Sample edge effects on tensile properties and sheet formability

Abstract

It is well known that the condition of the edge, whether sheared or milled, can affect the results in edge formability tests (i.e. a hole expansion test). In the current investigation, two experimental studies were performed to evaluate the effects of edge condition on the results of tensile tests and a laboratory formability test for sheet steels. The first study evaluated the tensile properties of a QP980 steel with samples prepared by four different machining methods: wire electro-discharge machine (EDM); mechanical milling; laser cutting; and waterjet cutting. It was found that the EDM and milled samples produced statistically similar values for yield strength, ultimate tensile strength, uniform elongation and total elongation. The yield strength and tensile strength from samples prepared by laser cutting were statistically different. Water jet cutting produced statistically different values for almost all four properties when compared to the EDM and milled samples. In the second study a DP600 steel was subjected to an angular stretch bend formability test with samples prepared by either shearing or milling. It was found that the sheared samples required higher forces but smaller displacements to reach the formability limit. Fracture for the sheared samples initiated on the edge of the sample whereas for the milled sample necking and fracture initiated in the center of the sample, close to the plane-strain condition. Results from both of these studies clearly show the importance of the edge condition on reported mechanical properties and on the formability of the sheet.