Dilip K. Banerjee

Finite Element Modeling of Deformation Behavior of Steel Specimens under Various Loading Scenarios

Lightweighting materials (e.g., advanced high strength steels, aluminum alloys etc.) are increasingly being used by automotive companies as sheet metal components. However, accurate material models are needed for wider adoption. These constitutive material data are often developed by applying biaxial strain paths with cross-shaped (cruciform) specimens. Optimizing the design of specimens is a major goal in which finite element (FE) analysis can play a major role. However, verification of FE models is necessary. Calibrating models against uniaxial tensile tests is a logical first step. In the present study, reliable stress-strain data up to failure are developed by using digital image correlation (DIC) technique for strain measurement and X-ray techniques and/or force data for stress measurement. Such data are used to model the deformation behavior in uniaxial and biaxial tensile specimens. Model predictions of strains and displacements are compared with experimental data. The role of imperfections on necking behavior in FE modeling results of uniaxial tests is discussed. Computed results of deformation, strain profile, and von Mises plastic strain agree with measured values along critical paths in the cruciform specimens. Such a calibrated FE model can be used to obtain an optimum cruciform specimen design.

A sensitivity study on the fire-induced heating of concrete slabs in composite floor systems

The fire resistance of composite floor systems depends on the interaction of the concrete slab with steel beams. To evaluate the fire resistance of composite floor systems, the temporal and spatial variations of temperatures must be accurately determined. The temperature profiles in a concrete section are a function of concrete thermophysical properties and boundary conditions. However, there can be considerable uncertainty in their values. A sensitivity study using an orthogonal full-factorial design approach was conducted to determine which of these parameters most significantly influenced the thermal response of the slab. The time–temperature data from a Cardington fire test were used to validate a heat transfer model of a representative section of the composite floor system. The orthogonal factorial design analysis indicated that the thermal behavior of the concrete slab is most sensitive to, in decreasing order, thermal conductivity, effective emissivity, convective heat transfer coefficient at the exposed surface, and enthalpy.

An integrated interactive visualization and analysis environment to study the impact of fire on building structures

In order to move away from the current prescriptive design methods towards performance based methods for the design of structures under fire, we need validated computer models. The next section describes our approach for modeling and analysis.

A Design of Experiments Approach for Determining Sensitivities of Forming Limit Analyses to Experimental Parameters

Forming limit testing is widely used to characterize materials for use in forming operations. Subset-based Digital Image Correlation (DIC) has become a common method used in research laboratories to measure the forming limit strain fields, but no consistent method exists for forming limit determination from these data. In this work, we apply a design of experiments approach along with Finite Element Analyses to exercise potential experimental measurement requirements numerically (especially varying expected image resolutions and frame rates) to quantifying the sensitivities of various proposed forming limit analysis methods to changes in experimental setup and DIC processing parameters. The main goal is to determine the appropriate hardware to produce benchmark experimental data, and possibly highlight which forming limit analysis methods may be either overly sensitive, or preferably insensitive, to subjective processing decisions.

Insights into Cruciform Sample Design

Four different cruciform sample designs, based on the work of Abu-Farha et al. (JOM 61:48, 2009) were studied. Key features of these designs are a recessed pocket with fillet and re-entrant corners. These samples were shown via digital image correlation to achieve widely differing strain values inside and outside the pocket. From the results of these tests, there are two competing failure mechanisms in the sample. The pocket region is affected by stress concentrations caused by the fillet, and re-entrant notches lead to strain-limited constraints similar to diffuse and localized necks in uniaxial samples. Balancing these two constraints determines the success or premature failure of the sample.

An Experimental and Numerical Study of Deformation Behavior of Steels in Biaxial Tensile Tests

Lightweighting materials are increasingly being used by automotive companies as sheet metal components to meet fuel economy targets by the year 2025. Consequently, accurate material model data need to be developed by applying biaxial strain paths with cross-shaped specimens, since traditional uniaxial stress-strain data cannot capture the deformation behavior in complex forming operations. This paper discusses the development of finite element models and verification of these models against experimental measurements. Such verification is the first step in developing procedures for making an optimum cross-shaped specimen design. Computed results of deformation, strain profile, and von Mises plastic strain in two different specimens agree with experimentally measured values along critical paths in the specimens. In addition, simulated results predict correctly the eventual failure location in the samples. Detailed analyses also suggest that specimen thickness has an influence on the eventual mode of failure. Further studies are being conducted to confirm this conclusion.

NIST Advances in Computer-Aided and Computational Methods in Wind Engineering

The paper summarizes recent research and development of computer-aided and computational methods in wind engineering at the National Institute of Standards and Technology (NIST). Specific topics to be covered include: 1. Time-domain database-assisted design of tall, flexible structures excited by turbulent winds inducing dynamic structural responses. 2. Computational Fluid Dynamics (CFD) estimates of aerodynamic pressure on low-rise buildings with geometrical dimensions comparable to the acrosswind integral turbulence length scale. 3. Development of synthetic directional wind speed databases covering on the order of 10,000 years, from measured wind speeds over periods of approximately 30 years. 4. Probability distributions of hurricane wind speeds, Gulf and Atlantic coasts. 5. Extreme wind speeds in non-hurricane prone conterminous United States. 6. Estimates of hurricane-borne missile speeds. 7. Disaster and Failure Events Repository – a new research tool for archival and analysis of post-disaster information and data.