Composites of polyurea (PU) reinforced with milled glass fiber (MG_f) were fabricated. The volume fraction and length of the milled glass fiber were varied to study their effects on the morphological and mechanical properties of the MG_f/PU composites. The morphological attributes were characterized with scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The SEM investigations revealed a uniform distribution and arbitrary orientation of milled glass fiber in the polyurea matrix. Moreover, it seems that the composites with longer fiber exhibit better interfacial bonding. It was found from the FTIR studies that the incorporation of milled glass fiber into polyurea leads to more phase mixing and decreases the hydrogen bonding of the polyurea matrix, while having a negligible effect on the H-bond strength. The compression tests at different strain rates (0.001, 0.01, 0.1, 1, 2000 and 3000 s⁻¹) and dynamic mechanical properties over the temperature range from −30 to 100 °C at 1 Hz were performed. Experimental results show that the compressive behavior of MG_f/PU composites is nonlinear and strain-rate-dependent. Both elastic modulus and flow stress at any given strain increased with strain rate. The composites with higher fiber volume fraction and longer fiber length are more sensitive to strain rate. Furthermore, the elastic modulus, stress at 65% strain and energy absorption capability were studied, taking into account both the effect of fiber volume fraction and mean fiber length. It is noted that an increase in fiber volume fraction and fiber length leads to an increase in elastic modulus, stress at 65% strain and absorbed energy up to ~103%, 83.0% and 137.5%, respectively. The storage and loss moduli of the composites also increase with fiber volume fraction and fiber length. It can be concluded that the addition of milled glass fiber into polyurea not only improves the stiffness of the composites but also increases their energy dissipative capability.

High-performance extruded aluminum alloys with complex textures suffer significant dimension variation under environmental temperature fluctuations, dramatically decreasing the precision of navigation systems. This research mainly focuses on the effect of the texture of extruded pure aluminum on its dimensional stability after various annealing processes. The result reveals that a significant increment in the area fraction of recrystallized grains with orientation and a decrement in the area fraction of grains with orientation were found with increasing annealing temperature. Moreover, with the annealing temperature increasing from 150 °C to 400 °C, the residual plastic strain after twelve thermal cycles with a temperature range of 120 °C was changed from −1.6 × 10⁻⁵ to −4.5 × 10⁻⁵. The large amount of equiaxed grains with orientation was formed in the microstructure of the extruded pure aluminum and the average grain size was decreased during thermal cycling. The area fraction of grain with crystallographic orientation of the sample annealed at 400 °C after thermal cycling was 30.9% higher than annealed at 350 °C (23.7%) or at 150 °C (18.7%). It is attributed to the increase in the proportion of recrystallization grains with direction as the annealing temperature increases, provided more nucleation sites for the formation of fine equiaxed grains with orientation. The main orientation of the texture was rotated from parallel to to parallel to after thermal cycling. The change in the orientation of grains contributed to a change in interplanar spacing, which explains the change in the dimension along the extrusion direction during thermal cycling.

Achieving broadband absorption of sound waves below 500 Hz with materials of sub-wavelength thickness is significant but still a great challenge in academia and industries. Here, we present and theoretically analyze an airtight sound-absorbing metasurface constructed by a membrane-covered and coiled Helmholtz resonator. It is discovered that the metasurface possesses a near-perfect absorption with a working wavelength approximately 33.6 times greater than the total thickness, which stems from synthetic modulation on acoustic reactance brought by the membrane, air gap formed behind the membrane, and a coiled channel. Furthermore, on-demand broadband absorption below 500 Hz is achieved by parallel assemblies consisting of four subunits. An excellent agreement between measurements and predictions confirms the validity of the proposed structures. The airtight construction also broadens its application scenarios compared to the common perforated absorbers with open pores directly exposed to external environments. Our design provides a new structure paradigm for low-frequency sound absorption.

The fatigue damage behavior and the failure characteristics of the T300/QY8911 laminate and 7050-T7451 aluminum single-bolt bolted joints were investigated by test and progressive damage analysis (PDA) in this article. The manufactured composites-to-metal bolted joints (CMBJs) with different thickness metal plates and lap-patterns have been tested in static and fatigue load case and the failure characteristics are compared. A modified Lemaitre fatigue damage model and a phenomenological composite fatigue damage model were proposed and applied simultaneously to capture the damage evolution in the laminate and the metal parts. The competition of the fatigue damage of the laminate and the metal parts in affecting the joint stiffness and the coupling of the damage accumulation were studied by PDA, and the results indicates that: the failing sequence of the CMBJs constructive parts may change between the static load case and the fatigue load case; the fatigue damage of the metal parts has little effect on the joint stiffness of the CMBJs, nevertheless, there is certain coupling of the laminates fatigue damage and metal fatigue damage in the CMBJs, the harden or soften of the metal parts by fatigue hardening or crack progression would promote or slow down the damage accumulation in laminate.

The change in material dimensional will lead to the decline of instrument accuracy and reliability. In this paper, the characterization and analysis of the lattice constant, precipitates, and dislocation density of the material by X-ray diffraction (XRD) and transmission electron microscopy (TEM) reveals the reason why the relative dimensional change in the rolled 2024Al is one order of magnitude lower than that of the as-cast 2024Al during isothermal aging. Compared with as-cast 2024Al, the dislocation density of rolled 2024Al is higher, the lattice constant decreases less before and after aging, and the precipitates have orientation and more content, resulting in the dimensional change in rolled 2024Al being smaller than that of as-cast 2024Al. In addition, two main reasons for decreasing the dimensional change in rolled 2024Al are discussed: the decrease in lattice constant, the formation and growth of the S phase before and after aging.