Akbar Abvabi

A STUDY ON MECHANICAL BEHAVIOR OF FUNCTIONALLY-GRADED CARBON NANOTUBE-REINFORCED NANOCOMPOSITES

In this study, we are focusing on the investigation of the effects of gradient patterns on mechanical behavior of functionally-graded carbon nanotube-reinforced nanocomposites and considering typical beams made of such nanocomposites. Both analytic and finite element-based numerical models were developed. Analytic model was developed based on the first-order shear deformation and Timoshenko beam theories meanwhile finite element models were developed using Abaqus in conjunction with user-defined subroutines for defining the continuously gradient material properties for different gradient patterns. Position-dependent elastic modulus equations for four continuously graded patterns were studied. A nongraded pattern was used for benchmarking with the same geometry and total carbon nanotube (CNT) contents. For validation and verification, the results on both deflection and stress of these nanocomposite beams were analyzed, which clearly showed high influence from gradient patterns on these mechanical behaviors of such beams.

Springback Investigation in Roll Forming of a V-Section

To have fuel efficient vehicles with a lightweight structure, the use of High Strength Steels (HSS) and Advanced High Strength Steels (AHSS) in the body of automobiles is increasing. Roll forming is used widely to form AHSS materials. Roll forming is a continuous process in which a flat strip is shaped to the desired profile by passing through numerous sets of rolls. Formability and springback are two major concerns in the roll forming of AHSS materials. Previous studies have shown that the elastic modulus (Young’s modulus) of AHSS materials can change when the material undergoes plastic deformation and the main goal of this study is to numerically investigate the effect of a change in elastic modulus during forming on springback in roll forming. Experimental loading-unloading tests have been performed to obtain the material properties of TRIP 700 steel and incorporate those in the material model used in the numerical simulation of the roll forming process. The finite element simulations were carried out using MSC-Marc and two different element types, a shell element and a solid-shell element, were investigated. The results show that the elastic modulus diminution due to plastic strain increases the springback angle by about 60% in the simple V-section roll forming analyzed in this study.

Development of an Inverse Routine to Predict Residual Stresses in the Material Based on a Bending Test

Bending with unloading and reverse bending are the dominant material deformations in roll forming and hence property data derived from bend tests could be more relevant than tensile test data for numerical simulation of the roll forming process. Recent investigations have shown that residual stresses affect the material behaviour close to the yield in a bending test. So, Residual stress introduced during prior steel processing may affect the roll forming process and therefore needs to be included in roll forming simulations to achieve improved model accuracy. Measuring the residual stress profile experimentally is expensive, difficult, time consuming and has limited accuracy. Analytical models are available that allow the determination of residual stress. However, for this detailed information about the pre-processing conditions is required; this information is generally not available for roll forming materials. The main goal of this study is to develop an inverse routine that generates a residual stress profile through the thickness of the material based on pure bend test data.