Failure Analysis of Advanced High Strength
Steels during Stretch-bending
EWI: W. Gan and S. S. Babu
OSU: R. Wagoner
Deform: W. Wu
1. Background & Current Status
Shear cracking has been observed during stamping of advanced high strength steels (AHSS),
without appreciable thinning before fracture. (See Figure 1, from the PI’s research, for the
difference between plastic localization and shear fracture in tension.) Review of literature shows
that such shear cracking is not typically encountered in traditional HSLA steels during forming.
Moreover, this type of early fracture cannot be predicted by conventional forming limit diagram of
AHSS. Therefore, a new failure criterion needs to be developed to predict shear cracking so that
product and tooling design can be performed correctly to avoid fracture during stamping.
The absence of shear failure in HSLA steels under
normal forming conditions as compared to AHSS
requires both fundamental understanding and practical
methods for designing forming operations. Assuming
that the lubricant condition and strain histories during
forming are similar for AHSS and conventional steels,
the obvious difference is the underlying microstructure
or possibly a dramatic difference of constitutive
response at high strain. While HSLA has only ferrite
and pearlite microstructure in the sheet, most AHSS
have multiple microstructures co-existing, such as
ferrite, bainite, martensite, and retained austenite. The
complexity of the microstructure may give rise to a
shear-localization
mode
that
involves
little
macroscopic necking under some forming conditions.
In order to address the needs of automotive sheet
Fig. 1 – Comparison of failure modes
forming with AHSS, a multi-faceted project is
in tensile deformation: shear failure
proposed.
It makes use of microstructural
of Mg AZ31B alloy (left), tensile
characterization, fracture analysis, macroscopic
property
determination,
practical
experiments
localization of Al 6013 (right). [1]
representing forming practice, and continuum
simulation. Two major outcomes are expected: a fundamental understanding of the shear
fracture mechanism, and an implemented fracture criterion in FEM that can predict both necking
and shear failures during forming.
1.1.
Microstructural Effects on Shear Cracking
Note: There is little published data for the fracture mechanisms of AHSS, particularly the
balance between shear failure and normal necking failure. One of the points of interest will
be identifying the conditions of forming and material microstructure that divide these
1
mechanisms. However, regardless of mechanism, the proposed draw-bend test results will
incorporate both kinds of failure such that they may be implemented in FEM codes.
AHSS are composed of soft ferrite and hard martensite microstructures, with widely varied strain
hardening behaviors. The heterogeneity in microstructure in these steels leads to non-uniform
deformation among the phases. The interaction between the phases can cause stress
concentration and strain partition [1]. Since sheet metal forming deals with strain limiting
conditions, small cracks can be induced in the surface with martensitic islands. With subsequent
forming conditions, these small cracks may grow and lead to early fracture.
It is well known that the strength and crack resistance of multi-microstructure steels depends on
their morphology , distribution, and hardening of individual microstructures [2-7]. Some steels
have high fracture toughness through local crack branching [8] or martensite transformation [911]. These findings show that microstructure plays an important role in the fracture properties of
material.
For example, the failure mode of dual phase steels, which consist of a ductile matrix of ferrite with
a distribution of hard martensitic phase, depends on the distribution and volume fraction of the
martensite. When the martensitic phase is a relatively low volume fraction and/or is finely
dispersed, the failure mode tends to be one of void nucleation, growth, and coalescence, where
voids nucleate around martensite phases, around other inclusions, or along martensite/ferrite
grain boundaries [12-14]. This is the dominant case for typical dual phase steels (Figure 1).
However, when the martensite phase is coarse, or when the volume fraction of the martensite
phase is sufficiently high that it forms an interconnected network along the ferrite grain
boundaries, the failure mode tends to be brittle cleavage along the martensite grains [13, 14].
Void nucleation and growth around a
martensite particle[3]
Void nucleation and growth at the
ferrite/martensite grain boundary[2]
Figure 1. Microvoid nucleation and growth mechanism of fracture in dual phase steels.
It has also been observed that material properties vary from the surface to sheet mid-thickness
[15]. Figures 2a and 2b show the microstructures of two AHSSs. There are bands of martensite
phase parallel to the sheet plane. The distribution of the bands across sheet thickness is not
uniform. Vicker’s hardness measurement revealed that the DP 780 material is harder near
surface than the mid-thickness region, while the TRIP steel shows opposite [15]. This type of
through thickness property variation has significant effect on springback [15]. It is believed that
the through thickness property variation will also have important effect on the formability of sheet
metals and perhaps on the failure mechanism. The stronger surface region may have less
ductility. The strength mismatch could also bring stress concentrations during forming. As a
result, the overall formability of the blank may be very different than its counterpart with uniform
properties through thickness. this hypothesis has not been proved either experimentally or
numerically, but is one aspect to be explored in the proposed project.
2
(a)
(b)
Figure 2. Banding microstructures in A) DP 780, b) TRIP 780
Recently, Kobe steel in Japan has developed a steel taking advantage of the concept of through
thickness property variation [16]. Its researchers designed a novel steel that has softer surface
than the mid-thickness region. Experiment validation showed that the bending and forming
properties of this new steel is improved significantly than other steels with similar overall strength.
The combined effect of microstructure heterogeneity and through thickness property variation can
have profound impact on the formability of AHSS. Both of the factors will increase the tendency
of stress strain concentration. The mechanism under which they determine sheet formability is
still unclear. The question will be answered using a novel formability test (introduced in section 2)
that replicates the most common sheet forming conditions in a precisely-controlled laboratory
environment. A new failure criterion based on the test results will then be developed and
implemented.
1.2.
Formability Testing and Failure Criteria
Fracture of the work-piece during stamping, extrusion, and drawing are a subject of concern
during large deformation cold and warm forming processes. For bulk forming processes, the
failure criteria are often based on ductile fracture. Details of these models can be found in [1720]. They are based on an integration of a stress measure over a strain range:
 f ( ) d   C
(1)
Where  represents the stress,  represent the strain, and C is the integrated damage value.
For many sheet forming process, Forming Limit Diagram (FLD) has been conventionally used for
the evaluation of failure in stretching operations. It is also based on necking and ductile failure.
By plotting the measured major and minor strains at a neck or fracture it is possible to construct
FLD, which gives the failure condition and the maximum allowable strain levels for the specific
forming operations. Most theoretical studies on FLD have been based on the localized necking
approach developed by Marciniak and Kuczynski [21]. In this approach, the existence of material
imperfections is assumed to lead the unstable growth of strain in the weaker regions due to the
presence of inhomogeneities in loading bearing capacity, and subsequently produce localized
necking and failure.
Since the material response of sheet metal relies on micro-properties, a recent trend of research
is to establish a link between micro-structure properties and failure. A fracture model for dual
phase steel, where the primary mechanism is one of void nucleation, growth, and coalescence,
3
was developed by Barlat et al for use in predicting FLD [12]. This model presumes that all microvoid nuclei occur at hard particle inclusions, and that all of the nuclei were created during the
rolling process of the dual phase sheet (i.e., none are created during the sheet forming process
itself). These micro-voids then grow and coalesce during sheet forming, increasing the relative
volume fraction of voids in the material. The kinetics with which this volume fraction of voids
increases is more rigorously determined in Barlat’s model than previous, more arbitrary models
[12]. This permits a defect-density damage-type computation which, applied to flow localization
(necking) in thin sheet, leads to strain limits which agree with experimental dual phase steel
forming limits. These limit strains calculated during necking depend heavily on the void aspect
ratio.
A fundamental drawback of strain based FLDs is that they are valid only during proportional
loading [22-24]. That is because the constitutive equations governing the stress strain relations
are path-dependent [25]. During metal stamping process, the stress strain state in the part
changes continuously and it differs from location to location throughout the component. A unique
FLD is required for every material point in-order to analyze the formability using strain base
criteria. In order to solve this complexity, promising stress base failure criteria have been
proposed [23, 26-29].
However, all the above failure criteria can be categorized to ductile failure models. They cannot
predict the shear cracking of AHSS during stamping. The apparent shear failure shows some
characteristics of the transition from ductile to brittle failure. While the crack initiation may be
ductile, its propagation can change to brittle under certain loading conditions [30, 31].
Furthermore, the shear failure may be dependent on both microstructural effects and strain path
effect.
Based on the above literature survey, one can conclude that a practical criterion for predicting
shear fraction is not currently available. Therefore, this limits the application of computational
predictive tools for the use in stamping design of AHSS in certain geometry. The development of
such a criterion will require both a novel formability testing method and the microstructure
analysis of the base material and cracked regions.
2. Research Tasks
The proposers will address the goals of Auto / Steel Partnership by first examining samples that
show cracking in typical forming operations. With that as the starting point, the goals of the ASP
will be addressed by the following three tasks:
(a) Characterization of the failed samples;
(b) Draw-bend-fracture (DBF) testing and characterization of failure
(c) Development of a fracture criterion to be implemented in FEA analysis.
The characterization task will focus on three activities. The first activity is related to the
characterization of failed part to be supplied by ASP will be performed. The second activity will
be related to characterization of samples that will be DBF tested. The third activity will be related
to characterization of failed stampings provided by ASP partners with candidate steels throughout
the project period. Characterization will be performed at EWI and OSU.
The second task will focus on the development of a draw-bend-fracture (DBFDBF) testing
methodology. This test will be performed on base line, ASP candidate steels and experimental
steels to be supplied by collaborating steel companies. This test will be developed and
performed at OSU under the supervision of Prof. R. Wagoner. The test results will be correlated
to the observed shear failures in existing parts provided by ASP partners. The DBF test was
developed and used successfully in the failure investigation of HSLA steels under draw-bending
4
conditions [19, 32]. The same testing methodology will be used in the analysis of shear cracking
of AHSS, with some modifications on the grip and tooling.
The third task will involve the development of a failure criterion based on the results from the first
two tasks. The criterion will be developed and tested with ABAQUS, LS-DYNA and Deform
software. The development and testing will be performed by EWI, Deform and OSU. The failure
criteria will be tested for other industrial stamping conditions to be provided by ASP. ASP
members will be encouraged to provide in-kind evaluation of the failure criteria to be developed in
this task. Furthermore, many of the continuum deformation properties needed for FE simulation
will be provided by ASP members who develop or use the candidate materials. This approach
will facilitate the demonstration of the concept and immediate technology transfer to the practicing
engineers.
The details of the research tasks are described below:
2.1.
Samples and Candidate Steels
The scope of test samples and new steels for comparative evaluation of shear failure tendencies
are tabulated in Table 1. The scope of the testing and samples can be changed based on the
input from the ASP partners who will be providing in-kind support. The proposal envisions a
close collaboration form the ASP members including Ford, GM and DCX. Discussions with key
individuals revealed a willingness to share their proprietary data to evaluate the concepts to be
developed as a part of this proposal. In addition, steel companies including Arcelor have shown
interest in providing experimental steels to evaluate the shear tendency.
Steels
ASP Samples*
HSLA
DP 780
TRIP 780
Candidate Steels**
Characterization
Yes
DBF Test
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Failure Criteria
yes
Yes
Yes
Yes
*The
team requests the ASP members provide the steels and failed parts. These will be referred
to as “ASP samples” throughout the proposal.
**The team has contacted Arcelor which has agreed to provide some experimental steels as costsharing for this research program.
2.2.
Characterization
The characterization work consists of base material and cracked sample characterizations. The
microstructure and mechanical properties will be examined for baseline information, then the
failure analysis of the failed samples will be performed to study crack initiation and propagation.
2.2.1. Base Material Characterization
Base material properties of ASP samples will be obtained through hardness measurement,
tensile testing and orientational imaging microscopy. The spatial hardness variation will be
measured through sheet thickness and along the transverse direction. Tensile tests will be
conducted for the full-thickness samples to get the baseline strength. Reduced-thickness
samples will be preparedby thinning the sheet metal to ¼ thickness, retaining either the center
part or the surface part. These samples will be tested and the results will be correlated with the
hardness measurements.
5
Orientation Imaging Microscopy (OIM) will be performed on sheet surface and mid-surface. The
texture information will be used with polycrystalline analysis [33-35] to simulate yield surface and
estimate material property differences through the thickness attributable to texture changes only.
The predicted yield stress, hardening rate, and r values from the polycrystalline simulations will
be compared with reduced-thickness tensile tests.
2.2.2. Microstructure Characterization of Cracked Samples
The microstructure of the failed samples in the surface and through thickness will be examined
using optical microscopy and OIM. The variation on the fracture appearance as a function of
through thickness will be characterized. The quantitative optical microscopy will be performed to
see the morphological distribution with reference to failure modes. In addition, orientational
imaging microscopy will be performed very close to the fracture surface. Based on the OIM, the
extent of strain partitioning will be evaluated near to the fracture surface in the surface and also
through the thickness. Microstructure characterization will provide a fundamental understanding
of process-material interaction that leads to cracking without thinning during stamping.
2.3.
Stretching-bending Fracture (DBF) Testing
Sheet metal fracture often happens near draw-bead and transitional radius areas. Material
experiences bending , unbending, and stretching simultaneously in these areas.
The
conventional forming limit diagram captures sheet failure in stretching-dominatedmode. (This is
the reason the FLD testing makes use of very large R/t ratios, where R is the radius of bending
and t is the sheet thickness.) In order to incorporate the bending-and-unbending deformation
history into failure criteria, along with the simultaneous imposition of these deformation modes, a
new fracture testing procedure is proposed in this program.
2.3.1. Concept of Testing:
Novel fracture testing is based on a modification of the draw-bend testing that has been used for
friction and springback testing [36-39] by R. H. Wagoner, Figures 3a and 3b. In this test, a strip
of sheet metal is constrained and bent around a roller by two grips. While one grip applies back
force, Fb, to the sample, the other grip pulls the sample downwards so that material flows over the
roller under controlled conditions.
In conventional friction and springback tests, Fb is kept constant throughout the test cycle. The
proposed DBF testing will use a varying back force that increases linearly with drawing distance.
The back force will keep increasing until cracking of the sample is detected. Thus, for a given R/t
ratio (fixed for a single test by the roller/pin radius and the sheet thickness), a critical value of
draw force, Fb*, will be determined. This procedure will detect both normal plastic localization /
necking and shear fracture. Therefore, any failure criterion constructed using this data will reflect
both modes and will be applicable to typical sheet forming practice because of the similitude
between the DBF and drawing over a die radius.
6
Lower
grip
Roller
Upper
grip
Strip
(a)
(b)
Figure 3. The DBF testing: (a) schematic, (b) fixture and grips. [REF]
Note: The DBF is not an untested concept, but rather has been applied successfully to
testing of HSLA steels [19, 31]. Because of the importance of details of this test to this
proposal, Reference 19 is provided as an Appendix to this proposal.
Two distinct types of failure are observed in
the tests [19, 32], as shown in Figure 4.
Bending failures occur near the unbending
region and are characteristically normal to the
specimen axis. Stretching failure usually
midway between the front grip and the
bending rod, and the fracture path is typically
inclined to the specimen axis.
BACK
MOTION
BENDING
FRACTURE
BENDING ROD
STRETCHING
HSLA and DQSK can be bent to zero radius
FRACTURE
under pure bending. With increasing back
tension force during the drawing process,
STRIP
fracture can occur during or after un-bending.
FRONT
a) However, the DBF response can not predicted b)
by any ductile failure criteria [19]. The results
suggest that a specific failure criterion for
Figure 4. Typical fracture locations during
stretch bending condition needs to be
DBF tests
developed.
The situation is even more complex for AHSSs. These materials have displayed shear fracture
without excessive thinning during stamping. Their ductility may be so low during bending and unbending that the only bending fracture can happen during DBF tests. However, the exact location
and characteristics of bending fracture will give us insights into the controlling mechanism of the
shear failure.
2.3.2. Testing Plan
The DBF tests will be performed at OSU using a special-purpose draw-bend machine designed
by OSU in collaboration with Interlaken Technologies. A range of tool radii, both fixed and rolling,
are available. A unique feature of the equipment is that the roller speed can be controlled
independently. Therefore, by adjusting the roller rotational speed to precisely match the draw
speed, the friction can be eliminated between the roller and the sample. The effect of friction
7
may be critical to initiating shear failure. Three friction conditions will be employed in the OSU
testing: regular lubricated friction, no friction and intermediate friction. They can be achieved by
using stationary roller with lubricant, rotating roller at the same speed as that of drawing, and
rotating roller at half of the drawing speed. These tests will allow the investigation of the relation
between sheet cracking and friction condition.
There is possibility that the shear failure of AHSS may depend on the strain rate. Hence it will be
interesting to conduct a few DBF tests at various drawing speed to see whether the differences
are significant. While a drawing speed of 40 mm/s was used in the conventional draw-bend tests
a drawing rate of 5 times lower or higher may be applied for the study of rate dependence on
cracking.
The ratio of R/t determines the bending
strain in the sheet. For each R/t, a critical
backforce will be measured. These will
correspond to equivalent tension stresses
and strains, either of which can be used to
construct a table of failure loci of R/t vs Fb,
* or *.
A curve of limiting loading
condition (stress or strain) versus R/t ratio
may be plotted, as shown in Figure 5.
This type of failure curves will serve as
guidance in shop floor operations, much
like FLD’s are currently used with grid
circle analysos. The gathered knowledge
will serve as the basis of the failure model
to be implemented for FEA analysis.
*
(or *)
R/t
Figure 5. An example failure curve for stretchbending operation
2.3.3. Microstructure Characterization of Failed DBF samples:
In order to determine how the failure initiates, scanning electron microscopy will be used to
examine the fractured surfaces. Through the use of characterization we can answer questions
like: if voids form near the martensite/ferrite interface or inside of ferrite matrix; whether the crack
is ductile or brittle; does it initial near sheet surface or interior, and so on. This information will
help us understand the correlation among shear fracture, loading conditions and material
microstructure, which is essential for the development of a new failure criterion for AHSS.
2.4.
Analytical Evaluation
As discussed before, FLDs are dependent on plastic strain only. However, the shear cracking of
AHSS appears to be dependent on both loading history and microstructure. EWI, OSU and
Deform will work together on the development of the new failure criterion. The approach consists
of three steps: (a) define the new criterion and its parameters; (b) implement the criterion in FEA
softwares; and (c) verification by comparison of simulations and experiments and practice.
2.4.1. Identification of a Failure Criterion
Once the DBF tests have been conducted and the microstructure characterization has been
finished, the important factors that affect shear cracking can be generalized. Based on a review
of available formability models, a new failure criterion will be proposed. It should take into
account of the loading history and material microstructure and will distinguish between plastic
8
localization and shear failure. Special treatment of ductile to brittle transition will also be applied
to simulate the premature cracking without localized thinning.
2.4.2. Coding of the Failure Criterion
Initially, the new failure criterion will likely be dependent on the microstructure of the material
investigated. To extend its application to other steels with different microstructure detailed
numerical analysis on meso-scale is needed. Material phase interactions under certain loading
conditions can be simulated explicitly. An equivalent stress or strain measure will be used to link
the response of the new material to those that have been tested.
An example of meso-scale FEA simulation is shown in Figure 8, where the void nucleation,
growth and coalescence around hard particles are simulated explicitly. The hardening behavior
of the material is one of work-hardening followed by saturated hardening and recovery after ~0.35
strain. In this simulation, microvoids nucleate at the interface between the ductile steel matrix
and a hard particle when the tensile stress at the interface exceeds 10% of the flow stress of the
steel. The top and bottom of the steel sheet are separated at a constant velocity of 1mm/s in
each direction (for a total velocity difference of 2mm/s between the top and bottom of the sheet).
Figure 8. DEFORM simulation of microvoid nucleation, growth, and coalescence during sheet
forming. (Left = zero elongation; Center ~14% elongation; Right ~ 37% elongation).
2.4.3. Nanomechanical Characterization;
The explicit simulation of the failure process will require the stress strain properties of individual
phases as inputs. The flow properties of the martensite and ferrite are indeed different. To
evaluate these variations, nano-indentation tests will be performed. The load displacement data
from the nano-indentation tests can be couple with OIM phase orientation results to derive the
critical shear stress and hardening properties of slip systems. Then through crystal plasticity
codes the macroscopic stress strain curves in arbitrary directions can be obtained.
2.4.4. Evaluation of the Failure Criteria
Once the failure criterion has been implemented in finite element programs such as Deform,
Abaqus or LS-Dyna, it’s ability to predicted shear cracking during DBF testing will be evaluated.
Some industrial examples will also be analyzed by the established failure criterion. For instance,
when ASP provides the geometry and forming conditions of the parts that experienced shear
fracture, the proposers will use FEA codes and the failure criterion to predict the cracking. This
9
type of benchmark tests will be conducted to prove the effectiveness of the new shear cracking
model.
3. Budget and Timeline
The project can be completed in three [TWO?] years through the collaboration among EWI, OSU
and DEFORM. The team proposes a gated approach to this task. After the first year, the results
of the characterization, testing and preliminary failure criteria developed will be presented to the
ASP committee. If the results show the potential, the tasks of using other candidate steels and
industrial evaluation at ASP partners can be continued. The budget also involves cost share from
participating partners and is expected from OSU (but application must be made and approved at
a later stage). The overall budget plan and schedules are summarized in Table 2. It is important
to note that these budgets and estimates are based on minimum amount of resources needed to
satisfy the goals of ASP. The investigators also welcome the comments from ASP on the
limitation or expansion of tests or activities during the final negotiations.
Table 2: Program budget and Schedules
Tasks
Characterization
ASP Steels
HSLA Steels
DP 780 & TRIP780
Candidate Steels
DBF Testing
Development
ASP Steels
HSLA Steels
DP 780 & TRIP780
Candidate Steels
Year 1
Gate
Review
Year 2
25 K
25 K
40 K
10 K
25 K
15 K
15 K
15 K
10 k
5K
5K
15 K
15 K
30 K
10 K
10 K
10 k
10 K
10 K
20 K
Total
170 K
150 K
Cost-Share
Deform
Arcelor
Overall
18 K
XX K
18 K
XX K
Fracture Criterion
Development
ASP Steels
HSLA Steels
DP 780 & TRIP780
Candidate Steels
10
4. Leverage:
EWI - It is important to note that there is a parallel internal research and development work at
EWI which focuses on the application of magnetic pulse processing to improve the formability of
advanced high strength steel sheets in terms of spring back and also formability which includes
minimization of fracture. The results of this work can be leveraged for the ASP and candidate
steels based on the award of this contract. In addition, the softwares developed for localized
heating and through-thickness effects will be provided.
OSU – The Transportation Research Endowment Program provides matching funds on a
competitive application basis for research that falls in its mission. An application of up to 33%
matching of the OSU part of the budget will be made for use at OSU to allow completion of the
proposed OSU tasks.
5. Summary
The proposal provides an innovative approach to consider through-thickness and other
microstructural effects of these advanced high strength steels which has been ignored in the
current sheet metal forming investigations. The collaborative program between EWI, OSU and
Deform focuses on (a) understanding the shear failures in AHSS during stamping; (b)
development of a novel test to characterize these steels to evaluate the tendency for shear
fracture; (c) and introduction and implementation of a fracture criterion suitable for a wide range
of FEA solvers. The proposed task focuses on implementation of these results in the industrial
practice immediately. The results from these tasks will meet the overall objective of the ASP
goals of fundamental understanding as well as industrial implementation.
ASP will benefit from cost-share from the participating members, steel companies and OSU /
TREP (to be requested), and by parallel research activities at EWI which focus on the methods to
improve the formability of advanced high strength steels through electromagnetic pulse
processing and warm forming.
6. References
New Reference 1 (for shear failure):
X.Y. Loua, M. Li, R.K. Boger, S.R. Agnew and R.H. Wagoner: Hardening evolution of
AZ31B Mg sheet, in press (currently available on-line at
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TWX-4JWMT594&_user=10&_coverDate=05%2F06%2F2006&_rdoc=1&_fmt=summary&_orig=brows
e&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md
5=7629461fa6cbc0551680050b84e8a154)
11
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6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
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