Key Engineering Materials Vol. 827

Abstract: The bending stress in beams may often be reduced by adding material to the cross section. In some paradoxical cases, however, the bending stress increases by adding material from zones far away, or close to, the neutral axis. Similarly, the bending stress of rectilinear or curved beams may often be reduced by adding ribs to the initial beam section. However, such ribs may sometimes cause a both undesired and unexpected stress increase, although they still produce a beneficial stiffening effect. The aim of this paper is twofold: a) to examine this unexpected result within the context of the paradoxical behaviour of some known beam sections, and especially of a recently noted paradox; b) to provide a preliminary rule of thumb for the mechanical design of ribs sometimes added to the outer surface of an eye, with particular regard to the small end of a connecting rod.

Abstract: Creep is defined as a time dependent component of plastic deformation. Creep tests can be performed either at constant load or at constant applied stress. Engineering creep tests carried out at constant load are aimed at determination of the creep strength or creep fracture strength, i.e. the data needed for design. The constant stress tests are important as a data source for fundamental investigations of creep deformation and fracture mechanisms and for finite element modelling of more complex stress situations. For some materials, the difference between the two type of testing can be very small, while for other materials is large, depending on the creep plasticity of the material under testing. The paper aims to compare the creep results of two different creep-resistant materials: the advanced 9%Cr martensitic steel (ASME Grade P91) and a Zr1%Nb alloy obtained by both testing methods and to clarify the decisive factors causing observed differences in their creep behaviour.

Abstract: This work is concerned with the study of the strength of nanocoated reinforcing fibers. In more detail, glass fibers were coated with an efficient thermoelectric (TE) ink in order to create multifunctional reinforcing fibers for advanced composite structural applications. The main scope is to evaluate the fracture properties of the TE-enabled hierarchical glass fibers. The hybrid nanocrystal TE ink was synthesized via a solvothermal reaction and further fully characterized in coating form. The morphology and wetting properties of the TE ink deposition onto glass fibers were evaluated via SEM and contact angle measurements. Enhanced values by 19.4% in tensile strength for the coated glass fibers compared to the reference are being reported, measured at single fiber level. The evaluated multifunctional glass fiber strength will be utilised during ongoing research for the interfacial shear strength determination.

Abstract: This study is focused on the effect of the nanomodification of the microcapsules healing agent on the healing efficiency. In detail, nanomodified epoxy resin with both carbon nanotubes (CNTs) and carbon black (CB) diluted with a non-toxic solvent was encapsulated into UF capsules. The morphology of the external surface and the mean diameter was investigated via Scanning Electron Microscopy (SEM). In addition, the thermal stability was estimated with Thermogravimetric analysis and healing efficiency was evaluated for the polymer epoxy matrix. A parametric study was performed at various solvent percentages and catalyst percentages. Results indicated an increase of the healing efficiency with nanomodified capsules against of the use of conventional microcapsules.

Abstract: A cross-ply fiber-reinforced composite in uniaxial tension is modelled using a mesoscale and a micro-scale approach comparing the results from both the analyses. The use of multi-scale modelling gives directly the macroscopic constitutive behaviour of the structures based on its microscopically heterogeneous representative volume element (RVE). In the meso-scale approach the material of each layer is modelled as a homogeneous transversely isotropic material whose properties resulted from a numerical homogenization analysis. One of the main advantages of micro-scale modelling is the ability to simulate damage mechanisms such as matrix cracking, delaminations of the matrix-fiber interface and fibre-damage. In the first part of this study, analytical and numerical homogenization schemes are compared. RVEs of continuous fibre and short-fibre reinforced composites are created, homogenized numerically and compared with the widespread analytical scheme of Mori-Tanaka based on Eshelby’s solution of the single inclusion problem. In the second part, results’ comparison between the simulations of both scales is performed. In the meso-scale model stochasticity has been introduced, assigning interfacial strength following a normal distribution, in order to predict cracking initiation, propagation and saturation at the matrix material. The stresses at the crack tips are compared with the stress fields around the cracks from the micro-scale analysis and the results are in good agreement.

Abstract: The water channels inside the Nafion membrane are considered as the elastic particles and the effect of the elastic modulus of the particles on the deformation behavior of the membrane is discussed with a particle-contained composite model, which is constituted based on the homogenization method. The results show that the deformation resistance of the membrane is quite dependent on the elastic modulus of the particles, whereas the strain softening behavior after the yielding of the membrane is almost derived from the onset and the propagation of the localized shear band in the matrix, which also leads to the dramatic disappearance of the entanglement points in the molecular chain network of the matrix.

Abstract: A computational interface damage model which takes into account crack initiation andgrowth along connections between parts of a multi-domain structure is proposed and is exposed tosituations where cyclic loading and its effects on the structure are noticeable, though the inertial effects are not considered. Modelling of damage takes into account various aspects of damage propagation and invoking of an interface crack. First, the degradation function of the interface layer controls the stressseparation relation on damage evolution. Second, the instant of triggering and cessation of damage propagation may in situations of cyclic loads depend on the actual state of the structure, influencing thus its endurance limit. Finally, the hysteretic character of damage provides together with loadingunloading conditions a fatigue-like character, where the crack appears for smaller magnitude of the cyclic load than for pure uploading. The numerical solution and a short parametric study is provided for a simplified situation of single damageable interface spring.

Abstract: In this research, the bullet-collision test was done in the different velocity. The influence of its velocity on the deformation shape after collision was discussed by comparison with experiment and simulation. The velocity of the bullet was chosen in several kinds of speed ranges at 80m/s-250m/s. The angle of incidence with the collision object is 90 degree (head-on collision). The deformation shape was measured by 3D measurement instrument and reconstructed with 3D-CAD based on the 3D digital data. The deformation mark of the polycarbonate plate and the penetration shape of acrylic plastic plate caused by the bullet's collision with the object were examined. In addition, the collision simulation of the bullet was done by using FE analysis code “LS-DYNA”, and these analytical results were compared with the 3D digital data which is the experimental results.

Abstract: In this paper a numerical methodology is proposed, which aims at predicting the fatigue behaviour of engine cylinder liners in an eight-cylinder V-type four-stroke turbocharged engine. A preliminary kinematic and dynamic study of the crank mechanism is fulfilled in order to properly identify the load cycle that involves the cylinder liner. Finite Element analyses, both thermal and thermo-mechanical, are performed to evaluate the stress and the strain of the component. In particular, non-linear models are developed to mimic the piston-liner interaction when subjected to different loading conditions. A simplified approach is proposed in order to reduce the computational effort of the simulations. FEM results are then processed employing the multiaxial Dang Van fatigue criterion.

Abstract: Three point bending and impact tests with sub-sized Charpy specimens were performed on the JRQ reference steel for reactor pressure vessels. Quasi-static and dynamic fracture toughness data were calculated and the fracture behavior in the ductile to brittle transition region was evaluated within the frame of the master curve method (ASTM E1921). Specimens with shallow and deep cracks were studied and the respective influence of crack length and loading rate on the reference transition temperature was determined. The force-time curves of specimens with shallow cracks presented significantly smaller oscillations with respect to the absolute force, making the fracture toughness evaluation more accurate.