Experimental and theoretical investigation on the role of friction in Nakazima testing. (November 2017)

• Record Type: Journal Article
• Authors: Zhang, Ling; Min, Junying; Carsley, John E.; Stoughton, Thomas B.; Lin, Jianping
• Is Part Of: International journal of mechanical sciences. Volume 133(2017)
• Page Start: 217
• Page End: 226
• Publication Date: 2017-11
• Subjects: Friction -- Lubrication -- Nakazima test -- Non-linear strain path -- Forming limit curve; Mechanical engineering -- Periodicals; Génie mécanique -- Périodiques; Mechanical engineering; Maschinenbau; Mechanik; Zeitschrift; Periodicals; 621.05
• DOI: 10.1016/j.ijmecsci.2017.08.020
• Languages: English
• ISSNs: 0020-7403

Abstract

Highlights: Friction has no effect on the strain path at specific locations in the test specimen used in the Nakazima test. The range of strain paths in the specimen are determined solely by the local bending strains and the specimen geometry. Friction changes the relative strain rate in different locations of the specimen geometry, changing the neck location. The difference in strain limits obtained for different friction conditions is caused by the strain path effects on necking. Strain limits compensated for non-linear strain paths are found to be identical under low and high friction conditions. Abstract: The Nakazima test is a commonly used method to experimentally measure the forming limit curve (FLC) of sheet metal. Various researchers have different opinions about the effect of tribological conditions at the interface of the punch and specimen in Nakazima testing, such that international industry test standards specify conflicting requirements to address friction conditions during testing. This paper provides analysis to resolve this conflict and demonstrates the effectiveness of an analytical procedure to account for and eliminate effects of friction in determination of the strain FLC as a material property that is independent of friction. With the aid of digital image correlation (DIC) techniques, Nakazima tests of a dual phase steel (DP980) were performed under two clearly different friction conditions: (1) friction was minimized between the punch and the specimen by conducting tests using a thin Teflon sheet, and (2) friction was maximized by conducting tests without any lubricant after chemically cleaning the punch and sheet surfaces. As expected, the strain FLCs obtained under these two friction conditions are different. However, analysis shows that the strain path at material points within the same test specimen are the same for both friction conditions. The only difference observed is that the relative strain rates at different material points are strongly affected by friction, which causes necking and fracture to initiate at different locations on test specimens. Furthermore, by accounting for the effects of local strain path, curvature, and contact pressure on the measured strain history at the location of necking using the method described by Min et al.[27], the corrected strain FLCs obtained under very low and very high friction conditions are shown to be indistinguishable. Graphical abstract: Forming limit strains measured for the same specimen geometry under low and high friction conditions are typically incompatible, suggesting incorrectly that the FLD might be a function of friction. It is demonstrated in this paper that friction does not change the strain path at material points within a given specimen geometry, but strongly affects the relative strain rates across the specimen. It is the friction effects on relative strain rates that causes different material points to reach different critical conditions based on the local stress state. Furthermore, by accounting for the nonlinear strain path, i.e., by computing the stress state for each critical condition, it is found that a unique stress-limit is found for both low and high friction conditions, and from this observation it is possible to obtain a single strain-based forming limit curve that is independent of friction condition.