Applied Mechanics and Materials Vols. 29-32

Abstract: Boiling heat transfer in vertical rectangular mini-channels with a width of 1mm and 0.1mm is studied in this paper. The course of bubble generation, growth and departure is numerically simulated and analyzed, and the influence of the movement of phase interfaces on pressure difference and average surface heat transfer coefficient is investigated by using geometry reconstruction and interface tracking. All the calculation is conducted taken into account the gravity, surface tension and wall adhesion. This paper presents that the width of channels plays a significant role in the course of bubble generation, growth and motion, which also leads to changes of critical heat flux. In addition, it is found that surface tension is much more important than gravity in the process of boiling heat transfer. As the channel size is decreased, the boiling heat transfer coefficient increases significantly, which proves that mini-channels can enhance the heat transfer. However, the boiling heat transfer coefficient obtained through numerical simulation is higher than the existing experimental data due to the ideal assumptions adopted in simulation.

Abstract: The split Hopkinson bar (SHB) technique has been widely used to determine dynamic plastic/damage properties of materials at high strain rates. But presently, a number of researchers used it to measure the so-called dynamic Young’s moduli. This paper aims at the feasibility to determine Young’s moduli by conventional SHB. The strain-stress curves of two kinds of metals, 45# steel and LY-12 aluminum, were measured by a conventional SHB facility. The corresponding numerical experiments were performed by ABAQUS/Explicit. The curves obtained by the both procedures agree well in elastic stage (strain range 0-0.5% assumed). But the slopes of the linearly fitted curves, i.e. the so-called dynamic Young’s moduli, are much different from the corresponding real values. The significant errors come from the non-uniformity of stress in specimen during the beginning stage of loading process. The results clarify that it is unfeasible to measure Young’s moduli by conventional SHB.

Abstract: When a structure is irradiated by a pulsed cold X-ray with high energy density, the instantaneous deposition of energy will induce melting, vaporization, and sublimation of the outer layer of material(s). As a result, the material(s) will blow off and hence lead to a so-called blow-off impulse. This kind of impulsive load will cause high-level structural responses. In order to investigate the effects, various test simulation techniques, such as the light-initiated high explosive (LIHE) technique, the spray lead at target (SPLAT) technique and the sheet-explosive technique, were developed due to the lack of proper X-ray sources. This paper presents a rod-explosive technique developed from the sheet-explosive technique. In this technique, the main property of the explosive, i.e. the specific impulse, is determined by using a pendulum test facility. The simulation load (equivalent to the cosine-distributed specific impulse on a conical shell induced by X-ray) is designed by load discretization and impulse equivalence. Numerical simulations of structural responses to both X-ray loads and rod-explosive loads were performed for validating the test simulation technique. An application example of testing a complex structure is briefly given in the end. The rod-explosive technique has the features of low costs and rather high fidelities. It provides a new approach for testing the structural responses induced by X-ray blow-off impulses.

Abstract: A new manner, named frequency band energy statistics method, has been proposed for the analysis of the spectra from the vibration signals. The recorded vibration signals were first divided into multi-segments. Then each segment was calculated via the FFT transformation and 1/3 octave spectrum to obtained the characteristics of energy distribution, by making the histogram maps of the obtained features of the frequency energies. Consequently, we can monitor the work conditions and fault diagnosis for the mechanical equipments by the compared analysis of the corresponding histogram maps of equipments with normal and abnormal work conditions. The results show that present method exhibits a very strong sensitivity to the changes of vibration signal, leading to the fine detection of the minor changes form the equipment work conditions. Current work might provide a novel and facile method for definitely monitoring the work conditions and fault diagnosis of the mechanical equipments.

Abstract: In this paper the structure response of quasi-statically loaded sandwich beams made of aluminum skins with open-cell aluminum foam cores is investigated experimentally. The experimental programme was designed to investigate the deformation and failure modes of sandwich beams, so a large number of experiments have been conducted, and the experimental results are reported and discussed systematically. It is found that sandwich beams under quasi-static punching loads can fail in several modes: face yield, face wrinkling, core shear, the bottom face fracture and interfacial failure between the core and the faces. Moreover, the effects of face thickness, cell size of foam material on the failure and deformation modes were discussed. The experimental results are of worth to optimum design of cellular metallic sandwich structures.

Abstract: This paper builds three-dimensional finite volume models for 4 different mufflers. Internal flow numerical analysis for mufflers is carried out with computational fluid dynamics software. Velocity field and pressure field are obtained to analyze the airflow influences on muffler performances. The corresponding regularities are obtained from simulation results, which provide a method for design optimization of mufflers.

Abstract: The three dimensional model of a van body truck is established, and then the aerodynamic characteristics of the truck is analyzed using finite element method with ANSYS software. Through the analysis, the velocity vector distribution charts, the velocity nephograms and the pressure nephograms are obtained respectively under two different vehicle speeds; then the results are compared which provide necessary guides for the optimal design of the van body.

Abstract: A new method of fracture controlled blasting was put forward—using different coupling medium (water and gas) within the hole at the same time. The paper studies the pressure of hole wall about the hydraulic pressure blasting and gas decoupling blasting by system experiment, and compares the strain with damage on different medium directions after blasting, which reflected the suffering force and distortion of medium. The blasting method can control the process of energy releasing and direction effectively, which makes it possible to destroy one side of the wall rock and protect the other side at the same time.

Abstract: This paper focuses on the situation of stress, strain and displacement of subgrade under the action of temperature and load. Carry out numerical simulation for strain problem of normal subgrade plane and compare the numerical results of stress field and displacement field to the analysis results obtained from predecessor, we can find that the distribution of every value is in conformity to its variation trend. The variation of stress and displacement shall be most violent at the top surface of subgrade, thus the effect of temperature and load on stress and displacement will decrease as the depth increases. Moreover, the capacity of calculation and visualization for program is developed greatly, which can supply beneficial assistance to engineering design and appliance.

Abstract: The realization of strain measurement based on digital image processing technique is discussed. Combined with the strain measurement principle and the optical imaging feature, considered the axisymmetrical and non-axisymmetrical distortion, the automatic surface-fitting calibration method based on region growing is adopted. Cubic polynomial is applied to establish the relationship between the object space and pixel coordinate system. The expectations based on wavelet coefficients are used for sub-pixel edge detection, measurement accuracy can reach 0.02 pixels. Accuracy of displacement for strain measurement achieves micrometer-level. The theoretic validity and practical feasibility of the system are proved by the strain measurement of a cantilever steel beam.