Key Engineering Materials Vol. 827

Abstract: This paper shows the applicability of a non-linear Finite Element (FE) methodology to analyse the elasto-plastic behaviour and the energy absorption of a padding noise-protection material applied to the vehicle interior components. This material is a sandwich built from alternating layers of polymeric foam and of glass fibre composite. The approach considers two design steps. The first one involves the experimental characterization of the material while the latter deals with the assessment of the numerical models validated for a full-vehicle crash analysis.

Abstract: Boride coatings on steels have an excellent combination of properties. They can significantly improve the hardness, the wear and corrosion resistance of steels. Boronizing of steels has been achieved using different methods such as pack cementation and paste boriding. On the other hand, fluidized bed technology has been successfully used in many surface engineering applications in the deposition of hard and / or corrosion resistant layers e.g. carburizing, aluminizing and chromizing. This method is simple, efficient and environmental friendly and is characterized by excellent heat and mass transfer, which results to improve quality of the as-produced coatings. As a result, fluidized bed technology can be considered as a useful alternative method for the production of boride coatings on steel substrates. In the present paper we used this method to deposit boride coatings on steels. The as-produced coatings were examined by means of optical microscopy, X-Rays diffraction, Vickers microhardness and pin on disk in terms of coatings thickness and morphology, phase formation and mechanical properties. It was found that they are characterized by good adherence and uniformity all over the substrate and showed improved tribological properties under dry wear conditions.

Abstract: It is well known that composite materials are founding increasing applications in the transport field thanks to their high strength to mass ratio. However, their use in primary structures is very challenging because of their high sensitivity to in-service damages and manufacturing defects. As a result, the current adopted damage tolerance approach leads to the oversizing of such structures. Structural health monitoring systems, aimed to the real time damage detection, can provide several benefits in terms of lightweight of the structures, maintenance operations and inspection costs. This paper deals with the use of the Probability Ellipse (PE) method, based on the propagation of ultrasonic guided waves on a composite winglet of a small aircraft. The PE method estimates the probability of the presence of the damage in the monitored area, starting from the knowledge of selected damage indexes for each sensors-path. The winglet, equipped with piezoelectric sensors, usable as both actuating and receiving devices, has been numerically and experimentally investigated under several configurations, varying the actuator location. Sensitivity analysis has been performed to assess the effectiveness of the PE method. The accuracy of the PE method in detecting both location and damaged area is herein discussed.

Abstract: A micro-scale-based approach for the numerical analysis of cement-based materials, subjected to low-and high-cycle fatigue actions, is presented in this paper. The constitutive model is aimed at describing the evolving microstructural changes caused by cyclic loading protocols. More specifically, statistically representative microscopic geometries are equipped with a fracture-based model combined with a continuous inelastic constitutive law accumulating damage induced by the cyclic stress. The plastic-damage-based model is formulated combining the concepts of fracture-energy theories and damage stiffness degradations, representing the key phenomena occurring in concrete under fatigue. The paper explores the potential of the technique for assessing fatigue microcracks formation and growth, and their influence on the macroscopic behavior.

Abstract: Ti-6Al-4V titanium alloy is a high strength-to-mass ratio material common in a lot of engineering fields. Its surface oxide can guarantee the protection of the substrate from various corrosive media. Unluckily, this film can be scratched in presence of mechanical and chemical loads and for this reason the corrosion resistance can decrease. The Structural Mechanics Laboratory (SM-Lab) is carrying out a characterization of the alloy in different environments under quasi-static loading. In this paper, a summary of the outcomes of the investigation and the description of the fracture surface of a specimen with EDM notches quasi-statically tested in methanol is provided

Abstract: Shallow cracks are often observed in dental enamel, however do not normally lead to deep fractures. Previous work has highlighted the toughening mechanisms that operate in enamel during crack propagation, but very little is known about the deformation and stress fields arising around the propagating cracks during realistic loading conditions. This work aims to elucidate how the stresses are distributed within human dental enamel when a pre-existing crack is subjected to opening and surface contact with in situ indentation. We present a synchrotron-based in situ analysis coupled with a linear elastic finite element method simulation. The experimental reconstructed stress fields identified a prominent residual stress within the enamel, accompanied by a visible pattern that appeared clearly associated with its underlying microstructure. The numerical modelling of the stress field and discerning of surface contact and crack opening caused by the indentation was subsequently possible, even if in this study the influence of the anisotropy induced by the presence of features at a smaller scale was neglected. The implications of these findings and directions for future research are discussed.

Abstract: The machinability of a steel workpiece through conventional and Laser-Assisted Machining (LAM) is studied by the help of the Finite Element Method (FEM). In LAM, the laser beam is applied as a heat source to ensure sufficient local heating of the workpiece at a certain distance from the cutting tool and the machinability of materials is increased since the values of the cutting forces are decreased. A thermostructural FEM model is developed to simulate the conventional and the LAM orthogonal cutting of AISI H-13 steel. The Johnson-Cook material model that takes into account the effect of plastic strain, strain rate and temperature, along with a fracture model, is used in the simulations. For varying feed rate, parametric simulations are carried out, for different test cases of the laser beam diameter and the laser heat flux. Key engineering parameters, like cutting forces, temperature distributions, Von Mises stresses and plastic strains, are compared for both cutting processes. This comparison leads to important notifications on the influence of the cutting and laser parameters to LAM. The obtained results indicate that LAM may improve the machinability of AISI H-13 steel by reducing the cutting forces to a maximum percentage of ~15%.

Abstract: During the production process, turbine blades are subjected to a solubilization heat treatment, followed by tempering treatment, in order to obtain better mechanical properties. It is observed that, in some cases, permanent distortion can occur during the high temperature treatment (austenitising temperature). In this work, a high temperature creep resisting steel blade with a simplified geometry is considered. A finite element model is developed considering: the material properties depending on temperature, phase transformation and viscoplasticity (Nabarro-Herring and bilinear kinematic models). A nonlinear transient thermo-mechanical analysis is performed to simulate a standard thermal cycle. Material properties are partially calibrated based on dilatometric tests and partially from data available in literature. Adopting a laser scanner system, the blades geometry is measured before and after the heat treatment to calculate the permanent deflection. Comparing numerical results with experiments, it has been observed that the distortion phenomenon is mainly affected by the low-stress diffusional creep. This effect is due to the fact that, during the heat treatment, the blade is held at high temperature for a relatively long time according to a particular supporting lay-out. To minimize the permanent distortion, the numerical model permits an appropriate supporting system to be set-up, whose validity has been confirmed experimentally.

Abstract: Equilibrium elements have been developed in hybrid formulation with independent equilibrated stress fields on each element. Traction equilibrium condition, at sides between adjacent elements and at sides of free boundary, is enforced by use of independent displacement laws at each side, assumed as Lagrangian parameters. The displacement degrees of freedom belongs to the element side, where an extrinsic interface can be embedded. The embedded interface is defined by the same stress fields of the hybrid equilibrium element and it does not require any additional degrees of freedom. The extrinsic interface is developed in the consistent thermodynamic framework of damage mechanics with internal variable and produces a bilinear response in a traction separation diagram. The proposed extrinsic interface can be modelled on every single element side or can be modelled only on a set of predefined element sides.

Abstract: This work is focused on the mechanical characterization and fracture surfaces analysis of thermosetting polymers reinforced with short, randomly oriented, recycled carbon fibres (rCFs). This work aims at evaluating fibre/matrix adhesion between recycled CFs - reclaimed via pyrolysis followed by controlled oxidation of the pyrolytic char - and different polymer matrices, namely epoxy and vinyl ester resins. The latter is the main focus in this work, being amongst the most widely used thermosetting resins in SMC processes, which are the typical target for short rCFs. The evaluation of the properties of this new recycled carbon fibre reinforced polymer (rCFRP) has been via thermogravimetric analysis, dynamic mechanical analysis, stress/strain tests in tensile mode, and a subsequent analysis of the fracture surfaces by means of images analysis obtained by macrophotography, Optical Microscopy and Scanning Electron Microscopy. The comparison amongst the results allowed to evaluate the influence of the polymer nature and of the adhesion quality between fibres and polymeric matrix, mainly on the mechanical properties of the rCFRPs.