Numerical and experimental evaluation of SIF for threaded connectors

This paper presents a methodology for fatigue crack growth analysis in tubular threaded connectors. A solution for stress intensity factor for semi-elliptical surface cracks emanating from a thread root in a screw connector is also discussed in the paper. The solution is based on a mixed approach incorporating weight function and finite element methods. The weight functions used are the universal functions for cracks in mode I and these are linked with a thread through-thickness stress distribution obtained from finite element analysis to produce a stress intensity factor for a crack at the critical tooth of a thread. The resulting crack growth data are then validated experimentally.     

Exploiting error-control coding and cyclic-prefix in channel estimation for coded OFDM systems

OFDM systems typically use coding and interleaving across subchannels to exploit frequency diversity on frequency-selective channels. This letter presents a low-complexity iterative algorithm for blind and semi-blind joint channel estimation and soft decoding in coded OFDM systems. The proposed algorithm takes advantage of the channel finite delay-spread constraint and the extra observation offered by the cyclic-prefix. It converges within a single OFDM symbol and, therefore, has a minimum latency.     

Energy-efficiency of MIMO and cooperative MIMO techniques in sensor networks

We consider radio applications in sensor networks, where the nodes operate on batteries so that energy consumption must be minimized, while satisfying given throughput and delay requirements. In this context, we analyze the best modulation and transmission strategy to minimize the total energy consumption required to send a given number of bits. The total energy consumption includes both the transmission energy and the circuit energy consumption. We first consider multi-input-multi-output (MIMO) systems based on Alamouti diversity schemes, which have good spectral efficiency but also more circuitry that consumes energy. We then extend our energy-efficiency analysis of MIMO systems to individual single-antenna nodes that cooperate to form multiple-antenna transmitters or receivers. By transmitting and/or receiving information jointly, we show that tremendous energy saving is possible for transmission distances larger than a given threshold, even when we take into account the local energy cost necessary for joint information transmission and reception. We also show that over some distance ranges, cooperative MIMO transmission and reception can simultaneously achieve both energy savings and delay reduction.     

Investigation of microstructure effect on fretting fatigue crack initiation using crystal plasticity

The onset of fretting fatigue is characterized by material microstructural changes in which the extent of the damage is comparable to grain size, and hence, the microstructure characteristics could have a significant effect on fatigue crack initiation. In this paper, a three‐dimensional finite element crystal plasticity framework is presented for simulation of the fretting fatigue. Controlled Poisson Voronoi tessellation (CPVT) method is employed to generate the polycrystalline region. In the CPVT method, regularity parameter controls the shape of grains. In this study, the impact of grain size and regularity parameter on crack initiation life and initiation site has been investigated. Cumulative plastic slip was used as a parameter of microstructure‐sensitive fatigue indicator. This parameter could effectively predict the location of crack initiation and its life. The results show that regularity parameter has a significant effect on the location of crack initiation. Furthermore, the effect of grain size on the fretting fatigue life of 316L stainless steel was investigated experimentally through testing different specimens with different grain sizes, to validate the simulation results.     

Analytical and experimental validations of a numerical band-limited Green’s function approach for modelling acoustic emission waves

Dynamic Green’s functions are essential for modelling acoustic emission (AE) wave propagation and for the quantitative characterisation of AE sources. In this work, a method for evaluating the Green’s function of a body using the finite element method is presented. The advantage of the proposed method is that it can be used to model realistic geometries, material properties and sources that cannot be treated analytically. The numerical results presented in this paper are compared with known analytical solutions of the Green’s function for an infinite isotropic plate and also with experimental measurements of AE waves generated by known artificial AE sources (ball impact and pencil lead break).     

Simulation of thermal shock cracking in ceramics using bond-based peridynamics and FEM

The effects of moderate intensity ‘hot’ or ‘cold’ shock in brittle solids have been extensively studied, while much less is known about thermal shock response during large temperature variations. In this study, a combined finite element – peridynamics numerical procedure is proposed for the simulation of cracking in ceramic materials, undergoing severe thermal shock. Initially, Finite Element nonlinear heat transfer analysis is conducted. The effects of surface convection and radiation heat exchange are also included. Subsequently, the interpolated temperature field is used to formulate a varying temperature induced action for a bond-based peridynamics model. The present model, which is weakly coupled, is found to reproduce accurately previous numerical and experimental results regarding the case of a ‘cold’ shock. Through several numerical experiments it is established that ‘cold’ and ‘hot’ shock conditions give rise to different failure modes and that large temperature variations lead to intensified damage evolution.     

An approximate method for simultaneous modification of natural frequencies and buckling loads of thin rectangular isotropic plates

In this paper, the first and second order derivatives of natural frequencies and buckling loads with respect to an arbitrary geometrical or physical property of a plate structure are calculated. Based on these eigenderivatives, and by using first and second order Taylor expansions, an approximate method is presented for simultaneous modification of natural frequencies and buckling loads. In the proposed method, the structural modification is considered as an inverse eigenvalue problem and calculated eigenderivatives are used for transforming this inverse problem to solving a system of algebraic equations. By solving this set of equations, the necessary changes in design parameters are calculated, in order to achieve predefined simultaneous shifts in natural frequencies and buckling loads. A plate with two boundary conditions is considered as a case study. The plate is divided into eight regions and the plate thickness in each region is considered as a design parameter. By considering several case studies, the required modification in design variables is calculated for achieving a predefined change in natural frequencies, buckling loads or simultaneous change of both. In each case, the calculated changes in design variables are implemented on the initial structure, and the changes in eigenvalues are computed using FE method. It is shown that the results from presented method compare very well with those obtained from a direct optimization procedure.     

Coupled modification of natural frequencies and buckling loads of composite cylindrical panels

This paper proposes a new method for performing predefined simultaneous modification of natural frequencies and buckling loads of composite cylindrical panels. The method is based on the fact that both natural frequencies and buckling loads are eigenvalues of an algebraic system of simultaneous equations. First- and second-order derivatives of these eigenvalues are calculated and the first two terms in Taylor expansion are used for developing a modification procedure that is defined as an inverse eigenvalue problem. A four-layered composite cylindrical panel with an arbitrary angle-ply stacking sequence is considered as a case study and several simultaneous modifications for natural frequencies and buckling loads are carried out. It is shown that the proposed method can perform the predefined modification with an acceptable accuracy even for large perturbations in objective functions.     

Strain based finite element for the vibration of cylindrical panels with openings

This paper investigates the effect of cut-outs on the dynamic behaviour of cylindrical panels, using a newly developed strain based finite element. The developed element is based on assumed strains and has only five necessary degrees of freedom at each corner node. The displacement fields of the element satisfy the exact requirement of rigid body displacements. The efficiency of the element is tested by applying it to the calculation of natural frequencies of shells. Investigations are first carried out to test the convergence of the element and to establish the mesh size to be used. The element is further applied to analyse cylindrical panels with cut-outs. Simply supported as well as clamped panels on their edges were considered and the effects of the size and location of the openings on the natural frequencies are investigated. The subspace iteration technique which is shown to have an economising effect, is used to obtain the natural frequencies and the associated modes of vibration.