Tao Jin

Ballistic resistance and energy absorption of honeycomb structures filled with reactive powder concrete prisms

Two kinds of innovative re-entrant and hexagonal cell honeycomb sandwich structures filled with reactive powder concrete were proposed, and the ballistic resistance and energy absorption of the sandwich structures were investigated by numerical simulations. The deformation and failure modes of the different structures were analyzed and evaluated in detail. The honeycomb sandwich structures filled with reactive powder concrete prisms improved the capacity of ballistic resistance and energy absorption significantly, compared to the normal reactive powder concrete plates and sandwich structures without reactive powder concrete prisms. The analysis shows that the auxetic re-entrant cell honeycomb sandwich structures have a better ballistic performance than the hexagonal cell honeycomb sandwich structures. The sandwich structures were subjected to impact by three kinds of projectiles: flat, hemispherical and conical nosed. The ballistic limit of the flat nosed projectile is the highest, while the impact performance of the conical and hemispherical nosed projectiles is obviously different from the flat nosed projectile, especially in a relative high velocity range. The sharper nose leads to a higher value of exit velocity and mass loss. In addition, effects of different design parameters on ballistic resistance were also studied by changing the thickness of honeycomb cell and face plates. Results indicate that the thickness of honeycomb walls and face plates have significant effect on the ballistic resistance and energy absorption in a relative low velocity range, while there are no big differences when the initial impact velocity exceeds 400 m/s.

Dynamic response of circular metallic sandwich panels under projectile impact

The dynamic response of circular sandwich panels with aluminium honeycomb and corrugated cores under projectile impact was investigated experimentally and numerically. Impulse loaded on the panel was controlled by projectile launching velocity and the deformation process of sandwich panels was recorded by a high-speed camera in the experiments. Typical deformation/failure modes of face-sheets and cores were obtained and analysed. The back face-sheet deflections and strain histories of face-sheets were measured and discussed. A parametric study was conducted by LS-DYNA 3D to analyse the effect of geometrical configuration on energy absorption mechanism and back face-sheet permanent deflection of circular sandwich panels. The results indicated that the impact resistance of the structure was sensitive to geometrical configuration. Increasing face-sheet thickness and core relative density significantly improved sandwich structure impact resistance. Increasing foil thickness improved the panel impact resistance more efficiently than decreasing wall side length. The results have important reference value to guide engineering application of the sandwich structure subjected to impact loading.

Size effects on the in-plane mechanical behavior of hexagonal honeycombs

Abstract In-plane uniaxial compression tests were experimented on seven kinds of honeycomb specimens with different sizes (cell numbers) under the loading rate of 10 mm/min, in order to investigate the size effects on four important mechanical parameters (the curve modulus, initial collapse stress, densification strain, and plateau stress) of the honeycomb structure. The results show that the curve modulus of honeycombs decreases with decrease in specimen size, indicating that the curve modulus in in-plane direction is sensitive to size. In addition, the initial collapse stress in x2-direction as well as energy absorption efficiency in x1-direction is slightly sensitive to the specimen size. However, the other mechanical parameters of honeycomb show significant size independence. The weak boundary cells are used to explain size effects on the in-plane mechanical behavior of honeycomb. Consequently, the minimum specimen size of hexagonal honeycomb specimens for measuring the effective in-plane mechanical properties of a bulk honeycomb is determined according to these results.

Experimental investigation on the yield behavior of PMMA

Quasi-static compression, tension and combined shear–compression tests were conducted to characterize the mechanical behavior of PMMA. The cylinder samples and dog-bone samples were used for uniaxial testing. The shear–compression samples of four different angles (15°, 30°, 45°, and 50°) were used to obtain the multiaxial stress state instead of complex loading equipment. The yield behaviors of PMMA cannot be described by the Tresca or Mises criterion due to the effects of both the first invariant of the stress and the third invariant of deviatoric stress. A criterion is proposed that includes the first invariant of the stress and the third invariant of deviatoric stress in this paper. In other word, the effects of hydrostatic pressure and Lode angle are considered. The proposed yield criterion has the capability to precisely capture the experimental yield loci of PMMA.

Shear properties of isotropic conductive adhesive joints under different loading rates

ABSTRACT Epoxy-based conductive adhesives have been widely used in the electronic field given the lead-free development of electronic packaging. The conductive adhesive joints must be subjected to shear loads during the service of electronic products considering the mismatch in mechanical properties between packaged chip and substrate. In this study, INSTRON 5544 universal material testing machine was used for tensile–shear tests of isotropic conductive adhesive joint specimens, which were prepared using pure copper plate adherend in the form of single-lap joints. Four loading rates, that is, 0.05, 0.5, 5, and 10 mm/min, were adopted. The relationship between shear load and displacement of two overlapping copper plates is deduced from a mechanical perspective. A mechanical model of the conductive adhesive shear specimen was developed by introducing dimensionless parameters, which are obtained from interfacial fracture energy and shear strength, to interpret the effect of loading rate on the shear properties of the conductive adhesive specimen considering the loading rate. Results show that this model can effectively reflect the relationship between shear load and displacement in the range of 0.05–10 mm/min.

CHARACTERISTICS OF TiNi THIN FILMS DEPOSITED BY MAGNETRON SPUTTERING SYSTEM WITH OPTICAL EMISSION SPECTROSCOPY MONITOR

TiNi composite thin films were fabricated using a closed-field unbalanced magnetron sputtering system equipped with optical emission spectroscopy monitor (OEM). The thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and nanoindentation. Results show that the TiNi films are amorphous, and their composition varies approximately linearly with the OEM value. Thus, the film composition could be controlled by in situ real-time OEM. The structure of the single B2 parent phase was observed in the annealed TiNi film. The hardness and elastic modulus of the films increased because of the precipitation of the Ti3Ni4 phase in the single B2 parent phase.