Evolving Material Porosity on an Additive Manufacturing Simulation with the Generalized Method of Cells

The effect of material porosity on final part distortion and residual stresses in a selective laser sintering manufacturing simulation is presented here. A time-dependent thermomechanical model is used with the open-source FEA software CalculiX. Effective homogenized material properties for Inconel 625 are precomputed using NASAs Micromechanics Analysis Code with Generalized Method of Cells (MAC/GMC). The evolving porosity of the material is estimated with each pass of the laser beam during simulation runtime. A comparison with a homogenous model and the evolving model shows that the evolving porous model predicts larger distortions with greater residual stresses

T650/AFR-PE-4/FM680-1 Mode I Critical Energy Release Rate at High Temperatures: Experiments and Numerical Models

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76240/1/AIAA-2007-2305-492.pd

Efficient and Robust Traction Laws for the Modeling of Adhesively Bonded Joints

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76317/1/AIAA-2008-1847-586.pd

Predictions of Delamination of a Stiffened Panel Using a Cohesive Zone Model

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/83573/1/AIAA-2010-2617-230.pd

Comparing modern and presettlement forest dynamics of a subboreal wilderness: Does spruce budworm enhance fire risk?

Insect disturbance is often thought to increase fire risk through enhanced fuel loadings, particularly in coniferous forest ecosystems. Yet insect disturbances also affect successional pathways and landscape structure that interact with fire disturbances (and vice-versa) over longer time scales. We applied a landscape succession and disturbance model (LANDIS-II) to evaluate the relative strength of interactions between spruce budworm (Choristoneura fumiferana) outbreaks and fire disturbances in the Boundary Waters Canoe Area (BWCA) in northern Minnesota (USA). Disturbance interactions were evaluated for two different scenarios: presettlement forests and fire regimes vs. contemporary forests and fire regimes. Forest composition under the contemporary scenario trended toward mixtures of deciduous species (primarily Betula papyriferaand Populus spp.) and shade-tolerant conifers (Picea mariana, Abies balsamea, Thuja occidentalis), with disturbances dominated by a combination of budworm defoliation and high-severity fires. The presettlement scenario retained comparatively more “big pines” (i.e., Pinus strobus, P. resinosa) and tamarack (L. laricina), and experienced less budworm disturbance and a comparatively less-severe fire regime. Spruce budworm disturbance decreased area burned and fire severity under both scenarios when averaged across the entire 300-year simulations. Contrary to past research, area burned and fire severity during outbreak decades were each similar to that observed in non-outbreak decades. Our analyses suggest budworm disturbances within forests of the BWCA have a comparatively weak effect on long-term forest composition due to a combination of characteristics. These include strict host specificity, fine-scaled patchiness created by defoliation damage, and advance regeneration of its primary host, balsam fir (A. balsamea) that allows its host to persist despite repeated disturbances. Understanding the nature of the three-way interaction between budworm, fire, and composition has important ramifications for both fire mitigation strategies and ecosystem restoration initiatives. We conclude that budworm disturbance can partially mitigate long-term future fire risk by periodically reducing live ladder fuel within the mixed forest types of the BWCA but will do little to reverse the compositional trends caused in part by reduced fire rotations

Dimensionless parameters in symmetric double lap joints: an orthotropic solution for thermomechanical loading

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76627/1/AIAA-2006-1959-520.pd

Analytical and Experimental Methods for Adhesively Bonded Joints Subjected to High Temperatures.

Recent advances in material systems have expanded the temperature range over which adhesively bonded composite joints can be used. In this work, several tools are developed for use in modeling joints over a broad range of temperatures. First, a set of dimensionless parameters is established which can be used for analysis of joint performance for an orthotropic symmetric double lap joint. A critical dimensionless ratio of mechanical and thermal loads is identified. The ratio predicts characteristics of the resulting stress distribution. A bonded joint finite element is also developed, wherein a joint-specific finite element is formulated based on an analytical solution. The resulting element allows for mesh-independent joint evaluation and multi-joint simulation at a system or vehicle level. As a mid-level analysis technique, the element has significant predictive and cost advantages over the previously available methods. An advanced analysis technique, the discrete cohesive zone method, is developed and demonstrated in a general element formulation. Initially, the element is examined from the perspective of computational efficiency and robustness. Two efficient traction laws are formulated and are compared to a traction law that is in common use. The element is subsequently used to investigate the interactions of adhesive parameters in standard adhesive characterization experiments. This quantification of experimental sensitivities allows for a deliberate mapping of cumulative experimental results to an appropriate set of model constitutive parameters. With knowledge of the parameter interactions, a set of experimental results are interpreted to determine a set of adhesive constitutive parameters for T650/AFR-PE-4/FM680-1, a high temperature material system of current interest.Ph.D.Aerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/60791/1/petegus_1.pd

Is there screwiness at the end of the QCD cascades?

We discuss what happens at the end of the QCD cascades. We show that, with just a few reasonable assumptions, the emission of soft gluons is constrained to produce an ordered field in the form of a helix. We describe how to modify the Lund fragmentation scheme in order to fragment such a field. Our modified fragmentation scheme yields results which are consistent with current experimental measurements, but predicts at least one signature which should be observable.Comment: 21+1 page

Ordered greed

Scheduling problems are among the most challenging and realistic problems application of problem solving heuristics, such as genetic algorithms (GAs). The naive greedy algorithm for scheduling simply assigns, in turn, each item to be scheduled the best yet untaken position for that item. We investigate using a genetic algorithm to search the space of orderings for this greedy algorithm. That is, the GA individuals are permuations that determine the permutations that are the schedules, rather than the GA individuals directly being the schedules. We have experimented with the classical N Queens problem anda realistic soccer tournament scheduling problem, comparing the GA individual as the assignment with our greedy hybrid algorithm ( ordered greed ). Warnsdorff\u27s heuristic is introduced to modify blind greed with excellent results. We also introduce the use of signatures in our GAs to represent permutations. Signatures are easy to create and manipulate in crossover and mutation operations

Covariate balance in a Bayesian propensity score analysis of beta blocker therapy in heart failure patients

Regression adjustment for the propensity score is a statistical method that reduces confounding from measured variables in observational data. A Bayesian propensity score analysis extends this idea by using simultaneous estimation of the propensity scores and the treatment effect. In this article, we conduct an empirical investigation of the performance of Bayesian propensity scores in the context of an observational study of the effectiveness of beta-blocker therapy in heart failure patients. We study the balancing properties of the estimated propensity scores. Traditional Frequentist propensity scores focus attention on balancing covariates that are strongly associated with treatment. In contrast, we demonstrate that Bayesian propensity scores can be used to balance the association between covariates and the outcome. This balancing property has the effect of reducing confounding bias because it reduces the degree to which covariates are outcome risk factors