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Additive manufacturing has made it possible to fabricate materials that were unachievable with traditional methods. This study focuses on understanding the deformation behavior and energy absorption mechanics of additively manufactured cellular materials with gradually varying densities. Foams have unique deformation behavior due to their intricate topology and composition, resulting in excellent energy dissipation capability. Varying the density can significantly influence their deformation response and improve energy absorption and impact resistance. Voronoi tessellation is employed to model the foams, as it effectively captures the cell morphology in foam structures and produces stochastic cellular topologies accurately. Resin-based additive manufacturing techniques are employed to fabricate cellular materials with varying density configurations for low-velocity and high-velocity impact experiments. The study demonstrates that density-graded foams effectively dissipate a broad spectrum of impact energies, surpassing uniform counterparts by transmitting reduced stress, especially at lower energy levels. This characteristic enhances their suitability for advanced energy absorption applications. The results also show that at high impact velocities, the direction of density gradation influences energy dissipation and peak stress transmission. • Density-graded foams fabricated through resin-based 3D printing are studied. • Local density distributions influence energy absorption and force transmission. • Graded foams dissipate a wide range of impact energies effectively. • Density-graded foams transmit reduced stress at lower energy levels. [ABSTRACT FROM AUTHOR]

The joining of thermoplastics through welding offers numerous advantages over mechanical joining. Most importantly, it eliminates the use of costly fasteners and has only a limited effect on the strength of the parts being joined. Since it does not require the introduction of holes, loading pins, and the associated stress concentrations, a specific form of welding, friction stir welding (FSW), was investigated for the creation of butt joints of unreinforced polyphenylene sulfide (PPS) and short carbon fiber (CF)-reinforced polyetheretherketone (PEEK) plates. Unlike metals, analytical models and experimental results show that the heat generated by the FSW tool is insufficient to produce the heat required to weld thermoplastic materials. Therefore, a second heat source is required for preheating these thermoplastics. In this research, a resistance type surface heater was placed at the bottom of two identical weld pieces to produce good quality welds. Two types of shoulder design such as rotating shoulder and stationary shoulder were developed in this study. Taguchi's design of experiment method was utilized to develop the welding process, where heating duration, material temperature, tool rotational speed, and tool traverse speed were used as the welding parameters. One of the process parameters, tool traverse speed, had significant influence on the tensile strength of PPS samples. While PPS sample showed relatively lower tensile strength with higher traverse speed, short CF-reinforced PEEK samples had higher tensile strength with a higher traverse speed. In addition to tensile tests, fracture toughness tests were performed for both PPS and PEEK samples to observe the influence of unwelded segments in the welded parts. In this study, joint efficiency of PEEK samples was found to be higher than that of PPS samples. Micrographs of PEEK samples showed uniform homogenous mixture of part materials. [ABSTRACT FROM AUTHOR]

Full-field deformation response of a copper bar impacted in a three point bending setup is studied. Ultra-high speed photography (frame rate: 5 × 10 6 fps) in conjunction with digital image correlation is utilized to study the in-plane deformation kinematics of the impacted specimen over 25.4 μs duration after the impact. Distributions of temporal derivatives, i.e. strain rate, velocity and acceleration, over the area of interest are computed using the in-plane displacement and strain fields measured from image correlation. Complex deformation patterns resulting from wave propagation and inertial loading are studied and discussed. Having deformed the specimen well into the plastic regime, the idea initially proposed by Pierron et al. (2011) is extended to identify visco-plastic constitutive response of the examined material by incorporating an analysis based on the method of virtual fields. Accordingly, a strain rate sensitive constitutive model for the copper specimen is identified by reconstructing the inertia and the impact forces using full-field acceleration and reaction force measurements, respectively. [ABSTRACT FROM AUTHOR]

The local deformation response of low carbon steel subjected to uniaxial tensile loading is investigated, and the local strain field at sub-grain scale is obtained using high-spatial-resolution digital image correlation. The implemented digital image correlation method enables the observation and study of inhomogeneous deformation response at microstructural levels. Detailed local deformation mechanisms including mesoscopic slip bands are captured. Furthermore, the local information is used for the determination of representative volume element size in polycrystalline low carbon steel. To obtain the representative volume element size, we proposed and successfully implemented a strain variation method. Further, the influence of global strain on the local deformation mechanisms and representative volume element size is discussed. The challenges associated with the local strain measurement using digital image correlation are also discussed. [ABSTRACT FROM AUTHOR]