Key Engineering Materials Vols. 340-341

Abstract: In this study, numerical simulations of flexural tests of sheet metals subjected to embossing and restoration process are carried out using LS-DYNA3D. Several models are created varying the number, position, and pitch of the emboss or restoration points. The emboss height and sheet thickness are also varied. Results show that improvement in rigidity of sheet metals can be optimized by taking into considerations several parameters as discussed in this paper.

Abstract: Failure of aluminium foams due to dynamic indentation and penetration is very common in their application such as light-weight structural sandwich panels, packing materials and energy absorbing devices. This requires a sound understanding of deformation and energy absorption mechanisms of the aluminium foams as well as the effect of impact velocity. In this study, a finite element analysis using ABAQUS is conducted for the dynamic indentation/penetration process of aluminium foams under a rigid flat-headed indenter. The indenter is pushed into the foam either at a constant velocity or with an initial velocity. Two mechanisms exist: compression of the foam ahead of the indenter and fracture along the indenter edge. Effect of impact velocity is noted on the size of a localized deformation and the total energy absorbed.

Abstract: Experimental, numerical and theoretical analyses are carried out to obtain the relationship between the stress and relative density of metal hollow sphere (MHS) materials during their large plastic deformation in order to estimate the energy absorbing capacity of these materials under uniaxial compression. Based on a numerical parametric analysis empirical functions of the relative material density are proposed for the elastic modulus, yield strength and ‘plateau’ stress for FCC packing arrangement. Analytical stress-strain dependences are suggested for the yield strength and material strain hardening properties as functions of the relative density of MHS materials under uniaxial compression.

Abstract: The experimental studies on the static and dynamic mechanical properties of aluminium foam material are presented first. Finite element models of four structures, including circular tube filled and bonded with aluminium foam, circular tube filled but unbonded with aluminium foam, single aluminium foam column and empty aluminium tube, under dynamic transverse compression are established by FEMB code. The dynamic mechanical behaviors of the structures are analyzed using LS-DYNA finite element code. The simulating results at certain cases are compared with experimental measurements and the satisfying consistency confirmed the validity of the model. The further numerical simulations are carried on the dynamic mechanical behaviors of four structures with outer tubes of different wall-thickness. It is found that aluminium foam filling can greatly improve the load-bearing capacity and energy-absorbing efficiency of structures. On the other hand, the effect of the aluminium outer tube on the structure is obvious compared with single aluminium foam column, in spite of the foam core and the tube are bonded together or unbonded. Another result can be seen that the bonding between the foam and outer tube affects the structure weakly for both thinner and thicker tubes. Finally, the simulating results show that the thicker wall of tube can improve the load-bearing capacity and energy-absorbing ability of the structure.

Abstract: The constitutive relation for open-celled metal foams with random characteristics of cells was constructed based on the mechanical behavior and the distribution of the cells, which implied the effect of the mesoscopic characteristics of the cells on the macroscopic behavior of the foam. The constitutive relation was able to represent the whole three phases of the stress-strain curve of the open-celled metal foam with merely one expression. Besides, the explicit expressions for the foam’s yield strain and yield stress were supplied. Experimental data was employed to check the constitutive relation. It was found that the constitutive relation was able to represent accurately the whole compression process of the foams, and the calculated yield points had a good agreement with the experimental results.

Abstract: This paper deals with the results of three dimensional compression tests carried out for high stiffness urethane foams (Penguin-foam, Sunstar Engineering Ltd.), and also deals with the constitutive modelling base on Shima-Oyane’s consolidation condition for the tested foamed urethane. Three kinds of urethane foams, relative densities of which were 0.1, 0.2 and 0.33, were employed in the experiments. Like metallic porous materials, the tested urethane foams show the strong plastic-compressibility. On the other hand, in modelling, unlike metallic porous materials, the identified material constants for different density foams do not take the same (or unified) values but take the different values when Shima-Oyane’s constitutive model is assumed. Furthermore, the experimentally derived stress-relative density curves could not be satisfactorily described by Shima-Oyane’s original constitutive model; the experimental stress-relative density curves show stronger work hardening as compared with the simulated ones especially in the large deformation stage. To avoid those inconvenience, in this paper, a modified Shima-Oyane type constitutive equation was also proposed, and it was shown that the proposed model could well express both the low work hardening area of the stress-relative density curves at the initial deformation stage and the strong work hardening area at the final deformation stage by supposing the stress restriction at initial deformation stage due to the buckling of cell walls of each foam, and the rapid stress increase at the large deformation stage caused by the successive contact and the friction between the bent cellular walls, respectively.

Abstract: Spacer method is excellent technique of processing porous metals with well-controlled pore characteristics such as porosity (up to 90%) and pore size (as small as several hundred micrometers). Compressive properties of porous aluminum fabricated by the spacer method are investigated. They were subjected to monotonic compression tests at room temperature, and showed less fluctuated flow stress during their compressive deformation than conventional porous aluminum alloy, reflecting their homogeneous pore characteristics. Also, shortening behavior of the porous aluminum fabricated by the spacer method during cyclic compression was significantly differed from that of conventional porous aluminum alloy. Therefore, it can be concluded that the homogeneity of pore characteristics is responsible for compressive properties of porous metals. Monotonic compression tests on porous copper specimens with various porosities, which were made by the spacer method, were also conducted. The yield stress of the porous copper with high porosity (or low relative density) depended on the relative density more strongly than that of the porous copper with low porosity (or high relative density). It is presumed that porous metals with high porosity and ones with low porosities have different deformation mechanisms.

Abstract: Fretting fatigue has grasped strong interest in last decades, some quantitative methods for the evaluation of fretting fatigue were developed. However, only very few studies have been reported on fretting wear, especially on its mechanical model and evaluation method. In this study, cumulative plastic strain is analyzed by FEM. To obtain accurate plastic strain, the shape change due to the plastic deformation has been taken into account. It is found that the cumulative plastic strain will be saturated after several hundred cycles at the initial fretting stage. Considering that fretting wear is very small during this early stage, as it can be observed from the fretting test, the wear at the contact interface before the saturation of cumulative plastic strain can be neglected. Since the saturated cumulative plastic deformation represents the stable deformation of the contact interface, it is proposed that the fretting wear can be characterized by the saturated cumulative plastic strain and accumulative shear stress. With this method, the wear profile of the specimen is predicted. By comparing FEM results with the experimental results of fretting wear, the proposed wear formulation is validated.

Abstract: The gray prediction theory has already been widely applied to solving all kinds of social, scientific and technical problems. It is also used for the prediction of engineering mechanical and material scientific problems. But the traditional gray prediction model is applicable only for equidistant time sequences. In order to extend the scope of application, an improved gray prediction model is put forward. By weighted summation, the new developed gray prediction model can also be applied to the situation of non-equidistant time sequence. The improved gray prediction model is used for the prediction of creep fracture time of rocks and gypsum. The deduced results are proven to be very accurate. The research work of the current paper opens many opportunities for further thoughts of the prediction of many other engineering mechanical problems.

Abstract: It is important for obtaining the relationship between seismic energies of single degree-of-freedom (SDOF) systems and multiple degree-of-freedom (MDOF) structures in engineering. In this paper, the formula of hysteretic energy between the MDOF structures and equivalent SDOF systems is developed. Here is also presented the procedure for estimating hysteretic energy of MDOF structures subjected to severe ground motions employing the energy relation equation based on equivalent SDOF systems. Eight examples for two regular and six irregular MDOF structures show that the procedure to obtain the hysteretic energy demands of MDOF structures may be used as a simple and effective energy estimation method.