Behrad Koohbor

Meso-scale study of non-linear tensile response and fiber trellising mechanisms in woven composites:

The non-linear deformation response of plain woven carbon fiber-reinforced composites is experimentally studied at meso-scales. Stereovision digital image correlation is utilized to capture the full-field strain distribution over a 10 × 10 mm2 area of interest located at the center of the specimens. The evolution of local strains on the fiber bundles and matrix-rich regions as a function of loading is extracted. The effect of fiber orientation angle on fiber bundles stretch ratio and their angle of rotation (fiber trellising) and the related underlying failure mechanisms are analyzed using the measured full-field displacement data. The results indicate that the local load-bearing mechanisms are different in on-axis and off-axis loading conditions, whereas the larger global failure strain noticed in off-axis conditions is attributed to the occurrence of fiber rotation. The fiber trellising is also shown to promote high local shear strain and consequently leads to the protrusion of the matrix material on the deformed specimen surface.

Investigations of the Failure in Boilers Economizer Tubes Used in Power Plants

In this study, failure of a high pressure economizer tube of a boiler used in gas-Mazut combined cycle power plants was studied. Failure analysis of the tube was accomplished by taking into account visual inspection, thickness measurement, and hardness testing as well as microstructural observations using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). Optical microscopy images indicate that there is no phase transformation during service, and ferrite-pearlite remained. The results of XRD also revealed Iron sulfate (FeSO4) and Iron hydroxide sulfate (FeOH(SO4)) phases formed on the steel surface. A considerable amount of Sulfur was also detected on the outer surface of the tube by EDS analysis. Dew-point corrosion was found to be the principal reason for the failure of the examined tube while it has been left out-of-service.

Thermomechanical Behavior of Work Rolls During Warm Strip Rolling

A mathematical model was developed to assess thermomechanical behavior of work rolls during warm rolling processes. A combined finite element analysis-slab method was first developed to determine thermal and mechanical responses of the strip being rolled under steady-state conditions, and then, the calculated roll pressure and temperature field were utilized as the governing boundary conditions for the thermomechanical problem of the work roll. Finally, the thermomechanical stresses within the work rolls were predicted by a thermoelastic finite element approach. The results of the model indicate that, in warm strip rolling, thermal and mechanical stresses developed in the work rolls are comparable, and thus, both thermal and mechanical aspects of the problem should be considered in such a problem. Besides, the model was shown to be capable of determining the effects of various rolling parameters on the thermomechanical behavior of the work rolls during warm rolling process.

On the Mechanical Response of Polymer Fiber Composites Reinforced with Nanoparticles

An experimental study was conducted on the effect of interply nanofiller on the mechanical response of fiber reinforced composite (FRC). Laminate samples were made by hot pressing of woven carbon fiber fabric prepregs. Two batches of samples are prepared, one using five plies of the basic prepreg, the other with silica nanofillers added between the plies during lay-up. Tensile specimen were cut from the laminate under 0, 15, 30, 45, 60, 75 and 90 degrees of fiber orientation are prepared from the laminate. DIC based tensile test is made and the effect of the nano fillers on the mechanical properties are analyzed. Appreciable improvement in strength and Modulus of Elasticity is obtained for fiber orientation of 75° and 60° and reversed response is observed for the fiber angle of 30° and 15°

On the Meso-Macro Scale Deformation of Low Carbon Steel

The multiscale deformation response of low carbon steel is investigated. The meso and macro scale displacement and strain fields for specimen subjected to pure tension are measured using in-situ multiscale digital image correlation technique. The specimen is specked with different scale pattern ranging from 5 to 500 μm size. The smallest scale, 5 μm, speckles are used for local meso-scale deformation measurement. In this case an optical microscope is used to record the local information within 1 mm square field of view. On the other hand, the larger size, 500 μm, speckles are used to measure the continuum level deformation. In this case two digital cameras with 5 megapixel resolution are used in 3D arrangement by considering the entire width of the specimen inside the field of view. Both the optical microscope and the digital camera systems are triggered simultaneously to acquire the deformation at the same time scale. The displacement and strain fields are extracted using digital image correlation. The effect of local deformation on the overall displacement and strain of low carbon steel is presented by comparing with the macro scale deformation and strain fields. Furthermore, microstructure images are obtained by optical microscope and used for the analysis of local strain field coupled with the strain field from digital image correlation.

Study on effect of residual stress distributions on kinetics of static strain aging after cold rolling

Abstract In the present research, the effect of residual stress distribution on the static strain aging (SSA) phenomenon in cold rolled steel was investigated. A three-dimensional model was employed to evaluate the residual stress distributions within the rolled strips, and hole drilling experiments were also performed to verify the data obtained from the mathematical model. Hardness and tensile tests were then performed on the cold rolled samples at different temperatures and aging periods, and the results of these tests were utilised to assess SSA behaviour after different rolling programs. The results show that SSA occurs within the cold rolled steel in the employed aging period, and its kinetics is affected by residual hydrostatic stresses. Additionally, the variation in residual hydrostatic stress distribution due to deformation path, e.g. single- and two-pass rolling layouts, slightly affects the activation energy of SSA and changes the kinetics of SSA after cold rolling.

Finite element modeling of thermal and mechanical stresses in work-rolls of warm strip rolling process

An integrated mathematical model was developed to study the thermo-mechanical behavior of strips and work-rolls during warm rolling process of steels. A two-dimensional finite element analysis was first employed to solve for the thermo-mechanical response of the rolled strip under steady-state conditions. The calculated roll pressure and temperature fields were then used to apply proper boundary conditions for solving the governing thermo-mechanical problem for the work-roll. The obtained results indicate that in warm strip rolling of steels, the thermal and mechanical stresses developed within the work-roll are comparable; however, the more significant influence is due to heating and cooling of the work-roll during the process, particularly in case of warm rolling operations of the strips with higher initial temperature. Besides, the utilized model was shown to be capable of determining the effects of various rolling parameters on the thermo-mechanical behavior of strips and work-rolls during warm rolling process.

Thermo-mechanical Properties of Metals at Elevated Temperatures

Results from novel DIC-based experiments focused on quantifying the high temperature thermo-mechanical properties of steel alloys at elevated temperatures are presented. In these studies, the vision-based high temperature measurement system is comprised of (a) an induction coil heating system, (b) blue and white light illumination of the specimen, (c) optical band-pass notch filters for each camera centered on the blue light illumination wavelength and (d)~a stereo-camera configuration for acquisition of synchronized image pairs at the desired temperature. The entire system is portable and is sufficiently flexible such that it can be employed with a wide range of specimen geometries. Each pair of stereo images acquired at a specified temperature is analyzed using three-dimensional digital image correlation (stereo-DIC). Results from laboratory studies confirm that the blue light-illumination with optical notch filtering allows high contrast images to be obtained even when the specimen is glowing white hot. The effectiveness of the system is demonstrated by successfully performing experiments to obtain the coefficient of thermal expansion of a steel alloy as a function of temperature. Additionally, technical challenges (e.g., sensitivity, spatial resolution, and repeatability of the measurements) when performing high temperature experiments with the proposed methodology will be discussed.

Impact Response of Density Graded Cellular Polymers

Full-field deformation response of density-graded cellular polymers subjected to high velocity impact is investigated experimentally. Recently developed experimental setup consisting of ultra-high speed imaging in conjunction with digital image correlation is used to measure in-situ full-field deformation on density-graded polymeric foam specimens. Loading of the specimens is performed using a direct impact configuration in a modified Hopkinson bar apparatus. Discretely-layered foam specimens made from three distinct layers each with a different bulk density are subjected to direct impact, while their deformation response is studied via ultra-high speed digital image correlation. Formation and propagation of compaction waves from the impact side to the support end of the specimen are observed and analyzed. Spatial distribution of inertia stress is determined from acceleration fields and density of the layers. Full-field stress distribution in the specimen is later used to estimate the stress gradients within compaction waves. Mechanisms associated with the energy dissipation in graded foam specimens are discussed.

Finite-element modeling of thermal aspects in high speed cold strip rolling

An integrated model based on finite-element method has been proposed to examine the mechanical and thermal responses of strips and work-rolls in tandem and reverse cold rolling operations. The model has been developed such that the influence of various process parameters, such as lubrication, rolling speed, frictional state and back-up rolls, can be examined. Thermal behaviors of the rolled material and the work-rolls have been analyzed using stream-line upwind Petrov–Galerkin approach, in order to make the model applicable to high-speed rolling processes, as well. The results have been compared to the actual on-line measurements and shown to be of acceptable accuracy. Such modeling approach can be considered as a useful means, providing a detailed insight on the thermo-mechanical response of strips, as well as work-rolls, during high-speed cold rolling of steel strips. Additionally, special attention has been drawn towards the prediction of the occurrence of the metallurgical phenomenon dynamic strain aging based on the results obtained from the numerical modeling in this work.