Key Engineering Materials Vol. 827

Abstract: A numerical methodology is presented which is based on Topology Optimization (TO) approach for designing continuous heterogeneous structures. This method is then exploited to reduce weight and to increase stiffness in mechanical components. Useful guidelines for the evaluation of equivalent material properties of metallic cellular structures are discussed, and they are applied for the optimization process of an engine mounting bracket under realistic loading and boundary conditions. The outcome of TO, which is related to the material density distribution into the design space, is critically reviewed for the definition of bulk, lattice and void regions within the component.

Abstract: The current research focuses on the characterization of the produced heat affected zone when laser heats AISI H13 steel, AISI 1045 steel and Ti6Al4V alloy workpieces via finite element simulations and experimental investigation. The surface roughness designedly varies on the surface of the samples and its influence on the absorption of laser light is investigated. Experiments are conducted at 1-4 W laser power and for two scanning speeds of 2 and 100 mm/min. A 3D transient thermo-structural finite element model for a moving Gaussian laser heat source is developed to simulate the micromachining process and predict the depth and width of the heat affected zone. The Johnson-Cook material model that takes into account the effect of plastic strain, strain rate and temperature, along with a fracture model, is adapted to the simulations. A good agreement between the experimental data and the simulation results is found. The depth and width of the heat affected zone strongly depend on the laser parameters and material properties of the irradiated samples. This study constitutes the basis to the optimization and improvement of the laser assisted micromachining process parameters and provides key insights on the roughness-absorptivity relation for the three metallic materials.

Abstract: In this contribution we present an application of the lowest order Virtual Element Method (VEM) to the problem of material computational homogenization. Material homogenization allows retrieving material properties through suitable volume averaging procedures, starting from a detailed representation of the micro-constituents of the considered material. The representation of such microstructure constitutes a remarkable effort in terms of data/mesh preparation, especially when there is not evident microstructural regularity. For such a reason, computational micromechanics may represent a challenging benchmark for showing the potential of VEM. In this contribution, polycrystalline materials are considered as an application. The proposed technique constitutes a first step towards modelling of damage processes in micro-structured materials

Abstract: A microscale formulation for low-cycle fatigue degradation in heterogeneous materials is presented. The interface traction-separation law is modelled by a cohesive zone model for low-cycle fatigue analysis, which is developed in a consistent thermodynamic framework of elastic-plastic-damage mechanics with internal variables. A specific fatigue activation condition allows to model the material degradation related to the elastic-plastic cyclic loading conditions, with tractions levels lower than the static failure condition. A moving endurance surface, in the classic framework of kinematic hardening, enables a pure elastic behaviour without any fatigue degradation for low levels of cyclic traction. The developed model is then applied to micro-structured materials whose micro-mechanics is analysed using a boundary integral formulation. Preliminary results demonstrate the potential of the developed cohesive model. The future application of the proposed technique is discussed in the framework of multiscale modelling of engineering components and design of micro-electro-mechanical devices (MEMS).

Abstract: Outstanding durability of plastic pressure and non-pressure pipes can cause difficulties, when a reasonable lifetime estimation is needed. It is impossible to prove the lifetime by testing, but there is a method of calculation that can provide a certain idea about the expected lifetime. The lifetime estimation is based on the assumption that the failure occurs as a result of the slow crack growth mechanism and it is calculated using the linear elastic fracture mechanics approach. Numerical simulations of crack growth in the pipe are necessary for this calculation. These simulations must consider various effects that can play a role in the lifetime. This paper deals with the lifetime calculations of a pressure and a non-pressure corrugated pipe considering the soil loads acting on pipes when they are buried. In the simulation of the pressure pipe, a combination of loads is applied that consists of internal pressure, residual stress and the soil loads. The influence of the loads is discussed. The non-pressure corrugated pipe is loaded by the soil loads only.

Abstract: The finite element method (FEM) is developed to analyse the size effect (flexoeletricity) for 2-D crack problems in thermo-piezoelectricity. Flexoelectricity is observed in micro/nanoelectronic structures, where large strain gradients destroy the symmetric structure of atoms in crystals and thereby causing polarization, even in dielectric materials. In contrast to using classical Fourier heat conduction theory, a finite speed of the thermal wave is considered in the higher order transport equation. The variational principle is applied to derive the FEM equations and C¹-continuous elements are employed in the implementation of the FEM. An example is presented to demonstrate the effect of the characteristic time parameter on the crack opening displacement and temperature distribution.

Abstract: The critical length a_Ic of a mode I crack that corresponds to a vanishing of the influence of the notch stress concentration can be approximately expressed by a formula reported by Lefort. This study aimed to generalize his approach to shear mode cracks by finding a criterion for a statistical compatibility of formulae for critical lengths of cracks. It revealed that the Lefort ́s formula describes the critical crack length a_Ic at which the relative level of the notch effect on the mode I SIF is less than 1%. Based on this criterion, a mathematically similar formula for the critical length a_IIIc was found. A part of this study was also a development of a transformation procedure suitable for fitting the obtained SIF data by simple analytical formulae and for clear related illustrative plots of results.

Abstract: Rigid low-density closed-cell polyurethane (PU) foams are used primarily as a thermal insulation material. The foams have to possess a sufficient strength and stiffness in order to ensure their mechanical integrity and dimensional stability in service. The mechanical characteristics of foams are enhanced by adding cellulose nanofibers to the polyol system, which both modify the foaming process and act as a reinforcement of cell struts and walls. A model of composite foam strength is developed based on a regular unit cell and assuming the onset of strut failure as the foam fracture criterion. The load-bearing capacity of foam struts is estimated by the modified Fukuda and Chou model considering the orientation of nanofibers along the strut axis. The model developed is shown to provide a reasonably accurate prediction for the nanofiber loading effect on the strength of composite foams.

Abstract: This paper contains results of a comprehensive experimental programme aimed at an evaluation of residuals stresses in and near laser welds of pressure vessels steels using X-ray diffraction method. Welds were prepared by the company RAPTECH s.r.o., where two types of steel plates, namely P355 and P460 of the thickness 5, 8 and 10 mm, were welded by a laser beam with specific parameters. Surface transversal and longitudinal stresses were analysed on both sides of the welds, namely upper and downside. The results, important for further applications of the used technology, are discussed particularly from the viewpoint of possible critical areas near the welds and their impact on fatigue loading.