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Fundamentals of Forming Limit Curves (FLC) Testing

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Introduction

The automotive industry is always evolving, providing us with more and better vehicles, but we may never have given much thought to how they are created. Any vehicle has numerous components, with the body structure being the most important. Most of the automotive bodywork is made of steel, particularly thin sheets of steel. Some body components contain aluminium, and that also is often in the form of thin sheets. Those sheets come from the material supplier as flat thin sheets, then they are formed into certain car body parts by the Stamping Process; a process in which sheets are pressed between a pair of dies with a specific shape and thus achieving that same shape. Before doing the stamping process, many parameters should be considered. Designers and manufacturers should have a reliable prediction for material behavior under a certain amount of press load, and for a specific die shape, to successfully fabricate the part without any problems. They simply need to know the limits by which a sheet material can be stretched before rupturing, or in other words, its “Formability”. We will explore the basics of formability testing, including theory, equipment, methods, results, and testing standards.

What is Sheet Formability?

Sheet Formability refers to the ability of a sheet metal to be formed into the desired shape without cracking or rupturing. For the simplest explanation, the tension test will be taken as an example.

In this test, a dogbone-shaped test sample undergoes a tensile load along its axis until rupture; from force and elongation measurements, the well-known “stress/strain” curve can be constructed. The x-axis here is strain, and it represents the change in length with respect to the original length of the dogbone sample (e=∆L/L0).

Formability in this case is equivalent to the “ductility” of the material; the latter can be arbitrarily taken at the point of necking (uniform ductility) or at point of rupture (total ductility), or somewhere in between.

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When moving from the simple tension test to actual sheet metal forming operations (such as the stamping of a door), the deformation of the material becomes more complicated, simply because the material is deformed in two directions at the same time.

Nevertheless, the definition of formability is similar yet scalable to this more complex case; i.e., sheet formability is a measure of the ductility of the material when deformed in two directions. If we take any particular small region within a stamped door, we would notice that the material is stretched/compressed in two directions by different amounts to generate the features and complex details of the door. If we were to measure the amount of stretching in the two directions, then the ratio between the smaller amount (minor strain) and the larger amount (major strain) is known as the “biaxial ratio”; determining the ductility limits of the material for various biaxial strain ratios is accomplished by “sheet formability testing”.

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Principles of Sheet Formability Testing

In sheet formability testing, a test sample with a particular geometry is clamped firmly between two binder rings and an axisymmetric punch (hemispherical or flat) is then driven against the test sample deforming it until rupture. The test sample geometry is designed intentionally to have a varying aspect ratio in order to induce different biaxial strain ratios.

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There are three main critical strain ratios (or loading paths) that are relevant to sheet formability testing: Uniaxial Tension (UT), Plane-Strain Tension (PST), and Balanced Biaxial Tension (BBT). Each one is characterized by different strain ratios. Uniaxial Tension (UT) describes a situation where stress is applied entirely along the major axis with no loading in the transverse direction. However, the since the material is stretching along the major axis, it must also contract along the minor axis to maintain a constant volume.

This leads to a typical UT strain ratio of -0.5 (the strain ratio is negative because the minor strain is compressive). Plane-Strain Tension (PST) describes a situation where there is stretching in just the longitudinal direction without any change in the width, thus producing a strain ratio close to 0.

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Balanced Biaxial Tension (BBT) describes an extreme situation where there is an even stretching in both the longitudinal and transverse directions, thus implying a strain ratio close to 1. The test samples are thus designed to achieve these three main strain ratios, as well as other ratios in between these critical values; this way one can guarantee coverage of the majority of strain ratios encountered during sheet metal forming operations. During testing of any particular sample, the minor and major strains accumulated as the material deforms are tracked and measured; the values corresponding to an “arbitrary point of instability” are then recorded. The latter is performed for each sample geometry, with at least three repeats per geometry for statistical reliability. By connecting all the recorded instability points, a “Forming Limit Curve” (or simply an “FLC”) takes shape. This FLC, typically a V-shaped curve, provides a general map for the formability of the material; any combination of minor/major strain accumulations higher than those described by the FLC curve are likely to lead to material failure, while combinations falling below the FLC curve are likely to lead to successful forming. Since necking is not easy to determine during biaxial stretching, the “instability” point at which an FLC point is taken during the test is arbitrary, and it is typically based on complex algorithms in order to lead to state somewhere between necking and rupture (when compared to uniaxial tension testing).

Testing Equipment

Based on the principles described before, formability testing requires two fundamental forces: a clamping force to hold the test sample in place, and a punch force that is responsible for deforming the test sample to failure. Any test machine capable of producing these two forces independently can be used. There are several ways available to achieve this, but typically a dual-acting hydraulic press with custom tooling is the most appropriate means to achieving this. The forces needed to deform materials, especially the latest generations of automotive steels, require large forces, and thus hydraulic power supply offers the highest forces for the footprint.

There are even specialized hydraulically-powered machines, known as “Universal Formability Tester”, that have been built for this purpose, and they are simply compact self-contained presses designed specifically for formability testing.

In terms of sensors, the only important type needed for FLC testing is a biaxial strain sensor; as the test sample stretches, the extensometer is to measure the major an minor strain accumulations across the sample. With the latest developments in FLC testing, and the need for complex algorithms to determine FLC points, optical metrology sensors using digital image correlation (DIC) are now the only practical means for satisfying the requirements of FLC testing.

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Formability Testing Results

An FLC is one of the most important tools used in sheet metal forming operations to provide guidance on how to design components and also select the appropriate materials with sufficient “formability limits” to accommodate the complex details of the component. As mentioned earlier, the exact algorithm by which an FLC point is determined is practically “arbitrary”, and in essence, it describes a particular state somewhere between necking and fracture. The ISO international standard on FLC testing dictates a particular approach for determining FLCs, and it is known as the Section-Based Method. Nevertheless, several manufacturers choose alternative methods and algorithms, or at least supplementary ones to the section-based method; that’s why it is not unusual to see two different FLCs extracted by two methods for the same material, as shown in the example below.

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Testing Standards

FLC testing is regulated primarily by the ISO 12004 testing standard.

Advanced Technical Topics in FLC Testing

DIC and Formability Testing
(Will be published soon)
Which Formability Testing Equipment to Choose for FLC Testing?
(Will be published soon)
Nakajima vs Marciniak for the Construction of FLCs
(Will be published soon)
FLC Determination Algorithms
(Will be published soon)
2D vs 3D DIC in FLC Testing
(Will be published soon)
Sensitivity of FLCs to Strain Rate
(Will be published soon)