Axinte Ionita

Controlling shockwave dynamics using architecture in periodic porous materials

Additive manufacturing (AM) is an attractive approach for the design and fabrication of structures capable of achieving controlled mechanical response of the underlying deformation mechanisms. While there are numerous examples illustrating how the quasi-static mechanical responses of polymer foams have been tailored by additive manufacturing, there is limited understanding of the response of these materials under shockwave compression. Dynamic compression experiments coupled with time-resolved X-ray imaging were performed to obtain insights into the in situ evolution of shockwave coupling to porous, periodic polymer foams. We further demonstrate shock wave modulation or “spatially graded-flow” in shock-driven experiments via the spatial control of layer symmetries afforded by additive manufacturing techniques at the micron scale.

Fracture model for cemented aggregates

A mechanisms-based fracture model applicable to a broad class of cemented aggregates and, among them, plastic-bonded explosive (PBX) composites, is presented. The model is calibrated for PBX 9502 using the available experimental data under uniaxial compression and tension gathered at various strain rates and temperatures. We show that the model correctly captures inelastic stress-strain responses prior to the load peak and it predicts the post-critical macro-fracture processes, which result from the growth and coalescence of micro-cracks. In our approach, the fracture zone is embedded into elastic matrix and effectively weakens the materials strength along the plane of the dominant fracture.

Finite Element Method Calculations on Statistically Consistent Microstructures of PBX 9501

We have used data from image analysis to guide us in creating a finite element mesh of PBX 9501. Information about the binder concentration at different length scales taken from micrographs allows us to create a mesh that naturally incorporates inhomogeneities of the microstructure in a manner that is statistically consistent with the observed microstructure. We then apply constitutive models that are consistent with the different binder concentrations and run finite element simulations. We will present our technique for incorporating the image analysis information into the mesh, our mechanical models, and results of the simulations.

Dynamic Response of PBX‐9501 through the β‐δ Phase Transition

The Gibbs free energies of the β and δ phases of HMX are constructed from zero pressure heat capacity data, specific volume measurements, numerical simulations, and diamond anvil experiments. The free energies provide input into a dynamic phase transition model developed for heterogeneous materials that undergo dynamically driven phase transitions. This model, which uses the method of cells analysis to treat the HMX‐polymer binder composite, is used to study dynamically loaded PBX‐9501 as it transforms from the beta to the delta phase.

NON‐RANDOM CRACK OPENING IN PARTIALLY CONFINED, THERMALLY DAMAGED PBX 9501 AND OBSERVATIONSON ITS EFFECTS ON COMBUSTION

In this work, we present evidence for how strong radial confinement can result in aligned macro‐scale crack opening. We damage cylinders in a tight‐fitting quartz sleeve, open on both ends, and observe the occurrence of aligned cracks opening normal to the longitudinal axis. This geometry and confinement is common in experimental arrangements such as strand burners and DDT tubes. Further, we observe, with high‐speed photography, how this non‐random crack opening affects combustion, and propose mechanisms, garnered from time‐lapse photography and elastic stress analysis, for how it occurs.

Modeling Aspects of the Dynamic Response of Heterogeneous Materials

In engineering applications, simulations involving heterogeneous materials where it is necessary to capture the local response coming from the heterogeneities is very difficult. The use of homogenization techniques can reduce the size of the problem but will miss the local effects. Homogenization can also be difficult if the constituents obey different constitutive laws. Additional complications arise if inelastic deformation occurs. In such cases a two-scale approach is preferred and this work addresses these issues in the context of a two-scale Finite Element Method (FEM). Examples of using two-scale FEM approaches are presented.