Key Engineering Materials Vols. 577-578

Abstract: Continuing upscaling trends in turbine height, rotor diameter, and rated power have resulted in massive, expensive tower structures. A modified guyed tower concept with struts (GTS) is proposed for saving material, and the basic design is made for a 2.5MW turbine. The tower and cable dimensions are optimized for lowest system cost given yield constraints. DACE (design and analysis of computer experiments) methods of sampling and surrogate model optimization are used for efficient parameter study and optimization of the ABAQUS finite element model using DAKOTA software. The resulting design is highly effective in transferring turbine loads from the tower to the cables. A mass savings of 41% is calculated vs. conventional structures, and further investigation of the GTS is recommended for both onshore and offshore applications. Additionally, the concept of retowering older turbines is introduced and proposed as an economic alternative to the common practice of repowering old wind farms with larger, new machines. The GTS is specifically suited to this application. Lastly, the design methodology developed for this study is shown to be effective and efficient; it can be applied for the massoptimization of similar cablesupported truss structures.

Abstract: This paper discusses the fracture-plastic material models for reinforced concrete and use of this model for modelling of reinforced concrete beams. Load-displacement relations and bearing capacity of reinforced concrete beams will be evaluated. A series of original (own) experiments - the beam and data from completed experiments - have been chosen for the numerical modelling. In case of the original experiments - reinforced concrete beams, stochastic modelling based on LHS (Latin Hypercube Sampling) will be carried out in order to estimate the total bearing capacity. The software used for the fracture-plastic model for reinforced concrete is ATENA.

Abstract: To reduce stress concentration near the loading point, Brazilian test, normally used for the indirect determination of tensile strength for rock or concrete quasi-brittle materials, is usually subjected to uniformly distributed pressure over finite width (or arc) of the Brazilian disc. As an extension, the same geometry, in which a central notch is introduced in the symmetrical plane of applied forces, can be used to determine fracture toughness of the materials. For this purpose, accurate expression for stress intensity factor (SIF) for central cracked Brazilian disc should be provided first. To this end, weight function method is used in this paper where weight function for central cracked disc is available in general SIF handbooks. Meanwhile, the exact stress solution in the intact Brazilian disc subjected to uniformly distributed pressure is derived by use of numerical method. In the end, some finite element analysis is carried out using the program package ABAQUS on the Brazilian disc, and the comparison results regarding the SIF between the weight function method and numerical method prove the validity of the proposed method.

Abstract: The drive shaft of the bi-modal tram has been designed and built into independent wheel drive; all driving wheels are driven by electric motor-gearbox sets independently, in order to make low floor to provide the old and the handicapped with easy access, and to remove differential gears to reduce turning radius. This paper was aimed to evaluate torques on drive shafts while the bi-modal tram was running on slope way in the test track, and at maximum speed in public roadway. On the basis of the torques measured on the drive shafts, torsion characteristics of the independent wheel drive were discussed in this paper.

Abstract: The giga-cycle property of a newly developed Al alloy, which contains 0.5wt.% excess Mg solute compared to a standard age-hardened 6061 alloy (6061-T6), was investigated by using smooth specimens subjected to ultrasonic fatigue. The fatigue strength of the new alloy was higher than that of a normal 6061 alloy particularly at relatively low stress amplitude level. Several analyses (surface crack observation, fractography, FIB cross-sectioning, etc.) were also conducted to reveal the micro-mechanism of the observed strength properties. The following results were obtained: i) No fatigue limit was confirmed for both 6061 and new alloy. ii) Total life (N_f) of 6061 and new alloys was determined by a single fatigue crack initiated from a surface PSB crack. iii) Crack initiation resistance defined by N₂₅ (number of cycles to reach ρ = 25 mm^-2, where ρ is the PSB crack number density) for new alloy was higher than that of 6061. iv) The higher fatigue strength of new alloy was explained by the effect of excess Mg solute which increased the resistance against the formation of PSB cracks.

Abstract: This paper is concerned with a second-order multiscale computational homogenization scheme for heterogeneous materials at small strains. A special attention is directed to the macro-micro transition and the application of the generalized periodic boundary conditions on the representative volume element at the microlevel. For discretization at the macrolevel the C¹ plane strain triangular finite element based on the strain gradient theory is derived, while the standard C⁰ quadrilateral finite element is used on the RVE. The implementation of a microfluctuation integral condition has been performed using several numerical integration techniques. Finally, a numerical example of a pure bending problem is given to illustrate the efficiency and accuracy of the proposed multiscale homogenization approach.

Abstract: An evaluation was carried out by applying a load to the underframe of the freight car to determine the structural strength of the carbody of a newly manufactured freight car for transportation. This study is aimed at evaluating the safety of the underframe of the freight car under the maximum load conditions. The freight car for use in rail freight was designed mainly with SM490A steel and ASTM A441 steel for production. The carbody for rolling stock is a principal structure supporting major underframe equipment and freight. Thus, strength evaluation of this structure is very important. Both structural analysis and loading test were performed under the loading condition. Prior to evaluating structural strength, finite element method software was used for structural analyses of stress distribution in a carbody of freight car. The strain gages were attached on the carbody based on the FEM results. A vertical load test, compressive load test, measurement of deflection amount and a natural frequency measurement test were conducted.

Abstract: Plastic zone with equivalent plastic strain ε above 0.02 PZ_0.02 and that above 0.12 PZ_0.12 in 304 stainless steel small punch specimen loaded at RT and 77K to various level to fracture were measured by the recrystallization technique. Martensite transformation was also checked magnetically. It is found that the behaviour of both PZ_0.02 and PZ _0.12 is at first stretching mode, then stretching and bending mode and then stretching mode. PZ_0.02 and PZ_0.12 propagated to the clamped region of the specimen.

Abstract: The partial transient liquid phase PTLP metals layer technique for ceramic-metal bonding was applied to surface coating of TiAl alloy. Ni/Al/Ni and Ni/Ti/Ni foils (10μm) layers were coated on flat surface of oxidation specimen OS and on notch root surface of notched compact tension specimen NCT of a TiAl alloy. It is shown that the oxidation resistance is significantly improved and that the resistance to the low cycle fatigue LCF crack initiation life is improved. A procedure of application of the present technique for three dimensional surface coating is proposed.

Abstract: This study investigates the influences of pre-existing cracks to simulate defects in structures using curved bi-layer system consisting of glass coating on a polymeric support layer, to illustrate the role of defects on the evolution of failure of hard coating bi-layer system under loading. The densities of pre-existing cracks were initiated near the margins and the relative orientations were varied. The pre-existing cracks were induced by using hard spherical indenter. The failure of these samples was investigated under single-cycle axial loading tests using soft indenter of low elastic modulus. The onset of fracture was observed in situ using a video camera. Margin cracks were observed to propagate from pre-existing cracks near the margins, while the pre-existing cracks in the compression zone under the soft indenter were arrested. Experimental trends suggested that critical loads were increased with specimens having no defects (pre-existing cracks). Finite element modeling was used to evaluate the stress distribution in the brittle coating and compared with the experimental trend. It is concluded that radial cracks can initiate and propagate anywhere in the tensile region. The larger the flaws the smaller the loads required for defects pre-existing cracks to propagate radials to failure.