Finite Strain Topology Optimization with Nonlinear Stability Constraints

This paper proposes a computational framework for the design optimization of stable structures under large deformations by incorporating nonlinear buckling constraints. A novel strategy for suppressing spurious buckling modes related to low-density elements is proposed. The strategy depends on constructing a pseudo-mass matrix that assigns small pseudo masses for DOFs surrounded by only low-density elements and degenerates to an identity matrix for the solid region. A novel optimization procedure is developed that can handle both simple and multiple eigenvalues wherein consistent sensitivities of simple eigenvalues and directional derivatives of multiple eigenvalues are derived and utilized in a gradient-based optimization algorithm - the method of moving asymptotes. An adaptive linear energy interpolation method is also incorporated in nonlinear analyses to handle the low-density elements distortion under large deformations. The numerical results demonstrate that, for systems with either low or high symmetries, the nonlinear stability constraints can ensure structural stability at the target load under large deformations. Post-analysis on the B-spline fitted designs shows that the safety margin, i.e., the gap between the target load and the 1st critical load, of the optimized structures can be well controlled by selecting different stability constraint values. Interesting structural behaviors such as mode switching and multiple bifurcations are also demonstrated.Comment: 77 pages, 44 Figure

Computational frameworks for homogenization and multiscale stability analyses of nonlinear periodic metamaterials

This paper presents a consistent computational framework for multiscale 1st order finite strain homogenization and stability analyses of rate-independent solids with periodic microstructures. Based on the principle of multiscale virtual power, the homogenization formulation is built on a priori discretized microstructure, and algorithms for computing the matrix representations of the homogenized stresses and tangent moduli are consistently derived. The homogenization results lose their validity at the onset of 1st bifurcation, which can be computed from multiscale stability analysis. The multiscale instabilities include: a) microscale structural instability which is calculated by Bloch wave analysis; and b) macroscale material instability which is calculated by rank-1 convexity checks on the homogenized tangent moduli. Details on the implementation of the Bloch wave analysis are provided, including the selection of the wave vector space and the retrieval of the real-valued buckling mode from the complex-valued Bloch wave. Three methods are detailed for solving the resulted constrained eigenvalue problem - two condensation methods and a null-space based projection method. Both implementations of the homogenization and stability analyses are validated using numerical examples including hyperelastic and elastoplastic metamaterials. Various microscale buckling phenomena are also demonstrated by examining several representative metamaterial examples. Aligned with theoretical results, the numerical results show that the microscopic long wavelength buckling can be equivalently detected by the loss of rank-1 convexity of the homogenized tangent moduli.Comment: 75 pages, 39 Figure

Multi-Scale Computational Simulation of Progressive Collapse of Steel Frames.

Progressive building collapse occurs when failure of a structural component leads to the failure and collapse of surrounding members, possibly promoting additional collapse. Global system collapse will occur if the damaged system is unable to reach a new static equilibrium configuration. The objective of this research is to identify and investigate important issues related to collapse of seismically designed steel building systems using multi-scale computational models. Coupled multi-scale finite element simulations are first carried out to investigate the collapse response of moment resisting steel frame sub-assemblages. Simulation results suggest that for collapse resistant construction, designers should strive to use a larger number of smaller beam members rather than concentrate resistance in a few larger members and should specify ASTM A-992 steel rather than specifying generic steels. Improved behavior can also be achieved by increasing the shear tab thickness or directly welding the beam web to the column. Using information gleaned from the sub-assemblage simulations, computationally efficient structural scale models for progressive collapse analysis of seismically designed steel frames systems are developed. The models are calibrated and utilized within the context of the alternate path method to study the collapse resistance of multistory steel moment and braced frame building systems. A new analysis technique termed “pushdown analysis” is proposed and used to investigate collapse modes, failure loads and robustness of seismically designed frames. The collapse and pushdown analyses show that systems designed for high seismic risk are less vulnerable to gravity-induced progressive collapse and more robust than those designed for moderate seismic risk. Motivated by a number of deficiencies in existing ductile fracture models for steel, a new micro-mechanical constitutive model is proposed. Damage mechanics principles are used and a scalar damage variable is introduced to represent micro-structural evolution related to micro-void nucleation, growth and coalescence during the ductile fracture process in steels. Numerical implementation and parametric studies are presented and discussed. Calibration and validation studies show that the proposed model can successfully represent ductile fracture of steels. The models and simulation methodologies developed herein can be extended in future work to address the collapse resistance of three-dimensional models.Ph.D.Civil EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/61630/1/kapilk_1.pd

Alcohol and Tobacco Use Among Undergraduate and Postgraduate Medical Students in India: A Multicentric Cross-sectional Study

Background: Substance use among medical fraternity is a well-known phenomenon among both undergraduate (UG) and postgraduate (PG) medical students. Yet, there have been very few multi-centric studies to estimate the actual burden of this problem in this important population group in India. This study was conducted to estimate the prevalence of alcohol and tobacco use, assess the knowledge and attitudes towards this issue, and identify factors associated with substance use among UG and PG medical students in India.Methods: A pre-tested, self-report, anonymous questionnaire was administered to medical undergraduates and post graduate medical residents of eight medical colleges across India. This study used a convenience sample of medical colleges with random selection of study participants within each college for each group, UG and PG.Results: Prevalence of alcohol and tobacco use among UG students was 16.6%, 95% CI [14.5, 18.9] and 8.0%, 95% CI [6.4, 9.6], respectively, whereas prevalence was 31.5%, 95% CI [26.3, 37.0] and 14.5%, 95% CI [10.7, 18.9], respectively for PGs. For both substances, males had a higher prevalence of use compared to females in both groups (p < 0.001). Positive family history of substance use (p < 0.001 for both groups) and early age of initiation (p = 0.011 for tobacco; p > 0.05 for alcohol) were associated with a greater difficulty to quit the habit. Over 90% of study participants felt that substance use adversely affected their skills and reported not using substances prior to managing their patients.Conclusions: Since substance use is a relatively common phenomenon among UG and PG medical students in India, future prospective studies and interventions are required to better understand the pattern of substance use and reduce its prevalence.

Morphological, Biochemical, and Molecular Characterization of Orange-Fleshed Sweet Potato (Ipomoea batatas [L.] Lam) Germplasms

The sweet potato is considered as an excellent source of β-carotene and anthocyanins and has a considerable value in the functional food market. In this report, 21 sweet potato (Ipomoea batatas [L.] Lam) germplasms were evaluated for genetic diversity using morphological and biochemical and molecular markers. Ten morphological traits were studied, and the mean squares due to germplasm were highly significant for storage root number per plant, individual root weight, storage root (fresh) per plant, storage root (dry) per plant, storage root yield, and storage root length. UPGMA cluster analysis based on morphological traits separated the germplasm into three groups. The similarity coefficient ranged from 0.00 to 0.50 with an average of 0.176. Biochemical analysis, viz. total phenol and antioxidant, was performed to find out superior genotype at biochemical level under given conditions. Maximum total phenol was observed in the genotype “V-12” (1.39 mg), whereas maximum total antioxidant was observed in “Samrat” (0.30 mg). RAPD analysis was carried out, and out of 15 RAPD primers, 10 primers produced 96 reproducible and polymorphic bands. UPGMA cluster analysis based on RAPD data also separated the genotypes into three clusters. The results of the present study can be used for sweet potato crop improvement through molecular breeding and marker-assisted selection for desired traits in future

Alcohol and Tobacco Use Among Undergraduate and Postgraduate Medical Students in India: A Multicentric Cross-sectional Study

Background: Substance use among medical fraternity is a well-known phenomenon among both undergraduate (UG) and postgraduate (PG) medical students. Yet, there have been very few multi-centric studies to estimate the actual burden of this problem in this important population group in India. This study was conducted to estimate the prevalence of alcohol and tobacco use, assess the knowledge and attitudes towards this issue, and identify factors associated with substance use among UG and PG medical students in India. Methods: A pre-tested, self-report, anonymous questionnaire was administered to medical undergraduates and post graduate medical residents of eight medical colleges across India. This study used a convenience sample of medical colleges with random selection of study participants within each college for each group, UG and PG. Results: Prevalence of alcohol and tobacco use among UG students was 16.6%, 95% CI [14.5, 18.9] and 8.0%, 95% CI [6.4, 9.6], respectively, whereas prevalence was 31.5%, 95% CI [26.3, 37.0] and 14.5%, 95% CI [10.7, 18.9], respectively for PGs. For both substances, males had a higher prevalence of use compared to females in both groups (p 0.05 for alcohol) were associated with a greater difficulty to quit the habit. Over 90% of study participants felt that substance use adversely affected their skills and reported not using substances prior to managing their patients. Conclusions: Since substance use is a relatively common phenomenon among UG and PG medical students in India, future prospective studies and interventions are required to better understand the pattern of substance use and reduce its prevalence

Establishing Relationship between Pressure & Normal Shocks for Convergent-Divergent Nozzle-

Objective of the present study is to analyze experimentally the pressure variation in a varying area circular passage at different supply pressures and compare these with the theoretical solutions. It has been observed that flow of stream of the normal shock is always supersonic while in downstream is always sub-sonic thus shocks slow down the flow rates by sudden increase in pressure ratios. Shock strength is determined uniquely by the mach no. higher the mach no. of upstream of supersonic flow , greater is the shock strength & lower is downstream subsonic mach no. Theoretically minimum pressure is always present at the throat during subsonic condition. But experimentally the position of minimum pressure might be varied because of variation in the stagnation properties of the fluid. This project also includes flow phenomenon over the entire length of nozzle for varying back pressure