Plastic Flow Instability Research Articles

Instability of plastic flow in the directions of pure shear: I. Theory

The localization of plastic flow (plastic instability) is a prelude to fracture initiation. Prior treatments are based mainly on the phenomenon of necking (load maximum) which has for its basis fluctuations in cross-sectional area. A second basic mode of localization has been confirmed experimentally. This is the localization of flow within a plastic zone along characteristic directions (direction of pure shear or zero extensional strain). The localization is effected by the activation of tangential velocity discontinuities along the characteristics. A necessary condition is that the material simulate the ideal plastic state. The conditions for this second mode of localization are formulated. A general condition for instability involving both modes is derived. In situations in which both modes are possible, necking is always expected to precede the localization along characteristics.

Instability of plastic flow in the directions of pure shear: II. Experimental

Two aspects of the phenomenon of plastic instability in direction of pure shear are examined, namely that the condition dσ = 0 (maximum in true flow stress) is necessary for localization of flow along characteristics as defined in continuum plasticity, and that fracture is initiated and propagates along characteristics. Two types of sheet specimens were employed, the standard-type flat sheet specimens, and specimens simulating both plane stress and plane strain. Grids were placed on gage sections and photographs were taken successively in the plastic range to enable strains to be calculated and instabilities to be observed and recorded. The principal variable in the flat specimen test was theW/T ratio (width to thickness). In the plane* strain specimens, both the gage length (constantW/T) and the strength level of the material (quenched and tempered AISI 4340 steel) were varied. A maximum in true flow stress is found consistently at the onset of instabilities. Fracture propagated consistently along the instability band-matrix interface. Variations in specimen geometry produces significant changes in stress state, directions of characteristics, and ductility. For a given specimen geometry, plane strain is more closely approached the higher the strength level of the material. In mixed mode fracture paths slant fracture is associated with the more embrittling stress state.

The Low‐Temperature Instability of Plastic Flow

physica status solidi (b)Volume 61, Issue 1 p. K45-K48 Short Note The Low-Temperature Instability of Plastic Flow G. A. Malygin, G. A. Malygin A. F. Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, LeningradSearch for more papers by this author G. A. Malygin, G. A. Malygin A. F. Ioffe Physico-Technical Institute, Academy of Sciences of the USSR, LeningradSearch for more papers by this author First published: 1 January 1974 https://doi.org/10.1002/pssb.2220610145Citations: 12AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume61, Issue11 January 1974Pages K45-K48 RelatedInformation

The Instability of Plastic Flow of Metals at Very Low Temperatures. II.

Evidence is presented to show that the load drops observed in most metals at very low temperatures arise from thermal instability rather than mechanical instability of the lattice. Measurements of the temperature rise are described.

The instability of plastic flow of metals at very low temperatures

The low-temperature unstable plastic deformation of aluminium alloys is described. It is shown that discontinuities in the stress-strain curve are caused by a localized temperature rise produced during the deformation. The calculated magnitudes of the drops in load and the transition temperature between smooth and discontinuous flow agree reasonably well with the experimental observations. It is believed that all metals should exhibit unstable deformation at sufficiently low temperatures.