Global vs Local Formability

Most sheet metals have different “Global” and “Local” forming capabilities, so it is critical to understand their meaning to optimize grade selection, processing, and usage.

Historically, most fabricators needed to consider only global formability when designing and stamping parts. Drawing, plane-strain tension, and stretch forming are “global” forming modes where deformation occurs in the plane of the sheet over relatively large regions of material. Tensile failures or necking failures, where the steel progressively thins during forming, are a characteristic of global formability failures. The strains across a stamped part start low and evenly distributed but begin to concentrate as the punch approaches bottom-dead-center. Critical thinning (also called necking) occurs if the strains induced by the part shape and forming process exceed the forming limit of the chosen sheet metal. Forming simulation software packages have reliably shown the ability to accurately predict global formability concerns and hot spots with the use of inputs like tensile test data and the correct forming limit curve.

Local formability failure modes are an entirely different failure condition, where fractures occur out of the plane of the sheet in response to concentrated deformation created when forming localized features like stretch flanges, extruded holes, or bends around a radius too small for the selected steel grade. These failures typically occur without any observable thinning or necking (Figure 1). Forming simulation software that considers only the forming limit curve or maximum thinning as the failure criteria cannot predict local formability failures.

Cutting conditions and edge stresses developed in blanking and slitting operations play a significant role in limiting local formability. Trim steel clearances, shear angles, trim steel materials, tool sharpness and design, steel rolling direction, and part design considerations are all important (Figure 2).

The absence of global thinning leading to the onset of localized necking highlights the importance of clearly defining the actual mode of failure before trying to identify possible solutions. Examination of the edge of the part where the failure is occurring is the best way to accomplish this. Figure 3 shows a photo of a typical local formability related edge fracture where no observable thinning/necking before fracture occurs.

Some AHSS grades are more prone to these local formability failures. A reduced hardness difference between microstructural phases appears to improve local formability. At a given tensile strength level, there may be grades with improved global formability, improved local formability, or a balanced approach available. Choosing the optimum grade requires understanding of the manufacturing die process and the functionality requirements associated with the part and its application.

Figure 4 shows an edge fracture on a DP980 rocker panel. This panel experienced edge fractures during production, occurring in embossments along the edge of the part, with no evidence of necking at the fracture site. This is a classic local formability failure, reinforcing the need to inspect the conditions at the fracture zone of AHSS to determine whether the root failure is global or local formability. Process and die solutions differ based on the mode of failure.

Grade dependency is highlighted in Figure 5, which compares edges of HSLA 50SK on the left and DP500Y/780T on the right after stretch-bend testing under conditions to produce fracture. The HSLA sample shows the characteristic thinning down associated with necking. The DP steel did not have a visible neck at the failure location.^S-11

Mechanical tests now being employed to better quantify and characterize the unique local formability related failure modes associated with AHSS include:

• Hole Expansion Test,
• Bend Under Tension (BUT) Test,
• 2D Edge Tension Test,
• Side Bending Test, and
• Half-Specimen Dome Test (HSDT).

Fracture-Limited Formability = Local Formability

In general, most sheet metal stamping failures result from exceeding the necking limit, and are termed global formability failures. However, there are important conditions which promote fracture before first developing a neck. These are of particular concern since such fractures cannot be predicted or anticipated based on conventional FLC and grid strain analysis techniques. Recent advances in metal forming software codes now allow input of appropriate data which substantially aids in these efforts. Still, there is no universally accepted shape of the Fracture Limit Curve.

Local Formability: Bending

Researchers discovered that all the steel through the sheet thickness must exceed the forming limit for necking to begin. In tight bends where the inner surface is compressed, strains never reach the critical strains in the conventional forming limit curve, which is why necking is not seen on the outer bend surface above a critical r/t level, or the ratio between the radius and the thickness.

However, as strength increases, the fracture limit is closer to the necking limit determined by the FLC. Advanced High Strength Steels are even more sensitive to local formability failures due to the localization of strains which occur at the interface between hard and soft phases in the microstructure.

On thin, wide sheet, bending strains on the metal surface plot along the axis of plane strain. The Fracture Limit Curve in this location is higher than the necking Failure Limit Curve, but since the critical strains only need to be reached on the outermost surface, higher-strength steels have a greater risk of experiencing bending fractures. In these cases, the material fracture limit becomes the effective forming limit in deformation modes with severe through-thickness strain gradients, and this is not considered in the traditional FLD.

Local Formability: Sheared Edge Expandability (Hole Expansion)

In a hole expansion test, the strains at the edge of the expanding hole follow a uniaxial strain path until reaching the fracture limit. In lower-strength steels with clean, machined (undamaged) edges, expansion ratios over 300% might be possible. As mentioned above, higher-strength steels have a lower fracture limit. Still, undamaged machined edges provide the best conditions for high hole expansion.

Cutting operations like blanking, shearing, and punching all damage the edge and lowers the fracture limit, meaning that the fracture limit might be reached at even lower strains. Strain localization occurring at the interface between hard and soft microstructural phases highlight why some grades have still lower critical strains.

Summary of Global Formability

• The resistance to localized thinning (necking) is the key to global formability.
• Stamped parts get progressively thinner as the press stroke approaches bottom-dead-center. Rapid thinning in critical areas lead to the onset of localized necking. Global formability promotes the ability to reach deeper draw depths before initiation of necking.
• Measures of global formability include the work-hardening exponent (n-value), the uniform elongation value, and the total elongation value determined in a tensile test, along with the forming limit curve (FLC).
• Global formability failures occur when a through-thickness volume of the formed sheet steel exceeds this forming limit and begins to neck.
  Necking failure typically should not occur if the global formability limit is exceeded on only the outer surface of steel bent over a radius or expanded at a cut edge and not through the full thickness of the metal.
• Uniform elongation measures the resistance to local necking. The global formability limit corresponds to Uniform Elongation in the strain state achieved in a tensile test sample. The Forming Limit Curve (FLC) encompasses all strain states.
• For mild and conventional high strength steels, there is a large difference between the necking limit and the ultimate fracture limit. This corresponds to the difference between Uniform Elongation and Total Elongation in a tensile test, where the value of total elongation is typically twice that of uniform elongation.

Summary of Local Formability

• The resistance to fracture is the key to local formability.
• Local formability promotes fracture resistance in response to creating local product features like stretch flanges, extruded holes, and tight-radius bends.
• Local formability tests which evaluate the stretchability of appropriately prepared edges by deforming them with a punch may have limited reproducibility between labs due to the influence of different sample preparation methods. Determining parameters like True Fracture Strain, Reduction in Area at Fracture, or Thickness Strain at Fracture with a well-defined tensile test typically results in repeatable and reproducible values.
• Local formability failure occurs when strains at the surface or edge exceed the ultimate fracture limit, a value that is higher than the necking limit determined by the Forming Limit Curve.
• Local formability failures occur more frequently on higher strength steels partly because of a smaller difference between the necking limit determined by the Forming Limit Curve and ultimate fracture limit.
• Work hardening and damage from edge preparation methods like shearing further reduce edge formability to values typically lower than indicated by the Forming Limit Curve, especially for AHSS. The strain localization at the interface between hard and soft phases in AHSS also contribute to an increased risk of local formability failures.
• The lack of a localized neck is a characteristic trait associated with of local formability failures.