![Defining Steels]()
Metallurgy
Basis
There are different ways to classify automotive steels. One is a metallurgical designation providing some process information. Common designations include lower-strength steels (interstitial-free and mild steels); conventional high strength steels, such as bake hardenable and high-strength, low-alloy steels (HSLA); and Advanced High-Strength Steels (AHSS) such as dual phase and transformation-induced plasticity steels. Additional higher strength steels include press hardening steels and steels designed for unique applications that have improved edge stretch and stretch bending characteristics.
A second classification method important to part designers is strength of the steel. This document will use the general terms HSLA and AHSS to designate all higher strength steels. The principal difference between conventional HSLA steels and AHSS is their microstructure. Conventional HSLA steels are single-phase ferritic steels with a potential for some pearlite in C-Mn steels. AHSS are primarily steels with a multiphase microstructure containing one or more phases other than ferrite, pearlite, or cementite – for example martensite, bainite, austenite, and/or retained austenite in quantities sufficient to produce unique mechanical properties. Some types of AHSS have a higher strain hardening capacity resulting in a strength-ductility balance superior to conventional steels. Other types have ultra-high yield and tensile strengths and show a bake hardening behavior.
AHSS include all martensitic and multiphase steels having a minimum specified tensile strength of at least 440 MPa. Those steels with very high minimum specified tensile strength are sometimes referred to as Ultra High Strength Steels (UHSS). Several companies choose 980 MPa as the threshold where “Ultra” high strength begins, while others use higher thresholds of 1180 MPa or 1270 MPa. There is no generally accepted definition among the producers or users of the product. The difference between AHSS and UHSS is in terminology only – they are not separate products. The actions taken by the manufacturing community to form, join, or process is ultimately a function of the steel grade, thickness, and mechanical properties. Whether these steels are called “Advanced” or “Ultra” does not impact the technical response.
Third Generation, or 3rd Gen, AHSS builds on the previously developed 1st Gen AHSS (DP, TRIP, CP, MS, and PHS) and 2nd Gen AHSS (TWIP), with global commercialization starting around 2020. 3rd Gen AHSS are multi-phase steels engineered to develop enhanced formability as measured in tensile, sheared edge, and/or bending tests. Typically, these steels rely on retained austenite in a bainite or martensite matrix and potentially some amount of ferrite and/or precipitates, all in specific proportions and distributions, to develop these enhanced properties.
Nomenclature
Historically, HSLA steels were described by their minimum yield strength. Depending on the region, the units may have been ksi or MPa, meaning that HSLA 50 and HSLA 340 both describe a High Strength Low Alloy steel with a minimum yield strength of 50 ksi = 50,000 psi ≈ 340 MPa. Although not possible to tell from this syntax, many of the specifications stated that the minimum tensile strength was 70 MPa to 80 MPa greater than the minimum yield strength.
Development of the initial AHSS grades evolved such that they were described by their metallurgical approach and minimum tensile strength, such as using DP590 to describe a dual phase steel with 590 MPa tensile strength. Furthermore, when Advanced High Strength Steels were first commercialized, there was often only one option for a given metallurgical type and tensile strength level. Now, for example, there are multiple distinct dual phase grades with a minimum 980 MPa tensile strength, each with different yield strength or formability.
To highlight these different characteristics throughout this website, each steel grade is identified by whether it is hot rolled or cold rolled, minimum yield strength (in MPa), minimum tensile strength (in MPa), and metallurgical type. Table 1 lists different types of steels.
Table 1: Different Types of Steels and Associated Abbreviations.
As an example, CR-500Y780T-DP describes a cold rolled dual phase steel with 500 MPa minimum yield strength and 780 MPa minimum ultimate tensile strength. There is also another grade with the same minimum UTS, but lower yield strength: CR440Y780T-DP. If the syntax is simply DP780, the reader should assume either that the referenced study did not distinguish between the variants or that the issues described in that section applies to all variants of a dual phase steel with a minimum 780 MPa tensile strength.
Another syntax issue is the presentation of the strength (yield or tensile), and whether it is rounded to the nearest 10 or 50 MPa. For example, consider DP980 compared with DP1000. Both forms represent essentially the same grade. In Europe, this steel may be described as having a tensile strength of 100 kgf/mm2, corresponding to 981 N/mm2 (981 MPa), and expressed as DP980. In Asia, the steel may be referred to as 100K (an abbreviation for 100 kgf/mm2). In other parts of the world, it may be rounded to nearest 50 MPa, as DP 1000. This naming approach applies to many grades, with some shown in Table 2. In some cases, although the OEM specification may list the steel as DP800 (for example), the minimum tensile strength requirement may still be 780 MPa. Furthermore, independent of the chosen naming syntax, the steel company will supply to the actual specification requirements, and will use different process controls to meet a 780 MPa minimum compared with an 800 MPa minimum.
Table 2: Syntax Related to AHSS Strength Levels
Press hardening steels sometimes require a different syntax. Some OEMs will use a similar terminology as described above. For example: CR-950Y1300T-PH (PH stands for Press Hardenable or Press Hardened) or CR-950Y1300T-MB (MB stands for Manganese-Boron steel) can describe the same cold rolled press hardening steel with 950 MPa minimum yield strength and 1300 MPa minimum tensile strength after completing the press hardening operation. Other specifications may show suffixes which highlight the forming process used, such as -DS for direct hot stamping, -IS for indirect hot stamping and MS for a multi-step process. Furthermore, sources may describe this product focused on its typical tensile strength as PHS1500T. The abbreviation PQS (Press Quenched Steel) is typically used for grades that do not harden after hot stamping. These may be noted as PQS450 and PQS550, where the numbers stand for the approximate minimum tensile strength after the hot stamping cycle (see the section on Grades With Higher Ductility on the linked page).
Graphical Presentation
Generally, elongation (a measure of ductility) decreases as strength increases. Plotting elongation on the vertical axis and strength on the horizontal axis leads to a graph starting in the upper left (high elongation, lower strength) and progressing to the lower right (lower elongation, higher strength). This shape led to the colloquial description of calling this the banana diagram.
Figure 1: A generic “banana” diagram comparing strength and elongation.
With the continued development of advanced steel options, it is no longer appropriate to describe the plethora of options as being in the shape of a banana. Instead, with new grades filling the upper right portion (see Figure 2), perhaps it is more accurate to describe this as the football diagram as the options now start to fall into the shape of an American or Rugby Football. Officially, it is known as the steel Global Formability Diagram.
Even this approach has its limitations. Elongation is only one measure of ductility. Other ductility parameters are increasingly important with AHSS grades, such as hole expansion and bendability. BillurB-61 proposed a diagram comparing the bend angle determined from the VDA238-100 testV-4 with the yield strength for various press hardened and press quenched steels.
Figure 3: VDA Bending Angle typically decreases with increasing yield strength of PHS/PQS grades.B-61
Figure 4 shows a local/global formability map sometimes referred to as the Hance Diagram named after the researcher who proposed it.H-16 This diagram combines measures of local formability (characterized by true fracture strain) and global formability (characterized by uniform elongation), providing insight on different characteristics associated with many steel grades and helping with application-specific material grade selection. For example, if good trim conditions still create edge splits, selecting materials higher on the vertical axis may help address the edge-cracking problems. Likewise, global formability necking or splitting issues can be solved by using grades further to the right on the horizontal axis.
Figure 4: The Local/Global Formability Map combines measures of local formability (true fracture strain) and global formability (uniform elongation) to highlight the relative characteristics of different grades. In this version from Citation D-12, the colors distinguish different options at each tensile strength level.
Grade Portfolio
Previous AHSS Application Guidelines showcased a materials portfolio driven by the FutureSteelVehicle (FSV) program, with more than twenty new grades of AHSS acknowledged as commercially available by 2020. The AHSS materials portfolio continues to grow, as the steel industry responds to requirements for high strength, lightweight steels. Table 3 reflects available AHSS grades as well as grades under development and nearing commercial application. The Steel Grades page provides details about these grades and their applications.
Table 3: Commercially available AHSS Grades and grades under development for near-term application. Grade names shown in Italicized Bold were available in FutureSteelVehicle. For all but PQS/PHS, Grade Name indicates the minimum yield strength, minimum tensile strength, and the type of AHSS. The min EL column indicates a typical minimum total elongation value, which may vary based on test sample shape, gauge length, and thickness. PQS/PHS grade name indicates nominal tensile strength.
Global automakers create steel specification criteria suited for their vehicle targets, manufacturing infrastructure, and other constraints. Although similar specifications exist at other companies, perfect overlap of all specifications is unlikely. Global steelmakers have different equipment, production capabilities, and commercial availability.
Minimum or typical mechanical properties shown on this web page and throughout this site illustrates the broad range of AHSS grades that may be available. Properties of hot rolled steels can differ from cold rolled steels. Coating processes like hot dip galvanizing or galvannealing subjects the base metal to different thermal cycles that affect final properties. Test procedures and requirements have a regional or OEM influence, such as preference to using tensile test gauge length of 50 mm or 80 mm, or specifying minimum property values parallel or perpendicular to the rolling direction.
Steel users must communicate directly with individual steel companies to determine specific grade availability and the specific associated parameters and properties, such as:
- Chemical composition specifications,
- Mechanical properties and ranges,
- Thickness and width capabilities,
- Hot-rolled, cold-rolled, and coating availability,
- Joining characteristics.
Formability, true fracture strain
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Fracture strain values derived from standard uniaxial tension tests can be used to evaluate automotive crashworthiness and forming behavior of aluminum alloys and Advanced High-Strength Steels (AHSS).Y-10, W-24, W-25, L-22, L-23, T-22, H-15, L-24 The true fracture strain (TFS) and similar concepts have emerged recently as intrinsic measures of local formability for AHSS. TFS is the true (logarithmic) strain associated with the “zero-gage-length” elongation at fracture (e0), where e0 is a conceptual engineering strain value based on an infinitesimal gage length in a tension test, where:
|
Equation 1 |
and Ao and Af are the cross-section area before testing and the fracture area after testing, respectively (constant volume assumed).D-13 It follows that TFS is defined as:
 |
Equation 2 |
TFS is related to percent reduction of area at fracture (Z, %), where:
 |
Equation 3 |
True Fracture Strain (TFS) Measurement Methods:
Fracture Area (Af)
Tensile test samples have a rectangular cross section before testing, indicated in the left image in Figure 1. The right image shows an idealized tension test specimen fracture surface, where five thickness measurement locations are indicated: two at the edges (ta, te); one at the center (tc), and two at the quarter-width positions (tb, td). Measured approximately at mid-thickness, wf is the fracture width. The dashed outline indicates the original specimen cross-section before testing (to·wo = Ao), and a 10-sided polygon (decagon) approximates the projected fracture area (Af), where the corners of the polygon correspond width-wise to the five thickness measurement locations.
Figure 1: Schematic representation of a tension test specimen before testing (left) and after fracture (right) viewed along the tensile axis.
From the dimensions portrayed in Figure 1, four possible methods to determine Af are indicated in Table 1—three lineal methods and one areal method.H-17 Method A uses a single thickness measurement at mid-width. Alternatively, for Method A the minimum thickness (tmin) may be used in cases where tc ≠ tmin. Method B uses a weighted three-thickness average A-24 —also known as the ASTM parabolic method.H-14, H-16 Method C uses a five-thickness average. For Method D, Af is the area of the polygon depicted in the left image of Figure 1.
Table 1. Fracture Area Measurement Methods.H-17
Dimensional measurements may be made quickly and conveniently with a calibrated digital microscope equipped with focal stacking capability and linked to image analysis software. Note that tf, wf and Af are respectively: the projected thickness, the projected width, and the projected area of the fracture surface. That is, to accommodate irregular and angled fracture surface features, measurements are made with respect to a virtual plane normal to the tensile axis. Alternatively, lineal measurements may be made with a conventional microscope equipped with a dial indicator or measuring stage. In this case, continual manual refocusing may be necessary to ensure true projected dimensional measurements.
Recently the effects of fracture area measurement method and tension test specimen type on fracture strain values were evaluated H-17. It was concluded that specimen type (i.e., width-to-thickness ratio) has a far greater impact on the consequent fracture strain value in contrast to that of measurement method. It was also advised that, when reporting fracture strain values derived from tension tests, the specimen type, the material thickness, and the fracture area measurement method be clearly indicated.
True Fracture Strain (TFS) Measurement Methods:
Fracture Types
Based solely on fracture appearance, various fracture types may be observed, with examples shown in Figure 2. H-17 The fracture type changes from Type 1 to Type 2 to Type 3 as the tension test specimen width-to-thickness ratio (wo/to) increases (left to right). Type 1 fracture is perpendicular to the tensile axis across the specimen width; Type 2 fracture is an irregular transition from Type 1 fracture to Type 3 fracture; and Type 3 fracture is aligned at an angle across the specimen width (~50-60° from the tensile axis).
Figure 2: Various fracture types observed in uniaxial tension testing, from left to right: Type 1 fracture (wo/to = 4.5—ASTM Standard Sub-size specimen); Type 2 fracture (wo/to = 12.8—ASTM Standard specimen); and Type 3 fracture (wo/to = 16.8—JIS No. 5 Standard specimen); The original width-to-thickness ratio (wo/to) is measured within the gage section of the test specimen.H-17
A similar fracture orientation dependence on specimen width was reported for Dual Phase (DP) steels more than thirty years ago by researchers at the Colorado School of Mines.S-49 A 2018 publication W-23 confirmed this behavior for other AHSS types, and illustrated that the transitional behavior—in terms of critical width-to-thickness ratio—is material dependent.
Figure 3 shows example Type 1 and Type 3 fractures in cross-section at the mid-width position (corresponding to position tc in Figure 1). Type 1 fractures typically run at an angle through thickness (~50-55° from the tensile axis). While most Type 1 fractures resemble that shown in Figure 2 (left) and that shown in Figure 3a, occasional through-thickness chevron profiles and “cup and cone” W-23 type fractures have been observed for Type 1 fractures. Nevertheless, Type 1 fractures are roughly symmetric about a plane normal to the sheet surface at the mid-width position.
Figure 3. Examples of (A) Type 1 fracture, and (B) Type 3 fracture; Polished through-thickness cross-sections at the mid-width position; Tensile axis is horizontal.
Type 3 fractures invariably show localized necking in through-thickness cross-section as in Figure 3b. Therefore, Type 3 fractures are roughly symmetric about a plane parallel to the sheet surface at the mid-thickness position. Type 2 fractures have both Type 1 and Type 3 characteristics at different positions across the width and thus have no overall plane of symmetry. Various degrees of damage (void formation) are observed in through-thickness cross-sections of fractured specimens— for example, Figure 3. Citation H-18 contains a detailed compendium with more information on this topic.
ElsewhereW-23, L-21 it was explained that the idealized fracture thickness profile—as depicted in the right image in Figure 1—is applicable only to smaller width-to-thickness ratios (thicker, hot-rolled materials or narrower gage sections). For thinner, cold-rolled materials, or for wider gage sections, there is often no clear fracture thickness minimum at the mid-width position. In fact, in some cases a fracture thickness maximum at mid-width has been observed. Furthermore, occasional mid-thickness delamination renders the volume-constancy assumption in question, with associated implications in Equation 1
Formability Classification and Rating System
In 2016 the foundation for a formability classification and rating system was introduced for AHSSH-14, where formability performance expectations are distinguished by the relationships between true fracture strain (TFS) and true uniform strain in a tension test. Such performance mapping concepts continue to be explored and modified by steelmakersH-16, W-23, L-21, D-12, W-22, V-5, R-6, S-48 by automakersH-18, H-19 and by international industry consortiums.G-20 Traditionally AHSS performance has been represented by the product of ultimate tensile strength and total elongation (UTS x TE) and relative position on the so-called “banana diagram” or Global Formability Diagram. While this conventional methodology discriminates behavioral extremes, much is lost regarding the nuances of local formability.
Intrinsic Formability Parameters
Widely considered an intrinsic measure of global formability, the true uniform strain (εu) is the logarithmic strain associated with uniform elongation (UE, %) in a uniaxial tension test, where:
 |
Equation 4 |
Example TFS and εu values are shown for a series of 980-class AHSS (980 MPa minimum tensile strength) in Figure 4H-14. In this analysis, TFS values ranged from less than 0.5 [DP 980 (LSi)] to more than 1.0 (CP 980), and εu values ranged from 0.05 (CP 980) to 0.15 (GEN3 980). The Third Generation AHSS materials (GEN3-type) have the largest εu values, and the Multi-Phase/Complex-Phase steels (MP/CP-type) have the largest TFS values. As a group the Dual Phase steels (DP-type) have intermediate εu values and a wide range of TFS values. As illustrated in Figure 4, TFS is far greater than εu. A similar disparity between fracture strain and uniform strain was shown in Citation D-14, and no consistent relationship between the two parameters was determined.
Figure 4: True uniform strain (εu) and true fracture strain (TFS) values for a series of 980-class AHSS; Error bars show the range among three test specimens for each material.H-14
The local/global strain ratio (L/GSR) and the formability index are key parameters to guide application-specific material selection and to help set targets for future AHSS grade developments.H-14, H-16 The L/GSR reflects the relative preponderance of local formability to global formability and is defined as:
 |
Equation 5 |
The L/GSR is useful in understanding relative intrinsic formability “character”. Materials with higher relative TFS values are naturally expected to perform better under flanging, edge stretching and tight-radius bending conditions [e.g., MP 980 (LCE) and CP 980 in Figure 4], while materials with higher relative uniform strain values (i.e., higher terminal n values) are better suited for stretch forming and are able to distribute strain more uniformly (e.g., GEN3 980 and GEN3 980-HY in Figure 4).
Furthermore, the formability index (F.I. in the formula) is defined as:
 |
Equation 6 |
This index represents an intermediate strain value between εu and TFS and provides a convenient measure of the overall formability expectation, where both local formability and global formability are considered. As an example, Figure 5 shows an exponential relationship between F.I., and the limiting bend ratio (r/t) determined from 90° V-bend testing for the same series of 980-class AHSS represented in Figure 4.H-14 It was reasoned that in the early stages of deformation, global formability (εu) dictates the strain distribution around the punch nose, while fracture resistance is governed by local formability (TFS) in the latter stages of deformation.
Figure 5. Correlation between the limiting bend ratio (f) and the formability index (F.I.) for a series of 980-class AHSS. A higher formability index corresponds to a lower (better) limiting bend ratio in 90° V-bend testing.H-14
Local/Global Formability Map
Figure 6 illustrates the essential framework of the local/global formability map—known eponymously as the Hance diagram.H-14, D-12, G-20 Here, the dashed lines represent the boundaries between global character (L/GSR < 5), balanced character (5 < L/GSR < 10), and local character (L/GSR > 10). The continuous curves represent arbitrary iso-F.I. contours corresponding to the values indicated (in parentheses). The qualitative assessments (Poor through Excellent) indicated for each formability level are also arbitrary; however, these performance level monikers were chosen to reflect real-world experience. Portrayed in this way (in contrast to Figure 4, for example), the relationships between true fracture strain and true uniform strain are more discernable, and both the formability character and the formability level become apparent.
Figure 6: Essential framework of the local/global formability map concept.H-16
Case Study Using the Local/Global Formability MapH-15
The basic utility of the local/global formability map concept was demonstrated for an automotive seating system development program.H-15 In this case study, stamping trials were conducted with two 980-class AHSS (980 MPa minimum UTS designation):
- 1.6mm 980DP(LSi): A classic Dual Phase (DP) ferrite/martensite steel with low silicon content (LSi), and
- 1.6mm 980MP(LCE)—a Multi-Phase (MP) steel with high yield strength and low carbon equivalent (LCE).
Basic formability parameters are summarized in Table 2 for the trial materials. Based solely on elongation values (UE, TE), one might have concluded that the formability of 980DP(LSi) would exceed that of 980MP(LCE).
Table 2. Formability Parameters for Two 980-Class AHSS.
However, the stamping trial results were counterintuitive and drastically different among the two trial materials. The 980DP(LSi) material exhibited severe edge-cracking in multiple locations, while the 980MP(LCE) material ran without issue. Both materials were free of necking-type failures as predicted by computer simulations (sufficient global formability). An example part overview is shown in Figure 7. Typical 980DP(LSi) edge cracks are shown in Figure 8 for a pierced/extruded hole (Location 1) and for a blanked/stretched perimetric edge (Location 2). Clearly, relative sheared-edge ductility may not simply be deduced from conventional tensile elongation values.
![Figure 7: Overview of a stamped automotive seating component [length ~ 480 mm (19 in.)]. H-15](https://ahssinsights.org/wp-content/uploads/2020/10/2.3.5a-Figure7.svg)
Figure 7: Overview of a stamped automotive seating component [length ~ 480 mm (19 in.)].H-15
![Figure 8: Close-up views of Location 1 (left) and Location 2 (right) identified in Figure 7 [material: 980DP(LSi)]; Location 1 is a pierced hole that was extruded during forming, and Location 2 is a blanked perimetric edge that was stretched during forming (underside with respect to Figure 7). H-15](https://ahssinsights.org/wp-content/uploads/2020/10/2.3.5-Fig8.jpg)
Figure 8: Close-up views of Location 1 (left) and Location 2 (right) identified in Figure 7 [material: 980DP(LSi)]; Location 1 is a pierced hole that was extruded during forming, and Location 2 is a blanked perimetric edge that was stretched during forming (underside with respect to Figure 7).H-15
The local/global formability map coordinates of the 980DP(LSi) and 980MP(LCE) trial materials are shown in Figure 9. With reference to the framework described in Figure 6, 980DP(LSi) exhibits global/balanced character with an overall borderline rating of Fair/Good (F.I. = 0.20); while 980MP(LCE) has decidedly local character with an overall rating of Good (F.I. = 0.26). Furthermore, the TFS value of 980MP(LCE) is more than twice that of 980DP(LSi).
Figure 9: Local/global formability map featuring three 980-class AHSS—980DP(LSi), a classic DP ferrite/martensite steel with low silicon content (LSi); 980MP(LCE), an MP steel with high yield strength and low carbon equivalent (LCE); and 980GEN3, a third generation AHSS. Image based on Citations H-15 and H-17.
As an independent confirmation of the local formability advantage of 980MP(LCE), the hole expansion ratio (HER, λ) is more than four times that of 980DP(LSi) (Table 2). A strong correlation between TFS and λ has been confirmed by several authors L-22 L-23 T-22 H-15 As the component featured in this case study is dominated by extremely challenging edge-stretching conditions, 980MP(LCE) is the clear wiser material selection. However, in applications with more demanding global formability requirements, other issues such as intolerable strain localization could arise, and a third generation AHSS might be the best choice. As an example, when contrasting 980GEN3 AHSS (third generation AHSS) to 980DP(LSi) in Figure 9, the intrinsic global and local formability parameters (εu and TFS), as well as the F.I, are approximately 50% greater.
True Fracture Strain (TFS): Alternatives to TFS
While the local/global formability map methodology was developed in the context of true fracture strain (TFS), there are other ways to represent intrinsic local formability (fracture resistance) with data derived from standard uniaxial tension tests. In the original conception (Hance diagram), true uniform strain (εu) and TFS are two points along a logarithmic strain continuum from zero to fracture, and the relationships between these values elegantly describe the formability character (local/global strain ratio) and the overall formability level (formability index). The “best” local formability parameter may be a matter of practicality or applicability, or simply a matter of preference. Each method has its strengths and weaknesses, and such fracture strain concepts continue to evolve.
True Thinning Strain at Fracture
The true fracture strain (TFS) value is an area-based measurement of fracture strain and thus reflects the tension test specimen width change as well as the thickness change, for better or for worse. Citation H-18 suggests that the true thinning strain at fracture (ε3f) is a more appropriate measure of local formability, where:
 |
Equation 7 |
and to and tf are the original thickness (before testing) and final thickness (after fracture). By convention ε3f is a positive value and represents the absolute value of the true thickness strain at fracture (a negative value). Furthermore, the post-uniform portion of the fracture strain may be isolated by subtracting the uniform component of thinning strain, where:
 |
Equation 8 |
and εu and r are the true axial uniform strain and the plastic strain ratio (normal anisotropy) measured during the tension test, respectively. Post-uniform fracture strain components might be more relevant to materials with high uniform elongation values such as TWIP steels H-18. In a similar way, the post-uniform portion of the area-based TFS value may be expressed as:
 |
Equation 9 |
Another study W-23 concluded that: (1) area-based fracture strain measurements such as TFS result in less experimental scatter when compared to thickness-based fracture strain measurements such as ε3f, and (2) area-based measurements show less dependence upon the method by which fracture strains are determined. It appears that a single thickness measurement may misrepresent the fracture strain and that a multiple-thickness (average thickness) or area measurement approach may be more stable.
Critical Fracture Strain
Perhaps a lesser-known and under-exploited representation of fracture strain is the so-called critical fracture strain value (CFS)—introduced in 1999 for aluminum alloys in Citation Y-10 and re-visited in 2007 in the context of High-Strength Steel in Citation W-24. In concept CFS is the estimated true thinning strain at fracture, where:
 |
Equation 10 |
In determining CFS, only the engineering stress-strain data from a uniaxial tension test are needed—that is, no post-fracture area or thickness measurements are required: eu is the engineering uniform strain value (% uniform elongation/100); sf is the engineering fracture stress; and su is the ultimate tensile strength or UTS. Figure 10 shows an example engineering stress-strain curve for a 980-class AHSS, where the parameters relevant to CFS are marked. In this example, CFS = -ln[(1-0.07/2)·(787/1015)] = 0.29.
Figure 10: Example engineering stress-strain curve for a 980-class AHSS. Here, eu is the engineering uniform strain, su is the ultimate tensile strength (UTS), sf is the engineering fracture stress, CFS is the critical fracture strain, and ε3f is the true thinning strain at fracture.
True Fracture Strain (TFS):
Correlation to Hole Expansion Ratio
In a recent studyL-5, tensile properties (80 mm gage length) and hole expansion ratio were measure for AHSS with minimum tensile strength designations ranging from 600 to 1200 MPa, and thickness between 1 and 2 mm. No particular correlation was found between the hole expansion ratio and conventional tensile properties such as uniform elongation, total elongation, n-value, and so on.
The most promising relationship was found between the hole expansion ratio (converted to logarithmic strain) and the true thinning strain at fracture in tension. This relationship is illustrated in Figure 11 for total thinning strain (ε3f, Equation 7) on the left, and for post-uniform thinning strain (ε*3f, Equation 8) on the right. In both cases, better correlation is shown for transverse tension tests rather than for longitudinal tension tests (linear fit through the origin).
Figure 11: Hole expansion ratio (logarithmic strain) as a function of true thinning strain at fracture. Left graph: total thinning strain (ε3f); Right graph: post-uniform thinning strain (ε*3f).L-5
While the above correlations are good, the inherent scatter associated with the hole expansion test, fracture strain measurements, and other local formability parameters may limit applicability in a production environment. Furthermore, various factors affect hole expansion in production environments, including hole preparation technique, edge condition, and cutting clearance.
 |
Thanks are given to Brandon Hance, Ph.D., who contributed this article. |
![Figure 7: Overview of a stamped automotive seating component [length ~ 480 mm (19 in.)]. H-15](http://ahssinsights.org/wp-content/uploads/2020/10/2.3.5a-Figure7.svg)
![Figure 8: Close-up views of Location 1 (left) and Location 2 (right) identified in Figure 7 [material: 980DP(LSi)]; Location 1 is a pierced hole that was extruded during forming, and Location 2 is a blanked perimetric edge that was stretched during forming (underside with respect to Figure 7). H-15](http://ahssinsights.org/wp-content/uploads/2020/10/2.3.5-Fig8.jpg)
