Abstract in English:Abstract The tube-array structures coupled both with the fluid and the axial load is focused. In a heat exchanger, the tubes are frequently affected by cold and hot fluid shock waves. Thus, tubes are often found to be vibrated and even to be worn out. This vibration may not only alter the performance of the tubes but also introduce the damage in the heat exchanger. In this work, the dynamic properties of coupled tube-array structures with the axial loads are investigated. Futher, the tube-array structures in a heat exchanger, consisting of periodic cooling tubes are also considered to simulate as a coupled periodic structure system. It is found that each tube of the system is weakly coupled to the adjacent tube via water. By using the Galerkin method, the equations of the coupled system with the fluid effect can be derived. Numerical results reveal that the axial load and fluid effects may influence the dynamic properties of a fluid coupled tube-array structures system of a heat exchanger significantly.
Abstract in English:Abstract The aim of this paper is to investigate the impact of the microscopic yield surface (i.e., at the single crystal scale) on the forming limit diagrams (FLDs) of face centred cubic (FCC) materials. To predict these FLDs, the bifurcation approach is used within the framework of rate-independent crystal plasticity theory. For this purpose, two micromechanical models are developed and implemented. The first one uses the classical Schmid law, which results in the formation of vertices (or corners) at the yield surface, while the second is based on regularization of the Schmid law, which induces rounded corners at the yield surface. In both cases, the overall macroscopic behavior is derived from the behavior of the microscopic constituents (the single crystals) by using two different scale-transition schemes: the self-consistent approach and the Taylor model. The simulation results show that the use of the classical Schmid law allows predicting localized necking at realistic strain levels for the whole range of strain paths that span the FLD. However, the application of a regularized Schmid law results in much higher limit strains in the range of negative strain paths. Moreover, rounding the yield surface vertices through regularization of the Schmid law leads to unrealistically high limit strains in the range of positive strain paths.
Abstract in English:Abstract First, the basic characteristics of macro and micro masonry models in numerical analyses of masonry structures are discussed. Afterwards, developed numerical macro and micro masonry models, intended for the nonlinear static and dynamic analysis of unreinforced and confined masonry walls, as well as of masonry-infilled frames, are briefly presented. The models are tested on previously performed experimental tests of masonry-infilled steel frame under horizontal static force and masonry-infilled steel frame on a shake-table. The precision of both models is compared. It is concluded that both numerical models can provide reliable results. However, the macro model has more advantages for wide practical application.
Abstract in English:Abstract Explosive devices represent a significant threat to military and civilian structures. Specific design procedures have to be followed to account for this and ensure buildings will have the capacity to resist the imposed pressures. Shrapnel can also be produced during explosions and the resulting impacts can weaken the structure, reducing its capacity to resist the blast pressure wave and potentially causing failures to occur. Experiments were performed by the Defence Science and Technology Agency (DSTA) of Singapore to study this combined loading phenomenon. Slabs were placed on the ground and loaded with approximately 9 kg TNT charges at a standoff distance of 2.1 m. Spherical steel ball bearings were used to reproduce the shrapnel loading. Loading and damage characteristics were recorded from the experiments. A finite element analysis (FEA) model was then created which could simulate the effect of combined shrapnel impacts and blast pressure waves in reinforced concrete slabs, so that its results could be compared to experimental data from the blast tests. Quarter models of the experimental concrete slabs were built using LS-Dyna. Material models available in the software were employed to represent all the main components, taking into account projectile deformations. The penetration depth and damage areas measured were then compared to the experimental data and an analytical solution to validate the models.
Abstract in English:Abstract This work discusses the experimental results of Acoustic Emission (AE) recordings during repetitive loading/unloading loops of cement mortar beams subjected to three point bending. Six repetitive loading cycles were conducted at a gradually higher load level until the failure of the specimens. The experimental results clearly show the existence and dominance of the Kaiser effect during each loading loop. Regarding the AE data, alternative analysis was conducted using the improved b-value, and the cumulative energy behaviour. Both quantities considered, show qualitative and quantitative characteristics that could be used as pre-failure indicators. In addition, a novel statistical physics analysis involving the AE interevent times was conducted by calculating the cumulative probability function P(>δτ) that follows a q-exponential equation. The entropic index q and the relaxation parameter βq of this equation show systematic changes during the various stages of the failure process. The last cycle led to a q value equal to 1.42, implying the upcoming fracture which is in good agreement with previous results obtained from a wide range of fractured materials.
Abstract in English:Abstract This paper presents a methodology for the conceptual design of a Maneuver Load Control system taking into account the airframe flexibility. The system, when switched on, is able to minimize the bending moment augmentation at a wing station near the wing root during an unsteady longitudinal maneuver. The reduction of the incremental wing bending moment due to maneuvers can lead to benefits such as improved pay-loads/gross weight capabilities and/or extended structural fatigue life. The maneuver is performed by following a desired vertical load factor law with elevators deflections, starting from the trim equilibrium in level flight. The system observes load factor and structural bending through accelerometers and calibrated strain sensors and then sends signals to a computer that symmetrically actuates ailerons for reducing the structural bending and elevators for compensating the perturbation to the longitudinal equilibrium. The major limit of this kind of systems appears when it has to be installed on commercial transport aircraft for reduced OEW or augmented wing aspect-ratio. In this case extensive RAMS analyses and high redundancy of the MLC related sub-systems are required by the Certification Authority. Otherwise the structural design must be performed at system off. Thus the unique actual benefit to be gained from the adoption of a MLC system on a commercial transport is the fatigue life extension. An application to a business aircraft responding to the EASA Certification Specifications, Part 25, has been performed. The aircraft used for the numerical application is considered only as a test case-study. Most of design and analysis considerations are applicable also to other aircraft, such as unmanned or military ones, although some design requirements can be clearly different. The estimation of the fatigue life extension of a structural joint (wing lower skin-stringer), located close to the wing root, has been estimated by showing the expected benefit to be gained from the adoption of such a maneuvering load control system.
Abstract in English:Abstract Currently, there are a lot of research activities dealing with gamma titanium aluminide (γ-TiAl) alloys as new materials for low pressure turbine (LPT) blades. Even though the scatter in mechanical properties of such intermetallic alloys is more distinctive as in conventional metallic alloys, stochastic investigations on γ -TiAl alloys are very rare. For this reason, we analyzed the scatter in static and dynamic mechanical properties of the cast alloy Ti-48Al-2Cr-2Nb. It was found that this alloy shows a size effect in strength which is less pronounced than the size effect of brittle materials. A weakest-link approach is enhanced for describing a scalable size effect under multiaxial stress states and implemented in a post processing tool for reliability analysis of real components. The presented approach is a first applicable reliability model for semi-brittle materials. The developed reliability tool was integrated into a multidisciplinary optimization of the geometry of a LPT blade. Some processes of the optimization were distributed in a wide area network, so that specialized tools for each discipline could be employed. The optimization results show that it is possible to increase the aerodynamic efficiency and the structural mechanics reliability at the same time, while ensuring the blade can be manufactured in an investment casting process.