Local Formability Archives - Page 2 of 2

Shear Fracture

Automotive product designers target small radii for springback control, sectional stiffness, packaging constraints, and design features. These small radii lead to new challenges as applications for AHSS grades continue to increase. One of these challenges is an increased sensitivity to crack formation in those designs with small die radius to material thickness (R/T) ratios. Cracks forming at small R/T in AHSS grades are known as shear fractures.

General forming limit curves or other press shop criteria cannot predict shear fractures, nor are they flagged when traditional approaches are used in forming simulation packages. However, these shear fractures do occur in die tryout. Shear fractures are another form of local formability failure associated with multiphase AHSS such as DP and TRIP.

Shear fractures on AHSS may exhibit similarities to edge fracture, specifically the absence of necking prior to failure. This is in contrast with global formability failures, which are characterized by significant thinning near the fracture. Shear fractures occur almost immediately (within 1 mm of displacement) after reaching maximum load, meaning there is essentially no post-uniform elongation. This is contrary to the tensile behavior where significant post-uniform ductility remains prior to fracture.^K-9

Figures 1 and 2 highlight the appearance of a crack on the bend radius caused by shear fracture. No thinning is observed, which is consistent with failures limited by local formability concerns.

$Figure 1: Shear fracture in DP780.F-5$

Figure 1: Shear fracture in DP780.^F-5

$Figure 2: Shear fracture in DP980.D-7$

Figure 2: Shear fracture in DP980.^D-7

Figure 3 shows a typical shear fracture on a DP780 part viewed from different angles. The shear fracture occurred on the sharp radius on the left whereas the larger radius on the right experienced no failure. Depth of draw and draw bead configuration were the same on both sides of the draw panel. Restraining force was also similar on both sides of the blankholder. The significant variable was the die radius.

$Figure 3: Different Views of a DP780 Part with Small R/T Leading to Shear Fracture at Bead Radius. An R/T = 1.67 led to shear fracture on the left side of the image, while the symmetric area on the right side had an R/T = 4.4, with no shear fracture even though it had the same depth of draw, draw bead configuration, and restraining force.M-5$

Figure 3: Different Views of a DP780 Part with Small R/T Leading to Shear Fracture at Bead Radius. An R/T = 1.67 led to shear fracture on the left side of the image, while the symmetric area on the right side had an R/T = 4.4, with no shear fracture even though it had the same depth of draw, draw bead configuration, and restraining force.^U-6

It is helpful to describe the characteristic differences between conventional tensile fracture and shear fracture, as shown in Figure 4. A conventional tensile fracture is called a Type I fracture, and has been the typical fracture type historically encountered. Type I fractures occur off the radius, and is preceded by necking or metal thinning. Successful prediction of this type of fracture occurs with conventional application of strain analysis and Forming Limit Diagrams. This is in contrast with shear fracture – categorized as a Type III fracture – which occurs within the die radius, with no thinning from necking (typical for local formability failures). Type II fractures occur at or near the tangent of the radius in metal drawn over the radius.

$Figure 4: Schematic descriptions of different fracture types ranging from shear fracture to conventional tensile fracture.S-19$

Figure 4: Schematic descriptions of different fracture types ranging from shear fracture to conventional tensile fracture.^S-19

Numerous studies show that radius to thickness ratios (R/T) are significant indicators of performance with respect to shear fracture on AHSS. This research led to the establishment of R/T ratio guidelines. While bend testing also categorizes products based on achievable R/T, the significant difference is that the ends are not restrained in a standard bend test. Shear fracture testing typically involves some type of restraining force, such as that seen in a Bending Under Tension test.

As with edge fracture, AHSS grades may be available at a similar strength but with improved minimum R/T ratio. Guidelines have been established for minimum R/T ratios based on bend test results as well as real world case studies. For DP340/590 and above, the R/T ratio should be at least 3T for product features such as embossments where there is relatively limited metal motion. Pulling these grades across a radius or through a draw bead under tension increases the minimum R/T ratio to at least 5T.

As strength levels increase, it is necessary to increase R/T ratios to avoid shear fractures. One study recommended a minimum R/T of 8 for DP800^W-4 while another has a critical R/T of at least 7T for DP980^S-19. Differences such as these are likely due to different test conditions such as the tension applied during bending, test speed and lubrication. Higher deformation rates (forming speed) and better lubrication tends to promote shear fracture and cause fracture on material close to the die radius.^S-19 A combination of high forming speed and back tension leads to DP590 and DP780 having a critical R/T value of 12, with DP980 having a critical R/T of over 16.^S-20

Shear fracture is sensitive to rolling direction. If the radius is running in the rolling direction, the bend will be transverse to the rolling direction which is the worst-case scenario when trying to avoid shear fracture. Understanding the directionality of minimum R/T ratios when designing the part to avoid shear fractures is therefore important. One study evaluated DP780, which discovered a critical minimum R/T of 5 to avoid shear fracture, although this varied with back tension and pulling speed. At all R/T ratios tested, the samples oriented so the bend radius was parallel to the rolling direction failed at lower stress than if the bend radius was perpendicular (transverse) to the rolling direction. Tighter R/T magnifies this effect, as does a higher strength grade. At 1.5 R/T, shear fracture with bends perpendicular to the rolling direction occurred when the stress reached 81% of the tensile strength, where in samples with the bend parallel to the rolling direction, failure occurred at 66% of the tensile strength.^S-21

Microstructure also plays a role. Phase distribution uniformity, fine microstructural phases, and a decrease in hardness ratio between martensite and ferrite all increase formability as measured by the smallest achievable R/T resulting in split-free panels.^{W-4, H-6} These are the same factors that result in improved hole expansion values.

Forming Limit Curves are the limiting strain states based on the onset localized deformation, or necking. Generating conventional FLCs typically involves using a 100mm diameter hemispherical punch to deform 1mm to 2mm thick sheet steels, resulting in R/T ratios of 25 or higher. Many decades of use have shown that conventional FLC approaches can be used to assess part robustness and predict failure in areas having large R/T ratios and failure modes affected by global formability. These show up as Type I fractures.

When the R/T ratio is within the shear fracture limit but above the simple bending limit (no stretching imposed to bending), failure may occur in the radius or outside the radius, depending upon the tension level applied during forming. If the failure occurs inside the radius, the failure limit derived from the shear fracture tests should be used as the criterion to predict failure. Finally, when the R/T ratio is smaller than the limit from the simple bending test, the failure occurs in the radius.

In addition to large R/T, most Forming Limit Curves are generated from tests are conducted at a very low strain rate, maintaining an isothermal condition (no heat generated from deforming the sheets). While these conditions are consistent with those adopted for the simulations, they differ significantly from those encountered in industrial practice.

Ignoring the role of deformation-induced heating is one of the most significant reason for the limited success of shear fracture prediction in conventional forming simulation. Strain rates for stamping are typically 1,000x to 10,000x greater than the strain rates for tensile testing. Contact between the sheet metal and the tooling at tight radii also drives up the local temperature. Maximum temperature at the die-sheet interface of 70°C has been found on DP590, but 105°C for CP800.^F-6 Stamping DP780 leads to die temperatures of 180°C and blank temperatures of 108°C.^P-11 Stamping DP980 produces contact temperatures of over 200°C in certain conditions.^F-28

Using a simulation model that accounts for the thermal effects of forming and the associated response in the sheet metal, along with the traditional mechanical deformation response, has been shown to dramatically improve simulation accuracy to predict when shear fracture will occur.^{K-9, S-20, S-22}

Therefore, it is important for the part designer to design an appropriate die radius for a given AHSS product and forming conditions / processes used to manufacture the part. Forming simulation is an excellent tool to derive an appropriate die radius for the specified part and forming process, recognizing that the failure criterion in the simulation must incorporate all conditions and failure modes encountered, including shear fracture.

Key Points

- AHSS grades are at risk of crack formation in designs with small die radius to material thickness (R/T) ratios. These cracks are known as shear fracture.
- The strains associated with shear fracture are below that which are associated with the Forming Limit Curve.
- As with edge fractures, shear fractures are also a function of microstructure, strength level, and rolling direction. Consider these variables when designing parts and conducting die try-outs with prototype steel.

Global vs Local Formability

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.

$Figure 1: A fracture related to insufficient local formability. Note the lack of thinning near the fracture.$

Figure 1: A fracture related to insufficient local formability. Note the lack of thinning near the fracture. ^H-5

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).

Figure 2: Cut edge uniformity influences edge quality.^U-6

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.

$Figure 3: A typical local formability edge fracture viewed from different angles, with no appreciable thinning prior to fracture.U-6$

Figure 3: A typical local formability edge fracture viewed from different angles, with no appreciable thinning prior to fracture.^U-6

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.

$Figure 4: Classic local formability related edge fracture on an embossment within a DP980 part.U-6$

Figure 4: Classic local formability related edge fracture on an embossment within a DP980 part.^U-6

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

$Figure 5: Left: HSLA 50SK, showing thinning at the fracture location, which is typical for global formability failures Right: DP 500Y/780T, showing no thinning at the fracture location, which is typical for local formability failures.S-11$

Figure 5: Left: HSLA 50SK, showing thinning at the fracture location, which is typical for global formability failures. Right: DP 500Y/780T, showing no thinning at the fracture location, which is typical for local formability failures.^S-11

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

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.

Figure 6 caption: Higher strength steels have a smaller gap between the necking Forming Limit Curve and the Fracture Limit Curve.

Figure 6: Higher-strength steels have a smaller gap between the necking Forming Limit Curve and the Fracture Limit Curve.

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.

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