Propulsion system development requires new, more affordable manufacturing techniques and technologies in a constrained budget environment, while future in-space applications will require in-space manufacturing and assembly of parts and systems. Marshall is advancing cuttingedge commercial capabilities in additive and digital manufacturing and applying them to aerospace challenges. The Center is developing the standards by which new manufacturing processes and parts will be tested and qualified. Rapidly evolving digital tools, such as additive manufacturing, are the leading edge of a revolution in the design and manufacture of space systems that enables rapid prototyping and reduces production times. Marshall has unique expertise in leveraging new digital tools, 3D printing, and other advanced manufacturing technologies and applying them to propulsion systems design and other aerospace materials to meet NASA mission and industry needs. Marshall is helping establish the standards and qualifications “from art to part” for the use of these advanced techniques and the parts produced using them in aerospace or elsewhere in the U.S. industrial base.

Additive manufacturing

We propose new approximate global multiplicative scaling factors for the DFT calculation of ground state harmonic vibrational frequencies using functionals from the TPSS, M06, and M11 functional families with standard correlation consistent cc-pVxZ and aug-cc-pVxZ (x = D, T, and Q), 6-311G split valence family, Sadlej and Sapporo polarized triple-ζ basis sets. Results for B3LYP, CAM-B3LYP, B3PW91, PBE, and PBE0 functionals with these basis sets are also reported. A total of 99 harmonic frequencies were calculated for 26 gas-phase organic and nonorganic molecules typically found in detonated solid propellant residue. Our proposed approximate multiplicative scaling factors are determined using a least-squares approach comparing the computed harmonic frequencies to experimental counterparts well established in the scientific literature. A comparison of our work to previously published global scaling factors is made to verify method reliability and the applicability of our molecular test set.

Harmonic Vibrational Frequencies: Approximate Global Scaling Factors for TPSS, M06, and M11 functional families using several common basis sets

In the past decade, 3D printing technologies have been adopted for the fabrication of microfluidic devices. Extrusion-based approaches including fused filament fabrication (FFF), jetting technologies including inkjet 3D printing, and vat photopolymerization techniques including stereolithography (SLA) and digital light projection (DLP) are the 3D printing methods most frequently adopted by the microfluidic community. Each printing technique has merits toward the fabrication of microfluidic devices. Inkjet printing offers a good selection of materials and multimaterial printing, and the large build space provides manufacturing throughput, while FFF offers a great selection of materials and multimaterial printing but at lower throughput compared to inkjet 3D printing. Technical and material developments adopted from adjacent research fields and developed by the microfluidic community underpin the printing of sub-100 μm enclosed microchannels by DLP, but challenges remain in multimaterial printing throughput. With the feasibility of 3D printed microfluidics established, we look ahead at trends in 3D printing to gain insights toward the future of this technology beyond the sole prism of being an alternative fabrication approach. A shift in emphasis from using 3D printing for prototyping, to mimic conventionally manufactured outputs, toward integrated approaches from a design perspective is critically developed.

3D Printing: An Alternative Microfabrication Approach with Unprecedented Opportunities in Design.

Objective To evaluate the effect of post-curing method, printing layer thickness, and water storage on the mechanical properties and degree of conversion of a light-curing methacrylate based resin material (IMPRIMO® LC Splint), used for the fabrication of 3D printed occlusal splints and surgical guides. Methods 96 bar-shaped specimens were 3D printed (Asiga MAX), half of them with a layer thickness of 100 μm (Group A), and half with 50 μm (Group B). Each group was divided in three subgroups based on the post-curing method used: post-curing with light emitting diode (LED) and nitrogen gas; post-curing with only LED; and non-post-curing. Half of the specimens from each subgroup were water-stored for 30 days while the other half was dry-stored (n = 8). Flexural strength and flexural modulus were evaluated. Additional specimens were prepared and divided in the same way for surface hardness (n = 96), fracture toughness, and work of fracture (n = 96). Five specimens were selected from each subgroup for evaluating the degree of conversion (DC). Data were collected and statistically analyzed with 1-way, 2-way ANOVA, and Tukey post-hoc analysis (α = 0.05). Results The 2-way ANOVA showed that the post-curing method and water storage significantly affected the investigated mechanical properties (P  Conclusion The post-curing method, water storage, and printing layer thickness play a role in the mechanical properties of the investigated 3D Printed occlusal splints material. The combination of heat and light within the post-curing unit can enhance the mechanical properties and degree of conversion of 3D printed occlusal splints. Flexural strength and surface hardness can increase when decreasing printing layer thickness.

Evaluation of the mechanical properties and degree of conversion of 3D printed splint material.

3D printing and post-curing optimization of photopolymerized structures: Basic concepts and effective tools for improved thermomechanical properties

3D printing preview for stereo-lithography based on photopolymerization kinetic models.

The existence of pre- and post-reaction complexes has been proposed to influence hydrogen abstraction reaction kinetics, but the significance still remains controversial. A theoretical study is presented to discuss the effects of complexes on hydrogen abstraction from 2-butanone by OH radicals based on the detailed PESs at the DLPNO-CCSD(T)/aug-cc-pVTZ//M06-2x-D3/may-cc-pVTZ level with five pre-reaction complexes at the entrance of the channels and four post-reaction complexes at the exit. The hydrogen bond interactions, steric effects, and contributions to the bonding orbital of the OH radical species and 2-butanone species in the complex structures were visualized and investigated by wavefunction analyses. Three kinds of mechanisms—the general bimolecular reaction, the reaction with the complexes considered, and the well-skipping reaction—were compared based on high-pressure-limit rate constants, predicted branching ratios, and fractional populations of reactants and products in the temperature range of 250–2000 K. The existence of complexes was proved to be crucial in the kinetics and mechanisms of the hydrogen abstraction from 2-butanone molecules by OH radicals.

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Ab initio kinetics predictions for the role of pre-reaction complexes in hydrogen abstraction from 2-butanone by OH radicals

With the continuous development of road construction worldwide, the construction sector is trying to find a versatile material for building roads in particular areas. Air-foamed lightweight soil, which is produced using low-cost raw materials and has a lower density varying from 300 to 1800 kg/m3, adjustable strength, excellent thermal isolation, and a more straightforward construction method, has emerged as a worthy candidate. This paper reviews air-foamed lightweight soil in terms of its composition as well as physical and mechanical properties, such as its compressive strength, flexural strength, and stability. This paper also generalizes the engineering applications of air-foamed lightweight soil and provides ideas for its wide use.

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Engineering Properties and Applications of Air-Foamed Lightweight Soil

A theoretical study for the role of complex in hydrogen abstraction of OH

High-energy-density materials (HEDMs) are crucial for explosives, propellants, and new energy materials. Designing high-performance HEDMs involves conflicting sub-objectives for molecular structure design. Previous research has focused on specific molecule types, limiting the understanding of molecular structural features’ influence on density. This work integrated the structure and density data of C2–C30 oxygen-containing compounds, expanded molecule types, and constructed a comprehensive database with 17,987 compounds’ SMILES and density values. Substructures such as ring frameworks, side chains, and functional groups were split using a molecular substructure splitting algorithm. A linear regression model was then established to examine the effects of substructures on density, with regression coefficients representing their contributions. The results showed that ring structures had a greater contribution to density. A decrease in C atoms and an increase in O atoms and their unsaturated bonds were significant positive features of density contribution in ring structures. Side chains and single alkane ring frameworks had similar density contribution distributions, and ring aromatic structures with three or more rings and cycloalkane structures with four or more rings had similar density contribution distributions, with ∼90% showing a significant positive contribution. Finally, the study examined the effects of the number of atoms in the skeleton of multi-ring alkane substructures and the average number of shared atoms on density contribution. The significant contribution of multi-ring alkane substructures first increased and then decreased when the average number of atoms in the ring and the variance of the number of atoms increased. 

Yi Gao

Papers

3D printing preview for stereo-lithography based on photopolymerization kinetic models.

Ab initio kinetics predictions for the role of pre-reaction complexes in hydrogen abstraction from 2-butanone by OH radicals

Engineering Properties and Applications of Air-Foamed Lightweight Soil

A theoretical study for the role of complex in hydrogen abstraction of OH

Analysis of Density Contributions of Molecular Substructures