Interphase characterization of composites under fatigue loadings

The effect of glass-fiber epoxy interface in cross-ply reinforced composites on the fatigue behavior by using load-increasing fatigue test is studied throughout this paper. The damage as measured by stiffness reduction is more significant for the composites with poor bonded fibers as was found for EP sized ones, dependent from test conditions. The loss energy is shown to be a sensitive tool to characterize the nature of fiber–matrix adhesion. The loss energy for composites with poor adhesion between fiber and matrix results in significantly higher amounts of consumed energy during a single stress-strain loop than those composites containing well-bonded fibers.     

考虑基底热传导的石墨烯薄膜热声理论

主要基于热声效应对石墨烯薄膜发声进行理论研究。首先建立了石墨烯薄膜耦合热振动模型,推导出了石墨烯薄膜发声器的声压表达式。在此基础上,进行了不同基底石墨烯薄膜发声器的声压测试,并将测试值与理论计算结果对比,二者随频率变化趋势基本吻合,测试值略低于理论值,验证了推导出的声压表达式的正确性。研究表明石墨烯薄膜发声器有很宽的频域响应,在低频段声压级随频率增大而增大,在高频段响应平稳,具有作为优秀的热致发声器的潜力。基底材料蓄热系数越小,石墨烯薄膜的声压值越大;声压级随薄膜热容量的增大而减小。研究结果对于石墨烯的发声机制探索及其在扬声器设计等方面的应用具有指导意义。     

Dynamic fatigue

Dynamic fatigue differs from quasi-static fatigue in the spall damage nature. A consequence of the spall nature is the alternate appearance of longitudinal cracks, each of which becomes the source of rarefaction. The focusing or interference of rarefaction waves specifies the sites of nucleation and growth of channel and ring cracks; therefore, initially existing surface defects are not operative. Another consequence of the spall damage nature is the determining effect of the orientation of the lateral faces of flyer plates having finite dimensions, since these faces specify the intensity of the appearing lateral rarefaction wave.     

Modeling nonlinearities of ultrasonic waves for fatigue damage characterization: Theory,simulation, and experimental validation

A dedicated modeling technique for comprehending nonlinear characteristics of ultrasonic waves traversing in a fatigued medium was developed, based on a retrofitted constitutive relation of the medium by considering the nonlinearities originated from material, fatigue damage, as well as the “breathing” motion of fatigue cracks. Piezoelectric wafers, for exciting and acquiring ultrasonic waves, were integrated in the model. The extracted nonlinearities were calibrated by virtue of an acoustic nonlinearity parameter. The modeling technique was validated experimentally, and the results showed satisfactory consistency in between, both revealing: the developed modeling approach is able to faithfully simulate fatigue crack-incurred nonlinearities manifested in ultrasonic waves; a cumulative growth of the acoustic nonlinearity parameter with increasing wave propagation distance exists; such a parameter acquired via a sensing path is nonlinearly related to the offset distance from the fatigue crack to that sensing path; and neither the incidence angle of the probing wave nor the length of the sensing path impacts on the parameter significantly. This study has yielded a quantitative characterization strategy for fatigue cracks using embeddable piezoelectric sensor networks, facilitating deployment of structural health monitoring which is capable of identifying small-scale damage at an embryo stage and surveilling its growth continuously.     

金属杆内疲劳裂纹与振动/超声相互作用及其定量表征

根据同步解调的原理,考察振动声调制检测输出信号中的调制成分,对金属杆内振动/超声在裂纹面上的相互作用进行了分析.制作了含有不同尺寸裂纹的铝杆试样,以扫频激励的方式得到合适的高频激励参数,分析输出信号中的调制信息,发现工件中的高频超声可分为两部分:一部分声波通过裂纹面,受到低频振动信号的调制,另一部分则保持不变。在此基础上对非线性调制模型进行了修正,并提出了一种用于裂纹检测的定量方法。与超声C扫描检测结果对比表明,此定量方法可用于估计金属杆中疲劳裂纹尺寸。     

Interaction of fatigue crack with vibration and ultrasound in metallic rod and its use for quantitative characterization

According to the principle of synchronous demodulation,the modulation information in the output signal of vibro-acoustic modulation test was investigated and the nonlinear interaction of sound and vibration with the crack interface in a metallic rod was analyzed.A swept signal excitation was used to obtain a proper high frequency parameter in the test of aluminum rod samples with different sizes of cracks and the modulation information in the output signal was analyzed.It was found that the ultrasound wave in the structure can be separated into two parts.One is the wave which passes through the crack interface modulated by the low frequency vibration,and other wave has no any changes.The nonlinear modulation model was modified and a quantitative method for crack detection was proposed.It is demonstrated by comparing with the ultrasonic C-scanning that this method can be used to estimate the fatigue crack size in the metallic rod.     

Scales of metal fatigue cracking

A new paradigm is proposed for considering metal fatigue cracking based on the principles of synergetics and physical mesomechanics. Fatigue cracking is described as a three-stage process. Metal evolution is studied with stress growth from the micro- (ultrahigh cycle fatigue) to meso- (high cycle fatigue) and then macroscale (low cycle fatigue). The notion of two effective stress concentration factors on the metal surface and in its bulk is introduced; their variation pattern with stress growth is discussed. In the general case, the propagation of through-the-thickness cracks is shown to also occur in three stages—on the micro- (shear), meso- (rotation with the formation of triangular fatigue striations) and macroscale (rotation plus shear which lead to the formation of fatigue striations of complex shape), consecutively.     

Luminescence fatigue in amorphous solids

We have investigated the fatigue in low temperature luminescence caused by prolonged exposure to strongly absorbed light for hydrogenated amorphous silicon and amorphous As₂S₃. We have measured the c.w. and time resolved spectra, and luminescence decay curves for exposures up to 2 × 10⁵J cm^-2 for a-Si(H) and 8 × 10³J cm^-2 for a-As₂S₃. The similarities and differences in the fatiguing behavior of these two amorphous solids are discussed.     

Hardening and softening during fatigue fracture

Summary The comparison of the change of hardness and plastic deformation amplitude at a constant stress loading or stress amplitude at a constant deformation loading during the fatigue process shows some singularity of the hardening and softening effects. These effects were investigated on mean carbon and low-alloyed steel and on globular cast iron.The fatigue fractures at cycle numbers 10⁴÷10⁶ under stresses below the yield strength predominate in the softening process, which arises after an inconsiderable hardness increase extends in the region to 0·2 from the fracturing cycle number. Under the stresses above the yield strength, which in some cases for annealed and coarse-grained states are below the fatigue limit, the hardening process predominates, followed by a hardness increase in the field up to 0·25 and above the fracturing cycle number.At low cycle fatigue fractures with cycle numbers < 10⁴ depending on the cyclic plastic properties of steels the fatigue process can be followed by a continuous hardening or softening till fracture. This process is characterized by the change of the deformation amplitude and a one-sided accumulation of plastic deformations at a constant amplitude of active stresses. The one-sided accumulation of deformations commonly ends in a quasistatic failure. Under loading with a constant deformation amplitude during softening a fatigue fracture takes place as a result of damage accumulation under the alternating stresses with amplitudes decreasing with cycle number.     

Optical fibre strength/fatigue experiments

New techniques for measuring and analysing the strength characteristics of optical waveguide fibres have evolved as the strength of the optical fibres improved. A recent life-fatigue experiment is described and the results analysed in terms of the most commonly used theoretical model which results in the power law V = AKⁿ. Ten m long fibre samples were tensilely stressed in a high humidity environment by both fixed (static fatigue) and constant rate (dynamic fatigue) loads. The respective test values for the power law exponent, the fatigue constant ‘n’, were 38 and 17. The results of the tests indicate the more conservative value, 17, for practical engineering design, and the need for incorporating an ageing term in the model.     

Experimental fatigue performance of laser-formed components

Much knowledge has been gained with respect to the forming of sheet material by laser technology since its inception during the mid-1980s. However, many press-formed sheet metal components are subjected to cyclic loading conditions during their service life. It is from this standpoint that the work reported in this paper originates. This work deals with some aspects of the structural integrity of laser-formed plate samples that were formed to a radius of curvature of approximately 125 mm. Furthermore, a comparison is drawn between laser-formed and stock plate samples fatigue tested under reverse-bending cyclic conditions. The results indicate that the fatigue life of the laser-formed samples was substantially enhanced when compared to that of the stock plate samples. This observed improvement in fatigue life is attributed to the laser-hardening mechanism, i.e., the phase transformation and rate of cooling of the material from above the A₃ temperature, i.e. approximately 1200 °C. This aspect of improved fatigue life is considered to be beneficial to the motor manufacturing industry that could result in the possible use of thinner gauge material.     

Dynamical fatigue of BaTiO3 monocrystals

The effect of fatigue in BaTiO₃ monocrystals was studied. Observed were: high degree and rate of aging of some specimens in fields E=3 to 4 kV/cm under the application of a certain type of sequence of pulses; identity of the processes of formation, and of regeneration of fatigue-failed samples; series of changes generated by aging in the mechnism of switching; increase of permittivity and reduction of residual polarization determined from pyroelectric effect in fatigue-failed samples. For the explanation of the fatigue the necessity of taking into consideration the nonhomogeneity of properties of monocrystals across their thickness is assumed.300-th Anniversary of the Reunion of Ukraine with RussiaTranslated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 92–96, June, 1972.     

Grain boundary engineering: fatigue fracture

Abstract

Grain boundary engineering has revealed significant enhancement of material properties by modifying the populations and connectivity of different types of grain boundaries within the polycrystals. The character and connectivity of grain boundaries in polycrystalline microstructures control the corrosion and mechanical behaviour of materials. A comprehensive review of the previous researches has been carried out to understand this philosophy. Present research thoroughly explores the effect of total strain amplitude on phase transformation, fatigue fracture features, grain size, annealing twinning, different grain connectivity and grain boundary network after strain controlled low cycle fatigue deformation of austenitic stainless steel under ambient temperature. Electron backscatter diffraction technique has been used extensively to investigate the grain boundary characteristics and morphologies. The nominal variation of strain amplitude through cyclic plastic deformation is quantitatively demonstrated completely in connection with the grain boundary microstructure and fractographic features to reveal the mechanism of fatigue fracture of polycrystalline austenite. The extent of boundary modifications has been found to be a function of the number of applied loading cycles and strain amplitudes. It is also investigated that cyclic plasticity induced martensitic transformation strongly influences grain boundary characteristics and modifications of the material’s microstructure/microtexture as a function of strain amplitudes. The experimental results presented here suggest a path to grain boundary engineering during fatigue fracture of austenite polycrystals.     

A statistical model of fatigue failure incorporating effects of specimen size and load amplitude on fatigue life

Among many contributing factors, the load range, number of load cycles and specimen geometry (including configuration and size) are three major variables for fatigue failure. Most existing statistical fatigue models deal with only one or two of these three variables. According to the statistical distribution of microcracks with respect to their size and spatial location, a weakest-link probabilistic model for fatigue failure is established to incorporate the combined effect of load range, number of load cycles and specimen size. The model reveals a compound parameter of load range and number of load cycles reminiscent of the empirical formulae of fatigue stress-life curve and its correlation with another compound parameter of cumulative failure probability and specimen size. Four sets of published fatigue test data are adopted to validate the model.     

Mean stress effect in fatigue crack propagation

Crack propagation rates in three different grades of mild steel and two types of age hardening aluminium alloys have been measured for different stress ratios. The results show a pronounced stress ratio effect for all these materials. A model of fatigue crack propagation is formulated in terms of the size of the cyclic plastic instability zone at the crack tip rather than the zone of plastic yielding. The micro-plastic instability zone is measured by a parameter involving the ratio of the maximum stress intensity and the stress level at which macro-plastic instability occurs in the $\text{S}\text{N}$ curve of plain fatigue test pieces. Such a parameter provides a means of normalizing crack propagation results obtained for various stress ratios.     

Probability of fatigue failure in glass fibers

Abstract

Fracture mechanics provides the background for making long-term failure predictions to assure the mechanical reliability of glass fibers. The fatigue parameters necessary for making these predictions can be obtained from static and dynamic fatigue strength experiments. The failure calculations are best understood by expressing the predictions in terms of a design diagram. The probability of failure in service as well as the proof test stress necessary to ensure a minimum lifetime in service can be obtained from a design diagram.