A note on the impact response of a cracked orthotropic material

An approach involving potential functions is used to simplify the equations of motion for an Orthotropic material to an elementary differential equation whose solution may be used for a wide variety of problems in elastodynamics of Orthotropic materials. Solutions for the dynamic mode I and II stress intensity factors for a Griffith crack in an Orthotropic material are obtained. The new approach allows the stress intensity factors to be determined with a minimal use of integral transforms. The solutions for the stress intensity factors are shown to be the same as those obtained by previous researchers.     

Estimating the impact of abnormal loads on network performance

The English Highways Agency is required to improve network performance including journey time reliability measured by average delay on the 10% slowest journeys along a specified set of primary routes in the strategic road network. Abnormal indivisible loads, which behave as moving bottlenecks, can cause significant delay, but the factors involved are complex, so methods have been developed to model their effect. These methods have been extended to estimate their impact through the Average Vehicle Delay (formerly PSA1 delay) measure, allowing mitigations through intelligent transportation system or policy options to be evaluated. Environmental impacts are also considered.     

Collapse of braced tubes under impact loads

The dynamic response of diametrically braced metallic tubes under impact loading conditions is investigated both experimentally and numerically. The effects of strain rate and the initial collapsing mode configuration on the resulting force (acceleration) levels at the impacted end of the tube are discussed. Differences in energy dissipation characteristics of the braced tubes under quasi-static and impact loading conditions are described as functions of the bracing orientation.     

Modelling fabric reinforced composites under impact loads

The paper describes recent progress on the materials modelling and numerical simulation of the in-plane response of fibre reinforced composite structures. A continuum damage mechanics model for fabric reinforced composites under in-plane loads is presented. It is based on methods developed for UD ply materials (Compos. Sci. Technol., 43 (1992) 257), which are generalised here to fabric reinforcements. The model contains elastic damage in the fibre directions, with an elastic–plastic model for inelastic shear effects. Test data on a glass fabric/epoxy laminate show the importance of inelastic effects in shear. A strategy is described for determining model parameters from the test data. The fabric model is being implemented in an explicit FE code for use in crash and impact studies and preliminary results are presented on a plate impact simulation.     

Model of inelastic impact of unit loads

In this study, the authors present a model of the inelastic impact of bodies that takes place during handling of unit loads (cubiform parcels). A modified non‐linear Kelvin model is proposed, in which the relationships between object strains and elastic and damping forces of the impact are represented by power functions. The results of analytical investigations on the proposed model were confirmed by experiments consisting in tests of free fall of the load on rigid ground and on elastic beam. The developed model allows the assessment of the influence of mechanical properties of loads and conveyor‐line deflection mechanisms as well as impact velocity, on the dynamic forces exerted on the manipulated objects during the impact. Copyright © 2008 John Wiley & Sons, Ltd.     

Delamination threshold loads for low velocity impact on composite laminates

An investigation of the low velocity impact load level at which a composite laminate will delaminate is presented. The delamination threshold load is described as the load level, obtained from the load–time history or load–displacement plot, at which a sudden load drop occurs due to specimen stiffness loss as a result of laminate level damage. Approximately 500 low velocity impact load–time histories from the Air Force Research Laboratory (AFRL) Low Velocity Impact Database are used to investigate the delamination threshold load. The database contains laminate impact test data for graphite/epoxy (AS4/3501-6), graphite/PEEK (AS4/APC-2), and graphite/BMI (IM7/5260) material systems. The delamination threshold load observations are compared to C-scan damage measurements of impacted specimens to determine if the sudden load drop corresponds to delamination development.     

Experimental and numerical modelling investigation on fracturing in coal under impact loads

In this paper, fracturing in coal under impact loads was studied using experimental and numerical approaches. Three-point beam bending tests were carried out on coal samples under impact loads. During the testing, cracking velocity in the samples was captured using a multi-spark high-speed photography system. Characteristics of the fracture surface were investigated using the scanning electron microscopy, 3D laser surface topography scanner and X-ray micro Computed Tomography (X-ray micro-CT). Differences between the fracture surface under impact loads and that in quasi-static test were analysed. Moreover, discrete numerical modelling was conducted to assess the influence of impact velocity, heterogeneity, and grain size on dynamic fracturing in coal. Based on observations from the testing and numerical simulation, it was concluded that the influence of heterogeneity and grain size was more pronounced in dynamic fracturing comparing to that under quasi-static loading.     

Load-carrying capacity of polymer tape rolls under impact loads

The appratus, method, and result of experimental investigation of load-carrying capacity of rolled magnetic tapes subjected to impact and thermal effects are described. Analysis of the basic regularities of loss of load-carrying capacity of the rolls (lamination of turns) has facilitated determination of the safe level of impact accelerations and thermal effects which do not cause damage of rolls under multiple impacts (10⁴...10⁵ cycles). It is established that breakdown in compactness of rolled magnetic tapes under multiple impacts occurs due to reduction in interturn pressures (up to 30 ... 50% of the original level). It is noted that, with increase in environmental temperature, there is sharp reduction in load-carrying capacity of rolls under the action of impact loads.Translated from Problemy Prochnosti, No. 4, pp. 112–116, April, 1990.     

Sandwich structures with textile-reinforced composite foldcores under impact loads

The mechanical behaviour of composite sandwich structures with textile-reinforced composite foldcores, which are produced by folding prepreg sheets to three-dimensional zigzag structures, is evaluated under compression, shear and impact loads. While foldcores made of woven aramid fibres are characterised by a rather ductile behaviour, carbon foldcores with their brittle nature absorb energy by crushing, showing extremely high weight-specific stiffness and strength properties. The impact damage under low and high velocity impact loads tends to be very localised. In addition to regular single-core sandwich structures, a dual-core configuration with two foldcores is also investigated, showing the potential of a two-phase energy absorption behaviour. In addition to experimental testing, finite element models for impact simulations with LS-DYNA have been developed. Despite the high degree of complexity of the models due to the various skin and core failure modes that have to be covered, the results correlate well with test data, allowing for efficient parameter studies or detailed evaluations of damage patterns and energy absorption mechanisms.     

A note on stress

Summary Consider an arbitrary stress system at a pointP in a continuum. AtP there exists at least one circle of directions of the unit vectorn for which the corresponding traction vectorst(_n) lie on a circle.     

Author collaboration and impact: A note on citation rates of single and multiple authored articles

The acceptance rate of articles which are collaboratively authored tends to be higher than that for single-authored papers, thereby suggesting a generally positive relationship between collaboration and quality. The analysis of ten-year citation rates of 270 randomly selected articles in three applied fields likewise shows a similar relationship, with somewhat higher citation frequencies for multi-authored papers than for single-authored ones. The relationships persist whether self-citations are included or excluded. However, these differences are not statistically significant for articles in clinical psychology or in educational measurement. Only multi-authored articles in management science show a statistically significant higher citation rate. Other aspects of the collaborative process and effects are discussed.     

基于重叠网格法的飞机水上降落水动力砰击载荷研究

为分析飞机在水上降落过程中的砰击载荷及运动规律,基于数值模拟研究NACA TN2929飞机水上降落过程,考查飞机的初始速度、初始仰角对降落过程的影响。求解雷诺平均纳维-斯托克斯方程(RANS)模拟飞机与水的相互作用,采用VOF方法捕捉自由面,采用重叠网格技术及DFBI模型模拟飞机与流体的耦合运动。通过计算楔形体和圆锥体自由入水过程验证模型的可靠性,通过网格和时间步长的收敛性分析确定合适的网格和时间步大小,与实验结果对比确定计算结果的正确性。模拟飞机以不同初始速度与不同仰角的着水过程,分析飞机底部压力的变化规律,研究飞机初始速度、初始仰角对砰击载荷及运动姿态的影响规律。数值计算结果表明飞机在入水0.2 s左右砰击载荷会达到峰值,载荷峰值与飞机着水的速度的平方线性相关;飞机机身底部压力峰值出现在水面与机身的正向交界处;飞机初始仰角增加,砰击载荷峰值会有所减小。     

Dynamic calibration methods of an installation for measuring impact loads

Conclusions The suggested method is sufficiently precise and leads to a wide application of strain-gauge measurements in impact testing. Moreover, it serves to use strain-gauge impact testing without any additional changes in the loading arrangement. In our cases transducers can be cemented directly to the base of the impact tester in a place which is subjected to the impact impulse. None of the existing calibration methods can be used in such a case, owing to the complicated stressing of the impact tester's stand.The suggested method can in principle be used for testing impact tension, compression, bending, twisting and other types of deformation.     

Dynamic response of laminated composite shells under various impact loads

Mechanics of Time-Dependent Materials - This paper investigates the dynamic behavior of laminated composite shells (LCS) under the effect of dynamic loads. The governing equations for composite...     

关于 Stirling公式的一个注记

记 rn=nne- n 2πn.这个注记证明 ,当 n≥ 1 0时rn 1 +11 2 n+ 12 88n2 +2 4 9.2 n 相似文献   

关于哑演算的一点注记

该注记指出了 [1]的定理 7推论 5中证明的不妥之处 ,并给出了一个严格的新证明 .