Standards Development

Scope

This test method (biaxial bulge test method) is a biaxial tensile test method for sheet metals. This document specifies the test method for measuring the biaxial true stress-plastic strain curves, forming limit strains, and forming limit stresses of a sheet metal by applying axial force and internal pressure to a cylindrical test piece (hereinafter referred to as test piece) made by bending and welding thesheet metal, and by generating the biaxial stresses in the axial and circumferential directions at the centre of the test piece.

Purpose

In order to curb global warming, transportation equipment with high CO2 emissions is required to
improve fuel efficiency through weight reduction. In order to reduce the weight of car bodies with
press-formed parts, it is essential to expand the use of lightweight and high strength sheet materials
such as aluminium alloys and high tensile strength steel sheets. However, lightweight and high
strength sheet materials have low ductility and are prone to have forming defects, therefore it is
necessary to predict forming defects with high accuracy using forming simulations and to determine
in advance whether forming is possible or not. To improve the accuracy of the numerical analysis, it
is necessary to improve the measurement accuracy of the mechanical properties of the material used
as input data.

As materials are formed in two orthogonal directions during press forming, determining the relationship
between stress and strain when sheet materials are deformed in similar directions is important for
developing models to improve the accuracy of the analysis and for use as input data for forming
analysis. In addition, the forming limit under biaxial stress states is also important as a fundamental
property of the material. These two properties can be measured by the biaxial bulge test method. In
this test method, a test piece made from a metal sheet material in the form of a cylindrical tube by
welding the material ends after bending is used. The test piece is subjected to internal pressure and
axial forces to apply strain in two orthogonal directions of the material.

The purpose of the proposal is to specify a biaxial bulge test method that enables the measurement
of the relationship between stress and strain in the axial direction and circumferential direction of the
test piece until the forming limit strain of the material in any stress path by measuring the internal
pressure and axial force applied to the metal sheet material formed into a cylindrical shape. The data
measured by this method can be used to improve the accuracy of analysis of sheet metal forming
simulations and to contribute to the development of material models that can be applied up to large
strains. As an effect of the standardisation of this test method, the use of lightweight and high strength
sheet materials will be promoted by increasing the accuracy of forming simulations. The resulting
reduction in CO2 emissions due to energy savings in the transportation sector will contribute to the
prevention of global warming.