Crashworthy optimization of skeleton-filled FRP tubes based on back propagation neural network.

Lightweight composite tubes have been widely used in vehicle safety systems as energy absorbers. To improve the crashworthiness of tubes, composite skeletons with a variety of cross-sectional profiles were ingeniously designed as internal reinforcements. Herein, a novel composite skeleton comprising cross-ribs and an inner circle (OS-skeleton) was proposed and integrally fabricated through the special assembling molds. The novel OS-skeleton presented a steady progressive failure mode under dynamic impact loads, leading to remarkable material utilization and energy absorption characteristics. Subsequently, finite element analysis (FEA) models were developed. The predicted response curves and deformation modes were consistent with the experimental results. Finally, a multi-objective optimization utilizing the back propagation neural network (BPNN) was then conducted to further enhance the mean crushing force (MCF) and specific energy absorption (SEA) by adjusting several structural parameters. The results showed that MCF and SEA increased with the increasing thickness of the skeletons and the number of circumferential ribs. By comparison, the diameter of inner tube and the number of circumferential ribs showed a non-linear relationship with the energy absorption characteristics due to their combined effects. In sum, the proposed composite tubes filled with OS-skeletons could maximize certain aspects of crashworthiness performance through proper structural design, demonstrating great potential for lightweight energy absorbers.

Deformation-activated recrystallization twin: New twinning path in pure aluminum enabled by cryogenic and rapid compression.

Bulk aluminum rarely forms deformation or annealing twins owing to its high stacking fault energy. We report a novel twinning mechanism mediated by dynamic recrystallization in 6N pure aluminum under high strain rate (∼1.3 × 104 s-1) impact at a cryogenic temperature (77 K). Discontinuous dynamic recrystallization occurs during rapid severe plastic deformation and generates inhomogeneous microstructures exhibiting low-angle and high-angle grain boundaries. Unexpectedly, Σ3 twin boundaries were able to develop during dynamic recrystallization. Although these recrystallization twins have similar morphology as that of annealing twins, their formation relies on deformation activation instead of thermal activation, which was suppressed by the cryogenic experiment. Besides, strong orientation dependence was observed for formation of these novel twins. Beyond annealing and deformation twin, deformation-activated recrystallization twin is a new path for pure aluminum twinning.

Influences of Printing Pattern on Mechanical Performance of Three-Dimensional-Printed Fiber-Reinforced Concrete.

Underperformed interfacial bond and anisotropic properties are often observed in three-dimensional-printed concrete, where the printing pattern is unidirectional. Such issues could be potentially alleviated by replicating microstructures of natural materials or applying different architectures, where printed layers are arranged into unique and unconventional patterns. Furthermore, the quest to develop printing methods for highly complex or self-support concrete architecture could benefit from these nature-inspired patterns. In this work, the influences of different architectural arrangements of layers on mechanical properties of hardened concrete on compressive and flexural strengths are investigated. Specifically, unidirectional (0°), cross-ply (0°/90°), quasi-isotropic (0°/ ± 45°/90°), and helicoidal patterns (with pitch angles of 10°, 20°, and 30°) are used to create unidirectional, bidirectional, and multidirectional layers in printed objects without and with 0.75% by volume of 6 mm-long steel fibers. The experimental results demonstrate considerable improvements in the flexural strengths of nontraditional specimens without steel fibers over the unidirectional control with a few exceptions. Among investigated patterns, the quasi-isotropic demonstrates significant influences in both compressive and flexural responses of printed concrete samples without steel fibers. The addition of steel fibers leads to noticeable improvement on both compressive and flexural strengths of samples in any pattern compared with their counterparts without fibers. Besides, the inclusion of steel fibers into unconventional layups (cross-ply, quasi-isotropic, and helicoidal patterns) leads to the alleviation of directional dependence of mechanical properties, which is a limitation of the unidirectional samples with fibers. Of all helicoidal patterns, the one with a 10°-angle layup is shown to be more beneficial to the flexural strength enhancement and damage resistance in bending. X-ray microcomputed tomography measurements are performed to visualize the direction and distribution of fibers.

Mechanical properties and energy absorption characteristics of tropical fruit durian (Durio zibethinus).

The paper presents for the first time the material properties and energy absorption capacity of durian shells with an attempt to use as an alternative sustainable material and mimic their structural characteristics to design a bio-inspired structure for protective packaging applications. A series of quasi-static compression tests were carried out to determine Young's modulus and bioyield stress of the durian shells as well as their energy absorption capacity. The mesocarp layers and thorns are interesting parts for investigating their energy absorption characteristics because they play an important role in protecting the flesh of durians during their drop impact onto the ground. The mesocarp layers of the shell were subjected to axial and lateral compression while the thorn specimens were compressed under axial loading with an increasing number of thorns. The results showed that the densification strain, plateau stress and specific energy absorption of the mesocarp layer under lateral loading is higher than that under axial loading. Furthermore, the compression tests on the thorns demonstrated that an increase in the number of thorns helped to absorb more energy and the specific energy absorption of the thorns was nearly two times higher than that of the mesocarp layer under the axial loading. In addition, the cyclic loading of the thorns showed that the extent of reversibility of deformation in the thorns decreases from 32% at the first cycle to around 10% at the 9th-cycle. Finally, the microstructure of the thorn and mesocarp layer was investigated to explain the experimental observation. The results indicated that the spherical shape associated with the thorns and mesocarp materials displayed an excellent energy absorption efficiency that can be mimicked to design an effective bio-inspired absorber for packing applications.

Finite Element Analysis of Aluminum Honeycombs Subjected to Dynamic Indentation and Compression Loads.

The mechanical behavior of aluminum hexagonal honeycombs subjected to out-of-plane dynamic indentation and compression loads has been investigated numerically using ANSYS/LS-DYNA in this paper. The finite element (FE) models have been verified by previous experimental results in terms of deformation pattern, stress-strain curve, and energy dissipation. The verified FE models have then been used in comprehensive numerical analysis of different aluminum honeycombs. Plateau stress, σpl, and dissipated energy (EI for indentation and EC for compression) have been calculated at different strain rates ranging from 10² to 104 s-1. The effects of strain rate and t/l ratio on the plateau stress, dissipated energy, and tearing energy have been discussed. An empirical formula is proposed to describe the relationship between the tearing energy per unit fracture area, relative density, and strain rate for honeycombs. Moreover, it has been found that a generic formula can be used to describe the relationship between tearing energy per unit fracture area and relative density for both aluminum honeycombs and foams.

Finite element modelling of nanoindentation based methods for mechanical properties of cells.

The viscoelastic properties of the living cells are for quantifying the biomechanical effects of drug treatment, diseases and aging. Nanoindentation techniques have proven effective to characterize the viscoelastic properties of living cells. However, most studies utilized the Hertz contact model and assumed the Heaviside step loading, which does not represent real tests. Therefore, new mathematical models have been developed to determine the viscoelastic properties of the cells for nanoindentation tests. Finite element method was used to determine the empirical correction parameter in the mathematical model to account for large deformation, in which case the combined effect of finite lateral and vertical dimensions of the cell is essential. The viscoelastic integral operator was used to account for the realistic deformation rate. The predictive model captures the mechanical responses of the cells observed from previous experimental study. This work has demonstrated that the new model consistently predicts viscoelastic properties for both ramping and stress relaxation periods, which cannot be achieved by the commonly used models. Utilization of this new model can enrich the experimental cell mechanics in interpretation of nanoindentation of cells.