Comparison of stiffening types of the cutout in tubular wind turbine towers

A very efficient type of wind turbine tower is the tubular steel tower configured as a cantilever cylindrical or conical shell, which is used considerably nowadays. A typical feature of such towers is the presence of a manhole cutout near the bottom. This cutout is of considerable dimensions and lowers significantly the tower strength including stress concentrations and increased danger of local buckling. Therefore, it is common to stiffen the region around the cutout aiming at compensating for this loss of strength. In this paper the most common types of stiffening are investigated by means of nonlinear finite element analyses calibrated via experimental testing and the efficiency of each type is highlighted. It is found that simple stiffening types consisting of either a peripheral frame or two vertical stringers and a ring are particularly efficient and can be used instead of more complex ones. Rules for dimensioning of the stiffeners are also proposed.     

An isogeometric numerical study of partially and fully implicit schemes for transient adjoint shape sensitivity analysis

Frontiers of Mechanical Engineering - In structural design optimization involving transient responses, time integration scheme plays a crucial role in sensitivity analysis because it affects the...     

Novel load responsive multilayer insulation with high in-atmosphere and on-orbit thermal performance

Aerospace cryogenic systems require lightweight, high performance thermal insulation to preserve cryopropellants both pre-launch and on-orbit. Current technologies have difficulty meeting all requirements, and advances in insulation would benefit cryogenic upper stage launch vehicles, LH₂ fueled aircraft and ground vehicles, and provide capabilities for sub-cooled cryogens for space-borne instruments and orbital fuel depots. This paper reports the further development of load responsive multilayer insulation (LRMLI) that has a lightweight integrated vacuum shell and provides high thermal performance both in-air and on-orbit.LRMLI is being developed by Quest Product Development and Ball Aerospace under NASA contract, with prototypes designed, built, installed and successfully tested. A 3-layer LRMLI blanket (0.63 cm thick, 77 K cold, 295 K hot) had a measured heat leak of 6.6 W/m² in vacuum and 40.6 W/m² in air at one atmosphere. In-air LRMLI has an 18× advantage over Spray On Foam Insulation (SOFI) in heat leak per thickness and a 16× advantage over aerogel. On-orbit LRMLI has a 78× lower heat leak than SOFI per thickness and 6× lower heat leak than aerogel.The Phase II development of LRMLI is reported with a modular, flexible, thin vacuum shell and improved on-orbit performance. Structural and thermal analysis and testing results are presented. LRMLI mass and thermal performance is compared to SOFI, aerogel and MLI over SOFI.     

回填式搅拌摩擦点焊接头多点拉剪行为研究

目的 研究焊点数量及分布对回填式搅拌摩擦点焊接头力学性能的影响。方法 选取2 mm的2219-O和8 mm的2219-CS铝合金板,采用试验研究和数值模拟相结合的方式,研究不同焊点数量（2点、3点和4点）及焊点分布对回填式搅拌摩擦点焊接头力学性能的影响,以及在拉剪力作用下接头的应力分布状态。结果 多点接头成形良好,无飞边、毛刺产生;随着焊点数量的增加,接头力学性能逐渐提高,并且当上板受力侧焊点数量分布较多时,接头承载力较高。结论 在拉剪过程中,靠近上板受力侧的焊点承受较大作用力。当受力侧仅有一个焊点时,该焊点附近应力集中比较明显;当受力侧有两个平行焊点时,焊点平分载荷。当焊点数为4且呈正四边形分布时,接头承载力较高。     

An enhanced beam-theory model of the asymmetric double cantilever beam (ADCB) test for composite laminates

The paper introduces a mechanical model of the asymmetric double cantilever beam (ADCB) test, usable to assess the mixed-mode interlaminar fracture toughness of composite laminates. The laminated specimen is represented as an assembly of sublaminates, each of which is modelled as an elastic beam partly connected to the other by a deformable interface, in turn considered to be a continuous distribution of elastic-brittle springs. Based on Timoshenko’s beam theory, a set of six differential equations, accompanied by suitable boundary conditions, governs the problem. By adopting the interfacial stresses as the main unknowns, the differential problem is solved analytically, and the contributions of the opening and sliding fracture modes are evaluated directly. Moreover, explicit expressions are determined for the interfacial stresses, internal forces, and displacements, as well as for the compliance, energy release rate, and mode-mixity angle. The predictions of the model are to some extent similar to those of analogous mechanical models in the literature and appear in good agreement with both numerical and experimental results.     

Quasi-static behaviour of composite joints with countersunk composite and metal fasteners

An experimental study of the quasi-static behaviour of composite joints has been carried out. Composite plates were joined by six fasteners of three different types: composite, titanium Torque-set, and titanium Huck-comp. Specimens were quasi-statically tested to obtain results of the joint tensile and compressive strength. Strain gauge and extensometer measurements were used to investigate the static behaviour of the joints. Strain gauge measurements were carried out to analyse strain distribution and load transfer between bolt rows. Extensometer measurements were done in order to observe bolt-movement behaviour. Generally, composite joints with titanium fasteners have a higher quasi-static strength than that with composite fasteners.     

Non-probabilistic reliability-based design optimization of stiffened shells under buckling constraint

Stiffened shells are affected by numerous uncertainty factors, such as the variations of manufacturing tolerance, material properties and environment aspects, etc. Due to the expensive experimental cost of stiffened shell, only a limited quantity of statistics about its uncertainty factors are available. In this case, an unjustified assumption of probabilistic model may result in misleading outcomes of reliability-based design optimization (RBDO), and the non-probabilistic convex method is a promising alternative. In this study, a hybrid non-probabilistic convex method based on single-ellipsoid convex model is proposed to minimize the weight of stiffened shells with uncertain-but-bounded variations, where the adaptive chaos control (ACC) method is applied to ensure the robustness of search process of single-ellipsoid convex model, and the particle swarm optimization (PSO) algorithm together with smeared stiffener model are utilized to guarantee the global optimum design. A 3 m-diameter benchmark example illustrates the advantage of the proposed method over RBDO and deterministic optimum methods for stiffened shell with uncertain-but-bounded variations.     

Study on atmospheric tribology performance of MoS2–W films with self-adaption to temperature

MoS₂ is easily oxidized into MoO₃ at elevated temperature, which leads to a sharp deterioration of lubricity and greatly limits the application. To improve the atmospheric tribology properties of MoS₂ at elevated temperature, the co-deposited MoS₂–W composite films are prepared by magnetron sputtering. Compared with pure MoS₂, the design of three kinds of MoS₂–W composite films not only maintains lower temperature-sensitivity, but also greatly improves the mechanical properties. In particular, the three MoS₂–W composite films embrace good tribology performance at all test temperature, especially 100 °C, at which physical adsorption of H₂O is prominently less than that of 25 °C and the oxidation is relatively slight, compared with that of the higher temperature. Individually, the MoS₂-8.2%W composite film maintains the optimal tribology performance at any test temperature and becomes self-adaptive to temperature variation, which originates from the optimized structure and good mechanical properties.     

Comparative study of optical and structural properties of electrospun 1-dimensional CaYAl3O7:Eu3 + nanofibers and bulk phosphor

We report the optical and structural studies of Eu^3 +-doped 1-dimensional CaYAl₃O₇ nano-fiber phosphor. CaYAl₃O₇:Eu^3 + phosphors were synthesized by electrospinning technique and the pristine nano-fibers were annealed at 900 °C to form well crystallized uniform fibers. Under ultraviolet excitation, the CaYAl₃O₇:Eu^3 + exhibited red emission, due to transitions in the 4f states of Eu^3 +. In order to explore the difference between the quantum efficiency of nano-fiber and bulk CaYAl₃O₇:Eu^3 + phosphor, detailed structural and optical analyses were carried out. The structural analysis of the CaYAl₃O₇:Eu^3 + nano-fibers indicates that the structural environment surrounding the dopant Eu^3 + ion was more unstable in nano-fiber when compared to a bulk sample. Decay curves for both the samples when fitted with double exponential decay model indicate that the nano-fiber has shorter decay time, arising from the larger contribution from the non-radiative decay, due to defect levels introduced in the host lattice.