Thickness and grain size dependence of limit strains in sheet metal stretching

To clarify the fracture mechanism of metal foils, the ductile fracture and the free surface roughening behaviors for pure copper foils are focused on in this study. Pure copper foils and sheets with various initial thicknesses of 0.05, 0.1, 0.3, and 0.5 mm are used. Fracture strain decreases with decreasing thickness under the uniaxial tensile state. In particular, the fracture strain decreases dramatically from a thickness of 0.3 mm to 0.1 mm. Dimples cannot be observed on the fracture surface of metal foils with thicknesses of 0.05 and 0.1 mm, whereas metal sheets with thicknesses of 0.3 and 0.5 mm had some dimples on the fracture surface. Free surface roughening can be observed for metal foils as well as metal sheets. The rate of increase in the ratio of surface roughness to thickness dramatically increases from a thickness of 0.3 mm to 0.1 mm because of the decreasing number of grains in the thickness, which is inversely proportional to the rate of increase in the ratio of surface roughness to thickness. In particular, when the number of grains in the thickness becomes less than approximately 5, the rate of increase in the ratio of surface roughness to thickness significantly increases in inverse proportion, leading to low fracture strain. Additionally, the fracture strain and the number of dimples on the fracture surface decreases for a metal sheet with a simulated surface roughness prepared by machining, which has nearly the same ratio of surface roughness and pitch to thickness as those of metal foil. Furthermore, ductile fracture criteria could not be used to predict the fracture of metal foil in stretch forming. Referring to these results, we discuss the ductile fracture mechanism of metal foils. Firstly, the surface roughness of metal foils increases with plastic deformation owing to free surface roughening phenomenon. Secondly, the ratio of surface roughness to thickness increases with plastic deformation. At the concave region formed by free surface roughening, local deformation occurs. Thirdly, it can be considered that the local deformation caused by free surface roughening leads to fracture. On the basis of this mechanism of ductile fracture resulting from the free surface roughening of metal foils, the metal foils have low fracture strain compared with metal sheets.

Ductile fracture and free surface roughening behaviors of pure copper foils for micro/meso-scale forming

Effects of microvoid nucleation and growth on the stress and strain fields near a crack tip are studied by employing the Gurson's constitutive model of porous plastic solids and by using a finite element method suitably formulated to admit large geometry change. It is shown that microvoids nucleate and grow rapidly in the region at a distance less than the blunted tip diameter away from the crack tip and have a great influence on the stress fields therein. Effects of a large void near a crack tip on crack initiation are also investigated by taking the shear localization into account. The critical value of the crack tip opening displacement predicted by the present investigation is shown to coincide with the published experimental data.

Effects of microvoids on crack blunting and initiation in ductile materials

Validation of FE simulation based on surface roughness model in micro-deep drawing

A Method of Determining Flow Stress under Forming Conditions

https://www.jstage.jst.go.jp/article/kikai1938/36/286/36_286_1017/_pdf

On the Roughening Phenomenon of Free Surface in Deformation Process

On Criteria for Ductile Fracture(Minor Special Issue on Plastic Working and Materials Characteristics)

The Effect of Deformation Speed on Friction and Lubrication in Cold Forging

Back Pressure Extrusion of Brittle Metals and Metallic Powder

The two-stage extrusion method, by means of which the metals can be easily extruded against high hydrostatic pressure, is developed. The effects of pressure on the extrusion ability of several brittle metals and the mechanical properties of the extruded metals are examined. It is found that the extrusion ability of magnesium and aluminum alloy AC2C-F increases with back pressure. The results are explained through slip line theory. The effect of back pressure on the mechanical properties of the extruded metals is significant for brittle metal such as magnesium, and is little for ductile metal such as the aluminum alloy 5052.

Moriya Oyane

Papers

On the Roughening Phenomenon of Free Surface in Deformation Process

On Criteria for Ductile Fracture(Minor Special Issue on Plastic Working and Materials Characteristics)

The Effect of Deformation Speed on Friction and Lubrication in Cold Forging

Back Pressure Extrusion of Brittle Metals and Metallic Powder

Improvement of Formability of Metals in Extrusion by Means of Two-Stage Extrusion