Analysis of Instability and Strain Concentration during Superplastic Deformation

Chung, Li Chuan; Cheng, Jung-Ho

doi:10.4028/www.scientific.net/msf.357-359.411

Cited by 1 publication

(2 citation statements)

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“…More recent work on the prediction of failure during SPF deformation has been presented by Chung and Cheng [13] on the basis of an instability criterion developed by Ding et al [14]. Ding et al considered a thin SP sheet under biaxial stretching and, using a procedure analogous to that of Marciniak and Kuczynski [15], analysed the conditions under which a local neck, assumed to form perpendicular to the major principal stress, σ 1 , becomes unstable.…”

Section: Failure Modelsmentioning

confidence: 99%

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A superplastic forming limit diagram concept for Ti-6Al-4V

Krohn

Leen

Hyde

2007

Proceedings of the Institution of Mechanical Engineers, Part L:

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The current paper is concerned with the development of a simplified method for predicting failure due to plastic instability during the superplastic forming (SPF) of titanium alloys. The rationale is that a key factor in the process of reliable failure prediction is the incorporation of a mechanisms-based model, which includes microstructural effects, such as static and dynamic grain growth and associated hardening, and which is also independent of the forming strainrate. Existing methods for predicting plastic instability during conventional metal-forming are discussed along with previous attempts at predicting failure during SPF. It is shown that no easyto-interpret method, such as the forming limit diagram (FLD) in conventional forming, exists for SPF. Consequently, an SPFLD concept in a major strain (ε 1 ), minor strain (ε 3 ), and equivalent strain-rate space (ε eq ) is presented on the basis of uniaxial SP ductilities across a range of strainrates along with the Hill-Swift instability criteria and using finite element-predicted ε 1 -ε 3 -ε eq paths for key points on the forming blank to predict failure. The predicted results are validated against measured data for Ti-6Al-4V at different strain-rates. important benefit is the low flow stress normally associated with SP deformation. The technology requires significant investment into part and tool design, die manufacture, and process modelling. A number of key challenges prevent the more widespread exploitation of SPF.1. The requirement for a controlled, slow strainrate prevents rapid manufacture of mass-produced parts, so methods and/or materials are required to speed up the process. 2. The associated high temperatures, especially for titanium SPF alloys, represent hostile conditions for materials, tooling, equipment, and personnel. 3. Since only specific alloys display SP behaviour, often requiring pre-conditioning, these alloys are normally costly to produce and purchase and they are costly and difficult to characterize. 4. Finally, because of the non-linear, large deformation, viscoplastic nature of SP material behaviour, it is normally necessary to employ computational methods, such as finite element (FE) analysis, to predict industrial forming processes, and there are JMDA150

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Section: Failure Modelsmentioning

confidence: 99%

“…with D ij as the plastic strain-rate tensor. On the basis of equation ( 22), Chung and Cheng [13] proposed the concept of a flow localization factor (FLF), ξ II , to enable a quantitative description of the localization process of unstable plastic flow as follows…”

Section: Failure Modelsmentioning

confidence: 99%