Investigation of surface instability in an elastic anisotropic cone by the use of surface waves of weak discontinuity

Summary The problem of surface instability of a right circular cone with an arbitrary opening made of a hexagonal single crystal (the cone axis coincides with the crystal's axis of isotropy) is investigated. The surface of the cone is free from normal and tangential stresses, but in the layer near the surface initial constant tensile or compressive stresses act in the hoop direction and in the direction of the cone's generators. Surface instability is analyzed by the use of weak nonstationary disturbances which propagate along the surface of the cone in the form of the two types of surface waves: the nonstationary Rayleigh waves polarized in the sagittal plane, and the nonstationary wave of the whispering gallery type polarized perpendicular to the sagittal plane. The weak nonstationary surface waves are interpreted as the lines of discontinuity (diverging circles) on which partial derivatives of the stress and strain tensor components with respect to coordinates and time have a discontinuity, but the components of these tensors are continuous. Each of the lines of discontinuity propagates with a constant normal velocity along the cone's surface in the direction of its generators and is obtained as a result of the exit onto the cone surface either of two conic complex wave surfaces of weak discontinuity intersecting along this line (Rayleigh wave) or of one real conic wave surface of weak discontinuity (wave of the whispering gallery type). The analysis is carried out within the framework of the theory of discontinuities based on the kinematic, geometric and dynamic conditions of compatibility; using them the velocities of the surface wave propagation and their intensities have been found. It has been shown that the surface wave velocities are dependent only on the initial stress acting in the direction of the propagation of a surface disturbance whereas the damping coefficients for the intensities of the surface waves are dependent not only on this stress but also on the initial stress acting in the hoop direction as well. The relationships for two critical magnitudes of the force compressive in the hoop direction have been obtained, and it has been shown that under the hoop compressive forces in excess of one of these magnitudes the intensity of the Rayleigh wave or the surface wave of the whispering gallery type begins to increase without bounds during its propagation, i.e., the surface of the cone loses stability with respect to either of two types of weak nonstationary disturbances.     

Precision submillimeter wave laser reflection measurements of high-temperature superconductors

Submillimeter wave laser reflection measurements of surface resistance can provide improved capability in the combination of sensitivity, spatial resolution, and frequency range. We have made reflectivity measurements on metals at 1630 GHz with an uncertainty of less than 0.3%. This sensitivity corresponds to a measurement sensitivity for surface resistance of 0.3 . Assuming anf ² frequency scaling of high-temperature superconductor surface resistance from the microwave to the terahertz frequency range, this sensitivity corresponds to about 1 ×10^–5 at 10 GHz. Capability for 10^–7 sensitivity could eventually be possible. Preliminary submillimeter wave reflection measurements of a YBCO thin film have been made with a sensitivity of 1%. Submillimeter wave reflectometry can make it possible to determine the spatial dependence of surface resistance in a wide range of material sizes and shapes. The spatial resolution could be on the order of 0.3–0.5 mm.     

Propagation of Surface Acoustic Pulses Generated by a Femtosecond Laser in Thin Films on Solid Substrates

The technique of phase velocity dispersion measurements of surface acoustic waves in a thin film-substrate system was demonstrated. The excitation of surface acoustic waves (SAWs) was quite efficient with femtosecond laser pulses, and the damage of the surface was minimized. The measurements were performed with films of Al deposited on silicon wafers. The errors in the determination of the phase velocity and absorption were analyzed. The temperature changes in the propagation velocity on bare Si wafers were also measured. The data obtained permitted estimation of the accuracy of the temperature determination from measurements with SAW pulses.     

A new point contact surface acoustic wave transducer for measurement of acoustoelastic effect of polymethylmethacrylate

A new acoustic transducer and measurement method have been developed for precise measurement of surface wave velocity. This measurement method is used to investigate the acoustoelastic effects for waves propagating on the surface of a polymethylmethacrylate (PMMA) sample. The transducer uses two miniature conical PZT elements for acoustic wave transmitter and receiver on the sample surface; hence, it can be viewed as a point- source/point-receiver transducer. Acoustic waves are excited and detected with the PZT elements, and the wave velocity can be accurately determined with a cross-correlation waveform comparison method. The transducer and its measurement method are particularly sensitive and accurate in determining small changes in wave velocity; therefore, they are applied to the measurement of acoustoelastic effects in PMMA materials. Both the surface skimming longitudinal wave and Rayleigh surface wave can be simultaneously excited and measured. With a uniaxial-loaded PMMA sample, both acoustoelastic effects for surface skimming longitudinal wave and Rayleigh waves of PMMA are measured. The acoustoelastic coefficients for both types of surface wave motions are simultaneously determined. The transducer and its measurement method provide a practical way for measuring surface stresses nondestructively.     

Calculation of ultrasonic surface waves from an extended thermoelastic laser source

A method is developed to calculate ultrasonic surface waveforms generated by an extended laser source, operating in the thermoelastic regime of laser-pulse energy density. This approach integrates over a suitably weighted distribution of point surface centers of expansion, for observation to within 1 mm of the edge of the source. Power spectra as well as both horizontal and vertical displacements are presented and discussed for ultrasonic waveforms on an aluminium surface, for incident laser pulses having Gaussian lateral profiles of various sizes. Far from the source, the waveform is dominated by a dipolar Rayleigh (R) wave, whose amplitude and spectral content depend on laser spot size. Weak, monopolar pulses also occur at the intersection of bulk pressure and shear wavefronts with the surface (denoted assP andsS, respectively). Close to the source, thesP wave amplitude approaches that for theR wave, and overlaps theR wave for large source sizes. The fall-off with distance for bothsP andR waves is given. Finally, the changes in pulse shape and amplitude are calculated when anR wave from an extended thermoelastic source is reflected or transmitted by a right-angled corner of an aluminium block.     

The elastic properties of ion-implanted silicon

The elastic properties of silicon implanted with As⁺ and Si⁺ in the dose range 10¹⁴ to 10¹⁵ ion cm^–2 has been investigated. Reflection scanning acoustic microscopy techniques have been used to determine changes in the velocity of surface elastic waves (Rayleigh waves) on ion-implanted silicon. With the aid of theoretical models for this mode of wave propagation, the experimental velocity changes have been interpreted in terms of changes in the elastic constants of the implanted layer. These changes have been found to be dependent upon the level of radiation damage produced by the implantation process. Decreases of 30% in the bulk and shear elastic constants have been deduced for damage levels present at the onset of implantation-induced amorphization.     

Effect of surface stress and irregularity of the interface on the propagation of SH-waves in the magneto-elastic crustal layer based on a solid semi space

The object of the present paper is to investigate plane SH waves through a magneto-elastic crustal layer based over an elastic, solid semi space under the influence of surface stress on the free surface of the crustal layer and irregularity of the interface. Two types of irregularities of the interface namely, rectangular and parabolic have been considered. Modulations of wave velocity due to the presence of surface stress, irregularity and the magnetic field have been studied separately. Their combined effect has also been investigated. Graphs are drawn to highlight some important peculiarities. It is observed that surface stress, irregularity and magnetic field have their respective role to play in the propagation of SH waves in the crustal layer. Further modulation of wave velocity occurs due to their combined effect.     

弯曲波对钢轨振动特性测试的影响研究

针对钢轨弯曲波的传播及弯曲波反射后对钢轨振动特性测试的影响,对钢轨的弯曲波波速进行了理论推导,并设计了弯曲波波速测试试验,对弯曲波传播波速及其反射波的波速等特性进行了试验验证,同时就钢轨弯曲波对振动特性测试的影响及其解决措施进行了研究。结果表明:钢轨弯曲波的波速不是常数,而是频率的函数,弯曲波的不同频率分量以不同的波速传播,其弯曲波的反射波与入射波波速相同;在有限长的试验段内进行轨道结构振动特性测试时,不可忽略钢轨弯曲波的影响;在钢轨端部安装阻尼设备可有效抑制反射波对钢轨振动特性测试的影响。     

Effect of Laser Shock Peening on the Microstructures and Properties of Oxide‐Dispersion‐Strengthened Austenitic Steels

Oxide‐dispersion‐strengthened (ODS) austenitic steels are promising materials for next‐generation fossil and nuclear energy systems. In this study, laser shock peening (LSP) has been applied to ODS 304 austenitic steels, during which a high density of dislocations, stacking faults, and deformation twins are generated in the near surface of the material due to the interaction of laser‐driven shock waves and the austenitic steel matrix. The dispersion particles impede the propagation of dislocations. The compressive residual stress generated by LSP increases with successive LSP scans and decreases along the depth, with a maximum value of ?369 MPa. The hardness on the surface can be improved by 12% using LSP. In situ transmission electron microscopy (TEM) irradiation studies reveal that dislocations and incoherent twin boundaries induced by LSP serve as effective sinks to annihilate irradiation defects. These findings suggest that LSP can improve the mechanical properties and irradiation resistance of ODS austenitic steels in nuclear reactor environments.

     

Methods for solving one-dimensional boundary-value problems in a nonlinear elastic medium

Summary One-dimensional problems connected with a mechanical exposure to the boundary of a nonlinear elastic half-space, which leads to a constantly accelerated motion of this boundary, are considered. The value , equal to the square root of the ratio between the velocity of the boundary motion and the velocity of longitudinal wave propagation in a linear elastic medium, is used as the value characterizing the intensity of this exposure. It is shown that as a result of such an exposure shock waves of small or finite amplitude may propagate in the half-space. The asymptotic matching principle and the ray method are used as methods of solution. The merits and demerits of each method are analyzed. It has been inferred that the matched asymptotic method can be applied to waves of small amplitude, and the ray method is usable when investigating the propagation of shock waves not only of small amplitude, but of finite amplitude as well if the time of a consideration of the shock process is not long. The results obtained by the two methods for shock waves of small amplitude are in close agreement. It has been demonstrated that the ray method is adaptable for solving more intricate boundary-value problems resulting in the propagation of several shock waves of finite amplitude at a time. The problem connected with the constantly accelerated motion of one of the boundaries of an initially deformed elastic layer provides an example.     

Picosecond laser heating and melting of graphite

Ultrashort laser pulses impinging onto solid, strongly absorbing surface deposit their energy within an absorption depth from the surface. This localized energy deposition may result in rapid and very efficient heating of the surface to temperatures far exceeding the melting or boiling point. Temperature evolution at the surface of samples and their electronic structure may be studied with nonperturbing, time-resolved optical diagnostic techniques. Picosecond laser pulses provide the fastest means for heating matter at high temperature, since the characteristic energy transfer times from photoexcited electrons to the lattice occur in this time scale. Surface evaporation is not affecting the observations in this time scale, simply because there is no time for the surface atoms to escape. As an illustration, measurements on graphite are presented here. The complex index of refraction of highly oriented pyrolytic graphite (HOPG) during picosecond laser irradiation has been measured at 1.06 m via time-resolved ellipsometry at angles of incidence up to 80. In particular, a value of the complex index of refraction for the liquid phase has been derived.Paper presented at the First Workshop on Subsecond Thermophysics, June 20–21, 1988, Gaithersburg, Maryland, U.S.A.     

Surface Hardening of Carbon Steel Using High Powered YAG Laser

A high powered YAG laser with kaleidoscope for surface modification was applied to the surface hardening of carbon steels containing 0. 18-0.54 wt% C without the absorbents, and the relationships between laser processing and surface hardening were investigated by hardness and microstructure. The structure of the hardened zone underwent complete martensitic transformation in all of the carbon steels tested, and its hardness increased with greater carbon content. Under identical irradiated conditions, the hardened zone expanded with increasing carbon content. A hardened zone extending from the surface to a depth of 1.0 mm was obtained at a laser power of 1.0 kW and a scanning speed of 1 mm/sec. It was found that in the surface hardening of carbon steels, a high powered YAG laser can be used to control the hardened zone by selecting the appropriate irradiation conditions, however, the hardened zone was affected by the assistant gas and the flow rate.     

Surface wave propagation in a fluid-saturated incompressible porous medium

A study of surface wave propagation in a fluid-saturated incompressible porous half-space lying under a uniform layer of liquid is presented. The dispersion relation connecting the phase velocity with wave number is derived. The variation of phase velocity and attenuation coefficients with wave number is presented graphically and discussed. As a particular case, the propagation of Rayleigh type surface waves at the free surface of an incompressible porous half-space is also deduced and discussed.     

Analysis of crack propagation during tooth interior fatigue fracture

Tooth interior fatigue fracture is a failure mode that is initiated as a fatigue crack in the interior of the tooth of a gear. TIFF cracks have been observed in case hardened idler gears. A fracture mechanical analysis of a TIFF crack is performed utilising FEA. A 3D TIFF crack is modelled at a position in the tooth that corresponds with an observed crack surface. The different material properties in the case and the core, determined by mechanical testing, are considered, as well as the residual state of stress due to case hardening. Various crack lengths are analysed to estimate crack propagation both into the core and into the case. The following results have been found:

• A TIFF crack initiated slightly under the case layer will propagate into the case layer where it stops.

• The main crack propagation will take place in the core.

• The crack propagation is only a small portion of the total life (order of 10⁵ cycles).

• After reaching the case layer the TIFF crack eventually deflects toward the tooth root and the upper part of the tooth falls off. The crack deflection is due to redistribution of contact loading. Several gear teeth pairs are simultaneously in contact and the cracked tooth is loaded less than the uncracked during this stage of life.

C60—C70涂层的激光增碳强化效应

研究了Ｃ６０－Ｃ７０涂层对４５＾＃钢的激光增碳强化机理，对强化层组织和性能的分析表明：Ｃ６０－Ｃ７０涂使４５＾＃钢产生增碳强化区，得到了挖主亚共晶白口铸铁的定向凝固组织，由细小的马氏体」Ｍ「及莱氏体「Ｌｅ」组成，Ｍ中分布着大量微米级的多边形弥散相，并生成了由不同位向的碳化物细针束构成的组织，淬火后达到的平均硬度为ＨＲＣ６３．８，深冷处理后，平均硬度可达ＨＲＣ６５．４．     

Distinctive features of the effect of laser treatment on the corrosion-fatigue fracture of structural steel

The dependence of the susceptibility of surface-hardened 40KhMFA steel (_u=950 MPa, _0.2=780 MPa) to crack initiation and propagation on the mode of laser treatment is studied by the methods of fracture mechanics. The influence of laser treatment on the initiation of a fatigue crack is ambiguous: in air, it results in a substantial increase in the fatigue threshold, while in a 3% NaCl solution, the fatigue threshold decreases. The growth rate of short fatigue cracks decreases in both air and corrosive media. The positive effect of laser treatment on the kinetics of crack growth and the threshold SIF becomes more pronounced as the yield limit of the surface layer _0.2 increases. The depth of the influence of laser treatment on crack growth is not restricted to the hardened surface layer due to the redistribution of residual stresses. Unlike the base material, short cracks in the surface layer hardened by a laser are characterized by a high degree of crack closure, which proves to be the main factor that provokes an increase in the fatigue crack-growth resistance.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 6, pp. 40–52, November – December, 1994.     

Love wave propagation in a solid with a cold-worked surface layer

In this paper, experiments show Love wave generation along the top surface of used railroad rail, where the shear wave velocity has been slightly reduced by the cold-working of wheel passage for years. The rf pulses used in the experiments have about 10 µs duration and a relatively narrow frequency spectrum. The group velocity of the Love wave is found to have a strong dependence on the carrier frequency over the tested range of 0.45–3.1 MHz. Application of the seismological one-layered model to the experimental measurements yields an NDE technique for the elastic properties and the thickness of the cold-worked surface layer. The results are interpreted on the basis of a destructive observation by micro-Vickers hardness testing.     

Predictive modeling of multi-track laser hardening of AISI 4140 steel

Laser hardening provides benefits over the conventional hardening processes, including minimum distortion in the parts and the absence of a quenchant. This process is also faster than conventional hardening processes and can be used for selective hardening of specific areas of components. One known problem with laser hardening in steels, however, is back tempering when a large area is hardened by multiple, overlapping passes. This study focused on the development of a numerical model to predict the back tempering in multi-track laser hardening. A tempering model was combined with existing models of thermal behavior and phase change kinetics, which were developed earlier in the authors’ group, to predict three-dimensional hardness profiles after multiple track laser hardening. The combined model was first validated through multi-track laser hardening tests and then used to predict and optimize the laser hardened case depth in multi-track laser hardening of AISI 4140 steel. The predictions and parameters optimized to obtain maximum case depth with the least variation along width of the hardened zone were experimentally verified. Case depths up to 2 mm were obtained with 5 mm overlapping of laser tracks.     

The propagation of Love waves in water-saturated soil underlain by a heterogeneous elastic medium

Summary The propagation of Love waves in water-saturated soil overlying a non-homogeneous elastic medium has been investigated in the present paper. The solution of the problem is evaluated with the help of Fourier transform technique. It has been found that the velocity of body wave depends on the direction of propagation, and the velocity of Love wave depends on the porosity of the layer and the non-homogeneity character of the medium as well. Further, the effect of porosity and heterogeneity has been shown graphically.With 2 Figures     

Two-dimensional wave propagation in a micropolar thermo-elastic layer with stretch

This paper deals with the propagation of 2-dimensional waves in a thermo-elastic micropolar solid layer which can stretch and contract. Thermo-elastic Rayleigh wave velocity equation in the micropolar medium with stretch has been deduced from the above theory. The wave velocity equation obtained is in agreement with the classical result of Rayleigh when the layer is unstretched and thermal and micropolar effects are negligibly small.