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The purpose of this paper is to measure the effect of process conditions on mechanical properties of laser‐sintered nylon 12 (Duraform^®) and to determine the range of conditions that provide consistent mechanical performance for additive manufacturing.

Tensile test specimens were fabricated over a range of well‐characterized process conditions including laser power, laser speed, scan spacing, layer thickness, build orientation, and build position. Tensile modulus, yield strength, ultimate tensile strength and elongation‐at‐fracture were measured and related to process parameters.

Tensile properties are strongly related to the amount of energy deposited during scanning. Strength and modulus approach their maximum values as the energy deposited exceeds the amount needed to fully melt the applied powder. Elongation‐at‐fracture does not reach its maximum until higher energy‐melt ratio. Performance of blends with reused powder matches that of virgin powder when blend composition is adjusted to a standard melt‐flow index. The volumetric energy density and the energy‐melt ratio are useful for correlating mechanical properties with multiple process parameters and material thermal properties.

This work presents the most extensive data to date on mechanical properties of nylon 12 (Duraform^®) as they relate to the full range of process parameters. These data show that mechanical performance correlates strongly with the volume energy density. In contrast to the area energy density (a.k.a. Andrews Number), this volumetric parameter includes the effect of varying layer thickness and can be related directly to the melting characteristics of the polymer material. Within the parameter range studied, this relationship allows adjustment of one scan parameter for improved speed or dimensional accuracy while ensuring good strength by an offsetting adjustment of another parameter. Such trade‐offs will be important in future manufacturing applications of the laser sintering process. Understanding the energy‐melt ratio provides insight into the relationship between scan conditions and the physics of powder melting and sintering, and offers a methodology to relate results at other bed temperatures and with other polymer powders.

Additive manufacturing of metallic scaffolds using laser powder bed fusion is challenging because of the accumulation of extra material below overhanging and horizontal surfaces. It reduces porosity and pore size and increases the effective strut size. These challenges are normally overcome by using volumetric energy density (VED) values lower than the optimum values, which, however, results in poor physio-mechanical properties. The purpose of this study is to assist scaffold manufacturers with a novel approach to fabricate stronger yet accurate scaffolds.

This paper presents a strategy for laser exposure that enables fabricating titanium-6–aluminum-4–vanedium (Ti6Al4V) alloy scaffolds with the required properties without compromising the geometric features. The process starts from computer-aided design models sliced into layers; dividing them into core (upper) and downskin (lower) layers; and fabrication using hybrid VED (low values for downskin layers and high values for core layers).

While exposing the core layers, laser remelted the downskin layers, resulting in better physio-mechanical properties (surface roughness, microhardness and density) for the whole strut without affecting its dimensional accuracy. A regression equation was developed to select the downskin thickness for a given combination of strut thickness and core VED to achieve the desired range of properties. The proposed approach was validated using microstructure analysis and compression testing.

This paper is expected to be valuable for the manufacturers of Ti6Al4V scaffolds, in achieving the desired properties.

This is probably the first time the hybrid VED approach has been applied for obtaining scaffolds with the desirable physio-mechanical and geometrical properties.

In recent years, additive manufacturing techniques have attracted much research attention because of their ability to fabricate customised parts with complex geometry. The range of composites suitable for laser-based powder bed fusion technique is limited, and has not been investigated yet. This paper aims to study the fabrication of AlSi₁₀Mg reinforced with nAl₂O₃ using the laser-based powder bed fusion technique.

An experimental approach was used to investigate the densification of AlSi₁₀Mg–nAl₂O₃ composites using laser-based powder bed fusion technique. Optimisation of the porosity was performed, and microstructure evolution was evaluated.

In this study, laser volumetric energy density (approximately 109 J/mm³) was found to be required for the fabrication of AlSi₁₀Mg–nAl₂O₃ composites with a relative volumetric density approximating 99%. The use of laser volumetric energy density resulted in larger grains. Columnar grain structure was observed via the use of electron backscatter diffraction mapping.

This paper examines the processing of new aluminium composite material suitable for the fabrication via the laser-based powder bed fusion technique.

Current methods for the preparation of composite powder feedstock for selective laser melting (SLM) rely on costly nanoparticles or yield inconsistent powder morphology. This study aims to develop a cost-effective Ti6Al4V-carbon feedstock, which preserves the parent Ti6Al4V particle’s flowability, and produces in situ TiC-reinforced Ti6Al4V composites with superior traits.

Ti6Al4V particles were directly mixed with graphite flakes in a planetary ball mill. This composite powder feedstock was used to manufacture in situ TiC-Ti6Al4V composites using various energy densities. Relative porosity, microstructure and hardness of the composites were evaluated for different SLM processing parameters.

Homogeneously carbon-coated Ti6Al4V particles were produced by direct mixing. After SLM processing, in situ grown 100–500 nm size TiC nanoparticles were distributed within the α-martensite Ti6Al4V matrix. The formation of TiC particles refines the Ti6Al4V β grain size. Relative density varied between 96.4% and 99.5% depending on the processing parameters. Hatch distance, exposure time and point distance were all effective on relative porosity change, whereas only exposure time and point distance were effective on hardness change.

This work introduces a novel, cost-effective powder feedstock preparation method for SLM manufacture of Ti6Al4V-TiC composites. The in situ SLM composites achieved in this study have high relative density values, well-dispersed TiC nanoparticles and increased hardness. In addition, the feedstock preparation method can be readily adapted for various matrix and reinforcement materials in future studies.

Density optimization is the first critical step in building additively manufactured parts with high-quality and good mechanical properties. The authors developed an approach that combines simulations and experiments to identify processing parameters for high-density Ti-6Al-4V using the laser powder-bed-fusion technique. A processing diagram based on the normalized energy density concept is constructed, illustrating an optimized processing window for high- or low-density samples. Excellent mechanical properties are obtained for Ti-6Al-4V samples built from the optimized window.

The authors use simple, but approximate, simulations and selective experiments to design parameters for a limited set of single track experiments. The resulting melt-pool characteristics are then used to identify processing parameters for high-density pillars. A processing diagram is built and excellent mechanical properties are achieved in samples built from this window.

The authors find that the laser linear input energy has a much stronger effect on the melt-pool depth than the melt-pool width. A processing diagram based on normalized energy density and normalized hatch spacing was constructed, qualitatively indicating that high-density samples are produced in a region when 1 < E* < 2. The onset of void formation and low-density samples occur as E* moves beyond a value of 2. The as-built SLM Ti-6Al-4V shows excellent mechanical performance.

A combined approach of computer simulations and selected experiments is applied to optimize the density of Ti-6Al-4V, via laser powder-bed-fusion (L-PBF) technique. A series of high-density samples are achieved. Some special issues are identified for L-PBF processes of Ti-6Al-4V, including the powder particle sticking and part swelling issues. A processing diagram is constructed for Ti-6Al-4V, based on the normalized energy density and normalized hatch spacing concept. The diagram illustrates windows with high- and low-density samples. Good mechanical properties are achieved during tensile tests of near fully dense Ti-6Al-4V samples. These good properties are attributed to the success of density optimization processes.

Development of military vehicles capable of surviving shallow-buried explosive blast is seldom done using full-scale prototype testing because of the associated prohibitively high cost, the destructive nature of testing, and the requirements for large-scale experimental-test facilities and a large crew of engineers committed to the task. Instead, tests of small-scale models are generally employed and the model-based results are scaled up to the full-size vehicle. In these scale-up efforts, various dimensional analyses are used whose establishment and validation requires major experimental testing efforts and different-scale models. The paper aims to discuss these issues.

In the present work, a critical assessment is carried out of some of the most important past efforts aimed at developing the basic dimensional analysis formulation for the problem of impulse loading experienced by target structures (e.g. vehicle hull) due to detonation of explosive charges buried to different depths in sand/soil (of different consistency, porosity, and saturation levels).

It was found that the analysis can be substantially simplified if only the physical parameters associated with first-order effects are retained and if some of the sand/soil parameters are replaced with their counterparts which better reflect the role of soil (via the effects of soil compaction in the region surrounding the explosive and via the effects of sand-overburden stretching and acceleration before the associated sand bubble bursts and venting of the gaseous detonation products takes place). Once the dimensional analysis is reformulated, a variety of experimental results pertaining to the total blast impulse under different soil conditions, charge configurations, charge deployment strategies, and vehicle ground clearances are used to establish the underlying functional relations.

The present work clearly established that due to the non-dimensional nature of the quantities formulated, the established relations can be utilized across different length scales, i.e. although they are obtained using mainly the small-scale model results, they can be applied at the full vehicle length scale.

Polyurea falls into a category of elastomeric co‐polymers in which, due to the presence of strong hydrogen bonding, the microstructure is of a heterogeneous nature and consists of a compliant/soft matrix and stiff/hard nanometer size hard domains. Recent investigations have shown that the use of polyurea as an external or internal coating/lining had substantially improved ballistic‐penetration resistance of metallic structures. The present work aims to use computational methods and tools in order to assess the shock‐mitigation ability of polyurea when used in the construction of different components (suspension‐pads, internal lining and external coating) of a combat helmet.

Shock‐mitigation capability of combat helmets has become an important functional requirement as shock‐ingress into the intra‐cranial cavity is known to be one of the main causes of traumatic brain injury (TBI). To assess the shock mitigation capability of polyurea, a combined Eulerian/Lagrangian fluid/solid transient non‐linear dynamics computational analysis of an air/helmet/head core sample is carried out and the temporal evolution of the axial stress and particle velocities (for different polyurea augmented helmet designs) are monitored.

The results obtained show that improvements in the shock‐mitigation performance of the helmet are obtained only in the case when polyurea is used as a helmet internal lining and that these improvements are relatively small. In addition, polyurea is found to slightly outperform conventional helmet foam, but only under relatively strong (greater than five atm) blastwave peak overpressures.

The present approach studies the effect of internal linings and external coatings on combat helmet blast mitigation performance.

This paper addressed some critical issues in the development of hybrid electric powertrains for aircraft and propose a design methodology based on multi-objective optimization algorithms and mission-based simulations.

Scalable models were used for the main components of the powertrain, namely, the (two stroke diesel) engine, the (lithium) batteries and the (permanent magnet) motor. The optimization was performed with the NSGA-II genetic algorithm coupled with an in-house MATLAB tool. The input parameters were the size of engine, the hybridization degree and the specification of the battery (typology, nominal capacity, bus voltage, etc.). The outputs were electric endurance, additional volume, performance parameters and fuel consumption over a specified mission.

Electric endurance was below 30 min in the two test cases (unmanned aerial vehicles [UAVs]) but, thanks to the recharging of the batteries on-board, the total electric time was higher. Fuel consumption was very high for the largest UAV, while an improvement of 11 per cent with respect to a conventional configuration was obtained for the smallest one.

The research used a simplified approach for flight mechanics. Some components were not sized in the proposed test cases.

The results of the test cases stressed the importance of improving energy density and power density of the electric path.

The proposed methodology is aimed at minimizing the environmental impact of aircraft.

The proposed methodology was obtained from the automotive field with several original contributions to account for the aircraft application.

The purpose of this paper is to address the problem of substitution of steel with fiber-reinforced polymer-matrix composite in military-vehicle hull-floors, and identifies and quantifies the associated main benefits and shortcomings.

The problem is investigated using a combined finite-element/discrete-particle computational analysis. Within this analysis, soil (in which a landmine is buried), gaseous detonation products and air are modeled as assemblies of discrete, interacting particles while the hull-floor is treated as a Lagrangian-type continuum structure. Considerable effort has been invested in deriving the discrete-material properties from the available experimental data. Special attention has been given to the derivation of the contact properties since these, in the cases involving discrete particles, contain a majority of the information pertaining to the constitutive response of the associated materials. The potential ramifications associated with the aforementioned material substitution are investigated under a large number of mine-detonation scenarios involving physically realistic ranges of the landmine mass, its depth of burial in the soil, and the soil-surface/floor-plate distances.

The results obtained clearly revealed both the benefits and the shortcomings associated with the examined material substitution, suggesting that they should be properly weighted in each specific case of hull-floor design.

To the authors’ knowledge, the present work is the first public-domain report of the findings concerning the complexity of steel substitution with composite-material in military-vehicle hull-floors.

The recently proposed concept solution for improving blast-survivability of the light tactical military vehicles is critically assessed using combined finite-element/discrete-particle computational methods and tools. The purpose of this paper is to propose a concept that involves the use of side-vent-channels attached to the V-shaped vehicle underbody. Since the solution does not connect the bottom and the roof or pass through the cabin of a light tactical vehicle, this solution is not expected to: first, reduce the available cabin space; second, interfere with the vehicle occupants’ ability to scout the surroundings; and third, compromise the vehicle’s off-road structural durability/reliability. Furthermore, the concept solution attempts to exploit ideas and principles of operation of the so-called “pulse detonation” rocket engines in order to create a downward thrust on the targeted vehicle.

To maximize the downward thrust effects and minimize the extent of vehicle upward movement, standard engineering-optimization methods and tools are employed for the design of side-vent-channels.

The results obtained confirmed the beneficial effects of the side-vent-channels in reducing the blast momentum, although the extent of these effects is relatively small (3-4 percent).

To the authors’ knowledge, the present work is the first public-domain report of the side-vent-channel blast-mitigation concept.