Probabilistic relations between thermo-mechanical response and microstructure of heterogeneous energetic materials for shock/nonshock loading

An approach is developed to predict the ignition sensitivity of heterogeneous energetic materials under shock and nonshock loading as a function of microstructure. The underlying issue of impact-induced initiation of chemical reactions is driven by the deposition of mechanical work into energetic materials in the form of localized heating or the development of hotspots. These hotspots govern the ignition of energetic materials. The aim of this study is to understand the mechanisms of hotspot evolution, computationally predict the ignition sensitivity, and analyze the effects of loading and microstructural attributes on hotspot development and material ignition sensitivity. A computational framework based on a Lagrangian cohesive finite element method (CFEM) is developed. This framework is used to statistically analyze the material sensitivity, accounting for microstructural attributes in terms of morphology, constituent properties, inclusions, and defects. Multiple samples with statistically similar microstructural attributes are generated in a controlled manner and used to obtain a quantitative measure for the statistical variation in ignition behavior due to material heterogeneity. To relate loading and microstructure to the onset of chemical reaction, a hotspot-based criticality criterion is established. The analysis involves the quantification of hotspots via the CFEM simulations. The approach yields criticality conditions in terms of the critical impact velocity, critical time required for ignition, and total energy required for ignition under a given loading rate. The stochasticity of the material behavior is analyzed using a probability distribution as a function of microstructural attributes including grain volume fraction, grain size, amount of metallic inclusions, and specific binder-grain interface area. A probability superposition model is proposed to delineate the effects of different sources of stochasticity. The ignition threshold for granular explosives (GXs) and polymer-bonded explosives (PBXs) under shock and nonshock loading are predicted. The particular thresholds predicted are the James-type ignition threshold and the Walker-Wasley ignition threshold. The dependence of the ignition probability on material and microstructure is analyzed for a wide range of loading conditions. The microstructure – ignition threshold relations with the probability envelopes developed in this study provide a guide for the design of new energetic materials.Ph.D

Effect of aluminization on ignition sensitivity of PBX

Thermomechanical response of aluminized HMX/Estain PBX under impact loading is analyzed. The study focuses on the effect of aluminum on the hotspot evolution and initiation of PBXs. This analysis utilizes mesoscale simulations which account for constituent elasticity, viscoelasticity, elasto-viscoplasticity, fracture, internal contact, frictional heating, and heat conduction. The probabilistic nature of heating and initiation is assumed to arise from stochastic variations in microstructures which have statistically similar attributes with HMX grain sizes ranging from 50 to 400 m. For the microstructure configuration studied, it is found that aluminization with particles 50 m in diameter delays the initiation of chemical reaction in the material as compared to that for the corresponding unaluminized PBX. To understand the mechanisms leading to the ignition delay, the differences in overall internal stresses, dissipations due to fracture and inelasticity, and hotspot field characteristics are quantified. The microstructure–response relations obtained can be used to assess the performance of PBXs

Modelling Fiber Orientation during Additive Manufacturing-Compression Molding Processes

The production of high-performance thermoplastic composites reinforced with short carbon fibers can be achieved by a novel “additive manufacturing-compression molding” technique. An advantage of such a combination is two-fold: controlled fiber orientation in additive manufacturing and less void content by compression molding. In this study, a computational fluid dynamics model has been developed to predict the behavior of printed layers during fiber-reinforced thermoplastic extrusion and subsequent compression molding. The fiber orientation was modelled with the simple quadratic closure model. The interaction between the fibers was included using a rotary diffusion coefficient that becomes significant in concentrated regimes. Finally, the second rank orientation tensor was coupled with the momentum equation as an anisotropic part of the stress term. The effect of different fiber orientation within printed layers was investigated to determine the favorable printing scenarios in the strands that undergo compression molding afterwards. The developed numerical model enables design of high-performance composites with tunable mechanical properties.Mechanical Engineerin

Experimental and Numerical Investigations on Dynamic Mechanical Properties of TPMS Structures

Triply Periodic Minimal Surface (TPMS) lattice structures have been of increasing interest due to their light weighting, enhanced mechanical properties, and energy absorption characteristics for automotive and biomedical applications. With the advent of additive manufacturing and geometric modeling software, TPMS lattices with complex geometries can be realized. In this work, TPMS lattice structures were fabricated with PLA using fused filament fabrication (FFF) and their dynamic properties are characterized through drop tower experiments. Although lightweight TPMS lattices are beneficial for their impact absorption capability, most of the existing works are limited to quasi-static compression, and dynamic impact tests are rarely performed. The current study investigates the stress-strain and energy absorption characteristics of TPMS lattices through drop tower testing and numerical modeling. Finite element modeling for TPMS lattices is carried out to validate the experimental responses. The mechanical properties, deformation, and failure mechanisms of TPMS lattices under dynamic impact are summarized for potential future applications.Mechanical Engineerin

Ignition thresholds of aluminized HMX-based polymer-bonded explosives

The ignition of aluminized HMX-based polymer-bonded explosives (PBXs) under shock loading is studied via mesoscale simulations. The conditions analyzed concern loading pulses of 20 nanoseconds to 0.8 microseconds in duration and impact piston velocities on the order of 400-1000 m/s or loading stresses on the order of 3-14 GPa. The sets of samples studied have stochastically similar microstructures consisting of a bimodal distribution of HMX grains, an Estane binder, and aluminum particles 50-100 µm in diameter. The computational model accounts for constituent elasto-viscoplasticity, viscoelasticity, bulk compressibility, fracture, interfacial debonding, internal contact, bulk and frictional heating, and heat conduction. The analysis focuses on the development of hotspots under different material settings and loading conditions. In particular, the ignition thresholds in the forms of the James relation and the Walker-Wasley relation and the corresponding ignition probability are calculated and expressed as functions of the aluminum volume fraction for the PBXs analyzed. It is found that the addition of aluminum raises the ignition thresholds, causing the materials to be less sensitive. Dissipation and heating mechanism changes responsible for this trend are delineated

Evaluating the Effect of Z-pinning Parameters on the Mechanical Strength and Toughness of Printed Polymer Composite Structures

Traditional Fused Filament Fabrication methods create a mechanically anisotropic structure that is stronger in the deposition plane than across successive layers. A recently developed pinning process deposits continuous pins in the structure that are orientated in the build direction across multiple layers. Initial studies of this technique have demonstrated the ability to increase inter-layer strength and toughness. The current study evaluated various z-pinning parameters for carbon fiber reinforced polylactic acid (CF-PLA) structures, including infill percentage, pin length, and deposition pattern. Each of these was found to affect the ability of the z-pin to mechanically bond with the existing lattice structure and had a resulting impact on the mechanical strength and toughness. Initial studies showed an increase in ultimate tensile strength in the Z-axis of around 3.5x. Upon expanding the pinning settings, further studies showed increases of over 35% from the X and Z axis ultimate tensile strength and improved mechanically isotropic behavior.Mechanical Engineerin

Design and Use of a Penetrating Deposition Nozzle for Z-Pinning Additive Manufacturing

Fused Filament Fabrication (FFF) involves depositing material layer-by-layer to create a three-dimensional object. This method often demonstrates high mechanical anisotropy in the printed structure, leading to a drop in the material strength of the part when comparing structures along the deposition plane (X/Y-Axis) versus across layers in the build direction (Z-Axis). Initial efforts to improve anisotropy led to the development of the Z-Pinning process, where continuous pins are deposited across layers in the Z-Axis. Z-pinning has demonstrated significant gains in toughness and inter-layer strength, particularly in fiber-reinforced materials. However, this process can also create flaws in the structure that increase in severity and frequency as the pins grow in length and diameter. To mitigate this, a penetrating nozzle has been developed that extends a fine-tipped extrusion nozzle deep into the pin cavity and simultaneously extrudes material as it retracts. This study investigates the printability of the penetrating nozzle for simple geometries and evaluates the resulting Z-pinning mesostructure. As a result of this study, the prototype penetrating nozzle design was analyzed and built. Through a pressure driven flow analysis it was determined that filament will flow through the penetrating nozzle as the system pressure drop of 9.3 Mpa is less then the minimum critical pressure of 12.07 Mpa. Additionally, it was after a transient thermal simulation, it was found that after a pause of 15 seconds the system can resume printing with no drop in heat at nozzle exit. This means the additional length of the penetrating nozzle, will not cause any clogs during any pauses in filament flow.Mechanical Engineerin

Modeling Thermal Expansion of a Large Area Extrusion Deposition Additively Manufactured Parts Using a Non-Homogenized Approach

Interest in the use of large area extrusion deposition additive manufacturing (LAEDAM) to create tools for creation of composites is on the rise, due to its ability to create complex shapes rapidly. To ensure the parts created from the tool meet geometric standards, it is important to understand the thermal expansion of the printed part. Which is a challenge as LAEDAM imparts a non-uniform fiber orientation to the deposited material. A non-uniform fiber orientation in the deposited material creates a non-homogeneous cross section at a given position. Due to this heterogeneity, the coefficient of thermal expansion (CTE) also varies according to the position in the cross section. Previous modelling attempts of LAEDAM parts have employed a homogenized approach. This work experimentally characterizes CTE variations across the cross section of a bead using thermomechanical analysis and uses this as a non-homogenized input at the bead level for a finite element model. Predictions from this finite element model are then be compared to strain maps measured using 2-D digital image correlation of large-scale printed parts (127 mm cubes).Mechanical Engineerin

Creating Toolpaths Without Starts and Stops for Extrusion-Based Systems

Toolpath generation for extrusion-based additive manufacturing systems, called slicing, involves operations on polygonal contours that are derived from an STL file. Slicing generates multiple paths per layer (both closed-loop and open-loop) that are designed to optimally fill the space outlined by the polygon(s). In the course of printing a layer, the extruder must start and stop, the tip must be wiped, and the extruder must travel between paths without printing. Any amount of time the printer spends moving without printing is considered wasted time because the part isn’t being constructed. In addition, the start/stop point, known as the seam, is often a blemish on the surface of the part that contributes to weaker material properties. Therefore, a single path for creating multi-bead walled structures is desirable because it would save machine time and create parts with better surface finish. This paper will cover one method of modifying the CAD file and slicing engine to allow for parts to be printed without starting and stopping the extruder.Mechanical Engineerin

Increasing the Interlayer Bond of Fused Filament Fabrication Samples with Solid Cross-Sections using Z-Pinning

The mechanical properties of parts made by fused filament fabrication is highly anisotropic, with the strength across layers (z-axis) typically measuring ~50% lower than the strength along the direction of the extruded material (x-axis). A z-pinning method has been developed in which material is extruded in the z-direction to fill intentionally aligned voids in the x-y print pattern. In previous studies that involved a sparse rectilinear grid cross-section (35% infill), the z-pinning approach demonstrated more than a 3.5x increase in strength in the z-direction. The current study expanded these efforts to evaluate the use of z-pins in a printed sample with a solid cross-section. Although a solid cross-section is more common in structural components, it is much less forgiving of instabilities that may occur in the z-pinning approach (such as over-filling). Even though this study utilized a low pin volume (~43% fill factor), the pinning approach demonstrated a 40% increase in z-direction strength for solid samples that had similar printing times.Mechanical Engineerin