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1.
Experimental Techniques - The time-of-flight method has been used to measure the longitudinal and shear wave speeds in six polymers (polyether ether ketone [PEEK], PEEK with 10% carbon fibers,... 相似文献
2.
A theory of plasticity without a yield surface is applied to obtain the simple wave solution of a thin-walled tube subject to the combined step loading. Four wave speeds are obtained in this analysis; all speeds involve the coupling of the longitudinal and the torsional motions as well as the loading and the unloading behaviors. Several numerical examples of our theoretical analysis and their comparisons with the experimental data and with the existing theory are also presented. The constant state region between slow and fast waves is not apparent in the present analysis. This is in good agreement with the experiment. Moreover, the predicted final state of shear strain is higher than the experimental, but less than Lipkin and Clifton's result. 相似文献
3.
We present examples of body wave and surface wave propagation in deformed solids where the slowest and the fastest waves do not travel along the directions of least and greatest stretch, respectively. These results run counter to commonly accepted theory, practice, and implementation of the principles of acousto-elasticity in initially isotropic solids. For instance, we find that in nickel and steel the fastest waves are along the direction of greatest compression, not greatest extension (and vice-versa for the slowest waves), as soon as those solids are deformed. Further, we find that when some materials are subject to a small-but-finite deformation, other extrema of wave speeds appear in non-principal directions. Examples include nickel, steel, polystyrene, and a certain hydrogel. The existence of these “oblique”, non-principal extremal waves complicates the protocols for the non-destructive determination of the directions of extreme strains. 相似文献
4.
Vernal H. Kenner 《Experimental Mechanics》1975,15(3):102-106
An important consideration in determining the veracity of data obtained from X-cut quartz crystals used to measure nominally one-dimensional load states is the role which spatial nonuniformity of the normal stress distribution on the crystal will play in determining the transducer output signal. This role was found to be minor both for crystals located 0.74 mm below a surface subjected to impact by spherical steel strikers and for 25.4-mm-diam crystals positioned in a 12.7-mm-diam Hopkinson bar through which a longitudinal wave was travelling. Load transducers utilizing two crystals to eliminate undesirable signals arising from bending of the crystal were also studied by attaching these devices to plates and subjecting them to dynamic loading by steel spheres. While bending effects were successfully eliminated, it was found that the method of transducer attachment may give rise to additional unwanted signals associated with other modes of crystal deformation. 相似文献
5.
Molecular dynamics simulation of crack-tip processes in copper 总被引:1,自引:0,他引:1
The crack tip processes in copper under mode II loading have been simulated by a molecular dynamics method. The nucleation,
emission, dislocation free zone (DFZ) and pile-up of the dislocations are analyzed by using a suitable atom lattice configuration
and Finnis & Sinclair potential. The simulated results show that the dislocation emitted always exhibits a dissociated fashion.
The stress intensity factor for dislocation nucleation, DFZ and dissociated width of partial dislocations are strongly dependent
on the loading rate. The stress distributions are in agreement with the elasticity solution before the dislocation emission,
but are not in agreement after the emission. The dislocation can move at subsonic wave speed (less than the shear wave speed)
or at transonic speed (greater than the shear wave speed but less than the longitudinal wave speed), but at the longitudinal
wave speed the atom lattice breaks down.
The project supported by the National Natural Science Foundation of China 相似文献
6.
Longitudinal wave velocity is used to characterize the point defects in crystalline solids. High purity Al single crystal was selected for both the finite element analysis and experimental work. Since the jog motions of dislocations caused by intersected slides such as cross slips induce point defects, the total amount of cross slips was calculated instead of calculating directly from the point defects. The effect of crystal orientations on total amount of cross slips under pure shear was also investigated via the finite element method. The result suggest that if the initial shear stress direction is located at the inner side of stereographic triangle, only single slip activities occurred at the beginning of plastic deformation and no effects due to point defects were present. However, as the shear stress direction rotates along the slip direction, point defects are induced by cross slips between primary and secondary slip systems due to work-hardening. This phenomenon was then examined by measuring longitudinal wave velocity changes propagating in Al single crystal subjected to the combination loads of equi-biaxial tension and compression (a pure shear state). Good qualitative agreement between the finite element result and measured data suggest that the longitudinal wave velocity can be used as an index to characterize point defects in crystalline materials. 相似文献
7.
Dynamic crack growth along the interface of a fiber-reinforced polymer composite-Homalite bimaterial subjected to impact shear loading is investigated experimentally and numerically. In the experiments, the polymer composite-Homalite specimens are impacted with a projectile causing shear dominated interfacial cracks to initiate and subsequently grow along the interface at speeds faster than the shear wave speed of Homalite. Crack growth is observed using dynamic photoelasticity in conjunction with high-speed photography. The calculations are carried out for a plane stress model of the experimental configuration and are based on a cohesive surface formulation that allows crack growth, when it occurs, to emerge as a natural outcome of the deformation history. The effect of impact velocity and loading rate is explored numerically. The experiments and calculations are consistent in identifying discrete crack speed regimes within which crack growth at sustained crack speeds is possible. We present the first conclusive experimental evidence of interfacial crack speeds faster than any characteristic elastic wave speed of the more compliant material. The occurrence of this crack speed was predicted numerically and the calculations were used to design the experiments. In addition, the first experimental observation of a mother-daughter crack mechanism allowing a subsonic crack to evolve into an intersonic crack is documented. The calculations exhibit all the crack growth regimes seen in the experiments and, in addition, predict a regime with a pulse-like traction distribution along the bond line. 相似文献
8.
Search for conditions of compressive fracture of hard brittle ceramics at impact loading 总被引:1,自引:0,他引:1
G.I. Kanel E.B. Zaretsky A.M. Rajendran S.V. Razorenov A.S. Savinykh V. Paris 《International Journal of Plasticity》2009
In this paper we discuss three different experimental configurations to diagnosing the modes of inelastic deformation and to evaluating the failure thresholds at shock compression of hard brittle solids. One of the manifestations of brittle material response is the failure wave phenomenon, which has been previously observed in shock-compressed glasses. However, based on the measurements from our “theory critical” experiments, both alumina and boron carbide did not exhibit this phenomenon. In experiments with free and pre-stressed ceramics, while the Hugoniot elastic limit (HEL) in high-density B4C ceramic was found to be very sensitive to the transverse stress, it was found relatively less sensitive in Al2O3, implying brittle response of the boron carbide and ductile behavior of alumina. To further investigate the effects of stress states on the shock response of brittle materials, a “divergent flow or spherical shock wave” based plate impact experimental technique was employed to vary the ratio of longitudinal and transversal stresses and to probe conditions for compressive fracture thresholds. Two different experimental approaches were considered to generate both longitudinal and shear waves in the target through the impact of convex flyer plates. In the ceramic target plates, the shear wave separates a region of highly divergent flow behind the decaying spherical longitudinal shock wave and a region of low-divergent flow. Experiments with divergent shock loading of alumina and boron carbide ceramic plates coupled with computer simulations demonstrated the validity of these experimental approaches to develop a better understanding of fracture phenomena. 相似文献
9.
Baljeet Singh 《European Journal of Mechanics - A/Solids》2011,30(6):976-982
Governing equations of thermoelastic diffusion material with voids are modified with the help of Lord and Shulman theory of generalized thermoelasticity. These governing equations are then solved in two-dimension to show the existence of four coupled longitudinal waves and a shear wave. The complex absolute values of the speeds of the coupled longitudinal waves are computed numerically against the frequency for Magnesium material. The reflection of these plane waves from a stress free thermally insulated boundary is also studied, where the dependence of the reflection coefficients on angle of incidence is shown graphically for the incidence of coupled longitudinal wave only. The speeds and reflection coefficients of plane waves are also computed numerically in the absence of voids and diffusion parameters, which are shown graphically to observe the effects of voids and diffusion. 相似文献
10.
通过实验手段研究了白云岩在三轴和单轴加载过程中的声波波速、幅度、频谱特性的变化特征. 结果发现:(1)白云岩在受压变形过程中,声波波速的变化较好地反映了岩石内部裂缝的闭合、产生、扩展和贯穿等变化. 总体而言,横波波速比纵波能更好地预测裂缝的产生,而纵波对裂缝的非稳定发展或贯穿有较敏感的反映. (2)随着轴压的增大,纵、横波波形的末端都有散射波的出现,且横波波形末端的"鱼尾"状散射波信号更为明显(此时应力点为极限强度的60%左右),预示着岩石内部裂缝的产生和稳定扩展. (3)频谱曲线也很好地反映了岩石内部结构变形情况. 随着岩石受力的增大,频谱曲线上的振幅都呈现增大趋势,标志着岩石的压实阶段;当频谱曲线上低频段较高频段活跃时,标志着裂缝的产生;更有甚者,低频还会取代高频成为主频. (4)在岩石受压裂缝闭合阶段,首波振幅和频谱主振幅都呈现上升趋势;而在裂缝不稳定扩展阶段,主振幅表现出比首波振幅低的上升趋势;振幅曲线达到峰值后都存在一突降拐点,预示着岩石的贯通破坏. 研究对于地层岩石的动态长期监测和工程岩体的稳定性评价都具有重要的理论参考价值. 相似文献