Study On The Fracture Mechanics Behavior Of Solid Rocket Motor Propellant Containing Defects

Solid rocket propellant, which is made up of soft, ductile matrix material reinforced by randomly scattered, stiff particles, is widely used in rockets, tactical and strategic missiles because of their light weight and stability. In general, solid rocket engines bear variable loads and temperature during production, storage, transportation and launch. Because of excessive load and stretching deformation under these loading conditions, it will generate crack and debonding in solid rocket grains. These defects will soften the mechanical behavior of the solid rocket propellant, the newly added combustion front caused by crack may be enough to over-pressurize and cause uneven burning of the propellant. Moreover, the formation of the crack can change the thrust characteristics of the engine significantly and decrease the range of the missile. In extreme cases, it may cause failure in the motor casing and cause an explosion and other catastrophic incidents. Thus, the crack growth laws and fracture toughness of the solid rocket propellants containing defects are important parts of the research on engine integrity and security.In this paper, for HTPB solid rocket propellant containing defects, the viscoelastic fracture mechanics and research status at home and abroad were combined with to study the fracture mechanics theory and test methods for the solid rocket propellant. The feasibility of the design of test device was tested by numerical calculation using ANSYS software. To study the fracture mechanics behavior of the HTPB solid rocket propellant tension specimens containing the unilateral crack and debonding specimens containing bilateral crack, the fracture performance tests were conducted by adopting the multi-sample method and J-testing method as shown in the followings:1. The HTPB solid rocket propellant fracture specimens containing defects, corresponding fixtures and the test device were designed and machined. A test program was established.2. The numerical methods to simulate the fracture behavior of viscoelastic material ware studied. The full-size finite element model of the actual test device was built by ANSYS software to inspect whether the experimental device can produce a homogeneous stress field on tensile specimen. The stress-strain relationship of viscoelastic material under a stretching rate of3mm/min was simulated, which was consistent with the results obtained by test.3. The strain rate conditions of motor grain during the launch process were calculated by LS-DYNA to provide the basis of loading rate range for the solid rocket propellant fracture test.4. For the HTPB solid rocket propellant tension specimens containing the unilateral crack, the multi-sample method and J-testing method were applied to conduct the fracture test. The crack initiation points of single edge notched tension (SENT) specimens were determined by using the high-speed microscope camera tracking method. The test showed that the passivation phenomenon of crack-tip was obvious, the passivation and initiation process of crack could be taken clearly by the high-speed microscope camera tracking method, which made it easy to determine the crack initiation point of crack propagation relatively accurately. The experimental results showed a large discreteness of the parameters such as the load value and the fracture energy at each crack initiation points and the positions of initiation points of the solid rocket propellent material. The least squares method could be used to deal with the discreteness of the fracture properties of the solid rocket propellant material.5. For the HTPB solid rocket propellant crack propagation specimens containing the unilateral crack, the multi-sample method and J-testing method were applied to conduct the crack propagation test. Using the high-speed microscope camera tracking method, the crack propagation process was photographed real-timely, and the crack growth amount (△a) was determined. The the least-squares method and the binomial fitting method were used to obtain the J resistance curve as well as the crack propagation velocity curve and blunting curve. The experimental results showed that the phenomenon of crack propagation was obvious.6. For the HTPB solid rocket propellant debonding specimens containing bilateral crack, the multi-sample method and J-testing method were applied to conduct the debonding test. The prefabricated method of interfacial crack was studied. According to the load-displacement experimental curves obtained by the test machine and the interface crack propagation phenomenon observed by the high-speed microscope camera tracking method, the crack initiation points were determined and the debonding critical J-integral value was calculated.