Papers by Author: J.Z. Liu

Abstract: Analytical studies were made on effect of size and location of a weld defect on fatigue life for argon-arc welded titanium alloy joint. In the analyses, a weld defect was assumed as an initial crack, and the crack growth life was taken as total fatigue life. By using the Isida and Noguchi’s stress intensity factor solution for a plate containing an embedded elliptical subsurface crack under tension, the life prediction code FASTRAN3.9 was revised. A small crack methodology based on the plasticity-induced crack-closure concept and the effective stress intensity factor range,
Keff , was used to predict the total fatigue life of welded joint, and to study the effect of the size and location of weld defect on fatigue life by means of the revised FASTRAN3.9 code. Limited amounts of experimental data were used to make comparison with the predictions. The predicted fatigue lives are in reasonable agreement with experiments, and the effect of both the size and location of the weld defect on fatigue life was found to be significant.

Abstract: Experimental and analytical studies were made on the fatigue behavior and life prediction for argon-arc welded titanium alloy joints, TA15. High cycle fatigue tests at two stress ratios, R=0.5 and 0.06, were carried out on smooth specimens with the argon-arc weld joint located at the specimen center section. Through macroscopic observation and SEM fractographic analysis, it was found that most of the cracks were initiated at weld defects such as voids and inclusions at the edge of weld and in the heat affected zone (HAZ). A small crack methodology based on the plasticity-induced crack-closure concept and the effective stress intensity factor range, ΔKeff , was used to predict the total fatigue life of the weld joints. Large crack growth curve for cracks in the HAZ area was employed as the da/dN-ΔKeff base-line of the TA15 alloy. From fractographic measurements, an average defect size of 100 microns was assumed as the initial small crack size in the life predictions. Predicted total fatigue life by solely considering small crack growth stage agreed well with the experimental data.

Abstract: Bone marrow mesenchymal stem cells (BMSCs) possess a high replicative capacity and have the capacity to differentiate into various connective tissue cell types. With the advance in cell culture technique, the BMSCs have been induced to differentiate to osteoblastics linage. To improve the situation of non-union in posterior spinal fusion (PSF), tissue engineering approach to combine BMSCs supported by the calcium phosphate ceramics was applied in PSF and its effect was investigated in the present study. Autologous BMSCs from 16-week-old rabbit tibiae were expanded and induced to differentiate into osteoblastic cells with defined medium and osteogenic supplement. Calcium phosphate ceramic (CPC) was used as the scaffold to deliver the cells to the standardized rabbit posterior spinal fusion model. The assessment of bone mineral and fusion mass volume was conducted at week 7 post-operation with quantitative computed tomography and micro-computed tomography. When compared with control, the composite of BMSCs with CPC enhanced the fusion mass volume by 40% (p<0.05) and bone mineral content in the CPC was 7% (p=0.05) higher. Our study showed that additional BMSCs at the fusion site of PSF did provide extra resource for new bone formation and enhanced the fusion rate.

Abstract: Research on low cyclic fatigue behaviors of two kinds of Zr-alloy(1#:Zr-1Sn-0.3Nb- 0.3Fe-0.1Cr.2#: Zr-1Sn-1Nb-0.4Fe-)at 400°C,The fractural characterization was investigated by SEM.