Key Engineering Materials Vols. 525-526

Abstract: A multilevel multiobjective platform for structural sizing reproducing the sequence of actions taken during design and structural sizing in industry is presented in this paper. This platform is integrated at two design levels labeled as Preliminary Design Level and Detailed Design Level. The set of design variables can be divided into a group of variables describing the main conceptual layout that affect the dimensions and architecture of the model and a second group of variables influencing the material and mechanical behavior. This kind of approach can be effective if it is possible to separate the constraints that are strongly dependent on the design variables of different design levels.

Abstract: The polycrystalline high purity 316H austenitic stainless steel has been thermo-mechanically treated to produce material with two layers of grain size, one of coarser and the other of finer grains. Small three point bend specimens containing a notch positioned in either the coarser or finer layer have been tested at a constant strain rate and a temperature of -196°C. The results are discussed with respect to the effect of grain size on the underlying deformation between the two layers of different grain size.

Abstract: Steel-concrete joints are often provided with welded shear studs. However, stress concentrations are induced in the structure due to the welding. Moreover, a reduction in toughness and ductility of the steel and a decreased fatigue endurance of the construction is observed. In this paper the shear bond strength between steel and ultra-high performance concrete (UHPC) without mechanical shear connectors is evaluated through push-out tests. The test samples consist of two sandblasted steel plates with a thickness of 10 mm and a concrete core. The connection between steel and concrete is obtained by a 2-component epoxy resin. Test samples with a smooth adhesive layer are compared with those with an epoxy layer, which is applied with a toothed paddle and/or gritted with small aggregates. In this research, specimens prepared with river gravel, crushed stone, and steel grit are compared and also two different epoxy resins are used. During the tests, the ultimate shear force is recorded as well as the slip between steel and concrete. All test specimens exhibited a concrete-adhesive or concrete failure. Furthermore, test results show that the use of a more fluid epoxy resin improves the anchorage of the gritted aggregates in the adhesive layer, resulting in higher shear bond stresses. No significant difference is found between specimens, gritted with river gravel or crushed stone. Applying the adhesive layer with the toothed paddle in horizontal direction slightly improves the bond behaviour. Finally, the experimental results of the test members with a smooth epoxy layer without gritted aggregates, provide test data for a fracture mechanics approach, which uses a 2D numerical model of the test specimen, composed of steel, epoxy resin, and concrete.

Abstract: The wedge-splitting test (WST) is a frequently used test configuration for performing stable crack fracture experiments on concrete specimens, thus allowing to determine the fracture process and crack propagation in the heterogeneous material. However, there are no standard rules regarding the wedge-splitting specimens geometry, groove dimensions or notch length. This paper concentrates on the influence of the initial notch length in geometrically identical, cubical specimens, cast from vibrated concrete. The experimental results of nine WSTs under monotonic loading, including F_sp-CMOD curves - splitting force versus crack mouth opening displacement - and fracture energy G_f, are presented. An important effect of the starting notch length on the fracture properties is observed.

Abstract: The growth of a small crack controlling the high-cycle fatigue life of a precipitation-strengthened 6061-T6 aluminium alloy was critically investigated. As the applied stress lowered, the small crack was arrested for a long period (over 10⁶ cycles) at grain boundaries before regrowth, which resulted in a significantly slow growth process. The morphological and crystallographic details of the small crack were then analyzed with focused ion beam and transmission electron microscopy. It was revealed that the small crack was formed along fine persistent slip bands (PSBs) whose structure was fairly different from that reported for other metals. The concept of PSB-limited fatigue strength may be extended to include the present material type.

Abstract: The solution treatment after nitriding (STAN) was performed to stabilize the γ- phase in a metastable austenitic stainless steel, type 304, and to improve the strength of type 304 by the solid solution of nitrogen. Plasma nitriding was conducted at 500°C for 8.5h, and then solution treatment was performed at 1200°C for 1h. As a result, the static strength and the hardness were improved by the STAN. Rotary bending fatigue tests were performed on the specimens with STAN (solid solution strengthened) together with the untreated and the nitrided ones in laboratory air and in 3%NaCl solution. In laboratory air, the fatigue strength of the solid solution strengthened specimen increased compared to that of the untreated specimens, where fatigue limits were 340MPa and 290MPa for the solid solution strengthened and the untreated, respectively. However, the fatigue limit of the solid solution strengthened specimen was lower than that of the nitrided specimen, that is, 380MPa. On the other hand, in 3%NaCl solution, the fatigue strengths of the nitrided specimens and the solid solution strengthened specimens decreased significantly compared to those in laboratory air. After the fatigue tests at the stress level of fatigue limit in laboratory air, the strain-induced martensitic transformation was examined by XRD. In the solid solution strengthened specimens, the strain-induced martensitic transformation was not detected during fatigue tests until 3x10⁷ cycles, indicating that the γ- phase was stabilized by the solid solution of nitrogen.

Abstract: Experiments have been conducted to investigate the crack growth characteristics of 7050-T7451 aluminium plate in L-S orientation. Two loading conditions are selected, i.e. constant amplitude and constant stress intensity factor range (ΔK). The effects of ΔK-levels and stress ratios (R) on crack splitting are studied. Test data shows that crack splitting could result in the reverse of crack growth rate trend with the increasing R ratio at high ΔK-level. The appearance of crack splitting depends on both ΔK and R.

Abstract: This paper presents structural studies of a medium scale composite wind turbine blade construction made of epoxy glass fiber for a 750kW rated power stall regulated horizontal axis wind turbine system. The complex geometry of the blade with a skin-spar foam sandwich structure was generated by utilizing commercial code ANSYS finite element package. Dimensions of twist, chord and thickness were developed by computer program. NREL S-series airfoils with different chord thickness are used along current blade cross-sections. The current design method uses blade element momentum (BEM) theory to complete satisfactory blade design and can be carried out using a spreadsheet, lift and drag curves for the chosen aerofoil. According to composite laminate theory and finite element method, optimal blade design was obtained. The focus is on the structural static strength of wind turbine blades loaded in flap-wise direction and methods for optimizing the blade cross-section to improve structural reliability. Moreover, the natural frequencies and modal shapes of the rotor blade were calculated for defining dynamic characteristics. Structural analysis was performed by using the finite element method in order to evaluate and confirm the blade to be sound and stable under various load conditions.

Abstract: An explicit extrapolating formula for a general case of the interfacial crack plane lying with an angle to the poling axis in transversely isotropic piezoelectric (TIP) materials is derived, which is very feasible to determine the intensity factors for numerical methods such as FEM or BEM. Additionally, a more concise extrapolating formula for the typical state of the interfacial crack plane lying perpendicular to the poling axis is also presented in this paper.

Abstract: The stitched composite T-joints and Z-pinned ones subject to bending load were investigated in this paper. A simple theoretical model characterizing the failure process of through the thickness reinforcement (TTR) during mode I delamination was presented. The experimental results showed that the initial damage load and maximum load of stitched specimens are higher than that of Z-pinned ones, while the energy absorption of stitched specimens during delamination is lower than that of Z-pinned ones. The energy absorption values predicted by the present model meet the experiments reasonably well. High friction force at the interface between TTR tow and matrix, with a long pull-out displacement of the tow, helps to improve the delamination resistance.