Papers by Author: Qing Yuan Wang

Abstract: Based on the design of specimen with standard hourglass shape, a modified dog-bone-shaped ultrasonic vibrational fatigue specimen was proposed and studied in this paper. The resonance length and the distribution functions of axial displacement and stress of such specimen were obtained analytically. A comparison of the very high cycle fatigue (VHCF) life of 6061 aluminium alloy with dog-bone-shaped specimen and hourglass-shaped specimen was done and it indicate that the forer is obviously lower than the latter and the data obtained by specimen with proposed shape are conservative. Then, the ultrasonic vibrational fatigue test of 6063 aluminium alloy specimen with dog-bone-shape was done, showing that the fatigue failure occured at the middle of the specimen with invariable section where has predicted maximum stress; the S-N curve descends continuously and there is no fatigue limit as the traditional fatigue conception describes; the fatigue crack grows in the form of shearing.

Abstract: Multiple interacting crack problems for 3-point bending specimen were studied in this article. Two symmetrical minor cracks were placed in the structure, besides a main crack at the middle, and using the finite element method program ABAQUS, the energy release rate (G) and the stress intensity factor (SIF) were evaluated based on the virtual crack closure technology（VCCT）in conjunction with finite element analysis(FEA). Then, effects of variation in relative lengths and locations of the minor cracks on the stress intensity factors of the main crack were obtained and analyzed. Finally, the approach was applied to dynamic analysis, and influences of interacting effects among the cracks on dynamic fracture parameters were also studied.

Abstract: The stress intensity factor (SIF) for tubular specimens were calculated based on virtual crack closure technique （VCCT）. The effects of geomet rical factors (inner radius, wall thickness and relative crack length) and external loads on the SIF were analyzed, respectively, by use of the single-variable approach. Finally, an approximate formula for SIF of compact-tensile tubular specimens was obtained as all factors mentioned were considered, which was presented as a reference for the tubular engineering design.

Abstract: In order to investigate the influence of grain size on notch sensitivities in fatigue of a fine-grained carbon steel, rotating bending fatigue tests were carried out using specimens with a V-grooved circumferential notch of commercial fine-grained carbon steel with grain size of 6.5µm. The results were compared with those of a larger grain sized carbon steel (grain size: 20 µm) and the notch sensitivities were evaluated based on Linear notch mechanics proposed by Nisitani. Notch sensitivities for both of fatigue limits for a crack initiation and its propagation of the fine grained steel were high. The results were discussed from the view points of the size of area related to crack initiation and the crack growth resistance.

Abstract: Rotating bending fatigue tests were carried out for maraging steels with different grain size in moist air in order to investigate the effects of humidity, grain size and reversion austenite on fatigue strength of the steel. Fatigue strength was decreased by humidity, and the decrease in fatigue strength was large in larger grain sized steel. Both of the initiation and propagation of a crack were promoted by humidity. Many cracks initiated at the specimen surface and intergranular cracks were observed at specimen surface and on fracture surface when humidity was high. The decrease in fatigue strength by humidity was suppressed by formation of reversion austenite.

Abstract: The importance of determining and understanding the very high cycle fatigue behaviors of materials has gained strength in recent years. Ti-alloys, in view of their high strength-to-weight ratio, have a range of structural applications. Of these, Ti-6Al-4V, belonging to the alpha-beta type is the most widely used. The present paper deals with investigations on the fatigue behavior of TC4, the Chinese equivalent to Ti-6Al-4V, up to very high cycles. Fatigue testing was carried out on a piezoelectric ultrasonic fatigue machine operating at 20 kHz frequency. Hourglass shaped resonant specimens were tested in ambient air at room temperature under completely reversed loading conditions (R = -1). Failure in the alloy was seen to occur right up to the gigacycle regime, with the fractures being found to initiate from the surface unlike in steels. The fracture surfaces exhibit brittle characteristics containing river patterns and cleavage facets, as well as striations.

Abstract: Very high cycle fatigue (VHCF) properties of high-pressure die cast Magnesium alloy AZ91HP have been investigated. Ultrasonic fatigue tests up to 109 cycles were conducted at the loading frequency of 20 kHz, under R=-1 condition and in ambient air. The experimental results show that specimens fail even after 107 cycles although the scatter seems to be large probably due to the presence of materials defects. However, there seems to be a fatigue limit at about 109 cycles. The fractures contain typical brittle features, with the fatigue cracks seen to initiate from the porosity in the material, either from the surface or beneath.

Abstract: In order to investigate the influence of loading frequency on the fatigue behaviors of the high strength steel, ultrasonic fatigue tests were carried out for a high-carbon-chromium steel and the results were compared with those of fatigue tests using conventional rotary bending fatigue test machine with a frequency of 52.5Hz. The different of fatigue strength at ultrasonic frequency level and conventional frequency level is very small and the S-N curve obtained from 20 kHz or 52.5 Hz shows the step-wise shape. The fatigue crack occurred from inclusions on the subsurface site in the long life regime and the typical surface fracture occurred in the short life one though the loading frequency level is different. It is indicated that ultrasonic fatigue method is an effective method to investigate the fatigue properties in super-long life region.

Abstract: Fatigue properties of Inconel 718 at 500°C were investigated under rotating bending. Fatigue strength was higher at 500°C than at room temperature. Fracture occurred by the initiation and propagation of a surface crack in short life region. The early propagation of a surface crack smaller than 20-30 μm was suppressed by oxidation at 500°C. This is a main reason for the higher fatigue strength at 500°C. However, the crack growth after the retardation of a small crack growth at 500°C was accelerated reflecting the lower static strength. On the other hand, in long life region, though a surface crack initiated at the early stage of fatigue life similar to the case in short life region, the crack did not propagate over 20-30 μm, and an internal fracture occurred. Consequently S-N curve at 500°C showed a duplex property and the fatigue limit was not recognized. The subsurface damage was initiated at the early stage of fatigue life.

Abstract: The stress-strain behavior of cast magnesium alloy (AM60) was investigated by strain-controlled cyclic testing carried out on MTS. In order to describe the cyclic stress and strain properties of AM60 by means of the energy storing characteristics of microstructure during irreversible deformation, a plastic constitutive model with no yielding surface was developed for single crystal by adopting a spring-dashpot mechanical system. Plastic dashpots reflecting the material transient response were introduced to describe the plasticity of slip systems. By utilizing the KBW self-consistent theory, a polycrystalline plastic constitutive model for Magnesium alloy was formed. The numerical analysis in the corresponding algorithm is greatly simplified as no process of searching for the activation of the slip systems and slip directions is required. The cyclic stress-strain behavior, based on this model, is discussed. The simulation results show good agreement with the experimental data for AM60.