Reza Mirzaeifar

Expansion of circular tubes by rigid tubes as impact energy absorbers: experimental and theoretical investigation

Abstract In this study, expansion of deformable tubes by a rigid tube is introduced as a new mechanism of dissipating energy. In this mechanism of dissipating energy, there is a specific clearance between the surfaces of the rigid and deformable tubes, and the rigid tube is press-fitted onto the top end of the deformable one up to 30 mm. when this arrangement of dissipating energy is subjected to axial compression, the rigid tube is driven into the deformable one; consequently, the impact energy is absorbed by the plastic expansion energy of the deformable tube and the frictional energy at the contact interface between rigid and deformable tubes. Experimental, numerical and analytical study of this process under axially quasi-static loading is presented in this paper. Through the experimental and theoretical studies, major crashworthiness parameters in design are identified. The influence of friction coefficient between the surfaces of deformable and rigid tubes on the value of mean crush load is studied, and typical expansion modes of deformation that may occur during axial compression are characterized. Also, expansion load-displacement history and mechanics of the expansion process as an impact energy absorber are studied. It is shown that this energy absorption method has high crush force efficiency and favourable crashworthiness characteristics both in uniform and non-uniform loading conditions.

Structural transformations in NiTi shape memory alloy nanowires

Martensitic phase transformation in bulk Nickle-Titanium (NiTi)—the most widely used shape memory alloy—has been extensively studied in the past. However, the structures and properties of nanostructured NiTi remain poorly understood. Here, we perform molecular dynamics simulations to study structural transformations in NiTi nanowires. We find that the tendency to reduce the surface energy in NiTi nanowires can lead to a new phase transformation mechanism from the austenitic B2 to the martensitic B19 phase. We further show that the NiTi nanowires exhibit the pseudoelastic effects during thermo-mechanical cycling of loading and unloading via the B2 and B19 transformations. Our simulations also reveal the unique formation of compound twins, which are expected to dominate the patterning of the nanostructured NiTi alloys at high loads. This work provides the novel mechanistic insights into the martensitic phase transformations in nanostructured shape memory alloy systems.

New insights into the collapsing of cylindrical thin-walled tubes under axial impact load

Abstract The current paper presents further investigations into the crushing behaviour of circular aluminium tubes subjected to axial impact load. Experiments prove that in order to achieve the real collapsing shape of tubes under axial loads in numerical simulations, an initial geometric imperfection corresponding to the plastic buckling modes should be introduced on the tube geometry before the impact event. In this study, it is shown that the collapsing shape of tube is affected by this initial imperfection and consequently it is shown that by applying an initial geometric imperfection similar to the axisymmetric plastic buckling mode, the tubes tend to collapse in a concertina mode. This phenomenon is studied for circular tubes subjected to axial impact load and two design methods are suggested to activate the axisymmetric plastic buckling mode, using an initial circumferential edge groove and using one- and two-rigid rings on the tube. In each case the broadening of the concertina collapsing region is estimated using numerical simulations. Experimental tests are performed to study the influence of cutting the edge groove on the collapsing mode. In order to optimize the crashworthiness parameters of the structure such as the absorbed energy, maximum deflection in axial direction, maximum reaction force, and mean reaction force, a system of neural networks is designed to reproduce the crushing behaviour of the structure, which is often non-smooth and highly non-linear in terms of the design variables (diameter, thickness, and length of tube). The finite-element code ABAQUS/Explicit is used to generate the training and test sets for the neural networks. The response surface of each objective function (crashworthiness parameters) against the change of design variables is calculated and both single-objective and multi-objective optimizations are carried out using the genetic algorithm.

Static and Dynamic Analysis of Thick Functionally Graded Plates with Piezoelectric Layers Using Layerwise Finite Element Model

In this paper, static and dynamic analysis of a functionally graded material (FGM) plate with surface-bonded piezoelectric layers is studied. A general finite element formulation based on the layerwise theory is developed for modelling an FG plate with piezoelectric layers or patches. The intermediate FG layer is assumed to be made of many sublayers. Each sublayer is considered as an isotropic layer with constant material properties calculated by the rule of mixtures at the bottom of sublayer. The developed FE model is used for analyzing the quasi-static, free vibration and response of plate to impulse loads. The effects of span-to-thickness ratio and the volume fractions of constituents on the natural frequencies, transverse and in-plane deflections, stress distribution and induced electric potential in piezoelectric layers are scrutinized and the results are compared with the previously reported analytical and numerical works in the literature, where available. It is shown that by using the proposed three-dimensional based finite element model, static and dynamic analysis of thin to thick FG plates with bonded piezoelectric layers can be performed with sufficient accuracy. Also, the non-linear distribution of electric potential in thick piezoelectric layers, which is ignored by most previous works, can be captured using the proposed model.

Active control of natural frequencies of FGM plates by piezoelectric sensor/actuator pairs

An optimization strategy is presented for modifying the dynamic characteristics of functionally graded material (FGM) plates which are actively controlled by piezoelectric sensor/actuator (S/A) pairs. A finite element (FE) model is developed for static and dynamic analysis of FGM plates with collocated piezoelectric sensors and actuators. In this model, the feedback signal to each actuator patch is implemented as a function of the electric potential in its corresponding sensor patch in order to provide active control of the FGM plate in a closed loop system. Using the proposed FE model, a method based on the first-order and second-order approximations in a Taylor expansion is developed to calculate the corresponding changes in the parameters which characterize the piezoelectric patches (i.e. the patch thickness and the feedback gain in each S/A pair) in order to achieve the desired eigenfrequency shifts in the FGM plate. An FGM plate with eight separate S/A pairs is considered as a case study. A sensitivity analysis is initially performed to identify the S/A pairs which have the most influence on the natural frequencies of the plate. The proposed method is used to find a sequence of feedback gains for shifting the natural frequencies to the desired level.

Nonlinear finite element formulation for analyzing shape memory alloy cylindrical panels

In this paper, a general incremental displacement based finite element formulation capable of modeling material nonlinearities based on first-order shear deformation theory (FSDT) is developed for cylindrical shape memory alloy (SMA) shells. The Boyd–Lagoudas phenomenological model with polynomial hardening in conjunction with 3D incremental convex cutting plane explicit algorithm is implemented for preparing the SMA constitutive model in the finite element formulation. Several numerical examples are presented for demonstrating the performance of the proposed formulation in stress, deflection and phase transformation analysis of pseudoelastic behavior of shape memory cylindrical panels with various boundary conditions. Also, it is shown that the presented formulation can be implemented for studying plates and beams with rectangular cross section.

Optimization of the Dynamic Characteristics of Composite Plates Using an Inverse Approach

This article presents an inverse formulation of the eigenvalue problem for computing the required changes in laminated composite plates in order to achieve desired dynamic characteristics in the structure. The stiffness and mass matrices of the plated structure are first derived using the finite element formulation based on the first-order shear deformation theory (FSDT) for laminated composite plates with arbitrary angle-ply stacking sequence. Based on this formulation and using the first- and second-order Taylor expansion both the direct and inverse eigenvalue problems are formulated to find the changes in eigenvalues due to an arbitrary change in physical or geometrical properties of the structure. An initial sensitivity analysis is conducted to identify the layer in which modification of design variables have the most influence on the structures dynamic characteristics. The design variables in this context are defined as the fiber angles in each layer and the layer thickness. The proposed algorithm is applied to several case studies to demonstrate the application and the accuracy of the proposed formulation in solving the direct and the inverse eigenvalue problems with one and two design variables. Dynamic behavior modification of a plate is performed using the inverse approach, and the required modified stacking sequence is obtained to shift the natural frequencies to desired values.

An investigation of intelligent tires using multiscale modeling of cord-rubber composites

ABSTRACT A computational model based on the multiscale progressive failure analysis is employed to provide the theoretical predictions for damage development in the cord-rubber composites in tires. Vulcanized rubber, reinforcing belts, and carcass used in tire structures cause the anisotropic behavior under different loading conditions. Steel reinforcement layers made of steel wires combined with rubber complicate the macro-scale finite element modeling of tires. This paper presents a new three-dimensional model of the cord-rubber composite used in tires in order to predict the different types of damage including matrix cracking, delamination, and fiber failure based on the micro-scale analysis. Additionally, intelligent tires have the potential to be widely used to enhance the safety of road transportation systems, and this paper provides an estimation of the effects of void volume fraction, fiber volume fraction, and stacking sequence of the cord-rubber composites on the acceleration profile of the tire measured at the inner-liner.

Numerical Investigation of Scale Factor in Composites Applying Extended Finite Element Method

In recent years, composite materials have been widely used in several applications due to their superior mechanical properties including high strength, high stiffness and low density. Despite the remarkable advancements in developing theoretical and computational methods for analyzing composites, investigating the effect of scale factor on the strength and damage behavior of composites still remains an active field of research. Some computational efforts has been reported to investigate how composites strength change relative to scale factor. Conventional continuum mechanic methods are not able to calculate the effect of scale factor on composites strength. Applying continuum damage mechanic methods can incorporate this factor. In this paper, it is shown that combining extended finite element and cohesive zone modeling methods leads to developing an efficient numerical framework to study the scale factor for composites strength. The procedure is starting from a two plies laminate and then increasing the laminate thickness to investigate how strength changes with the scale factor. Prediction of composites failure strength has always been an essential study in aerospace research areas. Defining this innovative combination of extended finite element and cohesive zone modeling methods can help researchers to reduce expensive experiments by replacing reliable results from computational analyses.

A review of fatigue and fracture mechanics with a focus on rubber-based materials:

Prediction of how cracks nucleate and develop is a major concern in fracture mechanics. The purpose of this study is to provide an overview of the state of the art on fracture mechanics with primary focus on different methodologies available for crack initiation and growth prediction in rubber-based materials under the static and fatigue loading conditions. The concept of fracture mechanics applied to rubber-based materials and concern of finite element analysis for J-integral estimation in elastomers are discussed in this paper. The strain energy release rate is commonly used to describe the energy dissipated during fracture per unit of fracture surface area and can be calculated by J-integral method, which represents a path-independent integral around the crack tip. As fatigue crack growth most commonly occurs in structures, the high-cycle fatigue life of components needs to be predicted by using extended finite element, strain energy density, finite fracture mechanics, and other techniques which will be covered in this review paper. In addition, some recent testing and numerical results reported in the literature and their applications will be discussed.