Strength Properties of Aluminum-Oxide Ceramics Prepared by the Additive Manufacturing Method under Shock-Wave Loading

Aluminum-oxide ceramic samples have been prepared by additive manufacturing with subsequent sintering. The Hugoniot elastic limit and spall strength of the ceramics are determined by analyzing the full wave profiles of the samples recorded using a laser interferometer upon their shock compression with amplitudes of 6.8 and 13.8 GPa.     

Investigation of the Process of Stability Loss for Smooth Thin-Walled Cylindrical Shells under the Local Action of a Radiation Pulse

It is shown that, under the action of a short-term pulse of external pressure caused by laser radiation, a shell suffers elastic deformation in the course of interaction with the pulse according to the short-wave form on the boundary of the region of loading. When the pulse action is terminated, the shell continues its motion, which is not axially symmetric. As soon as the limiting displacements are attained, the global stability loss is observed. The form of wave formation of the global stability loss in the circumferential direction is determined by the size of the region of loading.     

Pressure-shear stress wave analysis in plate impact experiments

Numerical results are presented for the combined longitudinal and shear wave propagation in an elastic-viscoplastic solid as it occurs in high strain-rate plate impact experiments. Special attention is paid to the initial stage of the impact experiment and the effects of the specimen thickness, elastic impedances of flyer-anvil plates, and viscoplastic properties of materials on the time to reach a homogenous stress and deformation state within a specimen. The simple interpretation of experimental results which assumes a homogenous stress and deformation state within a specimen is found in general to be valid only at a much later time after impact. It is recommended that the measurement of the stress wave profiles should be made at the back face of the specimen rather than at its impact face, and that a pressure-shear stress wave analysis of the plate impact experiment should be performed to evaluate the inertial effects on the early part of experimental recordings. The comparison between measured stress wave profiles and the numerical simulation of the experiment provides a critical assessment of advanced viscoplastic models developed for applications under impact loading.     

岩石冲击损伤试验的数值流形方法模拟

主要根据岩石在冲击载荷作用下的变形与破坏特征,对现有的仅适用于模拟基于线性本构关系和恒定载荷作用下物体变形规律的数值流形程序进行了扩展.程序的扩展主要集中在3个方面:添加了由程序本身决定的程序运行终止条件,把恒定的载荷作用扩展为三角波载荷作用,改变原来的单一线弹性本构关系为由冲击损伤本构关系和线弹性本构关系共同组成的复合本构关系.最后利用扩展后的程序对冲击试验结果进行了模拟,结果与试验符合较好.     

The effects of plastic deformation on stress wave propagation in multi-layer materials

The behavior of a multi-layer material at high strain rate and the effect of plastic deformation on stress wave propagation were investigated by a combination of experimental and numerical techniques. Plastic deformation effects were studied in multi-layer materials consisting of ceramic, copper and aluminum subjected to large strains under high strain rate loading. First, stress wave propagation behavior for the monolithic metals was studied, and then extended to multilayer combinations of these metals with each other and with a ceramic layer. The axial stress distributions were found to be non-uniform in the elastic deformation range of the specimen. The degree of non-uniformity was much more pronounced in the multi-layer samples consisting of different materials. The presence of a ceramic layer increased the magnitudes of stress gradients at the interfaces. It was also found that a major effect of plastic deformation is a tendency to produce a more homogeneous stress distribution within the components. The implications of these observations for practical systems are discussed.     

The effects of loading conditions and specimen environment on the nanomechanical response of canine cortical bone

Bone is a viscoelastic connective tissue composed primarily of mineral and type I collagen, which interacts with water, affecting its mechanical properties. Therefore, both the level of hydration and the loading rate are expected to influence the measured nanomechanical response of bone. In this study, we investigated the influence of three distinct hydration conditions, peak loads and loading/unloading rates on the elastic modulus and hardness of canine femoral cortical bone via nanoindentation. Sections from three canine femurs from multiple regions of the diaphysis were tested for a total of 670 indentations. All three hydration conditions (dry, moist and fully hydrated tissue) were tested at three different loading profiles (a triangular loading profile with peak loads of 600, 800 and 1000 μN at loading/unloading rate of 60, 80 and 100 μN/s, respectively; each test was 20 s in duration). Significant differences were found for both the elastic modulus and hardness between the dry, moist and fully hydrated conditions (p ≤ 0.02). For dry bone, elastic modulus and hardness values were not found to be significantly different between the different loading profiles (p > 0.05). However, in both the moist and fully hydrated conditions, the elastic modulus and hardness were significantly different under all loading profiles (with the exception of the moist condition at the 600- and 800-μN peak load). Given these findings, it is critical to perform nanoindentation of bone under fully hydrated conditions to ensure physiologically relevant results. Furthermore, this work found that a 20-s triangular loading/unloading profile was sufficient to capture the viscoelastic behavior of bone in the 600- to 1000-μN peak load range. Lastly, specific peak load values and loading rates need to be selected based on the structural region for which the mechanical properties are to be measured.     

A study of composite material damage induced by laser shock waves

A laser shock wave technique has been used to study the damage tolerance of T800/M21 CFRP (Carbon Fiber Reinforced Polymer) composite material with different lay_ups. Different levels of damage have been created according to various laser irradiation conditions. Several characterization methods such as Optical Microscopy, X-ray Radiography, or Interferometric Confocal Microscopy have been used to quantify these defects. The nature of the defects induced by the shock wave propagation has been studied. The sensitivity of the composite material damage to the shock conditions has been shown and quantified. Moreover, the experimental results gathered with each technique have been compared to each other and it leads to a better understanding of the CFRP behavior under high dynamic loading. These original results have enabled the definition of a specific damage criterion for CFRP under dynamic loading.     

Fatigue of asphaltic dike revetments under wave loading: validation and improved modelling

In the Netherlands, 600 km of the sea dikes are protected by an asphaltic revetment that has to resist considerable wave loads with a significant wave height of up to 4.5 m. The subsoil is normally sandy, and the asphalt layer acts as a protection against erosion. The asphalt layer can fail as a result of fatigue due to repeated loading under storm conditions. In case of very high wave loads, the asphalt can fail after a few large waves. Ageing of the asphalt has a large effect on the resistance against fatigue. Therefore, periodic monitoring is prescribed by law. This monitoring consists of: falling weight deflection measurements, lab testing, radar measurements, visual inspection and calculations with the software program ‘Wave impact’ (‘Golfklap’) in order to determine the strength under storm conditions. The subsoil bearing capacity determines how the asphaltic revetment deforms under wave loading. It is still unsure how the soil will behave and failure mechanisms may occur that are different from fatigue due to elastic bending, the latter being part of the current approach. A first step to validation consisted of finite element modelling that was compared with experiments on medium scale (scale 1:4). In these experiments, the wave attack was simulated by a mechanical system that was placed on a 5 cm thick asphalt plate on a sand layer. The deflection of the asphalt was measured. Crack development was detected by means of strain gauges. It was found that – as a first approximation and excluding extreme wave loads – the fatigue behaviour of aged asphalt on a typical sandy base under wave attack can be described with a combination of a relatively simple elastic–plastic response to wave loads and a special fatigue line that takes into account the flexural strength of the aged asphalt.     

载荷作用下EPS混凝土中弹性波传播特性研究

应用EPS混凝土来模拟含缺陷的岩石材料。对EPS粒径分别为1、2和3mm的三种EPS混凝土试样进行了载荷作用下不同频率的弹性波传播实验研究。采用单一频率脉冲叠合的方法来精确确定材料的波速,结果表明：EPS混凝土的p波波速随载荷增加在试件的开始压密实阶段有较明显的增大趋势,当试件相对密实,波速增加不是很明显;s波波速随载荷增加有一定程度增加,但幅度比p波波速增加得小得多。应用一种相对波速的方法,即将波速与当前载荷下材料的声波速度进行对比,可以较好地分析波速与载荷和频率的关系。最后对波速与载荷和频率的关系进行了理论模拟分析。此研究对于应用弹性波进行材料和结构的无损检测等技术方面有很好的参考意义。     

Estimation of the damage of a porous limestone using continuous wave velocity measurements during uniaxial creep tests

The damage of an oolitic limestone during quasi-instantaneous compressive tests and multi-step creep tests was studied using an experimental device which enabled the simultaneous and continuous measurement of strains and elastic wave velocities under mechanical loading and controlled hydrous conditions. The simultaneous measurements of five elastic wave velocities (3 P-waves and 2 S-waves) in different directions of propagation and polarisation allowed us to assess the acoustic tensor at any time during loading and thus continuously monitor the evolution of rock damage. The general trend observed in the experimental results was the strong influence of hydrous conditions on both the quasi-instantaneous and time dependent behaviour of this rock. As traditionally observed in the quasi-instantaneous behaviour of quasi-brittle rocks under deviatoric loading, typical anisotropic straining was observed, confirmed by the anisotropy of the wave velocity measured in axial and lateral directions. However, after full unloading, even if a decrease in all waves’s velocities and dynamic elastic moduli was observed, the quasi-isotropy of waves’s velocities was recovered. This recovery after unloading cycle was observed until stress levels close to the peak (i.e., failure) stress. A comparison of laboratory results with numerical predictions from micromechanical models showed that these observations could not be explained by traditional crack propagation theories and that other mechanisms of crack growth must therefore be considered. These trends were also observed during the loading/unloading stages between each creep step with one notable difference, namely that the anisotropy developed during the creep stage was not recovered during the full unloading. A combination of crack growth and crack nucleation combined with a mechanism of progressive crack sliding and closure seems to explain these unusual results. The nucleation of cracks is mostly related to fast loading while the crack propagation is mostly due to subcritical growth of favourable oriented cracks.     

Assessment of chemo-mechanical couplings in polymer matrix materials exposed to thermo-oxidative environments at high temperatures and under tensile loadings

The present paper is concerned with the experimental assessment of chemo-mechanical couplings in a polymer matrix material exposed to thermo-oxidative environments at high temperatures and under pure tensile loadings. A model developed within the thermodynamics of irreversible processes (TIP) states that oxygen reaction-diffusion in the polymer matrix should be affected by the strain tensor and its spatial gradient; the model motivates the design and the development of the experimental assessment, which is done by ultra micro-indentation (UMI) elastic modulus (EIT) measurements carried out - at room temperature - on unnotched and notched 977-2 polymer samples thermo-oxidized under tensile loadings.For the material and the loading conditions of the present research the EIT profiles are not influenced by the applied strain or by the strain gradients which develop close to the sample notches.     

Development of plastic nonlinear waves in one-dimensional ductile granular chains under impact loading

A modified split Hopkinson pressure bar (SHPB) was used to load one-dimensional granular chains of metallic spheres under impact loading rates. These homogeneous chains, comprised of brass spherical beads ranging from a single sphere to a chain of sixteen, are of interest because of their unique wave propagation characteristics. In the elastic range, for loads around 10 s of N, nonlinear elastic solitary waves have been observed to form. In this work, loading magnitudes spanning from 9 kN to 40 kN – considerably higher than most previous works on these systems which have been conducted in the elastic regime – cause the granular chains to severely deform plastically. The aim of this study is to identify whether a nonlinear solitary-type wave will be generated under such high load levels, and if so, under what conditions (e.g., chain length, load level, etc.) it will do so. The propagating pulse was found to assume a distinctive shape after travelling through five beads, similar to the elastic case where solitary waves are realized with a traveling wavelength of five bead diameters. The wave speed of the plastic pulses observed here was seen to depend on maximum force, indicating that indeed it is a nonlinear wave in nature and is comparable to the elastic solitary wave. Locally, the plastic dissipation at every contact point through the chains was studied by measuring the residual plastic contact area. It was found that after the formation of the plastic nonlinear solitary wave had occurred there is also decreasing plastic deformation along the chain length except at the end beads in contact with the SHPB, which rebound into the SHPB bar causing larger plastic dissipation locally. To our knowledge this research is the first effort to investigate in detail the development and evolution of solitary-like waves in the plastic regime and will form the basis of future work in this area.     

Excimer Laser Processing of Nodular Iron

Excimer laser processing is expected to improve the wear properties and the oxidation resistance of cast iron surfaces. In the present study, an XeCl laser prototype was used to change the surface morphology of nodular iron samples under high energy ultraviolet exposure. Roughness evolution results from ablation and, above all, from moving of the molten superficial layer, due to both high temperature and pressure involved during the laser shot. Conventional metallography was carried out to determine the mechanisms governing the modification of surface profiles. These were shown to be both a wide and a local phenomena, i.e. involved at two different scales: that of the laser spot and that of the nodule. The plasma high pressure applied to the surface moves the molten material towards the periphery of the spot, where it freezes by self quenching with a high cooling rate. The surface profile is also modified by the formation of a rim surrounding each solid heterogeneity such as graphite spheroids. The width of the rims did not depend on the sizes of nodules or inclusions. The study suggests a wave emission mechanism to interpret formation of the rims.     

7050铝合金激光冲击强化形貌光学表征与表面残余应力分布的研究

本研究涉及7050铝合金激光冲击强化形貌的定量光学表征和表面残余应力分布的测定。研究表明，随着激光脉冲能量的增加，冲击凹陷深度与冲击区表面粗糙度均有所增加；冲击区径向残余应力的分布与横截面凹陷深度的分布相似。冲击区径向残余应力分布与激光冲击产生的弹一塑性波分布有关；弹一塑性波产生的塑性变形越大，残余应力也越大。较大的激光脉冲能量可产生中心对称的塑性变形和相应的残余应力分布。     

Variation in flexural properties of photo-pultruded composite archwires: analyses of round and rectangular profiles

Prototype continuous, unidirectional, fiber-reinforced composite archwires were manufactured into round and rectangular profiles utilizing a photo-pultrusion process. Both 0.022 inch (0.56 mm) diameter and 0.021 × 0.028 inch (0.53 × 0.71 mm) rectangular composites were formed utilizing commercially available S2-glass® reinforcement within a polymeric matrix. Reinforcement was varied according to the number, denier and twists per inch (TPI) of four S2-glass® yarns to volume levels of 32–74% for round and 41–61% for rectangular profiles. Cross-sectional geometry was evaluated via light microscopy to determine loading characteristics; whereas two flexural properties (the elastic moduli and flexural strengths) were determined by 3-point bending tests. Morphological evaluation of samples revealed that as the TPI increased from 1 to 8, the yarns were more separated from one another and distributed more peripherally within a profile. For round and rectangular profiles utilizing 1 TPI fibers, moduli increased with fiber content approaching theoretical values. For round profiles utilizing 1 TPI and 4 TPI fibers, flexural strengths increased until the loading geometry was optimized. In contrast, the flexural strengths of composites that were pultruded with 8 TPI fibers were not improved at any loading level. Doubling the denier of the yarn, without altering the loading, increased both the moduli and flexural strengths in rectangular samples; whereas, the increases observed in round samples were not statistically significant. At optimal loading the maximum mean moduli and strengths equaled 53.6 ± 2.0 and 1.36 ± 0.17 (GPa) for round wire and equaled 45.7 ± 0.8 and 1.40 ± 0.05 (GPa) for rectangular wires, respectively. These moduli were midway between that of martensitic NiTi (33.4 GPa) and beta-titanium (72.4 GPa), and produced about one-quarter the force of a stainless steel wire per unit of activation. Values of strengths placed this composite material in the range of published values for beta-titanium wires (1.3–1.5 GPa). © 2000 Kluwer Academic Publishers     

On the crack propagation with variable velocity

The plane problem of propagation of a straight crack in an elastic medium under arbitrary variable loading is considered. The locations of the edges of the crack are specified as arbitrary smooth functions of time under the only restriction that crack speed at any instant of time is less than the velocity of Rayleigh wave. Solution for the distribution of plane stress components near the crack tip is obtained. In particular, expressions for stress intensity factors at the crack are given, which thus makes it possible to deduce the crack motion under given loading conditions.     

Deformation and failure of structural steels in pulsed loading

Measurements were taken of the dynamic elasticity limit and fracture stresses in uniaxial shock wave loading of 35Kh3NM, ÉP836, and 45 steels and a heavy alloy with a density of 17 g/cm³. Simultaneous loading of specimens was carried out by impacts of aluminum sheets 2 and 0.4 mm thick accelerated to a speed of 660 m/sec by explosive devices or by detonation of an explosion lens in contact with the specimen. Data on the strength properties of the materials were obtained from analysis of the profiles of the rate of the free rear surface of the specimens recorded using a laser doppler speed measuring device. The results show that in all examined materials the elastoplastic nature of the compressive wave is clearly evident, although the relationships governing their deformation in compression differ.Translated from Problemy Prochnosti, No. 3, pp. 42–48, March, 1992.In conclusion, the authors are grateful to L. G. Ermolov for help in preparing and carrying out experiments.     

灰岩和白云岩动态压缩力学性能的SHPB实验

为了得到爆破荷载作用下岩石的破坏特性及爆破振动波的传播特性,利用RMT-150C多功能实验机和改进后的ф50mm的分离式Hopkinson压杆装置,分别研究灰岩和白云岩试件的静态力学特性和在5种不同应变率等级下的动态力学性能。实验结果表明,随着平均应变率的增加,灰岩和白云岩试件的动态抗压强度、峰值应变、吸收能、比能量吸收值以及破碎程度都明显增加,表现出显著的应变率效应,而初始弹性模量对应变率的相关性不敏感。从岩石的动态抗压强度和能量吸收两个方面,对比分析灰岩和白云岩动态力学性能的共性和差异性,更合理地解释了岩石在动态冲击荷载下的破坏本质。     

The Influence of the Structure of a Magnesium–Aluminum Nitride Metal-Matrix Composite on the Resistance to Deformation under Quasi-Static and Dynamic Loading

The microstructure of an ML5 commercial magnesium alloy reinforced by aluminum nitride (AlN) nanoparticles with an average size of 80 nm at an amount of 0.5 wt % has been studied. Comparative data on strength and plasticity of the initial ML15 alloy and AlN-reinforced metal-matrix composite were obtained using Instron 3369 universal testing machine. The influence of material microstructure on the resistance of samples to high-rate deformation and fracture was determined by analysis of the full wave profiles measured using a VISAR laser Doppler velocimeter.     

激光烧蚀对石英/氰酸酯透波复合材料电性能的影响

研究激光烧蚀对石英/氰酸酯复合材料电性能的影响并揭示其影响机制,对极度恶劣热环境条件下石英/氰酸酯复合材料透波性能评估分析、热防护设计等具有重要意义。利用激光作为外热流加载手段,对石英/氰酸酯复合材料进行激光辐照烧蚀实验,对实验前后的介电常数进行了测试。为分析介电常数变化机制,对石英/氰酸酯复合材料激光烧蚀前后的表面产物进行了透射红外光谱、XRD测试,对实验后的石英/氰酸酯复合材料表面进行微观形貌观察,并对氰酸酯和石英纤维进行了热失重测试。结果表明：与初始状态相比,激光烧蚀后的石英/氰酸酯复合材料在7~18 GHz范围内的介电常数为6左右,增大近1倍。分析认为激光烧蚀对石英/氰酸酯复合材料电性能的影响机制为：在激光辐照作用下,材料吸收激光能量升温,使氰酸酯树脂基体发生热分解、裂解等变化,在表面原位生成具有导电能力和岛链状态的炭黑物质,致使发生烧蚀炭化石英/氰酸酯复合材料的介电常数增大,将增强对雷达波的吸收。同时烧蚀形成的粗糙表面状态和疏松状态对电磁波的反射、散射作用增强,可进一步削弱石英/氰酸酯复合材料的雷达波透射能。