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Open Access Publications from the University of California

Scholarly Works (2 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2013)
Numerical simulations of strong shock wave compression of Al-W laminate materials with different cell size ranging from 1 mm to 4 mm were conducted. Finite element simulations using LS-DYNA were employed with two different durations of loading pulse tailored by changing the length of the impactor. The results of the numerical simulations for homogeneous materials were used for validation of our code by comparing the results of the pressure, the temperature and the final volume with the available Hugoniot data. The focus of this work was on the study of the mechanical parameters and thermodynamic states reached through multiple shock reverberations. The mechanism of the formation of the steady leading front and final states were analyzed under different conditions of loading and laminate mesostructure. It was shown that a steady shock front can be reached even when the final steady state is not established. The results of the numerical simulations of the pressure, the specific volume and the temperature were compared to the values predicted by two models to assess whether these models correctly predict the final state. This work also includes a brief exploration of the effects of mesh refinement and shock viscosity on the simulated final state of the material. It is concluded that an increasing number of interfaces (through the reduction of the layer thicknesses) helps to achieve a steady shape of the leading front and the equilibrium state behind the shock faster. The investigated models correctly predict the final state (within the explored ranges of pressure) in the pressure-specific volume plane. It was also found that the final thermodynamic state is more sensitive to the composition of the layered materials and that the predictions of the models differ from our calculations even when a steady state behind the shock was reached
    Cover page: Finite Duration Shocks on Discrete Laminate Composite Al/W /
    • Thesis
    • Peer Reviewed
    UC San Diego Electronic Theses and Dissertations (2018)

    Heterogeneous materials under dynamic loading exhibit dramatic transformations resulting in self-organization and in situ microstructural changes due to the complex interaction of nonlinearity, dispersion, different mechanisms of softening, and fracture. We use numerical simulations to explain the diverse mechanisms of deformation, localization and in situ patterning exhibited by these materials and compare them to experimental results and phenomenological models.

    In Chapters 2 and 3 we study dissipative Al-W laminates with unit cells 1, 2 and 4 mm under high velocity impact loading of different durations. Depending on the duration of the loading pulse, a qualitatively new solitarylike wave, a train of such waves or a quasistationary shockwave is formed in these dissipative laminates. A phenomenological model based on the Korteweg-de Vries equation is formulated introducing important space/time scaling for solitary and shock waves depending on materials properties.

    In Chapter 4, Thick-Walled Cylinder experiments were conducted with highly heterogeneous porous mixtures of Al and W granular materials. Results were compared with numerical simulations to explore the effects of grain size and porosity on the mechanism of cavity instability. Numerical simulations demonstrated that a shear band like phenomena occurs along weak paths created by softer Al particles. The samples with initial porosity, delayed this mechanism of instability. Samples with grain sizes 400 and 100 micron demonstrated loss of cylindrical symmetry during collapse and sample with grain size 40 micron collapsed with mostly preserved cylindrical symmetry. This was consistent with experimental observation of cavity collapse in mixtures with 40 and 1 micron W particles.

    In Chapter 5, explosively driven Thick-Walled Cylinder experiments were conducted with samples of 4340 steel having variable microstructures achieved through heat treatment. It was observed experimentally that the change in the initial microstructure resulted in completely different dynamic behavior and self-organization of shear bands/cracks. The heat treated sample, catastrophically failed due the formation and interaction of shear bands developing into cracks. Numerical simulations are compared to the experimental results. It was shown that the inclusion of artificial defects is necessary to reproduce self-organized pattern of shear bands and cracks observed in experiments.

      Cover page: Addressing complexity of materials behavior under extreme dynamic conditions using numerical modeling
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