True stress (·)true strain (¾) relationships until just before fracture, i.e., the plastic deformation limit, were estimated by the stepwise tensile test and the Bridgman equation for various metals and alloys with different crystal structures. The estimated ·¾ relationships were different from the nominal stressstrain curves including the conventional tensile properties. In the relationships between the true stress (· pdl ) and true strain (¾ pdl ) at the plastic deformation limit, SUS304 and SUS329J4L indicated a better · pdl ¾ pdl balance. On the other hand, SUS329J4L, tempered martensite, and an ultrafine-grained steel showed superior results in the yield strength¾ pdl balance. The estimated ·¾ relationship for the ultrafine-grained steel suggests that grain refinement strengthening can improve · and ¾ up until the plastic deformation limit. The value of ¾ pdl became larger with increasing the reduction in area and a decrease in the fracture stress. The products of · pdl and ¾ pdl became larger with increasing work-hardening rate at the plastic deformation limit.
Circumferentially notched and smooth round tension tests were carried out at room temperature with and without compressive prestrain in a hot-rolled ferrite-pearlite SM490B steel, and the effects of the compressive prestrain and the notch on the plastic deformation limit (PDL) were investigated. The PDL was defined as a termination point in a true stress-true strain curve which was caused by a ductile or cleavage fracture. The notch caused variations in the average stress-true strain curve but did not affect the true stress-true strain curve. A true strain at the plastic instability was determined by a work hardening exponent, regardless of the notch. On the other hand, the true strain at the PDL differed according to the notch. An axial stress at the PDL was found to be about 1 300 MPa, regardless of the ductile and cleavage fracture. KEY WORDS: plastic deformation limit; circumferentially notched tension test; true stress; true strain; axial stress. Fig. 1. The circumferentially notched tension specimen.
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