Key Engineering Materials Vols. 417-418

Abstract: The fatigue properties of friction stir (FS) and TIG welded Al 6N01-T5 alloys were compared in the present study. The low cycle fatigue (LCF) test was performed under total strain amplitudes in the range of ± 0.6 ~ ± 1.5% and with a strain rate of 3 x 10-3/s. During low cycle fatigue, the base metal showed little cyclic hardening or softening, whereas both the FS and TIG welded zones showed a large amount of cyclic hardening until fracture, although the fatigue life of the TIG welded zone was much shorter than that of the FS welded zone. The fatigue crack growth rate (FCGR) was retarded in the heat affected zone (HAZ) on the advancing side of the FS welded zone, due to the compressive residual stress in this region. The differences in the fatigue properties in these two welded zones were discussed in terms of the microstructural changes during fatigue.

Abstract: In this paper, the mechanisms of propagation of the damage in aluminum panels repaired with bonded composite patches of different mechanical characteristics is analyzed. The aim of this study is to analyze analytically, experimentally and numerically the advantage of the use of bonded composite patches to increase the fatigue life and to reduce the state of tension at the crack tips. The experimental results show that both static strength and fatigue life of the repaired aluminum panel has significantly increased due to the bonded composite patches. The different patches and adhesive, used for cracked panels, have provided about a 100-110% improvement in the fatigue life and a 30-35% decrease in the stress intensity factor. A comparison between finite elements calculations and experimental data has been carried out. The good agreement between the experimental data and the numerical ones has demonstrated the possibility to obtain an optimized design of bonded patches with the numerical tools.

Abstract: Results of residual stress measurements on Inconel 718 turbine components after machining are here presented. The work is focused on the experimental detection of the residual stress state produced after turning (orthogonal cutting and standard) and milling. The aim of the experimental activity was to supply an experimental data base of proved reliability for milling and turning model validation. This activity was performed inside the EU STREP project VERDI. The residual stresses were detected by means of X-ray diffraction technique. The experimental plan of measurements was performed on components worked with different working parameters (cutting velocity and feed rate) and by means of new and worn tool. The trends of residual stress versus depth are presented.

Abstract: This paper is devoted to the analysis of the dynamics and vibrations of wing-like structures with bonded piezoelectric strips and subjected to unsteady aerodynamic loads for crack detection. Pairs of piezoelectric strips, acting as strain sensors, are bonded at the same locations on the opposite sides of a thin structure executing flexural oscillations. In this crack detection strategy, the measured voltage outputs of the two piezoelectric sensors forming a pair are conveniently subtracted in order to eliminate the voltage corresponding to the same level of strain on both sides. This differential voltage output is used to indicate the presence of a crack in the structure. The nonlinear mechanical behavior of the crack in the compression and extension phases of the oscillatory cycle increases substantially the sensitivity of this detection procedure. Furthermore, this crack detection method can take advantage of the aeroelastic oscillations of the wing structures, which are always present during normal flight evolutions of an aircraft. The numerical analysis of the dynamics of structure subjected to unsteady aerodynamic loads uses a finite element formulation for the structure and the piezoelectric strips and a panel method is used to compute the unsteady aerodynamic loads acting on the oscillating wing structure. Numerical simulation results are presented in the paper to explore the feasibility of this crack detection strategy by using the aeroelastic oscillations of the wing-like structures with bonded piezoelectric strips.

Abstract: Application of composite laminates was very wide in aerospace engineering, civil engineering, wind energy, auto industry, etc. Low cost glass fiber textile was often applied into composite laminates by many composites companies. It is of import that investigation of mechanical properties and damage mechanism of this composites laminates. Two types of composite laminates were studied in this paper. One type of composite laminates was made of glass fiber biaxial cloth. The other was made of glass fiber composite felt. Each type composite laminates has different direction aligned. Many specimen were tested in compression with universal testing materials machine model INSTRON 5500R. Strength of composite laminates and stress-strain diagram was obtained in these experiments. Effect of fiber different orientation on compression strength of laminates was found. Shear stresses between two laminas were calculated. Fracture mechanism of composites laminates was analyzed by macro-method. Fractography of laminates was applied into analysis of mechanism. SEM photo was acquired and observed in detail. The result is that strength and failure mechanism of different types composite laminates varied with fiber orientation and different textiles.

Abstract: With the increasing investigation on reliability and stability of engineering structures, it becomes important for reseach on mechanical properties of materials under fracutre conditions. Concerning to fracture problems of large deformation materials, deformation characteristics of the crack tip field is necessarily needed. This paper presents an experimental analysis on a double-cracked rubber disk under the I-II mixed-mode fracture condition. Deformation properties around the crack tip are described through analyzing displacement information acquired by digital moiré method of circular and radial gratings. Also the existence of sector division mode under mixed loading condition is verified, and it is also found experimentally that the shear component makes the sectors shift.

Abstract: Smoothed particle hydrodynamics (SPH) is a mesh-free numerical approximation technique for simulating various physical problems. A calculation system for transient heat transfer problem by SPH has been improved to deal with various boundary conditions and several model calculations are performed to verify it. As a practical application, the transient temperature field of a brake disk under emergency braking is analyzed. Both solid and ventilated disks are modeled with a moving heat source on the sliding surface. The numerical results show that the temperature sharply fluctuates because of the cyclic loading. Improvement of the calculation model is also discussed.

Abstract: Two hollow rectangular bridge columns with Reactive powder concrete (RPC) were tested under a cyclically reversed horizontal load. Based on the test results, the seismic behavior of such columns was presented. An analytical model was developed to predict the force-displacement relationship of specimens. The test results were also compared to the proposed analytical model. It was found that the ductility factors of the specimens are over 4.0, and the proposed analytical model can predict the force-displacement relationship of such columns with acceptable accuracy.

Abstract: In this research, damage spectra to quantify the damage of reinforced concrete (RC) bridges under earthquakes are proposed. The spectra are computed for hundreds of horizontal ground motions recorded during past several earthquakes. The damage spectra proposed are promising for performance-based seismic assessment of existing RC bridges as well as performance-based design of new systems.

Abstract: In this paper some results of 3D-finite element analyses of a modified CTS-specimen(MCTS) with an inclined crack front are presented. It will be shown, that through the inclination of the crack front, even under pure in-plane shear loading of the specimen, mixed-mode II and III loading conditions can be generated along the straight inclined crack front. Furthermore a superposition of all fracture modes I, II and III can be generated, if this MCTS-specimen is subject to an in-plane tension/shear loading. The computational fracture analysis is based on the calculation of separated energy release rates GI, GII and GIII along the crack front by the numerically highly effective modified virtual crack closure integral(MVCCI)-method and for the finite element(FE)-modelling the commercially available FE-code ANSYS is utilized.