Key Engineering Materials Vols. 535-536

Abstract: In this paper, element free Galerkin method (EFGM) has been applied to solve nonlinear solid mechanics problems using updated Lagrangian approach. The nonlinear equations have been solved using Newton Raphson method. An associative J2 flow rule and isotropic hardening has been used for the modelling of elasto-plastic material behaviour. Elastic predictor and plastic corrector algorithm has been used for the integration of incremental stress-strain relation. Few test problems involving large deformations have been simulated and the results obtained by EFGM have been compared with those obtained by FEM.

Abstract: In the study, a software program named “SUPER CE/SE” is developed for the simulation of hypervelocity impact problems with large deformations, high strain rates and spall fractures. In the software program, an Eulerian method consisting of an improved CE/SE (Space-time Conservation Element and Solution Element Method) scheme is used. A void growth model which takes the Bauschinger Effect (BE) into account and a newly proposed front tracking method are adopted in the simulation. The formation and propagation of a crack is described by a newly developed automatic crack growth algorithm. Numerical simulation of spall fracture in a plate when impacted by a spherical projectile at a velocity of 6.0 km/s is carried out. The numerical results are in qualitative agreement with the corresponding experimental data. It turns out that the BE has obvious influence on the length of the crack and better agreement with the experiment is obtained when the BE is considered. It is also validated that the newly proposed front tracking method is feasible and reliable for representing the cracks in the problems with large deformation and high strain rates. According to those research results, it is proved that the software program SUPER CE/SE is robust and effective in the simulation of hypervelocity impact problems.

Abstract: For automotive industries, weight of an automobile can be reduced if TRIP steel which has excellent mechanical properties dominated by strain-induced martensitic transformation (SIMT) can be applied to shock absorption members. However, strain rate sensitivity of TRIP steels has not been fully understood because a relationship between a strain rate and an amount of martensite produced by SIMT is still unclear. In previous studies, volume resistivity and impedance have been measured to obtain information on the amount of produced martensite, however, these studies have not been succeeded to clarify the relationship. Here, by focusing a property that martensite shows ferromagnetism, it is attempted that impedance of TRIP steel is measured at various strain rates during the deformation by using prototype coil and circuits.

Abstract: Based on the dynamic spherical cavity expansion theory of concrete and the analysis of experimental data, a mass abrasion model of projectile considering the hardness of aggregates, the relative strength of target and projectile and the initial impact velocity is constructed in this paper. The initial impact velocity is the most important factor of mass abrasion. The hardness of aggregates and the strength of projectile are also the significant factor of mass abrasion. But relatively speaking, the sensitivity of strength of projectile to mass abrasion is higher, which indicates that the effect of projectile material on mass abrasion is more dramatic than the hardness of aggregates.

Abstract: The dynamic behavior of a radiator support frame impacted by a free-falling engine hood was studied with the use of the 3-D finite element simulations. The engine room including an engine hood, a radiator support frame, and a hood ledge was modeled first. Taking computing efficiency and accuracy into account, approaches for selecting element size, cleaning up complicated features of CAD files were then studied. In order to make the simulations more efficient, the contact spot weld model was adopted. To further reduce the computing time, a theoretical model was also proposed in the present study to calculate the instantaneous angular velocity of the engine hood at the incipient impact to the radiator support frame. The engine hood free-falling tests were also conducted to validate the finite element simulations. The consistency between the experimental data and the simulation results confirms the validity of the finite element model constructed in the present study.

Abstract: This paper examines a projectile impact on a fully clamped sandwich beams with foam core. By modeling the sandwich beam into two perfect rigid-plastic beams connected by perfect rigid-plastic springs, different coupling mechanism for the responses of the two beams are constructed so that a complete solution considering small deformation is derived.

Abstract: The deformation/failure modes and dynamic response of fully clamped cylindrical sandwich shells with aluminum foam cores subjected to air blast loading were investigated experimentally. A four-cable ballistic pendulum system was employed to measure the impulse imparted to the blast-loaded specimen. The deformation/failure modes of specimens were classified and analyzed, the effects of face-sheet thickness, core relative density, specimen curvature and mass of charge on the structural response of metallic sandwich shells were examined. Experimental results indicate that both the deformation/failure modes and the dynamic response of the sandwich shells are sensitive to the structural configurations and blast impulse. The experimental results are useful for validating theoretical predictions, as well as in engineering applications of cellular metal sandwich structures.

Abstract: The strain rate effect of luffa sponge material is an indispensable property for it to be used for acoustic, vibration, and impact energy absorption. Compressive tests at different strain rates on cylindrical column specimens of luffa sponge material were conducted over a wide density ranging from 24 to 64 kg/m³. A photographic technique was applied to measure the section area of the specimen with irregular shape. The mechanical properties of luffa sponge material at various strain rates were obtained based on this measurement. The dynamic data were compared to those of quasi-static experiments. It was found that compressive strength, plateau stress and specific energy absorption of luffa sponge material were sensitive to the rate of loading. Empirical formulae were developed for strength, densification strain and specific energy absorption at various strain rates in the macroscopic level by considering the luffa fiber as base material.

Abstract: The paper presents the analysis of the dynamic response of a cooling tower to moderate earthquake. To represent inelastic behavior of the concrete material of the tower under dynamic loading, the concrete damaged plasticity constitutive model was assumed. The model consists of the combination of non-associated multi-hardening plasticity and scalar damaged elasticity to describe the irreversible damage that occurs during the fracturing process. Two different models of seismic excitation were used. Initially, a classical model of uniform kinematic excitation was applied. In this model it was assumed that excitation at all supports was identical. Then, a model of non-uniform kinematic excitation, typical for large multiple-support structures, was introduced. In that model the wave passage along the foundation ring was taken into account. It occurred that the assumption of asynchronous excitation led to the increase of the dynamic response of the tower with respect to the assumption of uniform ground motion. The tensile damage (cracking) in some parts of the tower appeared and the stiffness of the concrete was degraded when non-uniformity of excitation was considered. This was due to the quasi-static effects resulting from changes of subsoil geometry during the shock. The analysis indicated that the classical assumption of uniform excitation may lead to non-conservative assessment of the dynamic response of the shell described with concrete damaged plasticity model.

Abstract: With a phenomenon of strain-induced martensitic transformation, TRIP steel is expected to show excellent impact energy absorption characteristic. It is important for an improvement of a reliability of TRIP steel to evaluate an amount of martensite. In this study, AISI304, which is a kind of TRIP steel, is deformed plastically by a conventional material testing machine and the split Hopkinson pressure bar apparatus. During the deformation of TRIP steel, a circuit based on the Kevin double bridge measures change in volume resistivity which has a correlation with the amount of martensite. Experimental results show that the change in volume resistivity during the process of deformation at various strain rates.