I. Emre Gunduz

High speed X-ray phase contrast imaging of energetic composites under dynamic compression

Fracture of crystals and frictional heating are associated with the formation of “hot spots” (localized heating) in energetic composites such as polymer bonded explosives (PBXs). Traditional high speed optical imaging methods cannot be used to study the dynamic sub-surface deformation and the fracture behavior of such materials due to their opaque nature. In this study, high speed synchrotron X-ray experiments are conducted to visualize the in situ deformation and the fracture mechanisms in PBXs composed of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) crystals and hydroxyl-terminated polybutadiene binder doped with iron (III) oxide. A modified Kolsky bar apparatus was used to apply controlled dynamic compression on the PBX specimens, and a high speed synchrotron X-ray phase contrast imaging (PCI) setup was used to record the in situ deformation and failure in the specimens. The experiments show that synchrotron X-ray PCI provides a sufficient contrast between the HMX crystals and the doped binde...

Removing hydrochloric acid exhaust products from high performance solid rocket propellant using aluminum-lithium alloy

Hydrochloric acid (HCl) pollution from perchlorate based propellants is well known for both launch site contamination, as well as the possible ozone layer depletion effects. Past efforts in developing environmentally cleaner solid propellants by scavenging the chlorine ion have focused on replacing a portion of the chorine-containing oxidant (i.e., ammonium perchlorate) with an alkali metal nitrate. The alkali metal (e.g., Li or Na) in the nitrate reacts with the chlorine ion to form an alkali metal chloride (i.e., a salt instead of HCl). While this technique can potentially reduce HCl formation, it also results in reduced ideal specific impulse (ISP). Here, we show using thermochemical calculations that using aluminum-lithium (Al-Li) alloy can reduce HCl formation by more than 95% (with lithium contents ≥15 mass%) and increase the ideal ISP by ∼7s compared to neat aluminum (using 80/20 mass% Al-Li alloy). Two solid propellants were formulated using 80/20 Al-Li alloy or neat aluminum as fuel additives. The halide scavenging effect of Al-Li propellants was verified using wet bomb combustion experiments (75.5±4.8% reduction in pH, ∝ [HCl], when compared to neat aluminum). Additionally, no measurable HCl evolution was detected using differential scanning calorimetry coupled with thermogravimetric analysis, mass spectrometry, and Fourier transform infrared absorption.

Two-component additive manufacturing of nanothermite structures via reactive inkjet printing

With an eye towards improving the safety of the deposition of energetic materials while broadening the scope of materials compatible with inkjet printing, this work demonstrates the use of combinatorial inkjet printing for the deposition of energetic materials. Two largely inert colloidal suspensions of nanoaluminum and nanocopper (II) oxide in dimethylformamide with polyvinylpyrrolidone were sequentially deposited on a substrate using piezoelectric inkjet printing. The materials were deposited in such a way that the aluminum and copper (II) oxide droplets were adjacent, and overlapping, to allow for in situ mixing of the components. The alternating deposition was repeated to create a sample with multiple layers of energetic materials. Energetic performance was subsequently tested on samples printed with 3, 5, and 7 layers of materials using a spark igniter. This ignition event was observed with a high speed camera and compared to representative samples printed with pre-mixed nanothermite. High speed ther...

Effect of Strain Rate and Interface Chemistry on Failure in Energetic Materials

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.

Uncertainty Quantification in Nanoscale Impact Experiment in Energetic Materials

Finite element method is extensively used for the analysis of impact response in complex materials. The prediction from finite element model may exhibit significant difference from that of experiments due to uncertainties in model, experimental measurements, and parameters that are derived based on experiments for model development. The quantification of parametric uncertainties, such as parameters in constitutive relation, associated with the numerical model is an important aspect that needs to be investigated for a credible computational prediction. This work considers uncertainty quantification in finite element modeling of nanoscale dynamic impact problems. A viscoplastic power law constitutive model is obtained from nanoscale impact experiments on Hydroxyl-terminated polybutadiene (HTPB)-Ammonium Perchlorate (AP) samples. The constitutive model is used in a finite element model to simulate impact experiments. The measured response from impact experiment and FEM simulation is used to quantify the parametric uncertainties in the constitutive model for the analyzed HTPB-AP sample.

Interface Chemistry Dependent Mechanical Properties in Energetic Materials Using Nano-Scale Impact Experiment

Energetic materials are sensitive to mechanical shock and defects caused by a high velocity impact, which may result in unwanted detonation due to hot-spot formation. In order to understand the underlying mechanism, characterization of high strain rate mechanical properties needs to be studied. One of the key factors that can contribute to this type of defect is the failure initiated at the interfaces such as those between Hydroxyl-terminated polybutadiene (HTPB)-HMX (or HTPB-Ammonium Perchlorate (AP)). In this work, interface mechanical properties of HTPB-HMX (and HTPB-AP) interfaces are characterized using nano-scale impact experiments at strain rates up to 100 s−1. The experiments were conducted with impactor of radius 1 μm on the interfaces with varying amount of binding agent. For HTPB-AP samples, Tepanol is used as the binding agent. The impact response is determined in the bulk HTPB, HMX, and AP as well as at the HTPB-HMX and HTPB-AP interfaces. A power law viscoplastic constitutive model is fitted to experimental stress-strain-strain rate data which can be used in Finite Element Model simulation to predict the shock behavior of energetic materials. An in-situ mechanical Raman spectroscopy (MRS) setup was used to analyze the effect of interface chemistry on interface level stress variation. The stress distribution near the interface captures the effect of interface chemistry variation.

Investigation of Nanoparticle Reactions with Laser Heating by In situ TEM

Nanoscale materials have significantly different and enhanced properties compared to their micro or macroscopic counterparts. As a result, they have been investigated in many studies [1]. In particular, Al nanoparticles has attracted significant attention, especially in the study of propellants, explosives and pyrotechnics, due to their high energy density [2], non-toxicity, commercial availability and low cost [3]. Fluorine, which is often called as material of extremes because of its high reactivity, is generally preferred as an additive in Al nanoparticle systems. Their reaction produces one of the strongest bonds ever determined and improve the general performance of the propellants [4]. The structural changes in fluorine-based polymers during their reactions with nano-aluminum particles can be observed via transmission electron microscopes (TEM) [5].