Chapter 2 Effect of Strain Rate and Interface Chemistry on Failure in Energetic Materials Chandra Prakash, I. Emre Gunduz, and Vikas Tomar Abstract We study the failure at interfaces between Hydroxyl-terminated polybutadiene (HTPB)-Ammonium Perchlorate (AP) based energetic material. In this work, interface mechanical strength of a set of HTPB-AP interfaces is characterized using nano-scale impact experiments at strain rates up to 100 s 1. A power law viscoplastic constitutive model was fitted to experimental stress-strain-strain rate data in order to obtain constitutive behavior of interfaces, particle, and matrix. A mechanical Raman spectroscopy is used to analyze the effect of binding agent at different temperature. A tensile fracture experiment combined with In-situ Mechanical Raman Spectroscopy was used to obtain fracture properties. Stress maps are obtained near the interface using In-situ Mechanical Raman Spectroscopy to analyze the changes in the stress distribution around interfaces for different loads till failure. Cohesive zone model parameters were obtained from the consideration of local stress during failure and the cohesive energy required for delamination of AP from HTPB matrix. Effect of binding agent on the interface strength is found to be quite significant. The cohesive zone parameters and the viscoplastic model obtained from the experiment were then used in the cohesive finite element method to simulate the dynamic crack propagation as well as the delamination. Results show that interfacial properties are affected by the rate of loading and are also dependent upon the binding agent. Keywords Energetic material • Stress/strain relationship • HTPB • AP • NRS Energetic compounds are employed in a large number of applications, such as, explosive, propellant, and pyrotechnic formulations. An example of a heterogeneous solid propellant used in rocket industry is a crystalline oxidizer (e.g., ammonium perchlorate-AP) embedded in a polymeric binder (e.g., Hydroxyl-Terminated Polybutadiene or HTPB). Aluminum (Al) particles are sometimes added to enhance the propellant performance. A typical industrial solid propellant consists of 70% AP, 10% HTPB and around 20% Al by weight, [1]. Three main failure mechanisms in the composite material are identified as particle fracture, interfacial failure and the cavitation in the binder [2]. Palmer [2] investigated a number of polymer bonded explosives (PBXs) under tensile loading and observed finding a wide range of responses. They found that the crack propagation was mostly confined to binder and that the interface debonding was the dominant failure mode. Interface strength depends on the constituent material, i.e., particle, matrix and/or binding agents [3, 4]. Several experiments [5–7] have suggested a particle size effect on the performance of energetic materials. Yeager [8] has shown the effect of interface/interphase and the microstructure on the mechanical behavior of PBXs. Interfacial structure was altered by adding a plasticizer in the composite. The plasticizer was shown to inhibit the formation of a large interface/interphase and was more likely to have film delamination than the no-plasticized composite. The difference in interfacial properties was also shown to have significant effect on the crack initiation and explosive sensitivity. Two samples were prepared for analyzing the effect of functionalization on the interface mechanical properties. One consist of ammonium perchlorate (AP) particles embedded in hydroxyl-terminated polybutadiene (HTPB). In the second sample, a surface binding agent (Tepanol) was added at a mass ratio of 0.5 to fabricate samples with higher surface adhesion, while keeping the same index ratio. C. Prakash • V. Tomar ( ) School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN, 47907, USA e-mail: cprakash@purdue.edu; tomar@purdue.edu I. Emre Gunduz School of Mechanical Engineering, Purdue University, West Lafayette, IN, 47907, USA © The Society for Experimental Mechanics, Inc. 2018 J. Carroll et al. (eds.), Fracture, Fatigue, Failure and Damage Evolution, Volume 7, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-62831-8_2 7
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