Farid Abed

Comparisons of Constitutive Models for Steel Over a Wide Range of Temperatures and Strain Rates

This study investigates and compares several available plasticity models used to describe the thermomechanical behavior of structural steel subjected to complex loadings. The main purpose of this comparison is to select a proper constitutive model that can later be implemented into a finite element code to capture localizations (e.g., shear bands and necking) in steel and steel structures subjected to low- and high-velocity impact. Four well-known constitutive models for viscoplastic deformation of metals, i.e., Johnson–Cook (JC), Zerilli–Armstrong (ZA), Rusinek–Klepaczko (RK), and Voyiadjis–Abed (VA), have been investigated and compared with reference to existing deformation data of HSLA-65 and DH-36 steel conducted at low and high strain rates and various initial temperatures. The JC, ZA, and RK models reasonably describe the flow stress and the strain hardening behavior only in the certain ranges of strain, strain rate, and temperature for which the models were developed. This was attributed to the inaccurate assumptions used in developing these models. In contrast, the VA model most effectively describes the flow stress and strain hardening in which very good predictions are observed for the constitutive behavior of high strength steel over a wide range of strains, strain rates, and temperatures.

Shear characteristics of GFRP-reinforced concrete deep beams without web reinforcement

This article investigates the shear behavior of deep concrete beams reinforced with glass fiber reinforced polymer for flexure and without shear reinforcement. A total of 13 beams were tested under four-point loading until failure. Nine of which reinforced with glass fiber reinforced polymer bars and four with steel bars. The ultimate shear capacity along with the load–deformation relationship of all beams was studied. The effects of the shear span to depth ratio a/d, reinforcement ratio ρ, beam effective depth d, and concrete compressive strength on the ultimate shear capacity and mode of failure of all beams were also investigated. Results show that the stiffness of steel-reinforced concrete beams (slope of the ascending portion of load–deflection curve) is higher than that of beams reinforced with fiber-reinforced polymer bars as expected, due to the low axial stiffness of the fiber-reinforced polymer material. A slight variation in the ultimate shear capacity was noticed but no clear trend was observed. In addition, beams reinforced with fiber-reinforced polymer exhibited larger deformation at their ultimate failure load, after which a sudden failure occurred especially for beams having high shear capacity.

Multiple regression modeling of natural rubber seismic-isolation systems with supplemental viscous damping for near-field ground motion

AbstractThis work presents the development and implementation of the Multiple Regression Analysis (MRA) model to Seismic-Isolation (SI) systems consisting of Natural Rubber Bearings and Viscous Fluid Dampers subject to Near-Field (NF) earthquake ground motion. A model representing a realistic five-story base-isolated building is used. Several damper properties are used in creating an array of feasible combinations for the SI system. Two ensembles of seven NF earthquake records are utilized representing two seismic hazard levels. The key response parameters investigated are the Total Maximum Displacement, the Peak Damper Force and the Top Story Acceleration Ratio of the isolated structure compared to the fixed-base structure. Mathematical models for the key response parameters are established via MRA. The MRA models produced acceptable results with significantly less computation. This is demonstrated via a practical example of how the MRA models would be incorporated in the design process, especially at th...

Experimental Investigation of the Axial Capacity of Inelastically Pretwisted Steel Bars

An experimental investigation on the axial load capacity of fixed-end pretwisted steel bars of rectangular solid cross sections is described in this paper. The fixed-end bar test conducted in this study includes bars with both ends griped and embedded in cylindrical slips. More than 200 specimens are considered in this study containing bars with four different cross sections, widths of 20 mm or 30 mm and thicknesses of 3 mm or 6 mm, and three different lengths of 300, 400, and 500 mm. The samples are twisted beyond their elastic limit using torque machine and then subjected to axial load using a compression MTS machine. A set of twisting angles ranges between 0 and 225° is considered for each length. Three samples are tested for each angle then the average of the results is calculated. It is proven experimentally that the permanent twists have influenced the axial strength and the static performance of the pretwisted bars. This improvement is supported by the considerable increase in the critical buckling loads of the bars for certain angles of twists. Multiple regression analysis (MRA) is also conducted using the experimental results to develop mathematical models capable of predicting the critical loads of the tested bars. Results indicate that, with a minimal processing of data, MRA could predict well the critical loads of the pretwisted bars within 99% confidence interval.

Nonlinear Finite-Element Analysis of Buckling Capacity of Pretwisted Steel Bars

AbstractThis paper presents finite-element (FE) analysis to study the axial load capacity of pretwisted steel bars of rectangular cross sections. The FE simulations are conducted using the commercial software ABAQUS. The FE simulations include bars of 20- and 30-mm width, 3- and 6-mm thicknesses, and three different lengths of 300, 400, and 500 mm. The bar ends are gripped and embedded in cylindrical slips. A set of twisting angles, ranging between 0 and 270° with an increment of 15°, is considered for each length. Geometric imperfections as well as actual elastic-plastic behaviors have been implemented in nonlinear FE models. The column strengths, load-shortening curves as well as failure modes, were predicted. The FE model is initially verified by comparing the buckling capacity and mode of the simulated straight bars with the experiments and the AISC code. The bars are then twisted beyond their elastic limit, unloaded to remove elastic recovery, and subjected to axial displacement. FE simulations showe...

Damage Evolution in Structural Steel at Different Loading Conditions

This paper presents an experimental and numerical characterization of ductile damage evolution in steel subjected to large plastic deformations. The main objective of this research is to better understand damage initiation and evolution in structural steel throughout the deformation process at different strain rates. The proposed study relies on a continuum damage mechanics approach that involves characteristic parameters to describe the accumulation of plastic strain, the damage variable, and the strain rates. The work was divided into experimental, and simulation phases. The experimental phase involved testing under monotonic uniaxial tensile loading under varying strain rates. The obtained material parameters are then used as the basic data in the simulations that are performed afterwards. Finally, this model was implemented as a new user defined material in the finite element analysis software ABAQUS where damage was quantified. Initial results of this research showed that a simple model with substantial cost and time saving can be developed for damage assessment in steel. The rate of loading is a main sensitive parameter that affects both damage initiation and propagation, as they increased significantly with increasing loading rate. Beyond the ultimate load, the strain energy was sufficient to cause the damage to increase without any further applied load.

Buckling of slender columns with functionally graded microstructures

ABSTRACT The buckling of slender columns with functionally graded microstructures is studied. In such columns, the flexural modulus is varied in a controlled manner along the column length. The objective is to identify microstructures that maximize (and minimize) the critical buckling load when compared to a reference homogeneous column. Several microstructures are examined and a constraint is imposed so that the volume averaged flexural modulus remains the same in all columns. The buckling load is determined using both the linear perturbation analysis as well as the Rayleigh–Ritz method. A relationship between the material distribution and the corresponding mode shape is established.

Verification of a Thermoviscoplastic Constitutive Relation for Brass Material Using Taylor's Test

This paper presents a methodology to define and verify the dynamic behavior of materials based on Taylors test. A brass alloy with a microstructure composed mainly of two pure metals that have two different crystal structures, copper (face-centered cubic (fcc)) and zinc (hexagonal closed-packed (hcp)), is used in this study. A combined approach of different principal mechanisms controlled by the emergence and evolution of mobile dislocations as well as the long-range intersections between forest dislocations is, therefore, adopted to develop accurate definition for its flow stress. The constitutive relation is verified against experimental results conducted at low and high strain rates and temperatures using compression screw machine and split Hopkinson pressure bar (SHPB), respectively. The present model predicted results that compare well with experiments and was capable of simulating the low strain rate sensitivity that was observed during the several static and dynamic tests. The verified constitutive relations are further integrated and implemented in a commercial finite element (FE) code for three-dimensional (3D) Taylors test simulations. A Taylors test enables the definition of only one point on the stress–strain curve for a given strain rate using the initial and final geometry of the specimen after impact into a rigid surface. Thus, it is necessary to perform several tests with different geometries to define the complete material behavior under dynamic loadings. The advantage of using strain rate independent brass in this study is the possibility to rebuild the complete process of strain hardening during Taylors tests by using the same specimen geometry. Experimental results using the Taylor test technique at a range of velocity impacts between 70 m/s and 200 m/s are utilized in this study to validate the constitutive model of predicting the dynamic behavior of brass at extreme conditions.

Numerical investigation of hybrid FRP reinforced beams

This paper presents a numerical investigation of Hybrid FRP-reinforced concrete beams. A nonlinear finite element (FE) model is developed using the commercial software package ABAQUS to simulate the ductility and flexural behavior of steel, GFRP and Hybrid-reinforced beams subjected to 3 point bending. The present FE results including the flexural capacity, failure mode and load deflection curves are verified and compared with previously published experiments. The main purpose of the present FE model is to extend the numerical analysis and conduct a parametric study in the future. The present numerical analysis consisted of ten concrete beams of 4 meter length subjected to 3 point bending until failure. The effects of the adding steel reinforcement to FRP reinforced beams are investigated and compared to a reference GFRP and steel reinforced beams.

On the differences of dynamic localisations between different types of metals

This paper presents a numerical study of inelastic localisations in metals after utilising a thermodynamically consistent framework of viscoplastic formulation implemented into a commercial finite element code. The significance of using physically-based flow stress relations, instead of empirical relations, is examined by considering three microstructure-based constitutive equations developed previously for three different crystal types of metals. The same numerical values of material constants are used with a range of loading initial temperatures and velocity impacts. Differences of dynamic localisations between three body-centred cubic metals (niobium, tantalum and vanadium), one fcc metal (OFHC copper) and one hcp metal (titanium) are illustrated and discussed through studying the initiation and propagation of necking and shear bands in a circular bar and simple tension plane strain problems, respectively. Moreover, the effect of initial temperatures and strain rates on dynamic localisations is also scrutinised. Objective results using different mesh configurations are verified as a result of including viscosity in the constitutive models.