Key Engineering Materials Vols. 324-325

Abstract: Mode III fracture failure of a through crack in an orthotropic functionally graded strip is investigated. The shear moduli in two directions of the material are respectively assumed to vary proportionately as a definite gradient. Fourier cosine transform is used to reduce the problem to solving a Fredholm integral equation. The crack tip stress field is obtained by considering the asymptotic behavior of Bessel function. Energy density factor criterion is applied to obtain the maximum of the minimum energy density and direction of crack initiation. Numerical results are given graphically to illustrate the effects of the material property parameters and geometry criterion on the energy density factor.

Abstract: A vibration-based approach to detect crack damage in a cantilever composite wingbox is studied using the improved Hilbert-Huang Transform (HHT). The improved HHT is composed of HHT with Wavelet Packet Transform (WPT) and a simple but effective method for intrinsic mode function (IMF) selection. For different damage status, in order to obtain structural dynamic responses, which imply plentiful damage information, the composite wing boxes were excited by a contrived square wave signal. Then, the dynamic responses of intact wingbox and damaged wingbox are disposed using improved HHT. Finally, a feature index vector of structural damage, i.e. the
ariation quantity of instantaneous energy, is constructed. The obtained results show that the proposed damage feature index vector is more sensitive to small damage than those in traditional signal processing.

Abstract: Welded joint is a mechanical heterogeneous body, which is composed of base metal, weld metal and heat affected zone (HAZ). Many welded structures endure dynamic load in service. Mechanical heterogeneity has important influence on dynamic fracture behaviour of welded joint. In the present investigation, dynamic fracture parameter of J-integral of undermatched three-point-bending (TPB) welded joint specimens containing longitudinal crack with different geometry were computed. The strain rate near crack tip reaches 103 under the impact velocity of 5m/s, so dynamic properties under corresponding strain rate should be used in dynamic analysis. The results of instrumented impact experiment were used as the input parameters in the computation. Dynamic J-integral was evaluated using virtual crack extension (VCE) method of MARC finite element code in 3-D condition. Dynamic J-integral evaluated by VCE method is path independent. The value of dynamic J-integral curve increases with loading time smoothly, so inertia force has little influence on dynamic J-integral. The values of dynamic J-integral decrease with increase of weld width. When the weld width is bigger than 5mm the influence of base material on weld metal is not evident. The influence of initial crack length on dynamic J-integral is complicate. The value of dynamic J-integral of the computation model with initial crack length of a/W=1/4 is the highest, and that of the computation model with initial crack length of a/W=1/8 is the lowest. The results are helpful for dynamic fracture evaluation of macro-heterogeneous welded joints.

Abstract: A general-purpose useful parameter in data analysis is the intrinsic dimension of a data set, corresponding to the minimum number of variables necessary to describe the data without significant loss of information. Feature extraction, including linear or nonlinear mapping technique, is efficient to estimate the intrinsic dimension of the data set, which is a key issue to machine fault diagnosis. This paper presents a novel application of feature extraction using the nonlinear mapping technique called curvilinear component analysis (CCA) for gear failure detection. In the approach high-dimensional data are nonlinearly projected toward an output space with dimension equal to the intrinsic dimension. Hence, enough information is remained to describe correctly the original data structure, and feature extraction based on CCA reduces dimensionality of the raw feature space for machine failure detection. Gearbox vibration signals measured under different operating conditions are analyzed using the technique. The results indicate that the intrinsic dimension of the data set is estimated and a 2-D subspace is extracted by the CCA technique, then the high-dimensional original feature data are projected into the 2-D space and form several clustering regions, each indicative of a specific gear condition, respectively. Thus, the gear operating conditions including normal, one cracked tooth, and one broken tooth are classified and detected clearly. It confirms that feature extraction based on the nonlinear mapping is very useful and effective for pattern recognition in mechanical fault diagnosis, and provides a good potential for applications in practice.

Abstract: There are numerous applications of pressure vessels in aerospace field for storing liquid or gaseous media. Generally the metallic vessels have been manufactured by welding two hemispheres, which are machined or spin-formed. In this study, the solid state bonding method was utilized with blow forming to manufacture aerospace titanium tanks. This solid state joining technology replaced the welding process and without any secondary material or liquid phases in bonding process, homogeneous microstructure was obtained at bonding interface. Using this method, a titanium tank of a space vehicle was manufactured and during a hydraulic pressurizing test, the strain and acoustic emission signals are observed to investigate the effect of solid state bonding method on the failure mode and performance of the tank.. This result was compared with the one made by conventional method of spin forming and welding. The result shows that the pressurization rate and the acoustic emission signal increasing rate provide a similar tendency for a vessel of integrity, while the signal increasing rate is much higher than the pressurization rate for a vessel with welding defects. It is clear that the failure mode of the solid state bonded tank is different from the welded tank due to the completely united interface by diffusion process.

Abstract: This paper focuses on the study of the plastic zone in fatigue crack closure based on the results obtained by means of 3D Finite Element Analysis (FEA). These results show the crack behavior through the thickness. The plastic zone is visualized and quantified. It does not correspond to the classical shape. The plastic zone in the interior surface is similar to those obtained in 2D plane strain conditions and a reduced effect of closure is observed. However, close to the external surface, 2D plane stress results are not reproduced, the plastic zone size is smaller and an important change is observed. This transition is developed in a thin external portion of the specimen and it can only be captured if a fine mesh of the thickness is done.

Abstract: The probabilistic analysis of evolutive states of damage, as those induced by fatigue loads, has been the subject of a wide spectrum of researches in the most recent years, in order to widen the range of applications and to deal with large structures, especially in the aeronautical field, where the high safety requirements need the evaluation of very small probabilities of failure. However, the problem of maintenance policies and scheduling, together with the probabilistic behaviour of NDT, are difficult to introduce in risk assessment procedures, and are usually referred as if coming from the particular experience of the different authors, even if they constitute an essential part of damage tolerance techniques. In the present work the authors summarize the capabilities of an house-made code which is able to perform the risk assessment of aeronautical components of sheet type and the experiences obtained when introducing maintenance models, in order to begin to compare different scheduling techniques.

Abstract: In this paper, a plate containing a central hole was used to simulate gas turbine blade with cooling hole
. Numerical calculations based on crystal plasticity theory have been performed to study the elastic-plastic stress field near the hole under tension. Two crystallographic orientations [001] and [111] were considered. The distributions of resolved shear stresses and strains of the octahedral slip systems {110}<112> were calculated. The results show that the crystallographic orientation has remarkable influence on both von Mises stress and resolved shear stress distributions. The resolved shear stress distributions around the hole are different between the two orientations, which lead to the different activated slip systems. So the deformed shape of the hole in [001] orientation differs from that in [111] orientation.

Abstract: A series of uniaxial-compression tests were conducted on some representative brittle rock specimens, such as granite, marble and dolerite. A multi-channel, high-speed AE signal acquiring and analyzing system was employed to acquire and record the characteristics of AE events and demonstrate the temporal and spatial distribution of these events during the rupture-brewing process. The test result showed that in the primary stage, many low amplitude AE events were developed rapidly and distributed randomly throughout the entire specimens. In the second stage, the number of AE increased much slower than that in the first stage, while the amplitude of most AE events became greater. Contrarily to the primary stage, AE events clustered in the middle area of the specimen and distributed vertically conformed to the orientation of compression. The most distinct characteristic of this stage was a vacant gap formed approximately in the central part of the specimen. In the last stage, the number of AE events increased sharply and their magnitude increased accordingly. The final failure location coincidently inhabited the aforementioned gap. The main conclusion is that most macrocracks are developed from the surrounding microcracks existed earlier and their positions occupy the earlier formed gaps, and the AE activity usually becomes quite acute before the main rupture occurs.