The influence of biaxial stresses on high-cycle fatigue-crack propagation

Arising from a design study, an examination has been made of the problems associated with evaluating the fatigue behavior of an I-beam section joined to a transverse stiffener, loaded in biaxial bending. A laboratory test rig has been designed to study high-frequency (40–230 Hz) fatigue-crack propagation in the tensile flange of a composite aluminum I-section, for a range of stress biaxiality ratios (λ) from 0 to +1 (equibiaxial tension), λ being varied by adjusting the ratios of loading spans in the two orthogonal axes for the same amount of uniaxial deflection. The results obtained are considered in the context of the significantly contradictory information currently available in the literature. Crack geometry and test procedure (as influenced by load/stress measurement) are found to considerably influence the effect of biaxial stresses on fatigue. Thus, the growth rate of corner-initiated cracks increases with increasing stress biaxiality (monitored in terms of nominal applied stresses), but the reverse is true for center-cracked specimens. When tests are conducted in terms of combinations of local stresses, or data reduced using corrections, there is no significant effect of biaxial stresses on fatigue-crack growth.     

Ultrasonic detection of high-temperature fatigue-crack growth

An ultrasonic technique is described for continuously monitoring fatigue-crack growth at temperatures up to 300°C in wedge-opening-load type fracture-toughness specimens. The sound-wave energies reflected from the leading edge of the fatigue crack and from the bottom surface of the specimen are electronically compared and used to control the crack-measurement system.     

Estimating fatigue-crack lives for aircraft: Techniques

Some complex aircraft stress histories, steady-state spectra, are noted to induce fatigue-crack growth-rate behavior similar to that observed under constant-amplitude loading. The paper identifies the behavior induced by steady-state spectra, and attributes the behavior to spectrum-stress-event periodicity. Spectrum periodicity is subsequently defined by isolating the group of statistically repetitive stress events. The noted crack-growth-rate behavior that exists for steady-state spectra provides the analyst with new techniques for estimating crack lives. Several crack-life estimating techniques are compared; for the two steady-state spectra considered herein, one technique called the simple-crack incrementation-miniblock approach provides life estimates with the same accuracy as that given by the cycle-by-cycle life-prediction method studied but does so five times more efficiently. The reasons for associated accuracy and efficiency are discussed.     

Continuous monitoring of fatigue-crack growth by acoustic-emission techniques

The application of acoustic emission to the detection of fatigue-crack propagation in 7075-T6 aluminum and 4140 steel is investigated. The relationship between crack-growth rate, cyclic stress-intensity factor, load-cycling rate and observed acoustic-emission behavior is presented. Crack-growth rates of less than 10^?6 in./ cycle could be detected, and acoustic-emission counts per cycle were shown to be closely related to the energy released by crack extension per cycle. A quantitative relationship for the threshold conditions for detection of fatigue-crack growth is presented which agrees with experimental test results. The results also showed that fatigue-crack growth occurs in an accelerating and decelerating manner, even though the stress-intensity range remains uniform, and that the count rate posses through a peak that is believed to be associated with a plane strain-plane stress transition. The effects of instrumentation sensitivity and frequency bandpass are also investigated. The results obtained indicate that acoustic-emission techniques should be suitable for in-service monitoring of a variety of cyclically loaded structures, even in the presence of high background noises.     

Application of holographic interferometry to a study of wave propagation in rock

Holographic interferometry was utilized to determine the three orthogonal components of displacement in elastic surface waves. A pulsed ruby laser was used as the light source and techniques to improve its coherence properties are described. Procedures for the formation and reconstruction of the hologram, fringe interpretation, and data reduction and presentation are detailed. The elastic-wave velocities and material constants for pink westerly granite were obtained. Solutions for an explosively generated Rayleigh wave in a half space and its reflection from a free edge are presented.     

Effect of loading variables on the acoustic emissions of fatigue-crack growth

A series of experiments has been conducted to determine the effect of loading variables such as cyclic frequency, load ratio, and material on acoustic emission from fatigue-crack propagation. It is shown that the applied-stress intensity range (ΔK) is the controlling parameter for all materials studied while the other parameters have lesser effects. Two potential methods for engineering application of acoustic emission during fatigue loading are described.     

模糊-线性双模控制在平台稳定回路中的应用

为了克服单闭环系统平台稳定回路在抗干扰性能方面的欠缺,引入速度反馈的双闭环控制。在此基础上,通过多种模糊控制方法的仿真、分析、比较,采用模糊-线性双模控制方案对系统进行校正,模糊-线性双模控制的速度环以及位置环的线性控制器结构都与线性双闭环控制的相同,位置环的模糊控制器为修正因子自调整无量化模糊控制器,利用输出强度系数实现两种控制的平滑过渡,克服了常规模糊-线性双模控制器的切换点选择困难,在误差较大时,模糊控制器起主要作用,以提高系统的动态特性;误差较小时,PID控制器起主要作用,使线性控制和模糊控制的输出实现了平滑过渡,使系统在速度、精度尤其抗干扰能力方面均有良好的效果。     

Low-cycle fatigue-crack indications by strain gages operating in elastic strain fields

This paper describes the response of bonded electrical-resistance strain gages to low-cycle fatigue cracking of four, cyclically pressurized, boxlike structures. The strain-pressure “signatures” of gages installed on flexural parts of the structures were recorded semicontinuously. By observing the changes in signatures of certain gages, it was possible to follow indirectly the initiation and propagation of internal low-cycle fatigue cracks. The described technique appears feasible as a crack-detection system, or as an adjunct to some other type of damage-warning system. It is suggested that the sensitivity and accuracy of this technique make it possible to substitute “crack initiation” for “through cracking” as a criterion for fatigue failure in design.     

Application of finite difference method adopted in fluid mechanics to sound propagation in ducts

Researches on sound propagation problems in ducts done by the author and other investigators in recent years are briefly discussed in this paper. The finite difference method used in fluid mechanics is used for solving these problems thus presenting an aspect of the development of research work in this field at home and abroad.The project supported by National Natural Science Foundation of China.     

Costas环在无线扩频定位系统中的应用技术研究

分析了扩频定位系统中载波恢复和数据解调数学模型，设计了环路中应用的Butterworth低通滤波器和产生正交两路信号的Hilbert变换滤波器，在Simulink中建立了Costas环仿真模型。结果表明：该Costas环能够准确地实现扩频定位系统的载波恢复和发送端数据解调输出，理论仿真和实际工程实验基本一致。     

Application of asymptotic methods of the theory of nonlinear oscillations to the problem of wave propagation

The problem of the propagation of waves in an inhomogeneous medium is solved on the basis of the equation for a partial wave of the total field. After changing the independent variable x (the geometrical coordinate) to A(x) (the amplitude factor of a direct partial wave of the total field in the inhomogeneous medium) a modification of one of the asymptotic methods of the theory of nonlinear oscillations is applied.     

A methodology for studying fatigue crack propagation under spectrum loading: Application to rail steels

A testing procedure is applied to investigate the fatigue behavior of the detail fracture, in rail steel, under Mode I train stress spectrum. The results show that fatigue crack under spectrum loading depends on a “dynamic” accumulation of damage. This term is related to the load sequence in the spectrum and the gradient of the elastic stress intensity factor. It is theoretically very difficult to quantify this term. The rationale underlying the testing procedure is such that this dynamic term effect is implicitly included in the experimental results. The detail fracture was modelled by the penny-shaped crack geometry. This model has proved, in this study, to be satisfactory for simulating the fatigue growth of the detail fracture.     

Application of the method of spatial characteristics to the solution of axially symmetric problems relating to the propagation of elastic waves

We present a difference scheme, based on the method of spatial characteristics, for solving axially symmetric dynamical problems of the theory of elasticity. Consideration is given to the possibility of solving a Cauchy problem and a problem for a solid or a hollow cylinder which takes boundary conditions into account. It is suggested that linear problems may be solved by this method. An example is given in which the parameters characterizing the stress-deformation state of a semiinfinite cylinder are calculated, the points of the end of the cylinder being given an initial axial velocity. The calculation of these parameters was carried out on the BÉSM-6 computer.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 101–109, July–August, 1971.     

Application of the Discrete Element Method to crack propagation and crack branching in a vitreous dense biopolymer material

Crack propagation in a vitreous biopolymer material is simulated using the Discrete Element Method (DEM), which models the brittle material as an assembly of particles bonded together. The simulations are compared to experiments combining a high-speed camera monitoring of crack branching together with a micromechancial testing of samples where local mixture mode is generated by introducing a stress concentrator. Our experimental results show unstable crack propagation and branching occurring upon crack deviation by the action of the stress concentrator. The validity of the DEM simulations is checked by comparing its result to the Finite Element Method (FEM) and to an analytical expression under similar conditions. DEM results show a higher sensitivity to mixed mode compared to FEM and a better match with the analytical formulation. Finally, crack branching is correctly predicted using DEM without any specific criterion for the initiation of secondary cracks.     

Neck propagation

Propagation of a neck along the length of a tensile specimen as occurs in certain polymeric materials and in a few metals is studied. Two material models are considered: a nonlinear elastic solid, and an inelastic flow theory solid with both rate-dependent and rate-independent behaviors. For the elastic solid the states ahead and behind the neck transition can be obtained fairly simply from just the jump conditions governing continuity of mass, momentum and energy. For the inelastic solid a full three-dimensional analysis must be performed to obtain the same information, and an analysis of axisymmetric neck propagation is carried out.     

Reflective photoelasticity and holo-interferometry applied to stress wave propagation

The principal stress distribution over a section of a polyester model of a semi-infinite plate during the propagation of a stress wave has been determined experimentally. The stress pulse was generated by the impact of a pendulum hammer. Two external suitable trigger mechanism were used to trigger a double-pulse Ruby laser (0.5 Joule per pulse) at pre-determined intervals. One surface of the model was made fully reflecting so that by means of reflective holo-interferometry of that surface and reflective photoelasticity through the other surface of the model the isopachic and isochrometic fringe patterns were recorded at different time delays after impact.     

Anisotropic wave propagation and its applications to NDE of composite materials

This paper utilized anisotropic wave propagation theory to measure the elastic constants of a unidirectional fiber-reinforced composite specimen. For plane waves propagating in the composite specimen, the deviation of the propagational directions between the energy and phase velocities were measured. It is found that in such a sample, the deviations may be as large as 60 degrees. The measured energy velocities were transformed to the phase velocities of the plane waves by employing a numerical scheme. It is demonstrated that the elastic constants of a unidirectional fiber-reinforced composite can be determined by conducting ultrasonic experiments in two principal symmetry planes.