plastic instability
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Author(s):  
Ali Bandizaki ◽  
Asghar Zajkani ◽  
Saeed Moulood

In this paper, the influence of functional elastomeric substrate-supported layers for enhancing potential resistance capability against localized plastic failure of advanced high strength steels is considered based on a localized necking model of vertex theory. Application of this structure leads to postponing the plastic instability of the metallic part. By defining diffuse and localized modes of deformation in a general framework, the theoretical models are developed to predict necking limits at several stress states. In addition, the results of the Hookean and neo-Hookean elastomers are compared in terms of strain hardening with the anisotropy parameter of Hill’s yield criteria. Since necking band angle (NBA) is a principal factor for the necking prediction, its effect on bifurcation events is evaluated specifically for different ratios of stress rate, and quadratic and non-quadratic yield criteria. This analysis is performed by proposing a supported and yield-dependent necking bound angle (YD-NBA). All considerations are done by providing equilibrium conditions governed over the NBA. Finally, obtained results indicate good agreements between several theoretical considerations and experimental data.


2021 ◽  
Vol 904 ◽  
pp. 525-529
Author(s):  
Ning Ning Li ◽  
Zhi Juan Zhao ◽  
Xiao Yuan Xie ◽  
Qiang Dai ◽  
Jing Miao Li ◽  
...  

In this paper, the physical and chemical analysis method was used to analyze the leakage of a regeneration gas heater device accident. The result shows that this device was heated seriously at local area, and oxidized severely. The carbon content around the leakage is obviously lower than the limit of standard, and the grain deformation at crack tip of the leakage is obvious.It shows that reason of device failure is caused by typical plastic Instability, and low carbon content leads to insufficient mechanical properties at high temperature. According to data from manufacturer, the unreasonable arrangement of thermo-couples may be the root cause of this leakage.It can be considered that replace the higher grade materials conservatively, while optimizing the arrangement of thermo-couples.


2021 ◽  
Vol 2021 ◽  
pp. 1-12
Author(s):  
Yue Guo ◽  
Yi-xiong Liu ◽  
Yun-wu Wu ◽  
Hang Cao ◽  
Da Mo

The increasing demand for power, fuel efficiency, and safety of aeroengines has called for weight reduction and structural integrity examination of the critical components. This paper is aimed at performing a systematic investigation on the design of a high-speed Ti2AlNb blisk, including disc geometry optimization and burst speed prediction. Incorporating the design of the experimental approach and the commercial software has guaranteed that the optimization could be accomplished. Six key parameters were defined as variables with regard to the geometric dimensions whereas the safety factors were set as constraints to make the disc feasible. Sensitivity analysis has been conducted to study the effects of the variables on the safety factors and disc weight. Bore width, web width, and bore angle are identified to be the dominant factors regarding optimization. Results reveal that the bore width and web width are positively related to the safety factors at the cost of increasing the disc weight. On the contrary, the effects of the bore angle show the opposite trend. Finally, the achieved minimum disc weight is 15.2 kg with all the safety factors meeting the requirements. Upon completing the disc shape optimization, the burst speed was estimated using three elaborated methods. The comparisons between the numerical results and the experimental results indicate that the mean stress method is accurate when the correction coefficient is chosen properly. The local stress and strain method and the global plastic instability method also offer a precise prediction on the burst speed with errors of less than 5%. It could also be concluded that the predicted web failure in the radial direction of the disc is in good agreement with the experimental results.


2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Abdallah Shokry ◽  
Aylin Ahadi ◽  
Per Ståhle ◽  
Dmytro Orlov

AbstractImprovement of structural efficiency in various materials is critically important for sustainable society development and the efficient use of natural resources. Recently, a lot of attention in science and engineering has been attracted to heterogeneous-structure materials because of high structural efficiency. However, strategies for the efficient design of heterogenous structures are still in their infancy therefore demanding extensive exploration. In this work, two-dimensional finite-element models for pure nickel with bimodal distributions of grain sizes having ‘harmonic’ and ‘random’ spatial topological arrangements of coarse and ultrafine-grain areas are developed. The bimodal random-structure material shows heterogeneities in stress–strain distributions at all scale levels developing immediately upon loading, which leads to developing concentrations of strain and premature global plastic instability. The bimodal harmonic-structure material demonstrates strength and ductility significantly exceeding those in the bimodal random-structure as well as expectations from a rule of mixtures. The strain hardening rates also significantly exceed those in homogeneous materials while being primarily controlled by coarse-grain phase at the early, by ultrafine-grain at the later and by their compatible straining at the intermediate stages of loading. The study emphasises the importance of topological ultrafine-/coarse-grain distributions, and the continuity of the ultrafine-grain skeleton in particular.


Materials ◽  
2021 ◽  
Vol 14 (16) ◽  
pp. 4609
Author(s):  
Hai Qiu ◽  
Rintaro Ueji ◽  
Tadanobu Inoue ◽  
Yuuji Kimura

Inhomogeneous plastic deformation damages the surface quality of a product in the metal forming process. Therefore, it is necessary to investigate the plastic instability of a metal. Tempered martensite is a common microstructure of medium-carbon steel. Plastic instability (Lüders phenomenon, Portevin-Le Châtelier phenomenon) in this phase was investigated by a uniaxial tension test performed at room temperature. The formation and propagation of a plastic band were analyzed via two-dimensional digital image correlation, and the strain and strain-rate fields were experimentally evaluated. The results obtained are as follows: (1) there was no clear yield plateau on the stress–strain curve; (2) Lüders phenomenon was present, but the Portevin-Le Châtelier phenomenon was not found; (3) in the Lüders deformation process, local strain distribution in tempered martensite is more complicated than that in ferrite.


Author(s):  
Sang Kyu Woo ◽  
Risheng Pei ◽  
Talal Al-Samman ◽  
Dietmar Letzig ◽  
Sangbong Yi

Author(s):  
Sangkyu Woo ◽  
Risheng Pei ◽  
Talal Al-Samman ◽  
Dietmar Letzig ◽  
Sangbong Yi
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