Papers by Author: Yong Xiang Zhao

Abstract: Measurement on fatigue cracking thresholds is investigated to the grade B cast steel for China railway rolling wagon bogie frames. Compact tensional specimens are fatigued under a continuous decreasing loading mode for present study. Results reveal that the minimum test values have different crack growth rates without meeting the code-based rate requirement. Therefore, the test results can not be directly used for measuring the thresholds. A probabilistic modeling in the form of a non-linear law is developed for reflecting the scattered test data. The thresholds at specified reliability levels are then derived in meeting the threshold definition with a specified growth rate. The measurement for the present material has verified the availability of the present modeling.

Abstract: Compact tensional specimens are fatigued for modeling the fatigue crack growth rates of the cast steel for Chinese railway rolling wagon bogie frames. Typical fracture surface observations indicate that the fatigue cracks grew with distinct fatigue striations, river-like flowers, and second cracks in perpendicular to the fatigue crack path. Lots of dimples appeared in the transient fracture district to indicate that present material is ductile. Similar to the previous NASGRO’s exploration, a full modeling is proposed from fatigue cracking threshold to the transient fracture corresponding to the material fracture roughness. Availability has been verified to be applied for the present material. In addition, this modeling is very simple that the modeling can be performed by the conventional tests of fatigue cracking thresholds, growth rates, and fracture roughness values.

Abstract: Measurement on fracture roughness values is investigated to the grade B cast steel for China railway rolling wagon bogie frames. Due to be limited by production bogie frame geometry, a ductile fracture roughness measuring method is employed for the present study. Experiments reveal that, when crack growth increments are very scattered, the code based measurement of 0.2 mm crack growth increment for gauge ductile fracture roughness may be not reasonable. And a measurement of 90 % maximum ductile fracture roughness should be suggested to be an appropriate measurement. The new suggested 90 % maximum fracture roughness measurement is verified to be effective for the fracture roughness values of the present material.

Abstract: Cyclic deformation characters and description method are investigated to the grade B cast steel for Chinese railway rolling wagon bogie frames. Incremental step test method with 8 strain amplitude loading mode were employed. Results reveal that the material acts as non-Masing behaviour with a significant Bausinger’s effect and appears a slight cyclic softening. Great of dimples in the transient fracture district indicate that present material is ductile. Significant discrepancy between the monotonic and cyclic beaviour was revealed to indicate that there is different monotonic and cyclic deformation behaviour. The scattered cyclic stress-strain relations indicate that appropriate description for the cyclic deformation of present material should be a probabilistic modeling. And then, a probabilistic modeling with measurements of survival probability and confidence is developed to give a good description to the cyclic deformation behaviour.

Abstract: Chemical composition, metallurgical photos, macro- and micro-hardness values, macrostructures, inclusions, denseness, mechanical properties are experimentally investigated on the grade B cast steel for the bogie frames of Chinese railway rolling wagons. Results reveal that the material consists of ferrites and pearlites. Micro-hardness values of the ferrites are significant lower those of pearlites to indicate the ferrites is the weakest phase of material. Casting flaws are mainly point-network like eutectic sulfur compounds and polygonal sphere like Al2S3 compounds. Lower Young's modulus, higher proportion of yielding to ultimate strengths, higher reduction of area, and a lower elongation than those of conventional ferrous alloys are exhibited. These characters are contributed to the non-sharp cast flaws. The flaws play roles of two effects. One is to benefit anti-vibration capacity and fracture roughness. And another is to decrease the mechanical strengths.

Abstract: Damage modes, causes, and effects of railway wheel set are investigated from wide production fields. And then, the reliability strategies are classified as service reliability and development reliability. The development one includes design, material and manufacturing, quality approval/authentication, which is a basis of the service one. The service one is the true product quality, which has not only the basis of development reliability but also the effects of non-destructive monitoring level, damage analysis and assessment capacity, and repairing and renewing policies. Reliability of wheel set is so important that a series of code-based regularities have been issued by professional societies or associations. Optimal design is a most effective way to improve wheel set reliability. And in service, non-destructive inspection and repair opportunities are valuable to be synthetically programmed. The critical damage defect/flaw sizes should be scientifically determined. Three advances in strength reliability field should be introduced. They are affordable and appropriate method for reliability assessment and fatigue life prediction including the super long life regime, no-Paris type crack growth models which include crack initiation, stable growth, and approach fracture, and fatigue S-N relation derived from short cracking growth behaviour.

Abstract: Surface rolling effect on effective short fatigue cracks density, which reflect the affecting capacity on the initiation firstly and then growth of the dominant short crack result finally in specimen failure, is experimentally studied by a replica technique. Two groups of smooth hourglass shaped specimens of LZ50 axle steel with/without rolled surfaces were tested. The crack density of surface rolled specimens was much lower than that of the other group. This indicates surface rolling technology having the effect of hardening surface layer material to introduce compressive residual stresses. The effect appears to restrain the short crack nucleation and propagation and then, to extend the fatigue life.

Abstract: Scale-induced effect is investigated on the fatigue properties to the grade B cast steel for China railway rolling wagon bogie frame. Three kinds of structure-like specimens with production structural surfaces and different scales were fatigued in a manner of group tests at same stressing amplitude. Their fatigue properties are compared with those of material specimens. Fracture surface observations reveal that the fatigue cracks were initiated from the material casting shrinking cavities with/or hot ties. Distinct fatigue striations, river-like flowers, and second cracks appeared in perpendicular to the fatigue crack path. Lots of dimples appeared in the transient fracture zone to indicate a ductile fracture. Slight deference was observed that more dimples are for material specimen. To consider the scattered fatigue properties, effective sectional area-fatigue life relations are established for the structure-like specimens. And then, a probabilistic model is established for the scattered effect factors on fatigue lives. An effective application is given for constructing structural probabilistic S-N relations including super-long life regime from material S-N curves.

Abstract: , was experimentally investigated by a replica technique to seven smooth hourglass shaped specimens for railway LZ50 axle steel. Character of two-stages, i.e. the micro-structural short crack (MSC) stage and the physical short crack (PSC) stage, was revealed for the crack initiation and growth. Most importantly, the crack growth rate exhibited decelerations twice in MSC stage. This behavior was corresponding to the ferrite grain boundary firstly and then to the pearlite banded structure. The boundary appeared a barrier because there were pearlites around with significant higher micro-hardness values. The banded structure appeared a barrier because each band was rich in hard layered pearlites for the crack to cross. In PSC stage, the crack propagated with a decreasing resistance of micro-structural barriers as the crack length increased. The two barriers are inherent in the material and the crack initiation and growth are subjected to an evolutionary process under competition between the inherent resistances from the barriers and the increasing driving force from the growing crack size. This provides a prehensive understanding of the crack initiation and growth.

Abstract: For an engineering structure with an actual fatigue life over that corresponding to a so-called fatigue limit, appropriate reliability assessment and fatigue life prediction are essential for developing the structure and sustaining its high quality in service. Basic clues are explored. A competition fatigue initial mechanism is shown to provide a requirement of material primary quality management. Affordable deduced material and structural probabilistic S-N curves are presented by fitting into material mid-and-long life S-N data and fatigue limits and, then, comparing to structural fatigue limits. Random cyclic stress-strain relations are depicted for constructing random stressing history of structures. Reliability assessment and fatigue life prediction are established to synthetically consider the interference of applied stresses deduced from the random cyclic stress-strain relations and capacity strengths derived from the structural S-N relations with an expected life. Affordable and appropriate method has been then developed to realize the reliability assessment and fatigue life prediction including the super long life regime. Availability of the present method has been indicated through a reliability analysis to the velocity related reliabilities and fatigue lives of a railway axle.