Impact behaviour of hollow sphere agglomerates with density gradient

This paper presents a study on the influence of the density gradient profile on the mechanical response of graded polymeric hollow sphere agglomerates under impact loading. Quasi-static, standard split Hopkinson pressure bar (SHPB) tests as well as higher speed direct impact Hopkinson bar tests and Taylor tests are performed on such hollow sphere agglomerates with various density gradient profiles. It is found that the density gradient profile has a rather limited effect on the energy absorption capacity from those tests. It is because the testing velocity performed (<50 m/s) is rather small with respect to its average sound wave speed (around 500 m/s) and the equilibrium stress state can be reached rather quickly. The high impact tests allow to generate a non-equilibrium state condition and the influence of density profiles is clearly observed. Besides, in order to extend this study to the situation beyond our testing limitations, a numerical model is built on the basis of the experimental behaviour data. It confirms the important influence of the density gradient profile under a non-equilibrium stress state situation. This study shows that placing the hardest layer as the first impacted layer and the weakest layer as the last layer has some benefits in terms of maximum energy absorption with a minimum force level transmitted to the protected structures.     

Ultrasonic Velocity as a Tool for Physical and Mechanical Parameters Prediction within Geo-Materials: Application on Cement Mortar

This paper discusses the ability of ultrasonic wave velocity to evaluate some physical parameters within mortar. The behavior of ultrasonic pulse velocity within mortar subjected to incremetal stress was also studied. For experimentation, we carried out ultrasonic measurements on mortar samples before and during uniaxial compressive strength, perpendicularly to the stress direction. The water/cement ratios were varied in order to contribute certain specific characteristics. A set of expressions was obtained linking the initial velocities of longitudinal ultrasonic waves with compressive strength, density, porosity and load at elastic limit.The evolution of ultrasonic velocity through mortar samples under continuous incremental uniaxial stress were also investigated. The results illustrated the behavior of ultrasonic pulse velocity as a function of the applied stress. It was observed that velocity did not decrease under initial loading and until about 70% of the ultimate stress, where sudden decrease occurred, followed by the failure of the material.     

Parametric study and numerical analysis of empty and foam-filled thin-walled tubes under static and dynamic loadings

In this paper the crushing behavior of thin-walled tubes under static and dynamic loading is investigated. First, a finite element (FE) model for empty thin-walled tube was constructed and validated by available experimental and numerical data. The comparison between the FE results and the existing numerical solutions as well as the available experimental results showed good agreements. Next, a model for the foam was adopted and implemented in an in-house FE code. The implemented isotropic foam model was then used to simulate the behavior of foam-filled tubes under both static and dynamic loadings. Good agreement was observed between the results from the model with those obtained by analytical relations and experimental test data. The validated FE model was then used to conduct a series of parametric studies on foam-filled tapered tubes under static and dynamic loadings. The parametric studies were carried out to determine the effect of different parameters such as the number of oblique sides, foam density and boundary conditions on crushing behavior of rectangular tubes. The characteristic included deformed shapes, load–displacement, fold length and specific energy absorptions.     

Propagation of Rayleigh waves in a rotating orthotropic material elastic half-space under initial stress and gravity

This paper aims to investigate the influence of rotation, initial stress and gravity field on the propagation of Rayleigh waves in a homogeneous orthotropic elastic medium. The government equations and Lame??s potentials are used to obtain the frequency equation which determines the velocity of Rayleigh waves, including rotation, initial stress and gravity field, in a homogeneous, orthotropic elastic medium has been investigated. The numerical results analyzing the frequency equation are discussed and presented graphically. It is important to note that the Rayleigh wave velocity in an orthotropic elastic medium increases a considerable amount in comparison to the Rayleigh wave velocity in an isotropic material. The results indicate that the effects of rotation, initial stress and gravity field on Rayleigh wave velocity are very pronounced.     

A study of spur gear pitting formation and life prediction

In this study, a numerical prediction on pitting formation is carried out in spur gear made from austempered ductile iron. General two-dimensional rolling sliding contact situations are considered for the development of the analytical model. The problem is assumed under linear elastic fracture mechanics and the finite element method is used for numerical solutions. Mixed mode stress intensity factors K_I and K_II for cyclic loading are evaluated and related to crack extension by a Paris-type equation. The maximum tangential stress criterion is used to determine the crack-turn-angle during crack propagation under cyclic loading.A series of experimental study is also carried out to determine the pitting formation life. Test specimens were first austenitized in salt bath at 900 °C for 90 min after which they were quenched in salt bath at 325 and 425 °C, for 60 min. A comparison is carried out between numerical and experimental results.     

Computation of a turbulent natural convection in a rectangular cavity with the low-reynolds-number differential stress and flux model

A numerical study of a natural convection in a rectangular cavity with the low-Reynoldsnumber differential stress and flux model is presented. The primary emphasis of the study is placed on the investigation of the accuracy and numerical stability of the low-Reynolds-number differential stress and flux model for a natural convection problem. The turbulence model considered in the study is that developed by Peeters and Henkes (1992) and further refined by Dol and Hanjalic (2001), and this model is applied to the prediction of a natural convection in a rectangular cavity together with the two-layer model, the shear stress transport model and the time-scale bound model, all with an algebraic heat flux model. The computed results are compared with the experimental data commonly used for the validation of the turbulence models. It is shown that the low-Reynolds-number differential stress and flux model predicts well the mean velocity and temperature, the vertical velocity fluctuation, the Reynolds shear stress, the horizontal turbulent heat flux, the local Nusselt number and the wall shear stress, but slightly under-predicts the vertical turbulent heat flux. The performance of the model is comparable to that of the low-Reynolds-number differential stress and flux model except for the over-prediction of the horizontal turbulent heat flux. The two-layer model predicts poorly the mean vertical velocity component and under-predicts the wall shear stress and the local Nusselt number. The shear stress transport model predicts well the mean velocity, but the general performance of the shear stress transport model is nearly the same as that of the two-layer model, under-predicting the local Nusselt number and the turbulent quantities.     

航空发动机通风器金属泡沫性能仿真研究

为研究航空发动机轴心通风器中的核心部件—开孔金属泡沫的阻力性能与油滴穿透率性能,采用X光断层扫描技术获取金属泡沫的扫描模型.通过对模型的样本截取,分析不同样本的压降性能;对其中一个样本采用DPM模型进行油气混合物的穿透率分析.结果表明:泡沫样本截面积与样本厚度对泡沫单位压降性能有一定的影响,但当截面积和厚度大于某一个值...     

Diffraction of sound by a poroelastic cylindrical absorber near an impedance plane

Acoustic scattering from an infinitely long fluid-saturated porous elastic circular cylinder located near a planar boundary with locally varying surface impedance is studied. The formulation utilizes the novel features of Biot dynamic theory of poroelasticity, the appropriate wave field expansions, the classical method of images and the translational addition theorem for cylindrical wave functions along with a simple local surface reaction model involving a complex amplitude wave reflection coefficient applied to develop a closed-form solution in the form of infinite series. The analytical results are illustrated with a numerical example in which a cylindrical plastic foam absorber is located near a layer of foam material set on an impervious rigid wall. The numerical results reveal the important effects of incident wave frequency, angle of incidence, interface local surface reaction, cylinder poroelasticity and position on the acoustic field quantities. The proposed model can lead to a better understanding of acoustic scattering from two-dimensional near-interface porous absorbers or targets which are commonly encountered problems in outdoor acoustics, noise control engineering, and ocean engineering. The presented solution could eventually be used to validate those found by numerical approximation techniques.     

Diagnostics of Creep of Heat-Resistant Steels on the Basis of Measurements of the Ultrasonic Wave Velocity in Nondestructive Testing of Energy Equipment: I. Probes and Tools for Measuring the Velocity of Ultrasound

The designs of probes for measuring the time of propagation of bulk and surface ultrasonic waves and examples of their use for diagnostics of the process of metal creep are described. The results of measurements of the surface wave velocity in the elastic and plastic domains are given. It is shown that in the elastic domain (up to the yield limit of the metal) change in the surface wave velocity is not greater than 0.02%. In the plastic domain, a decrease in the surface wave velocity is observed and the magnitude of the decrease depends linearly on the loading rate.     

Determining the Speed of Longitudinal Waves in an Isotropic Homogeneous Welded Joint Using Echo Signals Measured by Two Antenna Arrays

A technique is proposed for measuring the speed of longitudinal ultrasonic waves in a homogeneous welded joint, based on comparing measured and computed echo signals reflected from the bottom of a test object when using two antenna arrays mounted on prisms and operating in the double scanning mode. The effect of errors in setting the values of such parameters as the distance between the antenna arrays, test-object thickness, and others on the accuracy of calculating the wave velocity in the weld has been analyzed. Results of numerical and model experiments on calculating the wave velocity in the welded joint are presented. In a model experiment, the technique has made it possible to measure the speed of longitudinal waves in the weld model with an error of less than 0.7%. The method can be used to find the initial approximation in a nonlinear inverse problem of tomographic inspection of welded joints in the wave approximation.     

Stress wave effects in a finite element analysis of an impulsively loaded articular joint.

A dynamic contact finite element formulation was used to study transient stresses in the impulsively loaded rabbit knee, an established experimental model of mechanically induced osteoarthrosis. The computations were used to test the hypothesis that stress wave propagation and reflection, from juxtarticular interfaces of material property discontinuity, could be responsible for markedly increased levels of transient local cartilage stress. The finite element results demonstrated intuitively credible stress wave propagation and interfacial reflection phenomena. However, the magnitude of these waves was not nearly large enough to appreciably alter the quasi-static stress distributions otherwise prevailing. Thus, local stress wave reflection from interfaces of modulus discontinuity (for example the cartilage/subchondral plate) probably does not contribute appreciably to the heightened tissue sensitivity to impulsive loading experimentally observed in this animal model.     

Numerical Analysis and Verification of the Gas Jet from Aircraft Engines Impacting a Jet Blast Deflector

The process of the gas jet from aircraft engines impacting a jet blast deflector is not only a complex fluid–solid coupling problem that is not easy to compute, but also a safety issue that seriously interferes with flight deck environment. The computational fluid dynamics (CFD) method is used to simulate numerically the impact effect of gas jet from aircraft engines on a jet blast deflector by using the Reynolds-averaged Navier-Stokes (RANS) equations and turbulence models. First of all, during the pre-processing of numerical computation, a sub-domains hybrid meshing scheme is adopted to reduce mesh number and improve mesh quality. Then, four different turbulence models including shear-stress transport (SST) k-w, standard k-w, standard k-ε and Reynolds stress model (RSM) are used to compare and verify the correctness of numerical methods for gas jet from a single aircraft engine. The predicted values are in good agreement with the experimental data, and the distribution and regularity of shock wave, velocity, pressure and temperature of a single aircraft engine are got. The results show that SST k-w turbulence model is more suitable for the numerical simulation of compressible viscous gas jet with high prediction accuracy. Finally, the impact effect of gas jet from two aircraft engines on a jet blast deflector is analyzed based on the above numerical method, not only the flow parameters of gas jet and the interaction regularity between gas jet and the jet blast deflector are got, but also the thermal shock properties and dynamic impact characteristics of gas jet impacting the jet blast deflector are got. So the dangerous activity area of crew and equipments on the flight deck can be predicted qualitatively and quantitatively. The proposed research explores out a correct numerical method for the fluid–solid interaction during the impact process of supersonic gas jet, which provides an effective technical support for design, thermal ablation and structural damage analysis of a new jet blast deflector.     

智能压路机的动力特性分析

根据智能压路机的现场施工作业特点,建立压实系统的动力学模型,采用不同压实阶段的动力特性参数,以压实系统垂直方向的最大压振力作为评判跳振的依据,系统分析动力学响应特性。结果表明,压实中后期容易产生振动轮跳离地面现象,应适当调整施力方向以保证压实路面质量。     

梁杆碰撞激发瞬态波传播的动态子结构模型

针对梁杆碰撞问题,通过推导物理坐标和模态坐标下的动力学控制方程,采用黏结接触模型和单轴压缩局部接触模型处理碰撞产生的接触约束,提出一种适用于梁杆碰撞激发瞬态波传播研究的动态子结构模型。与理论解的比较表明,该动态子结构模型具有良好的数值收敛性和较高的计算精度。数值计算结果显示,该模型能够有效地分析碰撞激发的杆中轴向波和梁中弯曲波的传播、反射和相互干涉。借助该模型,分别研究轴向波和弯曲波通过碰撞接触约束的传播效应,研究结果表明,柔性构件碰撞的一个显著特征是波传播效应直接影响碰撞力响应,并使碰撞力响应形式变得异常复杂。     

泡沫金属材料的动力压缩响应

提出的一维质量-弹簧-阻尼单元模型用于计算模拟泡沫金属材料在冲击载荷作用下的动力响应.模型考虑泡沫金属材料微观密度不均匀及胞室流体对动力压缩特性的影响,并分别由单元质量块的密度和阻尼器在压缩过程中产生的应力描述.单元非线性弹簧的应力-应变关系取决于单元泡沫金属材料的压缩特性,包括胞缘弹性屈曲、胞壁或胞室的塑性坍塌及材料压实过程,并服从加、卸载定律.动力平衡方程采用显式积分算法求解.对30 mm×30 mm×100 mm铝泡沫材料块进行冲击模拟计算,结果表明铝泡沫材料的相对密度、密度分布的均匀性、胞室结构、胞室流体及脉冲压力强度、脉冲压力作用周期对泡沫金属在冲击载荷作用下压缩变形、冲击波扩展及能量吸收的影响.     

KBM-HB分析方法在工程振动中的应用

以具有代表性的Bouc-Wen模型系统为算例,采用Krylov-Bogolyubov-Mitropolsky（KBM）法和谐波平衡（HB）法相结合的方法分析滞回系统在简谐激励下的动力响应特性.在一阶近似情况下,非对称滞回模型的系统动力响应中存在偶次和奇次谐波成分,而对称滞回模型的系统动力响应中仅存在奇次谐波成分.以智能振动压实系统为工程实例,分析现场施工过程中振动轮具有非对称滞回特性的垂直方向和具有对称滞回特性水平方向的振动响应.试验结果与理论定性分析相吻合,验证了KBM-HB方法可用于识别与检测工程振动中滞回系统特性分析.     

Analytical and Numerical Models for Tangential Stiffness of Rough Elastic Contacts

The paper considers elastic contact of rough surfaces and develops a simple analytical expression for the stiffness of the contact under tangential loading, which predicts that the contact stiffness is proportional to normal load and independent of Young??s Modulus. The predictions of this model are compared to a full numerical analysis of a rough elastic contact of finite size. The two approaches are found to be in good agreement at low loads, when the asperity spacing is large, but the numerical approach predicts much lower stiffnesses at medium and high loads. It is shown that the overall stiffness cannot exceed that of the equivalent smooth contact, and a simple means of modifying the analytical approach is proposed and validated.     

Surface damage under dynamic loading

Experimental studies of surface damage resulting from high velocity impacts on a metal target, detonation spraying and impulsive laser loading of coatings reveal common features that can be understood within the framework of shock wave mechanics. The nature of dynamic damage is spallation caused by interference of rarefaction waves following the compression impulse and forming tensile stress regions. Focusing of rarefaction waves from the sides of impacting particles leads to longitudinal cracks. A system of less deep circular cracks is generated around the focusing spall crack during repeated loading of the contact zone. The removal of debris from the zone of multiple longitudinal spallation results in valleys on the target. The interference of rarefaction waves occurring on valley walls and face causes transverse face spallation. In this paper it is assumed that, since similar damage morphologies are reported in the erosion wear of ductile metals at lower velocities, in these cases too a shock physics analysis, rather than the commonly used quasi-static plasticity approach, is valid.