Key Engineering Materials Vols. 417-418

Abstract: A new stress-state dependent cohesive zone model for thin walled structures is proposed. The model incorporates the stress-state explicitly within the traction-separation law using basic elasticity-plasticity equations combined with a model parameter. The numerical implementation of the model is able to reproduce ductile fracture observed in a pre-cracked C(T) specimen as well as a notched plate specimen of the same material.

Abstract: This work presents a numerical simulation of the behavior of stress guided waves (GWs) propagating in a multilayer system composed of porous materials. To this end, the damped Semi-analytical Finite Element (SFE) formulation proposed in [1] is extended to take the dry porosity into account. An application of waves propagating in two squared aluminum bars jointed by a weaker layer of porous adhesive is presented. Numerical results show that variations of the adhesive's porosity modify the GWs spectro-temporal patterns. These changes could be exploited to detect the level of adhesive porosity in a total nondestructive manner for the above structures.

Abstract: In this study, a new wireless smart sensor for fatigue damage detection of a single steel bar is proposed. A modified stress intensity factor is used to introduce a master curve of fatigue crack growth. Acoustic Emission signals are also measured and incorporated in the procedure for the structural health monitoring.

Abstract: Realistic Failure Process Analysis (RFPA3D) was used to simulate the concrete-filled rectangular steel tube columns which are subjected to axial loads. The ultimate bearing capacity and the load-strain figures were presented. The Numerical simulation results were verified by experimental data of the paper [5]. Meanwhile, the development process of crack in the specimen was described. The whole failure process of rectangular steel tube columns was reproduced. The failure principles of concrete-filled rectangular steel tube column were discussed deeply.

Abstract: It is not easy to simulate realistic mechanical behaviors of elastically deformable objects with most existing mass-spring systems for their lack of simple and clear methods to determine spring constants considering material properties (e.g. Young's modulus, Poisson’s ratio). To overcome this obstacle, we suggest an alternative method to determine spring constants for mechanical simulation of deformable objects under compression. Using the expression derived from proposed method, it is possible to determine one and the same spring constant for a mass-spring model depending on Young's modulus, geometric dimensions and mesh resolutions of the 3-D model. Determination of one and the same spring constant for a mass-spring model in this way leads to simple implementation of the mass-spring system. To validate proposed methodology, static deformations (e.g. compressions and indentations) simulated with mass-spring models and FEM reference models are compared.

Abstract: Ultrasonic and rotating bending fatigue tests were carried out using plain specimens and specimens with a small blind hole for an extruded and age-hardened Al alloy 7075-T6 in different environments in order to investigate the effect of humidity on fatigue strength and fracture mechanism. Fatigue strength was decreased by high humidity under both tests. The effect of humidity on fatigue strength was larger in ultrasonic fatigue. The humidity affected both of crack initiation and propagation processes. Crack propagated in tensile mode then changed to shear mode macroscopically in all environments under ultrasonic fatigue, though it was only in tensile mode under rotating bending fatigue. These differences in fracture mechanism related to the difference in environmental effect on fatigue strength in both tests.

Abstract: In this paper, transient dynamic crack analysis in two-dimensional, linear magnetoelectroelastic solids is presented. For this purpose, a time-domain boundary element method (BEM) is developed and the elastodynamic fundamental solutions for linear magnetoelectroelastic and anisotropic materials are derived. The spatial discretization of the boundary integral equations is performed by a Galerkin-method while a collocation method is implemented for the temporal discretization of the arising convolution integrals. An explicit time-stepping scheme is developed to compute the discrete boundary data and the generalized crack-opening-displacements. To show the effects of the coupled fields and the different dynamic loading conditions on the dynamic intensity factors, numerical examples will be presented and discussed.

Abstract: Piezoelectric ceramics have recently become one of the most used materials in all kinds of electromechanical systems. However, the presence of defects in such materials prevents them from fulfilling their function. A number of numerical, analytical and experimental works are recently being developed to understand the behaviour of piezoelectrics with presence of damage, but very few aimed at locating defects. One of the current challenges in monitoring piezoelectrics is the correct interpretation of the readings from sensors, in order to reliably recover the defect characteristics minimizing uncertainties due to noise and model. An inverse problem strategy is proposed for this reconstruction, starting from the electromechanical response measurement as input data, and incorporating a numerical model that simulates that response. This model is solved using a Boundary Element Method (BEM), whose formulation is developed for the 2D static case. The damage identification inverse problem is solved using genetic algorithms for the minimization of the discrepancy or cost functional. The effect of noise on measurements and uncertainties in the model is studied in detail through a sensitivity analysis for some simple cases of defect.

Abstract: Knowledge of the stress distribution is the first and necessary step for the reliable assessment of construction with a geometrical or material discontinuity. General geometry and orthotropic material characteristics of both material components lead to singular stress distribution with general stress singularity exponents different from ½. For the final stress field determination both analytical and numerical approaches are utilised. The results of the theoretical approaches are compared to results from finite element method.

Abstract: The stress criterion of multiaxial high cycle fatigue is a type of non-linear equation of high-order. It is used to predict the failure of fatigue in proportional torsion and bending loads. Soon-Bok Lee presented a new design criterion for fully reversed out-of phase torsion and bending. The values are randomized in different random distributions in Lee’s criterion formula. The correlations among random variables are considered and limit state equation is also established. This paper attempts to use First Order Reliability Method (FORM) and Second Order Reliability Method (SORM) to calculate the reliability of material fatigue in torsion and bending loads. The example is calculated and it is found that the failure probability estimated by using the SORM is more reliable than those of the FORM in multiaxial high cycle fatigue.