Dynamic versus quasi-static loading of X65 offshore steel pipes

Anchors or trawl gear occasionally impact offshore pipelines, resulting in large local and global deformations. Impact velocities are typically less than 5?m/s, but local strain rates may be very high. In this study strain rate effects in an X65 offshore material was characterised by split Hopkinson bar tests, while the cross-section homogeneity and possible anisotropic behaviour were determined by quasi-static material tests. Further, dynamic impact tests at prescribed velocities were carried out on simply supported full scale X65 steel pipes. Next, deformation-controlled quasi-static tests with the same boundary conditions were conducted. The level of deformation in the quasi-static tests was set to be equal to that attained in the dynamic tests. Finally, an assessment of the differences between the dynamically and quasi-statically loaded pipes was made in terms of force-displacement response, energy absorbed, and fracture. An optical light microscope and a scanning electron microscope were used to investigate fracture surfaces arising from the various tests.     

锥头弹丸穿透薄钢板的实验研究

 为了研究延性薄靶的穿透性能,进行了90°锥头弹丸穿透薄钢板的准静态和动态实验。通过准静态实验,得到了4种不同厚度钢板的载荷-压入位移曲线及破坏模式;结合膜力理论模型及纯弯静塑性破坏模型,对实验结果进行了分析,得出：对于薄靶,大变形膜力起主导作用,随着靶厚增加,弯距在板整体变形过程中的影响逐渐增大。通过动态穿透实验,得到了弹丸穿透4种不同厚度钢板的入射速度、残余速度及靶板破坏模式,计算得到穿透耗能。最后,将准静态实验与动态实验结果进行了对比讨论。     

The forced motion of viscoelastic thick cylindrical shells

This paper presents a theoretical analysis of a dynamic boundary value problem of the axially-symmetric motion of isotropic, homogeneous, linearly-viscoelastic, thick, cylindrical shells subjected to time-dependent surface tractions and/or time-dependent boundary conditions. Williams' modal-acceleration method has been used to treat the time-dependent boundary conditions. Two forms of the correspondence principle are used to obtain the governing differential equations and the quasi-static solutions. A numerical example is given to study the transient response of a cylindrically hollow rod subject to longitudinal impacts.     

Threshold fracture energy in solid particle erosion

Abstract

The effect of geometrical shape of eroding absolutely rigid particles on the threshold rate of failure has been studied. The Shtaerman–Kilchevsky theory of quasi-static blunt impact, which generalizes the Hertzs classical impact theory, is used for modelling the frictionless contact interaction of an axially-symmetric particle with an elastic half-space. The incubation time fracture criterion is applied for predicting surface fracture. It is shown that there exists a critical value of the particle shape parameter such that for all its lower values, the fracture energy possesses a non-zero minimal value.     

Ground-borne vibration due to static and dynamic axle loads of InterCity and high-speed trains

In predictions of railway-induced vibrations, a distinction is generally made between the quasi-static and dynamic excitation. The quasi-static excitation is related to the static component of the axle loads. The dynamic excitation is due to dynamic train–track interaction, which is generated by a large number of excitation mechanisms, such as the spatial variation of the support stiffness and the wheel and track unevenness. In the present paper, the quasi-static excitation and the dynamic excitation due to random track unevenness are evaluated by means of numerical predictions. A solution strategy is presented that allows for the evaluation of the second-order statistics of the response due to dynamic excitation based on the power spectral density function of the track unevenness. Due to the motion of the train, the second-order statistics of the response at a fixed point in the free field are non-stationary and an appropriate solution procedure is required. The quasi-static and dynamic contribution to the track and free-field response are analysed for the case of InterCity and high-speed trains running at a subcritical train speed. It is shown how the train speed affects the quasi-static and dynamic contribution. Finally, results of numerical predictions for different train speeds are compared with field measurements that have been performed at a site along the high-speed line L2 Brussels–Köln within the frame of homologation tests.     

聚四氟乙烯和氟化乙丙烯共聚物复合膜的压电性

以多孔PTFE膜为骨架,而以致密（非多孔）FEP膜为储电介质层的孔洞结构复合压电驻极体膜的制备方法.利用正压电效应,测量了复合膜的准静态压电系数d₃₃;研究了压电系数的热稳定性和复合膜中空间电荷的动态特性;并通过介电谐振谱的分析,比较了这类复合膜的准静态和动态压电系数.结果表明：FEP和PTFE复合膜压电驻极体的准静态压电系数d₃₃可以达到300 pC/N.经90℃老化20 h 后的d₃₃仍保持在初 关键词： 压电驻极体 压电性 多孔聚四氟乙烯 致密氟化乙丙烯共聚物     

对气体等温气压公式的修正

用理想气体的多方过程方程取代等温过程方程，推导出气体处于多方平衡过程时的气压公式，该公式是对气体等温气压公式的一种很好的修正．对多方指数”等于1，1．1，1．2，1．3，1．4等5种不同情况分别给出了大气压强随高度的变化曲线．此外还讨论了考虑到重力加速度随离地面高度的变化后对气压公式的修正．     

On the calculation of nonstationary mechanical stresses formed in solid objects upon laser energy absorption by the thermoelastic mechanism

A general approach to calculating nonstationary thermoelastic stresses generated in solid objects as a result of absorption of the laser radiation energy is worked out in the quasi-static approximation. In the 3D model, analytic expressions are derived for the radial and tangential stress tensor components on the surface of the object.     

Experimental study of energy accumulation and dissipation in iron in an elastic-plastic transition

Modern experimental studies of the temperature evolution at the surface of plastically deformed metals show that processes of energy accumulation and dissipation in the material are nonlinear even in the quasi-static case and obviously depend on the loading conditions as well as past history of deformation. Considerable intensification of investigations in this field, which has been observed during the last decade, is due to the fact that this class of problems has wide application apart from its fundamental importance. High-sensitivity IR detectors used in such experiments make it possible to develop new effective methods of nondestructive control, which are based on analysis of thermal precursors of deformation localization and failure. In this study, the features of propagation of thermal waves on the surface of pure iron under an elastic-plastic transition are investigated and time dependences of energy accumulation rate are obtained during quasi-static deformation.     

SCRATA's practical approach to vibration-white finger in foundries : A. Champion, (17 pages, 7 figures, 17 references)

Linearized, quasi-static, fluid force coefficient data obtained from wind tunnel tests are used in an analysis of the fluidelastic stability of a double row of flexible circular cylinders subject to a cross-flow. Although the analysis is quasi-static, frequency dependent terms are obtained in the aerodynamic stiffness and damping matrices; the origin of these terms is twofold: firstly, because of the time lag between flow leaving an upstream row and arriving at a downstream row, which becomes important at low values of the non-dimensional flow velocity $\text{U}\text{fd}$; secondly, because of retardation of the flow approaching the cylinder, which is particularly important when small displacements of the cylinder result in large changes in the fluid force coefficients. This analysis is used to investigate the effects of a number of parameters on the critical flow velocity and the theoretical results are compared with those available in the literature. In general, agreement between theory and experiment is reasonably good, indicating the validity of this analysis.     

Spall fracture of coarse-grained and ultrafine-grained aluminum under nanosecond relativistic high-current electron beam

The results of investigations of fracture behavior in coarse-grained and ultrafine-grained aluminum under the action of a nanosecond relativistic high-current electron beam in a SINUS-7 accelerator and under conditions of quasi-static tensile loading are reported. It is shown that for both types of deformation, irrespective of the grain size, the fracture is ductile both in deformation and structural features. Based on the examination of the fracture surface, it is found out that under quasi-static loading decohesion of coarse-grained and ultrafine-grained aluminum occurs through shear, and under spalling condition by rupture. It is shown for both grain structures that the thickness of the separated layer increases with the irradiated specimen thickness. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 32–38, December, 2007.     

Hysteresis loss component under non-sinusoidal flux waveforms

Separation of total energy dissipation per magnetisation cycle into a frequency-dependent dynamic component and a frequency-independent hysteresis component is a common practise in evaluating electromagnetic losses in Si–Fe electrical steel sheet. The assumed frequency-independent hysteresis component is defined by a coefficient C₀ (J/kg). In this work, the value of C₀ was determined using a linear extrapolation method and quasi-static hysteresis energy loss per cycle. The extrapolation method gave a considerable error when applied to non-sinusoidal excitation voltages (pulse width modulation and square) in a frequency range from 25 to 100 Hz. For this reason the coefficient values obtained from the quasi-static measurements at 0.01 Hz were assumed.     

Electrical contact resistance and dynamic contact stiffness for a cluster of microcontacts: cross-property connection in the low-frequency range

An explicit cross-property connection for a rough interface between the electrical contact resistance and dynamic contact stiffness has been established in the low-frequency limit. The present analysis is based on the first-order asymptotic model of multiple dynamic contact between small flat-ended indenters on an elastic half-space, which is a dynamic analogue of the quasi-static Greenwood model. The obtained results can be used in developing a vibration method for measuring the interface contact stiffness in tribological systems, and in estimating the surface-roughness effect in oscillation indentation tests.     

Stress distribution and surface instability of an inclined granular layer

Static granular materials may avalanche suddenly under continuous quasi-static drives. This phenomenon, which is important in many engineering applications, can be explained by analyzing the stability of the elastic solutions. We show this for a granular layer driven by its inclination angle in gravity, where the elastic problem can be solved generally and analytically. It is found that a loss of stability may occur only at the free surface of the layer. This result is considered to be relevant for understanding surface avalanches and the flows observed experimentally.     

Piezoelectric coefficients of cross-linked polypropylene films stretched at elevated temperatures

Cross-linked polypropylene (XPP) foams are modified by a hot-stretching process and charged to be piezoelectric. The results show that a piezoelectric d₃₃ -coefficient of 308pC/N is obtained for sample with an elongation ratio of 200%. The dynamic d₃₃ value is smaller than the quasi-static value, which is probably due to the rise of Young’s modulus with increasing frequency. All the samples show pressure-independent d₃₃ in the range up to 30 kPa. Improved d₃₃ was obtained in the sample rough surface exposed to the corona during charging. Compared to linear PP, the stretched XPP shows pressure independence of d₃₃ in larger range.     

激光入射角对靶面方向超热电子发射的影响

实验研究了超短超强激光脉冲与薄膜靶相互作用中产生的超热电子角分布随激光入射角的变化.在靶面方向观测到一束方向性很好的高能超热电子.该高能超热电子束的电子数目随着激光入射角的增大而增大.对结果的分析表明，表面准静态磁场是导致表面电子产生的主要原因. 关键词： 超热电子 表面准静态磁场 超强激光脉冲与等离子体相互作用     

A new class of anisotropic nonexchange spin waves

It is shown, using a thin magnetic film of a tetragonal antiferromagnet as an example, that indirect spin-spin exchange via the long-range field of quasi-static magnetoelastic deformations with allowance for the magnetocrystalline anisotropy results in qualitatively new types of nonexchange propagating spin waves, both surface and bulk. The case of an isolated magnetic film and a magnetic film with a one-sided or two-sided coating is studied.     

不同应变率下两种岩石的压缩破碎特征试验研究

为了研究不同岩石在不同应变率下压缩时裂纹的产生规律及破坏模式,将石灰岩和红砂岩制成试件,研究其在不同应变率和受力模式下裂纹的形成模式。开展了两种岩石的准静态压缩和动态压缩试验,采用高速摄影机记录了裂纹的产生和破坏模式。对两种岩石试件的裂纹形态进行对比,基于岩石的物理性质、受力状态、能量演化分析,得到了在不同应变率下压缩时产生差异性的原因。结果表明:准静态压缩下岩石试件受压的破坏模式也会因应变率的不同而存在差异,并且破坏模式的差异对岩石试件的抗压强度将产生显著的影响;从能量演化的角度分析,入射能量的大小将会决定岩石试样动态抗压强度曲线是否出现起伏;动态压缩时,裂纹的周向扩展速度与岩石抗压强度呈正相关。     

Flux pinning by a barrier on the surface of YBaCuO grains studied in dynamic regime

The model of a surface barrier was found to explain well the observed behaviour in quasi-static magnetization experiments on highT_csuperconductors. We study this phenomenon in dynamic conditions in an AC magnetic field with frequencies ranging from 3 Hz up to 115 kHz. The experimental data recorded on a polycrystalline YBaCuO sample are explained by a model supposing the existence of an intergrain critical state and the barrier on the surface of the grains. The most striking feature we found is the decrease of the surface barrier with increasing frequency.     

On-line detection of fatigue cracks using an automatic mode tracking technique

Experimental fatigue tests usually require large testing times. In addition to the resulting increased time-to-market, the large fatigue test time also implies that any structural health monitoring technique that is used should be automatic. When using the modal parameters as damage indicators, an important amount of user interaction is still needed to separate physical poles from computational ones. In this paper, an experimental framework will be developed to automatically track the health of the structure on-line with the performance of fatigue tests. The modal parameters are tracked using a combination of the maximum likelihood estimator and an auto-regressive model. Since confidence levels on the modal parameter are available it is possible to detect if damage is present. In addition, the quasi-static stiffness with computed confidence levels is also used as a damage indicator. The proposed techniques are demonstrated on a steel beam with a propagating fatigue crack.