tow splitting shear failure of composite in the dome areas 16

17 Acknowledgments The author sincerely appreciates DST Weapons Propulsion Group staff, in particular Dr Ian Johnston for various professional advice, and Dr Greg Yandek of US Air Force Research Lab for S&T advices in composite rocket case design and manufacture. 17