Improved contact algorithm for the material point method and application to stress propagation in granular material

Journal ArticleContact between deformable bodies is a difficult problem in the analysis of engineering systems. A new approach to contact has been implemented using the Material Point Method for solid mechanics, Bardenhagen, Brackbill, and Sulsky (2000a). Here two improvements to the algorithm are described. The first is to include the normal traction in the contact logic to more appropriately determine the free separation criterion. The second is to provide numerical stability by scaling the contact impulse when computational grid information is suspect, a condition which can be expected to occur occasionally as material bodies move through the computational grid. The modifications described preserve important properties of the original algorithm, namely conservation of momentum, and the use of global quantities which obviate the need for neighbor searches and result in the computational cost scaling linearly with the number of contacting bodies. The algorithm is demonstrated on several examples. Deformable body solutions compare favorably with several problems which, for rigid bodies, have analytical solutions. A much more demanding simulation of stress propagation through idealized granular material, for which high fidelity data has been obtained, is examined in detail. Excellent qualitative agreement is found for a variety of contact conditions. Important material parameters needed for more quantitative comparisons are identified

Shear deformation in granular materials

An investigation into the properties of granular materials is undertaken via numerical simulation. These simulations highlight that frictional contact, a defining characteristic of dry granular materials, and interfacial debonding, an expected deformation mode in plastic bonded explosives, must be properly modeled. Frictional contact and debonding algorithms have been implemented into FLIP, a particle in cell code, and are described. Frictionless and frictional contact are simulated, with attention paid to energy and momentum conservation. Debonding is simulated, with attention paid to the interfacial debonding speed. A first step toward calculations of shear deformation in plastic bonded explosives is made. Simulations are performed on the scale of the grains where experimental data is difficult to obtain. Two characteristics of deformation are found, namely the intermittent binding of grains when rotation and translation are insufficient to accommodate deformation, and the role of the binder as a lubricant in force chains

Generalized Interpolation Material Point Approach to High Melting Explosive with Cavities Under Shock

Criterion for contacting is critically important for the Generalized Interpolation Material Point(GIMP) method. We present an improved criterion by adding a switching function. With the method dynamical response of high melting explosive(HMX) with cavities under shock is investigated. The physical model used in the present work is an elastic-to-plastic and thermal-dynamical model with Mie-Gr\"uneissen equation of state. We mainly concern the influence of various parameters, including the impacting velocity $v$, cavity size $R$, etc, to the dynamical and thermodynamical behaviors of the material. For the colliding of two bodies with a cavity in each, a secondary impacting is observed. Correspondingly, the separation distance $D$ of the two bodies has a maximum value $D_{\max}$ in between the initial and second impacts. When the initial impacting velocity $v$ is not large enough, the cavity collapses in a nearly symmetric fashion, the maximum separation distance $D_{\max}$ increases with $v$. When the initial shock wave is strong enough to collapse the cavity asymmetrically along the shock direction, the variation of $D_{\max}$ with $v$ does not show monotonic behavior. Our numerical results show clear indication that the existence of cavities in explosive helps the creation of ``hot spots''.Comment: Figs.2,4,7,11 in JPG format; Accepted for publication in J. Phys. D: Applied Physic

On higher order gradient continuum theories in 1-D nonlinear elasticity. Derivation from and comparison to the corresponding discrete models

Higher order gradient continuum theories have often been proposed as models for solids that exhibit localization of deformation (in the form of shear bands) at sufficiently high levels of strain. These models incorporate a length scale for the localized deformation zone and are either postulated or justified from micromechanical considerations. Of interest here is the consistent derivation of such models from a given microstructure and the subsequent comparison of the solution to a boundary value problem using both the exact microscopic model and the corresponding approximate higher order gradient macroscopic model.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42682/1/10659_2004_Article_BF00043251.pd

The Seniors' outdoor survey : an observational tool for assessing outdoor environments at long-term care settings

Purpose of the Study: To describe the development and psychometric testing of the Seniors\u2019 Outdoor Survey (SOS), an instrument for evaluating how well the outdoor space in a long-term care setting supports the preferences and outdoor usage of residents. Design and Methods: Content validity of the main SOS items initially was based on relevant literature and preliminary studies in diverse long-term care settings. After conducting a multiregional pilot study with 152 outdoor spaces at 68 assisted living facilities, the instrument was substantially revised and tested for interrater and test\u2013retest reliability with 22 outdoor spaces at 12 long-term care settings, using 2 raters. Validity was examined using content analysis of resident survey responses (N = 1,128) from the multiregional study and specific item validation by subject matter experts (N = 53). Results: The final instrument contains 60 ratable items organized in 5 domains: access to nature (14 items), outdoor comfort and safety (15 items), walking and outdoor activities (14 items), indoor\u2013outdoor connection (11 items), and connection to the world (6 items). Intraclass correlation coefficient (ICC) estimates of interrater reliability were .91 for the overall instrument, ranging from .83 to .98 for the 5 domains. Interrater reliability (ICC) was above .70 for more than 79% of individual items. Test\u2013retest reliability (ICC) was .92, ranging from .81 to .98 for domains. Implications: The SOS tool fills a gap in the available environmental assessment instruments, providing a reliable way for researchers, providers, and designers to evaluate and compare the supportive potential of outdoor spaces for long-term care residents

Viscoelastic models for explosive binder materials

An improved model of the mechanical properties of the explosive contained in conventional munitions is needed to accurately simulate performance and accident scenarios in weapons storage facilities. A specific class of explosives can he idealized as a mixture of two components: energetic crystals randomly suspended in a polymeric matrix (binder). Strength characteristics of each component material are important in the macroscopic behavior of the composite (explosive). Of interest here is the determination of an appropriate constitutive law for a polyurethane binder material. This paper is a continuation of previous work in modeling polyurethane at moderately high strain rates and for large deformations. Simulation of a large deformation (strains in excess of 100%) Taylor Anvil experiment revealed numerical difficulties which have been addressed. Additional experimental data have been obtained including improved resolution Taylor Anvil data, and stress relaxation data at various strain rates. A thorough evaluation of the candidate viscoelastic constitutive model is made and possible improvements discussed

The Seniors’ Outdoor Survey (SOS Tool) : comparing ratings and reliability between Italy and the USA

To test applicability of an environmental assessment instrument in new settings, the ratings and reliability of the SOS Tool were compared in diverse settings with a similar climate: Milan, Italy; and Bryan/College Station, Texas, USA. This instrument evaluates physical environment support for outdoor usage by residents at senior facilities. Two trained raters assigned ratings (using a 1\u20137 scale) to the 60 instrument items at each setting (Italy: 94 outdoor spaces, 67 facilities; US: 22 outdoor spaces, 12 facilities). Overall mean ratings were similar across both sites (6% difference), and three of the five domains had comparable mean ratings. Mean inter-rater reliability was good in both settings but higher in Italy (ICC\ua0=\ua0.97 versus.82). Despite geographic/cultural differences, mean ratings for most items and domains were surprisingly similar between Italy and the US. This finding, and the high inter-rater reliability found in both settings, suggest this instrument may be useful in a wide range of settings