For the design and the safety assessment of wood structures, an adequate knowledge of wood fracture behavior is important. Due to the complexity in its structure, wood is naturally anisotropic in the macroscopic scale, which makes it more di-cult to study wood failure based on fracture mechanics than to study isotropic materials. Therefore, only fractures of pure mode I and pure mode Ⅱ were given a considerable attention. Wood structural members are, however, often subject to the mixed mode fracture in reality, and the mixed mode fracture is now one of hot topics in the fracture studies. This paper gives a literature review on the wood mixed fracture from two aspects:the fracture criterion and the testing method. After an introduction of some empirical and physical models to predict the mixed mode fracture failure of wood, a comparison is made among these models using the test data in the literature. Various test methods are reviewed, with focus on fractures of mode I and mode Ⅱ combined. Some issues for future studies are highlighted.
The transfer orbit for the human lunar mission means that between the trans-lunar orbit and the trans-earth orbit, and it has the characteristics of long flight time, complex dynamics model, and strong nonlinear and variable coe-cients. The controllability of the inevitable deviation afiects the result of the task execution, even the success or the failure of the engineering task directly. This paper reviews the studies of the transfer orbit of human lunar mission, the analysis method of the deviation propagation, the mid-course correction optimal strategy and the mid-course correction aiming algorithm. At last, some suggestions are made or the mid-course correction of the Chinese human lunar mission in the future.
The VFE-2 65° delta wing with sharp leading edge in subsonic flows at Mach number of 0.4, angle of attack of 20.3°, and Reynolds number of 2×106 is numerically simulated by using an unstructured hybrid mesh based flnite volume method, with emphasis on how to extract the key flow features for visualization both on surface and in space for such type of complex flows. Approaches for the advanced flow visualization techniques are used for qualitative and quantitative analyses, as a solid foundation for elaborated analysis of complex flow structures of delta wing. With these approaches, the complex vortex flow structure of the subsonic delta wing at high angle of attack is analyzed, and results are consistent with experiments. It is shown that the flow over delta wing at high angle of attack has a complex physical nature with a strong viscous coupling efiect, and the evolutions of the primary and secondary vortices can be accurately captured only by a Navier-Stokes equation based simulation.
Transferring the aerodynamic load to FEM (flnite element method) nodes is a hard work but is very important. In order to transfer the two-dimensional aerodynamic load, a thin plate spline surface of the pressure is constructed according to the existing pressure on the aerodynamic nodes. Then the pressure on the FEM nodes can be computed. At last the concentrated force on the FEM Nodes can be obtained by integration of the pressure on the FEM elements. The large airplane often has complicated high-lift devices and their aerodynamic load may be three-dimensional. For the three-dimensional aerodynamic load, with the integration of the pressure on the aerodynamic webs, the concentrated force on the aerodynamic nodes can be obtained, and these forces can be transferred to the FEM nodes using the quadratic program method.
During the combustion testing of the scramjet engine, early failure of pipes may occur, due to the presence of fractures followed by catching flre of aviation kerosene in the pipes as the fuel and coolant. This paper investigates the failure mechanism of the pipes. First, the fractography is analyzed by using both the stereomicroscopy and the scanning electron microscopy. It is indicated that the cracks are initiated exactly at the root segment of the welding line of the pipe connections, as a classical multi-source origin type. Besides, a half-circle shape with blue color is visible in the initiation area, indicative of the welding cracks. Second, fatigue striations are observed on the fracture surface with the sub micron-sized interval. According to the striation interval, the pipe thickness and the experiment period, the frequency of the fatigue loading can be estimated, which is well in agreement with the natural vibration frequency estimated based on the elastic theory and the modal analysis. Finally, it is concluded that the failure is due to the vibration fatigue. Based on the analysis mentioned above, a few flx points are added in order to change the natural vibration frequency of the pipe and to reduce the vibration amplitude. The welding process is also improved to avoid initial damages. As a result, the early failure is efiectively avoided.
This paper presents a damage constitutive model for masonry subjected to axial monotonic and cyclic compressive loading, based on related theories and test results. In order to establish flne lever models, double parameters consisting of the peak compressive strain and the strength are adopted to adjust the deformation curve. The uni-axial model presented is consistent with the existing elastic-plastic models, and the mechanical property of masonry materials that a higher strength generally leads to a lower ductility. Assuming that a unloading path of the cyclic constitutive model is linear, a bilinear deterioration function of the unloading compressive stifiness is established by utilizing two test results, and a linear interpolation of the compressive strength for the degradation rate that is permitted.
The seismic fragility of a RC (reinforced concrete) continuous girder bridge is analyzed, with consideration of damages of bridge piers, bearing components and the collision of the main beam. In view of the structural parameter randomness of the bridge structure, 20 near-fault ground motions are selected from the PEER(Paciflc Earthquake Engineering Research database), and a large number of random structure samples are established. Using the bridge damage index under difierent damage states, the fragility curves of the bridge components and the bridge system are obtained based on the structure of the capacity and the demand. According to the actual seismic demand of similar as-built bridges, the damage states can be obtained in the sense of probability, and provide a reference for future seismic damage predictions.
The blasting excavation is an important influencong factor for the instability of engineering rock mass. In view of the characteristics of combined mining for open-pit and underground, catastrophe theory models of the underground chamber roof instability induced by open blasting disturbance are built by taking the underground chamber roof as a horizontal simply supported beam based on the non-linear theory. The instability criteria and the critical safe thickness of the underground chamber roof are derived according to the catastrophe theory models. At the same time, the important influencing factors for the critical safe thickness of the underground chamber roof, such as the amplitude and the key frequency of the blasting vibration, and the characteristics of the surrounding rock are discussed. The critical safe thickness of the roof between the open-pit and underground stopes is obtained by a safe thickness analysis of Changba Pb-Zn deposit group mined-out area roofs in Gansu province as an example. It is shown that the dynamic instability of the underground chamber roof is obviously nonlinear and discontinuous, while the critical safe thickness of the underground chambers roof not only depends on engineering geological characteristics of the roof, but also on other multiple factors, for example, the amplitude and the key frequency of the blasting disturbance, as is consistent with the actual situation.
The application of the difierence curvature index for the damage detection is limited because of the loading mode. In this paper, a loading mode based on the truck is proposed. The working condition of the concentrated load can be obtained after the twice truck loading. A damage detection method for bridges of simply supported beams is established by a new indicator called the truck load difierence curvature indicator (TLDCI). According to the measured vertical displacement data, a graph of the TLDCI can be established. The occurrence and the localization of the damage then can be identifled and the early warning for the bridge damage can be realized. The TLDCI method involves a simple calculation and does not require the prior information of the intact bridge. Its efiectiveness is validated by an example. Finally, how to improve the stability of this new method is discussed.
In view of the similarities in the slope evolution between the isotropic compression line of the structured soil and the strain softening curve, a method of applying the dynamic line in modeling the strain softening curve is proposed in modeling the isotropic compression behaviors of structured soil. In the presented model, the normal compression line (NCL) is extended to a dynamic one as a reference line. Making the reference line decrease with the loading and the compression line of the structured soil follow the reference line all the time, the constitutive model describing the structured soil in the isotropic compression is formulated. Through the comparison between the test data and the model simulation results, the presented model is verifled in describing the compression behaviors of the structured soil smoothly and continuously.
Based on the variational principle of unknown boundaries formed in the forming process of an ideal elastoplastic straight beam of rectangular section, this paper carries out the variational calculation of the potential energy to derive the deflection curve equation of a cantilever beam bending mode, and to obtain the solution to the bending of the proflling forming straight beam. According to the calculation results, the deflection equations of the proflling forming straight beam can be derived with the application of the variational principles of potential energy and complementary energy.