Zheng Xiulin

Fatigue crack propagation in steels

Keywords: 108/ICOM Reference ICOM-ARTICLE-1983-005doi:10.1016/0013-7944(83)90070-XView record in Web of Science Record created on 2008-01-24, modified on 2016-08-08

Vanadium dioxide films with good electrical switching property

A novel method is described for preparing high-quality thin films which exhibit an abrupt resistivity change of 4 - 5 orders of magnitude at about ; this is the best result among those so far prepared on non-crystal substrates. To show the difference from the ordinary sol - gel method which uses vanadium alkoxide as precursor, this method has been called (by the authors) the inorganic sol - gel method, for it uses powder as precursor. This newly developed method shows some advantages in that it produces high-quality thin films (adherent, crack-free, and with good switching properties) in a cheap, simple, and reproducible way. The method consists of four processes: quenching, coating, drying and vacuum heat treatment. These processes show strong effects on the properties of the final films. In this paper, the effects of the drying process on the film blistering and the effects of film thickness, quenching temperature, and heat treatment time on the resistivity switching property were studied, and DSC, XRD, ESCA, and SEM were utilized to study the drying process and characterize the films. It was found that when the drying temperature was and heating rate was in the drying process, the final films would be blister-free and crack-free (on non-crystal substrates). It was also found that the magnitude of resistivity change at the phase transition of the samples treated at for 10 h under a pressure of 0.8 Pa increased with increasing thickness (in the thickness range 0.4 to ). When the film thickness was about , the magnitude of resistivity change of the samples treated under the same conditions was found to decrease with increasing quenching temperature. In a certain range of treatment time the magnitude of resistivity change of the samples treated at under a pressure of 0.8 Pa was found to increase with increasing heat treatment time.

A further study on fatigue crack initiation life — mechanical model for fatigue crack initiation

Abstract In the present study, a new formula for the fatigue crack initation life is developed based on recent progress in the study of fatigue damage and crack initiation, and is substantiated experimentally. The new formula reveals a correlation between the fatigue crack initiation life, the notched element geometry, the cyclic loading condition, the tensile properties and the fatigue crack initiation threshold, which is indicated as an important parameter in describing the fatigue damage and crack initiation. The correlation between the fatigue crack initiation threshold, the endurance limit and the tensile properties is also given. The threshold for fatigue crack initiation can be obtained by the regression analysis of the fatigue crack initiation life test data without any additional test.

Solid-state reaction of Al/CuO couple by high-energy ball milling

Abstract The solid state reaction of Al/CuO induced by high-energy ball milling was studied systematically. When the amount of Al was 20 wt%, only reduction occurred. When the amount of Al exceeded 20 wt%, along with the reduction, a synthesis reaction occurred simultaneously. As the amount of Al increased, the reaction products were Cu9Al4, CuAl2 or a Al(Cu) solid solution, respectively. The mechanism of these two types of reactions occurring during ball milling is discussed.

Predicting fatigue crack growth rates and thresholds at low temperatures

In this paper an attempt was made to offer a new method for predicting fatigue crack growth (FCG) rates and thresholds of metals in low temperature structural applications. The experimental results available have indicated that a decrease in temperature can influence the FCG mechanism, as well as the FCG rates, and the fatigue ductile-brittle transition (FDBT) may occur in some metals. Accordingly, metals can be divided into two groups, i.e. alloys without an FDBT and those with an FDBT. In the former, the fracture mode of FCG is ductile transgranular for temperatures down to 4 K and the crack growth rates in the intermediate region (da/dN = 10−8−10−6 m cycle−1) depend mainly upon Youngs modulus. In the latter, the FCG behaviour above the FDBT temperature is similar to that of the alloys without an FDBT. When the temperature is below the FDBT temperature, the FCG rates (especially at higher ΔK levels) will be enhanced by the brittle transition of the FCG mechanism and both the strength and the ductility of the alloys will have significant effects on the FCG rates. The expression previously proposed by one of the present authors and a coworker for FCG rates not only explains the low temperature FCG behaviour of the metals mentioned above but also predicts the low temperature FCG rates in both the near-threshold region and the intermediate region. Finally, a new method to predict ΔKth at low temperatures is tentatively proposed based on the stratic fracture model.

Effects of applied bias voltage on the properties of a-C:H films

Abstract The effects of the applied negative bias voltage on the hydrogen content, refractive index, extinction coefficient, internal stress, Vickers microhardness, adhesion and growth rate of a-C:H films deposited from a C 2 H 2 + Ar mixture by a d.c.-r.f. plasma-enhanced chemical vapour deposition process have been investigated. The results showed that the properties of a-C:H films strongly depend on the applied bias voltage. In the investigated bias range (−400 to −1000 V), it was found that, apart from the growth rate and extinction coefficient (which were found to increase with increasing bias), all the other properties studied showed a maximum at around −800 to −900 V. Compared with those studies reported previously on self-bias voltage effects, these results indicated a slight difference between the effects of the self-bias voltage and applied bias voltage. The experimental results are discussed.

Fatigue tests and life prediction of 16 Mn steel butt welds without crack-like defect

Fatigue test results of 16 Mn steel butt welds without crack-like defect under both constant and variable amplitude loads are reported and new procedures are used to predict fatigue crack initiation (FCI) life, fatigue crack propagation (FCP) life and total life of the butt welds. The results indicate that the FCI life and FCP life should be calculated separately and the total life is the sum of the FCI life and FCP life. For the butt welds investigated, stress cycles to initiate a crack of engineering size may occupy more than 70 percent of the total life of the butt welds and it is more suitable to express the total life as a power function of the equivalent stress amplitude {ie275-1}. In predicting the FCI life, the expression of FCI life obtained from the test results of notched specimens is used but the effects of microstructure, surface condition, macro- and micro-geometrical discontinuities at weld toe should be taken into account. In predicting the FCP life, the formula developed by Zheng and Hirt is used and the stress ratio is taken as 0.6 to account for the residual stresses effect on the FCP rate. Because overload produced by the maximum load in a load spectrum has no effect on the FCI life of 16 Mn steel and weldment of the steel, according to the procedures outlined in the paper, one can use the FCI life expression mentioned and the linear damage accumulation rule proposed by Miner to predict the FCI life of 16 Mn steel butt welds under variable amplitude loads. A good agreement is achieved between the predicted results and the test data.

On the corrosion fatigue crack initiation model and expression of metallic notched elements

Abstract In the present study, attempts are made to extend the application of the mechanical model for the fatigue crack initiation (FCI) and the FCI life formula of metallic notched elements in laboratory air to those in the corrosive environment. The test results and analysis of the corrosion FCI (CFCI) life of aluminum alloys and Ti6A14V show that the expression of the CFCI life obtained by modifying the FCI life formula in laboratory air can give a good fit to the test results of the CFCI life. The salt water (3.5% NaCl) environment has no effects on the CFCI resistant coefficient compared with the FCI resistant coefficient in laboratory air. However, 3.5% NaCl environment greatly decreases the CFCI threshold of aluminum alloy, but has little effect on the CFCI threshold of Ti6A14V. The loading frequency ranging from 1 Hz to 10 Hz has no appreciable effect on the CFCI life, and thus, the CFCI threshold of aluminum alloys investigated. Hence, the expression for the CFCI life of metallic notched elements proposed in this study is a better one, which reveals a correlation between the CFCI life and the governing parameters, such as, the geometry of the notched elements, the nominal stress range, the stress ratio, the tensile properties and the CFCI threshold. However, this new expression of the CFCI life needs to be verified by more test results.

An expression for fatigue life and probability distribution of fatigue strength under repeated torsion

Abstract An expression for the fatigue life under repeated torsion (torsion fatigue) is developed and checked by using the test results of metals in the available literature. The shear fatigue strength coefficient, A ′, and the theoretical shear fatigue limit, τ ac , in the expression mentioned above are the material constants. The statistical analysis of the data of A ′ and τ ac obtained from the regression analysis of test results shows that A ′ and τ ac follow the log-normal distribution and that τ ac also follows the normal distribution. Using the expression for the torsion fatigue life developed in this study and the distribution of A ′ and τ ac , one can easily obtain the expressions for the torsion fatigue life with given survivability, which are in good agreement with test results. The probability distribution of the shear fatigue strength can also be obtained from the above-mentioned expression and the distribution of A ′ and τ ac . It is shown that the shear fatigue strength follows a log-normal distribution in the medium and long-life region. Therefore, the log-normal distribution of the shear fatigue strength can be applied in the fatigue reliability assessment of elements subjected to repeated torsion.

Effects of microstructure on fatigue crack initiation and propagation of 16Mn steel

In the present study, the effect of microstructure of 16Mn steel on fatigue crack initiation (FCI) life and fatigue crack propagation (FCP) rates was experimentally investigated under two different conditions,i.e., as-received condition and high-temperature normalized (H.T.N.) condition. The microstructure of 16Mn steel under the as-received condition is ferrite and pearlite, which corresponds to that of the base metal of welded elements, and the microstructure under the H.T.N. condition is mainly coarse Widmanstätten structure, which can be thought of as the simulated microstructure at the weld toe. The fatigue test results show that the high-temperature normalization results in the increase of FCP rates in near-threshold region and the decrease of both FCI and FCP thresholds, and FCI life of 16Mn steel. Little effect of the microstructure is observed on the FCP mechanism in the intermediate range (da/dN=10−8 to 10−6 m/cycle). Based on the test results and analysis, the general expressions are given for both FCI life and FCP rates under the two conditions. It is pointed out that which of the test results should be applied to prediction of FCI life and FCP life depends upon the FCI location and FCP path in the welded elements.