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The development of two multiaxial ductility factor predicting models based on creep cavity growth theory

Wu Dongquan, Li Yupeng, Liu Zixiang, Li Dinghe

Journal of Theoretical and Applied Mechanics

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2023

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Vol. 61 nr 3

481--494

EN

In this study, the multiaxial ductility factor was analyzed based on the power-law creep grain-boundary cavities growth theory under multiaxial stress states. Based on this theory, the theoretical cavities growth rates under a multiaxial stress state were discussed and the predicting model of a stress-state parameter α was revised by using an empirical fitting expression denoted as αWu, which exhibited a good agreement to analytical results of the stress-state parameter α and multiaxial cavities growth rates. Then, according to the re- lationship between uniaxial and multiaxial creep failure strain, a new empirical predicting model of multiaxial ductility factor MDFWu was established which involved the multiax- ial parameter αWu and uniaxial parameter α0. Besides, the theoretical model of multiaxial ductility factor MDF could also be established. By fitting the theoretical values of MDF, an- other predicting model MDFWM was proposed. The development of two multiaxial ductility factor predicting models could be achieved. Finally, predictions of these two novel multiax- ial ductility factor models and the Cocks-Ashby as well as Wen-Tu model were compared with experimental data, and the prediction accuracy of MDFWu and MDFWM models was significantly improved, especially for the latter one.

2

Study on the stress intensity factor of a compact specimen under the pre-compressed load condition

Wu Dongquan, Liu Zixiang, Li Dinghe, Zhang Zhiqiang, Chen Jianguo

Journal of Theoretical and Applied Mechanics

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2023

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Vol. 61 nr 1

37--47

EN

Structural components are often operated under combined stress conditions (primary and secondary stresses), but the stress levels generated by residual stress (or secondary stress) is hardly ever evaluated. Hence, stress intensity factors at the crack tips of a compact tension (CT) specimen under a pre-compressed load condition are analyzed using the finite element method. Then, the average residual stress intensity factor is calculated and analyzed. As the crack length α0/W increases, the average residual stresses σave/σ0 grows under the same pre-compression load. σave/σ0 increases rapidly at a low range of the pre-compression load but tends to a constant in a high range of the load. The distribution of the average residual stress intensity factors Kave and Κave/σ0 of the CT specimen with same crack length under different pre-compression loads have the same tendency. Additionally, the distribution of Κave and KFEM under different pre-compression loads are also similar. Nevertheless, Kave estimated by the average residual stress is too conservative and not accurate, and the method is complex, which depends on the analysis of simulation. Therefore, a simple method for calculating Mode I stress intensity factor K for this model is presented. A group of examples is presented to verify the accuracy of the method.

3

Study on constraint effect and creep crack initiation of plate containing elliptical embedded cracks

Wu Dongquan, Liu Zixiang, Li Yupeng, Li Dinghe, Zhang Zhiqiang

Journal of Theoretical and Applied Mechanics

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2023

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Vol. 61 nr 3

545--557

EN

In this study, a plate structure containing elliptical embedded cracks loaded under high temperature is studied. The constraint effect and creep crack initiation of the plate con- taining embedded cracks are discussed by using the finite element method based on the creep ductility exhaustion model. It is indicated that the highest constraint level or the load-independent parameter Q* is observed at the endpoint of the ellipse major axis of an elliptical embedded crack, and the constraint levels increase with crack length or depth, which represents a worse condition for the structure, such as higher stress concentration and greater danger of failure for a larger crack depth ratio a/t or crack length ratio a/c. Moreover, under the creep condition, the embedded crack with a larger a/t, a/c or loadings is accompanied with a higher crack driving force, which can accelerate creep damage, creep cracking initiation (CCI) and shorten the creep crack initiation (CCI) time. Additionally, an empirical prediction equation and engineering approach to the constraint parameter and the CCI time for elliptical embedded cracks are proposed, and the engineering approach to the CCI time is validated.

4

Numerical study on compression properties of semi-reentrant filled tubular structures

Wu Dongquan, Li Dinghe, Zhang Zhiqiang, Chen Jianguo

Journal of Theoretical and Applied Mechanics

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2023

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Vol. 61 nr 2

233--244

EN

In this study, a semi-reentrant structure (SR) filled with different tubular structures, includ- ing tube, triangular and rectangle structures were designed. The tubular structures were perfectly assembled into semi-reentrant cells to avoid swaying in the semi-reentrant cell. The geometric relations and relative density for these structures were established. For the out-of-plane and in-plane compressions, SR filled tubular structures exhibited different de- formation patterns compared to those of SR or pure fillers. A constraint effect was found between the filler tubular and container SR. With fillers contained inside the SR structures, the plateau stresses for three conditions were all promoted compared to those of SR. The best out-of-plane compression resistance occurred in the SR filled rectangle which might be caused by larger interaction areas between the SR and rectangular structures. The (specific) energy absorption of the SR filled tube compressed out-of-plane was the largest. The peak and plateau stress of the SR filled triangle was the largest compared to other structures when compressed in plane due to stability of the triangle. It was found that the plateau stress, energy absorption and specific energy absorption of SR filled triangle was the largest, while that of SR filled rectangle was the lowest.