Papers by Author: Yong Xiang Zhao

Abstract: Fracture surface observations and statistical deriving are applied for the present investigation on the random fatigue crack growth rates of China grade B cast steel wheel. Results reveal that: the crack grows with relative regular initial tip companied fabric like stripes along the growth direction but few of fatigue striations, and then, cleavage flowers appearing on the district of transient fracture with few of dimples. It indicates that sharp notched structural crack behaviour acts as a few of plastic dissipation process. Non-linear statistical modeling covering entire three pieces of fatigue crack growth is constructed for describing the random crack growth rates. Scattered randomness of the growth rates has been well characterized.

Abstract: Fracture surface observations and statistical deriving are applied for investigating the random critical fracture toughness values of China grade B cast steel wheel. Results reveal that: the crack grows to show fabric like stripes along the growth direction with few of dimples. Cleavage flowers appear under higher magnification. Cracked structural damage process is verified with few of plastic dissipation. At the same time, code based evaluated results indicate that significant scatter exists for the toughness values. Lognormal modeling is constructed appropriately with a comprehensive statistical comparison method. It is verified that random characters and quantitative measurements have been well depicted for the present critical fracture toughness values.

Abstract: Fracture surface observations and statistical deriving are applied experimentally for the present investigation on random fatigue cracking thresholds of China grade B cast steel wheel. Results reveal that: the crack grows with relative regular initial tip companied fabric like stripes along the growth direction but few of fatigue striations and then, cleavage flowers appearing on the district of transient fracture with few of dimples. This indicates that sharp notched structures may be subjected to a little plastic dissipation process. In the same time, non-linear statistical modeling is constructed for describing the random crack growth and then, random fatigue cracking thresholds are measured at specified growth rate. The measurements hold a same growth rate which can not be obtained directly from test results.

Abstract: Synthetic method is studied on assessing scale and surface machined effect on fatigue lives of China railway grade B cast steel wheel. Three kinds of specimens are applied for revealing the non-linear effect with scale kinetics at a least of cost. And 8 to 9 samples are applied for each kind of specimen to reveal the random fatigue behaviour. Assessing work is carried out on a phase of fatigue lives with scale kinetics at a specified strength. Results reveal that fatigue cracks were initiated from the specimen surface having a bigger site and the fatigue damage process companied with less plastic deformation than the material specimens. Random effects on fatigue lives and effect factors are constructed for measuring effects. Statistical character of the test data has been well described.

Abstract: Four grouped rectangle specimens with thirty four samples, plus eighteen samples for fatigue limits, are tested for constructing the fatigue reliability curves of China grade B cast steel wheel. It reveals that fatigue cracks commonly initiated from the surface weakest phases and grew with typical fatigue striations and sub-cracks perpendicular to the growth direction. Cyclic plastic seems to be strengthened as cyclic stressing level increasing. Fatigue reliability curves are constructed on a consideration of addressing the real super long life. They are evaluated by a concurrent probability extrapolation method. And the measured curves verified that appropriate description has been reached on the test data.

Abstract: Random fatigue limits are experimentally studied on China grade B cast steel wheel. Test data were obtained by employing a so-called small sampling up-and-down test method. Results reveal that fatigue cracks of the present material were commonly initiated from the casting flaw of shrinkage cavities and grew with typical fatigue river-like flowers and striations. To address the scattered test data, randomness of fatigue limits are comparably measured by the existent four approaches, i.e. conventional, Dixon-Mood, Zhang-Kececioglu, and Zhao-Yang approaches. It is verified that Zhao-Yang approach is appropriately applied for the present assessment on random fatigue limits of China grade B cast steel wheel.

Abstract: Cyclic deformation behaviour is experimentally investigated using an incremental step method under a spectrum straining block loading mode for China grade B cast steel wheel. Results reveal that the material acts cyclically as non-Masing behaviour with a Bausinger effect. Different from monotonic tensile situation, material under fatigue loads was subjected to a plastic dissipation process without a yielding sign. Fatigued areas of specimen fracture surfaces are around 35%. Cyclic stress-strain (CSS) curve exhibits constant lower than monotonic tensile curve. Difference is enlarged as strain level increasing in elastic regime of the monotonic deformation. And this difference sustains a level of around 100 MPa as over yielding point. In addition, randomness is also observed for the present cyclic deformations. It indicates that random CSS relations should be comprehensively applied even if under stress based fatigue analysis.

Abstract: Random mechanical properties are experimentally studied on China grade B cast steel wheel. Significant dimples and fabric-like tearing signs were observed firstly and then, lotus flower-like cleavage fracture faces were appeared to indicate the material subjected to a plastic dissipation process under monotonic load. Randomness appears in the deformation process companied with a similarity in total and significant differences in local. Appropriate statistical description is explored with a comprehensive statistical comparison method. Lognormal distribution is excavated out by comparison from seven possible statistical models. Random characters and quantitative measurements have been given for the mechanical properties of China grade B cast steel wheel.

Abstract: Fatigue S-N relations are investigated experimentally on the grade B cast steel for Chinese railway freight car bogie frames. Group test method was employed. Results reveal that the fatigue cracks were initiated from the material cast shrinking cavities with hot ties. Distinct fatigue striations, river-like flowers, and second cracks appeared on the fracture surface in perpendicular to the fatigue crack path. Lots of dimples in the transient fracture district indicate that present material is ductile. Considering both the scattered S-N relations and no fatigue limits under variable loads, the S-N relations are measured probabilistically in terms of continuous declined probabilistic S-N curves including super-long life regime (SLLR) of >107 cycles. Affordable novel approach, i.e. concurrent probability extrapolation method, was applied. The test S-N data in mid-long life regime (MLLR) and conventional fatigue limits are used. The curves are established by extrapolating the conventional S-N curves crossing the fatigue limits, in a concurrent probability manner, into the SLLR. Appropriate measurements have been given for the test data of the present material.

Abstract: Fatigue limit measurement is investigated experimentally on the grade B cast steel for Chinese railway freight car bogie frames. Small sample up-and-down test method was employed for the present study. Results reveal that fatigue cracks initiated mostly from the material cast shrinking cavities. Distinct river-like flowers and second cracks appeared on fracture surface in perpendicular to fatigue crack path. Lots of dimples are distributed in transient fracture district to indicate that present material is ductile. Maximum likelihood approach is applied for measuring the probabilistic fatigue limits, in which the limits are defined as the fatigue strengths following normal distribution at an expected fatigue life. Statistical parameters are then estimated by a likelihood function method. A comparison analysis to the existent conventional, Dixon-Mood and Zhang-Kececioglu approaches indicates that the maximum likelihood approach is the approach meeting the definition.