R.J. Asaro

University of California, San Diego, San Diego, California, United States

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Publications (137)215.65 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The extreme loads generated by blast events, both man-made and accidental, can cause devastating consequences for structures and their components. Typically, the development of analysis methodologies, design procedures and hardening strategies is driven by conclusions that have been obtained experimentally via field testing with live explosives. These types of experiments, although generally effective, are often expensive and due to the extreme environment, do not provide clear visual evidence and quantitative data of structural response throughout the explosive event. In 2006, a blast simulation facility was designed and constructed in order to provide blast-like loadings on structures and components in a controlled laboratory setting. The Blast Simulator, located at the University of California, San Diego, was the first facility to utilize ultra-fast, hydraulically drive, computer-controlled actuators to generate impulsive loadings on full-scale structures. Because the experiments do not involve explosives and the subsequent fireball, high speed cameras and other instrumentation can be effectively used to produce quantitative, as well as high resolution visual data in a repeatable environment. Since 2006, over 500 experiments have been conducted using the Blast Simulator for both validation studies and exploratory research. This paper serves to summarize the current methodology for imparting blast and shock-like loads, as well as to validate the Blast Simulator’s effectiveness in producing accurate, controllable and repeatable loading on a range of civil and military structures.
    Engineering Structures 07/2014; 70:168–180. DOI:10.1016/j.engstruct.2014.03.027 · 1.77 Impact Factor
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    ABSTRACT: The increasing use of composites in high performance structural applications subjected to dynamic loading drives the need for understanding basic material failure properties and structural response as a function of loading rate. Balsa core sandwich beams having woven carbon/vinylester composite facesheets were investigated in both shear and bending failure at quasi-static and dynamic loading speeds producing strain rates ranging from 10−5 to 100 s−1. Short sandwich beam tests show weak rate dependency, with a slight decreasing trend in the balsa core shear failure strain for increased strain rate. Wide scatter in core shear failure strain was observed due to the highly variable nature of the balsa material, as well as its heterogeneous tiled construction within the sandwich panel. Excitation of facesheet compressive failure in sandwich beams with weak core of 50.8 mm thickness required the use of 2.43 m long sandwich beams subject to high speed loading (10 m/s) in order to achieve 100 s−1 strain rate. Facesheet compressive failure strain in the long beam specimens under dynamic loading was measured to be 10% higher than for quasi-static loading. Comparison of long sandwich beam bending results to shorter solid laminate bending strength measurements show a different trend in rate dependency, and thus it is recommended that full-depth sandwich beams (and panels) be used to characterize the failure of these types of sandwich structures.
    Journal of Sandwich Structures and Materials 07/2012; 14(4):365-396. DOI:10.1177/1099636212452054 · 0.84 Impact Factor
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    ABSTRACT: Cyclic nano- and microindentation, along with indentation creep, were performed on nanotwinned Cu with two twin structures, and on nanotwinned Ag. The results provide evidence that nanotwinned face-centered cubic (fcc) structures are more stable than their nanocrystalline counterparts. The results are put in the important context of the available body of theoretical study of nanotwinned fcc metals, and in particular in the context of the theoretical forecasts of Kulkarni and Asaro [Kulkarni Y, Asaro RJ. Acta Mater 2009;57:2711]. It is shown, for example, that, as predicted, nanotwinned Ag displays performance comparable, if not superior, to nanotwinned Cu.
    Acta Materialia 06/2012; 60(11):4623–4635. DOI:10.1016/j.actamat.2012.03.020 · 3.94 Impact Factor
  • Jiddu Bezares, Asaro, Robert, J., Lubarda, Vlado, A.
    01/2012; 39(4):343-363. DOI:10.2298/TAM1204343B
  • Zhangli Peng, Robert J. Asaro, Qiang Zhu
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    ABSTRACT: To quantitatively understand the correlation between the molecular structure of an erythrocyte (red blood cell, RBC) and its mechanical response, and to predict mechanically induced structural remodelling in physiological conditions, we developed a computational model by coupling a multiscale approach of RBC membranes with a boundary element method (BEM) for surrounding Stokes flows. The membrane is depicted at three levels: in the whole cell level, a finite element method (FEM) is employed to model the lipid bilayer and the cytoskeleton as two distinct layers of continuum shells. The mechanical properties of the cytoskeleton are obtained from a molecular-detailed model of the junctional complex. The spectrin, a major protein of the cytoskeleton, is simulated using a molecular-based constitutive model. The BEM model is coupled with the FEM model through a staggered coupling algorithm. Using this technique, we first simulated RBC dynamics in capillary flow and found that the protein density variation and bilayer–skeleton interaction forces are much lower than those in micropipette aspiration, and the maximum interaction force occurs at the trailing edge. Then we investigated mechanical responses of RBCs in shear flow during tumbling, tank-treading and swinging motions. The dependencies of tank-treading frequency on the blood plasma viscosity and the membrane viscosity we found match well with benchmark data. The simulation results show that during tank-treading the protein density variation is insignificant for healthy erythrocytes, but significant for cells with a smaller bilayer–skeleton friction coefficient, which may be the case in hereditary spherocytosis.
    Journal of Fluid Mechanics 11/2011; 686:299 - 337. DOI:10.1017/jfm.2011.332 · 2.29 Impact Factor
  • Antony Chen, Hyonny Kim, Robert J. Asaro, Jiddu Bezares
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    ABSTRACT: A dynamic loading method for simulating explosive blast was developed using a crushing foam projectile launched by a gas gun at velocities ranging from 30 to 60m/s. The objective of this test method is to dynamically load “small-scale” composite beam specimens so as to allow for the dynamic failure characterization of these materials subject to blast-type loads. Such non-explosive test methods are desirable since they are more easily implemented, less expensive, and safer than actual explosive blast tests. Furthermore, instrumentation and visual documentation of the experiment is more easily achieved in the absence of “spatially extended” blast pressures, fireballs, and other destructive phenomena associated with the use of explosives that obscure observation. This paper focuses on the description of the pressure pulse generating projectile development and the dynamic pressure profiles produced. Control of the pressure pulse is achieved via selection of foam density used as a crushable media in the projectile, developing peak pressures ranging from 3 to 6.5MPa. Application of this projectile onto beam specimens show dramatic rate dependency of the composite compressive failure strain (143% higher for strain rate of 101s−1 relative to quasi-static), whereas the tensile failure strain showed less, yet still significant, increase (29% for 101s−1 strain rate relative to quasi-static) for dynamic loading rates. Sandwich beams all failed in core shear for the configuration tested, with no significant rate dependency for the shear failure strain of the balsa core material.
    Composite Structures 10/2011; 93(11). DOI:10.1016/j.compstruct.2011.05.027 · 3.12 Impact Factor
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    Pei Gu, Ming Dao, Robert J. Asaro, Subra Suresh
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    ABSTRACT: We present a unified mechanistic model to rationalize size-dependent flow stress, activation volume and strain-rate sensitivity for metals with either nanocrystalline grains or nanoscale twins. The non-uniform partial dislocation model for flow stress [Asaro and Suresh, Acta Mater, Vol. 53, pp. 3369–3382, 2005; Gu et al., Scripta Mater, Vol. 62, pp. 361–364, 2010] is generalized here to consider both grain-size dependence and twin-thickness dependence of nanotwinned metals. A non-homogeneous nucleation model is proposed to predict the dependence of activation volume on both grain-size and twin-thickness. With the activation volume predicted from the non-homogeneous nucleation model and the flow stress obtained via the non-uniform partial dislocation model, strain-rate sensitivity as a function of characteristic structural length scale is also evaluated. This provides a unified approach from envisioning partial dislocation emission for the three size-dependent parameters characterizing the plastic deformation mechanism, flow stress, activation volume and strain-rate sensitivity, so that each one of these parameters is predicted when the geometry of the grains or nanotwins is known. The model predictions are shown to be consistent with a variety of available experimental data.
    Acta Materialia 10/2011; 59(18):6861-6868. DOI:10.1016/j.actamat.2011.07.019 · 3.94 Impact Factor
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    ABSTRACT: Characterizing structural responses and applied loads during the entire course of a blast event is problematic due to the harsh conditions of the explosive environment. A procedure for the distribution of blast-like pressures to structures of complex geometries using custom water bladders has been developed using the University of California, San Diego’s (UCSD) Blast Simulator. The methodology was motivated by an effort to test the blast resistance of structures subject to internal, or external, blasts where attention would be focused on areas such as joints, corners, or other areas within occluded geometry.Three series of experiments were conducted in an effort to characterize the use of water bladders for blast simulations. Bladder material, geometry, use of baffles and strapping methods were varied along with Simulator input parameters such as impact velocity and impacting mass geometry. The effects of these variables have been quantified through the comparison of measured pressures, pulse durations and impulses. The experimental methodology demonstrates the ability to tailor load curves to simulate a wide range of blast scenarios.
    International Journal of Impact Engineering 07/2011; 38(7):546-557. DOI:10.1016/j.ijimpeng.2010.06.002 · 2.01 Impact Factor
  • Ming Dao, Bimal K. Kad, Robert J. Asaro
    MRS Online Proceeding Library 01/2011; 364. DOI:10.1557/PROC-364-1029
  • Bimal K. Kad, Kenneth S. Vecchio, Robert J. Asaro
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    ABSTRACT: Plasma-sprayed microstructures of MoSi2 have been studied by electron microscopy. The as-deposited microstructures are metastable and inhomogenous. Two new dissociations of [001] and 1/2[331] dislocations have been observed for the first time. The 1/2[331] is dissociated on the (lTO) plane bounding a superlattice intrinsic stacking fault (SISF). The reaction is given as: 1/2[331] = 1/4[331] + SISF + [331]. The [001] is also dissociated on (110) plane into two symmetrical components, with the reaction being given as [001] = 1/2[001] + 1/2[001].
    MRS Online Proceeding Library 01/2011; 288. DOI:10.1557/PROC-288-1123
  • Pei Gu, R. J. Asaro
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    ABSTRACT: This paper presents an analytical solution of skin wrinkling for sandwich polymer matrix composite panels in a combined thermal–mechanical condition. The thermal gradient in the transverse direction is induced by one-sided fire exposure, and the mechanical load is the in-plane compression. Due to low thermal conductivities of polymer matrix composites, the thermal gradient exists for a long period of time. The material properties of polymer matrix composites are degraded as temperature rises. These behaviors induce mechanical properties' gradients along the transverse direction. The general solution for the wrinkling load in the thermal–mechanical loading condition is investigated. The solution is characterized in terms of two non-dimensional parameters that represent material properties and dimensional lengths of the skin and the core. The wrinkling load is presented for fairly complete ranges of the two non-dimensional parameters. The wrinkling load is also derived from Winkler model for non-homogeneous materials. An example of thermal-mechanical simulation to design the wrinkling load-bearing capacity of a panel exposed to fire is given.
    Thin-Walled Structures 01/2011; 51. DOI:10.1016/j.tws.2011.10.008 · 1.43 Impact Factor
  • MRS Online Proceeding Library 01/2011; 322. DOI:10.1557/PROC-322-49
  • Jiddu Bezares, Robert J Asaro, Marilyn Hawley
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    ABSTRACT: Direct experimental probes of the mechanical response of the biopolymer framework of nacre extracted from the shell of the gastropod Haliotis rufescens have been performed. Both monotonic tensile, and time dependent relaxation, tests revealed that the tissue comprising the interlamellar layers within nacre obeyed a simple constitutive model conforming to the visco-elastic standard linear solid, with time constants in the range tau=140+/-4s. We conclude that the behavior is essentially that imparted by the chitin core of these layers. Interestingly we find that the chitin network of the core appears to be connected over multiple CaCO(3) tiles. A simple composite model is formulated and used to interpret the observed behavior.
    Journal of Structural Biology 06/2010; 170(3):484-500. DOI:10.1016/j.jsb.2010.01.006 · 3.37 Impact Factor
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    Zhangli Peng, Robert J Asaro, Qiang Zhu
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    ABSTRACT: To quantitatively predict the mechanical response and mechanically induced remodeling of red blood cells, we developed a multiscale method to correlate distributions of internal stress with overall cell deformation. This method consists of three models at different length scales: in the complete cell level the membrane is modeled as two distinct layers of continuum shells using finite element method (Level III), in which the skeleton-bilayer interactions are depicted as a slide in the lateral (i.e., in-plane) direction (caused by the mobility of the skeleton-bilayer pinning points) and a normal contact force; the constitutive laws of the inner layer (the protein skeleton) are obtained from a molecular-based model (Level II); the mechanical properties of the spectrin (Sp, a key component of the skeleton), including its folding/unfolding reactions, are obtained with a stress-strain model (Level I). Model verification is achieved through comparisons with existing numerical and experimental studies in terms of the resting shape of the cell as well as cell deformations induced by micropipettes and optical tweezers. Detailed distributions of the interaction force between the lipid bilayer and the skeleton that may cause their dissociation and lead to phenomena such as vesiculation are predicted. Specifically, our model predicts correlation between the occurrence of Sp unfolding and increase in the mechanical load upon individual skeleton-bilayer pinning points. Finally a simulation of the necking process after skeleton-bilayer dissociation, a precursor of vesiculation, is conducted.
    Physical Review E 03/2010; 81(3 Pt 1):031904. DOI:10.1103/PhysRevE.81.031904 · 2.33 Impact Factor
  • Pei Gu, R.J. Asaro
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    ABSTRACT: This paper discusses the influence of material nonlinearity on thermal distortion of polymer matrix composite panels. The load is transverse temperature gradient introduced by one-sided heat exposure, e.g., fire. For such low thermal conductivity materials, when there are no external mechanical loads, the transverse thermal gradient induces non-uniform thermal expansion along the thickness that results in transverse deformation field, known as thermal distortion. The power form stress–strain relation and the Ramberg–Osgood form stress–strain relation are discussed to include the temperature dependent behavior of polymer matrix composites. The degradation of polymer matrix composites at elevated temperature, thermal softening, is discussed. The variations of the reference stress and the reference strain with temperature are specified to describe the temperature dependent constitutive relationships. Semi-analytical simulation and finite element simulation are carried out for panels with roller end support condition. Results suggest that, while the material nonlinearity has insignificant influence on the transverse displacement of the panel, it has strong influence on local stress and strain. The stress and strain can go beyond the yield stress and the yield strain into plastic stage in certain circumstances.
    Composites Part B Engineering 01/2010; 41(1):58–66. DOI:10.1016/j.compositesb.2009.06.006 · 2.60 Impact Factor
  • Qiang Zhu, Zhangli Peng, Robert J. Asaro
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    ABSTRACT: Erythrocyte (red blood cell, or RBC) possesses one of the simplest and best characterized molecular architectures among all cells. It contains cytosol enclosed inside a composite membrane consisting of a fluidic lipid bilayer reinforced by a single layer of protein skeleton pinned to it. In its normal state, this system demonstrates tremendous structural stability, manifested in its ability to sustain large dynamic deformations during circulation. On the other hand, it has been illustrated in experiments that triggered by mechanical loads structural remodeling may occur. A canonical example of this remodeling is vesiculation, referring to the partial separation of the lipid bilayer from the protein skeleton and the formation of vesicles that contain lipids only.
    ASME 2010 First Global Congress on NanoEngineering for Medicine and Biology; 01/2010
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    Yashashree Kulkarni, Robert J. Asaro
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    ABSTRACT: Here we investigate whether certain face-centered cubic metals display a superior behavior of nanotwinned structures compared to others. We also address the question of an optimal lamella thickness that yields maximum strength and stability. Our analysis of the intrinsic stacking fault energies, c sf , and the unstable stacking fault energies, c us , of Al, Pd, Cu and Ag, as well as our atomistic simu-lations of dislocation–twin boundary interactions in these metals, suggests an optimal behavior of nanotwinned Pd and Ag as compet-itive to Cu, and hence a special utility in their synthesis and further exploration. Our results also indicate that the influence of twin–twin interactions may lead to a loss of strength below a critical value of twin lamella thickness.
    Acta Materialia 09/2009; 57(16). DOI:10.1016/j.actamat.2009.06.047 · 3.94 Impact Factor
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    Pei Gu, Ming Dao, R. J. Asaro
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    ABSTRACT: Development in advanced composite fabrication technology offers the clear prospect of cost effective application of polymer matrix composites for large load-bearing structures. However, polymer matrix composites can be severely degradated under the thermal condition caused by fire. This paper addresses the compressive load-bearing capacity for polymer matrix composite panels in naval structures and civil infrastructures under the combined thermal–mechanical condition. The failure modes arising from structural instability for single skin and sandwich panels in such combined thermal–mechanical condition are the focus in this study. The thermal field under fire heating and the degradation of mechanical properties with elevated temperature are discussed. Analytical solutions for these mechanical failure modes are presented for design considerations. The approach to the development of a quantitative methodology for fire protection design is discussed in the context of the analyses and the experiments. Design diagrams are constructed to design mechanical loads for given fire protection time, and on the opposite, to design fire protection time for given mechanical loads.
    Marine Structures 07/2009; 22(3):354-372. DOI:10.1016/j.marstruc.2009.04.001 · 1.24 Impact Factor
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    Pei Gu, R.J. Asaro
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    ABSTRACT: This is the continuation of our published paper [Gu P, Asaro RJ. Designing polymer matrix composite panels for structural integrity in fire. Compos Struct 2008;84:300–9]. This paper addresses the compressive mechanical load bearing capacity of sandwich polymer matrix composite panels under transverse thermal gradients caused by fire. An example of such combined loading condition is that of panels in ship and vessel structures under transverse thermal gradients caused by fire. Specifically, we discuss the mechanical design considerations, failure modes that lead the panels to fail in the combined thermal–mechanical condition. An approach to assess critical load among the failure modes is discussed for design purpose. A computer program is developed based on the approach to evaluate sandwich panel’s load bearing capacity in fire. Examples are presented to design the panel for given thermal and mechanical requirements. Due to the core’s low bending stiffness, there is a large decrease of the panel’s load bearing capacity when the fire-exposed skin is degradated.
    Composite Structures 05/2009; DOI:10.1016/j.compstruct.2008.05.006 · 3.12 Impact Factor
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    Yashashree Kulkarni, Robert J. Asaro, Diana Farkas
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    ABSTRACT: The optimality of nanotwinned structures in imparting maximum strength and stability has been investigated by performing atomistic simulations of dislocation interactions with various types of grain boundaries. By developing an understanding of the mechanism of the instability, we predict that nanocrystalline metals with grain sizes below 50 ± 10 nm or 70 ± 10 nm (in Cu) are inherently unstable at 0 or 300 K, respectively, in contrast to nanotwinned face-centered cubic structures which are stable and optimal.
    Scripta Materialia 04/2009; 60(7):532-535. DOI:10.1016/j.scriptamat.2008.12.007 · 2.97 Impact Factor

Publication Stats

6k Citations
215.65 Total Impact Points

Institutions

  • 1990–2012
    • University of California, San Diego
      • Department of Structural Engineering
      San Diego, California, United States
  • 1997
    • National Tsing Hua University
      Hsin-chu-hsien, Taiwan, Taiwan
  • 1977–1993
    • Brown University
      • School of Engineering
      Providence, RI, United States
  • 1989
    • Sandia National Laboratories
      Albuquerque, New Mexico, United States
  • 1982–1983
    • University of Rhode Island
      Кингстон, Rhode Island, United States