Formability tests that let you fully leverage stronger automotive steels

The problem with tensile testing

Ultra-high strength steel is an established material in the automotive industry and is commonly used in selected structural body reinforcements, bumper reinforcements, door impact beams as well as seat frames and mechanisms.

Ultra-high strength steel is responsible for helping to achieve 5-star crash ratings and reducing component weight by up to 40 percent. It also allows OEMs to cut costs and become more efficient in production, while developing more innovative component designs that are more competitive on the market.

Despite the benefits, many OEMs are still choosing softer grades of steel and missing out on a competitive advantage. The reason is that they are relying solely on the elongation results of tensile tests when thinking about formability.

“The tensile test is the most frequently used test,” explains Dr. Lars Troive, a SSAB Senior Forming Specialist. “The idea is to pull the specimen apart until fracture. Then you measure how much it has extended in length. This is considered its elongation. For example, if the test piece measures 80 millimeters and then becomes 88 millimeters before cracking, it represents a 10 percent elongation.”

He goes on to say: “While the tensile test has long been the most common practice for judging the formability of steel, modern, stronger steel grades are not correctly represented by this method. This is because these stronger steels behave differently, having more local plastic deformation compared to conventional softer grades.”

A more accurate way of predicting the behavior of ultra-high strength steel is through the creation of a forming limit diagram (FLD), also known as a forming limit curve. One single FLD provides a graphical description of several material failure tests performed, i.e., punched dome tests, using different specimen geometries. Each specimen (i.e., steel blank) has a unique width-to-length ratio resulting in different deformation modes until failure. They will deform differently, having their own strain-path.

Before a FLD-test is performed, each specimen is first painted in white and then covered by black dots randomly distributed by spray-painting in a “speckle pattern.” The white base color is applied in order to get a good contrast to the black pattern.

During the tests, the speckle pattern is photographed by two cameras built into the press. The cameras capture the movements of every dot during the entire forming operation, which allows the estimation of the strain path until failure. As you perform the punched dome test on each of the different (blank) geometries, you will get two values for each test: the main and minor strains. The FLD is then drawn in a X- and Y-diagram, with a line connecting all of the obtained strain values. This curve represents the forming limit at which the steel is at a high risk for splitting (cracking).

Figure 1: the random dot speckle pattern (left image) and the major and minor strains plotted onto a forming limit diagram (FLD; right image).

In other words, the formability test determines how far you can go when forming before the steel will crack, depending on the strain state and which way the material is deformed.

To see evidence that ultra-high strength steel can be formed far beyond what the elongation values are telling you, look at the drawn cups in Figure 2.

Figure 2: drawn cups made from a range of very soft to ultra-high strength steels, e.g., 1400M having a tensile strength of 1400Mpa.

More accurate UHSS forming test results

“Visually, a slim specimen from the FLD-test forms nearly in the same way as the tensile test specimen,” says Troive. “It narrows in the middle when pulled, just like the tensile test specimen; this is called ‘uniaxial deformation.’ So why does the elongation test result differ from the FLD-test result?”

“We do a simple test, applying a square grid-pattern, 2mm by 2mm in size, on a tensile-tests specimen, measured after failure,” continues Troive. “What happens over a distance of 2 millimeters, in percentage terms, is much greater when compared to what happens over a length of 80 millimeters — the measure used in tensile tests, where the total extension in millimeters is divided by 80 millimeters, which means an average elongation over this length.”

Figure 3: for example, a 20% local strain over 2 millimeters (the grid) is much greater in percentage terms than what is indicated by testing the same UHSS steel over an expanse of 80 millimeters, as commonly done in tensile (pull) tests.

This explains why the two tests (tensile and FLD) have such a big difference in test results, leading to very different conclusions regarding how much one can form a UHSS steel.

Interpreting an FLD

Since an FLD provides the most accurate data on how a specific grade of ultra-high strength steel can be formed, understanding how to interpret the results is crucial.

Today, finite-element (FE) simulations of the forming process are very commonly used by the automotive industry. Based on this, the FLD is a very important tool as it is able to show if calculated strains are within the safe forming region — or if they are close to failure.

The FLD can be divided into three parts:

• Equibiaxal (stretching) to the right.
• Plane strain at the center.
• Pure shear (drawing) to the left.

Troive explains: “The FLD diagram attempts to provide a graphical description of a number of material failure tests with different strain paths. Basically, the area below the forming limit curve is considered to be safe for forming operations. It is common to lower the curve somewhat in order to have a margin for possible scatter, due to minor variations in the stamping process or material properties. FLDs are widely used as fracture criteria for forming simulations or strain measurements.”

“However, there are a few cases where the FLD is not able to predict failure. One of them is cut edges. The ductility of cut edges depends a lot on how the blank was cut. For example, was the correct cutting clearance used? Were the tools sharp? And so on. In such a case, we instead rely on a practical test and compare results with the strain level on the edge,” says Troive. (For more information, check out the Docol^® on-demand webinar “Problem-solving approaches to AHSS edge ductility.”)

Different types of shapes and forming will force the material to deform in different ways. In general, the worst scenario is when a part is formed at pure plane strain condition. Simple bending is an example of this type of forming operation, resulting in the shortest strain-path to failure. Sometimes it is possible to change a strain-path. One way can be as simple as optimizing the geometry of the blank so as to prevent the material from becoming stuck, so the material is drawn instead of stretched.

Comparing the results of tensile and FLD tests

Historically, automotive manufacturers have worked a lot with softer steel and the results between tensile and FLD tests were quite similar. That said, the tensile test was — historically — more established and therefore more widespread. The risk with only using the tensile test is that opportunities to use stronger steel are missed. Lars Troive explains:

“If you only look at the tensile test data, you might think everything is impossible. If you instead look at the formability, we are talking almost a 100 percent increase, for example from 10 to 20 over the actual area intended for a forming process. For an automotive application, a variety of possibilities arise by looking at the forming limit diagram instead of only the elongation.”

Figure 4: A₈₀ tensile pull test (white squares) and FLD test 2mm (gray squares) (results in %).

When plotting both elongation results from a tensile strength test and results from a forming test, the difference can easily be seen as the strength of the steel increases.

Real-world proof of UHSS formability

Many automotive OEMs already rely on FLD data when choosing materials. Therefore, the evidence that ultra-high strength steel with extremely high tensile strength can be formed into automotive applications already exists.

For example, Shape Corp. has created lighter, stronger, and more space-efficient roof rail tubes and A-pillars by 3D roll-forming Docol^® 1700MPa martensitic steel. These more compact designs increase interior space and driver visibility while optimizing the placement of airbags in the Ford 2020 Explorer and 2020 Escape.

Additional benefits of leveraging higher strength steels

Besides allowing for higher crash performance and weight reduction, the optimized choice of ultra-high strength steel can supply automakers with other valuable advantages:

1. Less materials used: The unique strength and technical properties of ultra-high strength steel can allow OEMs to reduce the amount of material needed for making a car component by utilizing thinner component walls.
2. Less costly materials: Ultra-high strength steels can be much more cost-effective than other high strength lightweight materials, based on both material costs and forming costs.
3. Less costly to form: Although you might need to invest in stronger tooling components than for softer steels, UHSS steels are typically formed using conventional production equipment, allowing you to leverage machinery you already own.
4. Faster forming, using less energy: You may be able to replace hot-stamped boron steel with cold-formed AHSS steels. You save money by not needing complex hot-stamping dies (that also require a lot of energy for heating and cooling), while speeding up your production time.
5. Weldability: Many ultra-high strength steels can be welded using standard welding processes due to their lean chemical composition.

Maximize your automotive parts design potential

When choosing ultra-high strength steel for automotive components, the possibilities for innovation are great. However, relying only on elongation data from tensile tests for judging formability will lead to choosing softer steel and missed opportunities for improvements. Instead, look to the forming limit diagram to ensure that you are getting the most out of your choice of ultra-high strength steel.

Would you like SSAB’s expertise in helping you to determine if a specific UHSS steel is formable enough for your automotive application? Then please contact your local Docol^® representative.