A Brittle Fracture-Based Experimental Methodology For Ductile Damage Analysis

MT07.39

Mickaël Pradelle

Student: Mickaël A.P. Pradelle

Supervisors: C.Cem Tasan, J.P.M.Hoefnagels, M.G.D.Geers

Department of Mechanical Engineering, TU/e, Eindhoven, the Netherlands

Department of Materials, Polytech’Grenoble, Grenoble, France

Period from the 2/05/07 to the 22/08/07

Sciences et

Génie des

Matériaux

Sciences et

Génie des

Matériaux

1

ABSTRACT

One of the great motivations for research in the automotive industry is the weight

reduction of the components. Several strategies were studied to reach this objective. The

replacement of the conventional steels (HSLA steel) by Dual-Phase steels promises good

results in terms of weight reduction and strength/weight ratio. However, the increase in

specific strength is done in expense of ductility, and unexpected ruptures were observed

during working operations of these new steels. It’s believed that these ruptures depend on the

progressive damage evolution in these materials. The main goals of the project “Forming the

limits of damages prediction” which is carried out in TU/e are to understand the physical and

mechanical mechanisms responsible of the evolution in ductile damages and to develop new

experimental methods that are able to provide the necessary parameters for the modeling

efforts. In this paper a new experimental method is proposed to observe the progression of the

damage inside some sheet of steels. A brittle fracture is done in the length of our samples to

characterize the ductile fracture due to tensile tests. The goal of this work is to determine the

evolution of voids, and their volume, generated during a deformation in a Dual-Phase steel

(DP600) and in an Interstitial Free (IF) steel using a new method that involves the use of

experimental tools such as SEM, confocal microscope and a new device adapted for the

Charpy impact test machine.

2

CONTENTS

Introduction p.3

Materials and Experimental Methodology p.4

Results p.10

Discussion p.14

Conclusions p.15

Recommendation p.16

Acknowledgments p.17

Appendix A p.18

Appendix B p.23

References p.25

3

Introduction:

The recent decades witnessed the introduction of materials with higher specific strength

(such as dual phase (DP) or transformation-induced plasticity (TRIP) steels) into the

automotive industry. In despite of their higher strength, these materials are also susceptible to

develop unexpected failures during forming operations, mainly due to the microvoid

evolution as they are being deformed. This weakness requires a better understanding of

physical mechanisms of ductile damage evolution to obtain numerical tools with higher

predictive capabilities.

Observation of ductile damage mechanisms is an experimentally difficult task due to the

small scale in which the process takes place. Nevertheless, there are several different methods

that are used for this purpose. Lemaitre classified these methods into two groups: direct and

indirect methods [1]. Direct methods such as micro-tomography and electron microscopy

allow the visualization of damage. Whereas indirect methods aim to determine a damage

variable through the measurement of change in young’s modulus, hardness, electrical

potential or other physical properties of matter.

The most commonly used direct method is electron microscopy due to its ease of specimen

preparation and observation capabilities. However, the specimen preparation methods for this

type of analysis may also cause some unwanted artifacts. For example, a detailed examination

of microvoids requires the use of consecutive grinding and polishing steps which may cause a

smearing effect even if the final polishing step is as fine as 1mu. This is due to the mechanical

character of this type of polishing procedure. An example of such a smearing effect is shown

in figure 1.

Figure 1: SEM picture of a smeared surface

To overcome this smearing effect, an electrochemical polishing is commonly used.

Electropolishing removes mechanically deformed layer on the surface of the material

revealing out the microvoids successfully. However, this method also has its limitations. One

of these limitations is the fact that there is an almost unavoidable edge rounding effect. More

importantly, the size of the microvoids may be exaggerated due to the material removal

around the void (Figures 2a, 2b).

4

Figure 2a: SEM image of an Figure 2b: SEM picture of

electropolished (smooth) surface an exaggerated void

One of the suggested ideas to overcome these problems is to develop a methodology that

does not involve any plastic deformation in examination of the voids [2]. This can be

achieved through brittle fracture of deformed specimens into two parts. Shi et.al. in their work

compared many of the possible specimen preparation methods for SEM evaluation and

concluded that this method was the most suitable. However, they do not present detailed

examples of void morphologies obtained by the use of this method. The critical point of such

a strategy is to differentiate between the deformation due to the tension test and that due to the

brittle fracture itself.

In this work, a microscopical and topographical characterization of brittle fracture surfaces

of IF and DP600 steels are carried out for the evaluation of ductile damage evolution in these

materials. In the following section the details of the set-up and the materials are explained.

Afterwards, obtained results are analyzed and discussed. Some fundamental information

about brittle and ductile fracture and fracture mechanisms of IF and DP steels are provided in

Appendix A.

Materials and Experimental Methodology:

I. Materials:

IF steel is ferritic steel with a very low carbon concentration. It is used for the body

structure of cars. The IF steel has no carbide precipitates at the grain boundaries. Its

microstructure is given in figure 3. It’s only composed of ferrite grains (their size is roughly

equal). It has second phase particles of AlO and TiN (example figure 4).

Figure 3: (SEM picture) Microstructure of IF steel

5

Figure 4: (SEM picture) Second phase particle in IF steel

DP600 steel is composed of two phases: martensite and ferrite. These type of steels are

produced by annealing at the austenite plus ferrite phase field, followed by cooling at a

sufficient rate to transform the optimum amount of austenite to martensite. The microstructure

is presented in figure 5. Figure 6 shows that a high concentration of martensite formation has

occurred in the center of the sheets.

Figure 5: (SEM picture) Microstructure of DP600 steel. The dark part is the ferrite and the brighter part the

martensite

Figure 6: (SEM picture) Concentration of martensite

The samples have a classic geometry. The specimen dimensions for both of these steel are

given in figure 7a and 7b consecutively.

6

Figure 7a: dimensions of IF steel specimen Figure 7b: dimensions of DP600 steel specimen

II. Experimental Methodology:

a) Tensile tests:

The objective of the tensile tests is to obtain the elastic and plastic properties of the steels of

interest. The tests are done with a tensile stage at a strain rate of 20 µm/s, in rolling direction.

b) The longitudinal brittle fracture:

The goal is to observe the voids caused by the tensile test inside the DP and IF steel

samples. For this objective, specimens have to be fractured in the longitudinal direction. The

goal is to observe only these types of voids, so the longitudinal fracture is required not to

create dislocation slip leading to plastic deformation. The idea is to develop a new adaptable

tool on the Charpy impact test machine. The Charpy machine is traditionally used for testing

of samples in the transverse direction, and requires no clamping. But for this project this setup

is modified, as it is necessary to cut the samples in the longitudinal direction (figure 8a, 8b).

Figure 8a: Direction of cutting after the tensile stage Figure 8b: Observation sense

� The design of the tool:

A massive block is designed to be fixed on the Charpy impact test machine (figure 9). On

the right side there are two cylinders to slip a "removable clamp". This component (figure10)

is divided into two parts. The first is inserted and maintained by the massive block thanks to

two holes for the cylinders. The other is to tighten the sample with the first by three screw.

Two pieces allow doing the same experiment for samples having various thicknesses. Each

piece of the clamp is serrated for better tightening.

With the values which are obtained by the tensile tests, it is possible to determine what

maximum weight our samples can bear without plastic deformation (calculation shown in

appendix B). Consequently, the idea is to develop a clamp which will be maintained in the air

only by tightening on the specimen. The "block in the air" is based on the same principle that

the mobile block: two grooved parts to tighten the sample by four screws (figure11).

7

Figure 9: Massive block for the Charpy machine

Figure 10: Schema of the “removable” clamp

8

Figure 11: Schema of the bloc just tightened on the sample

� Principle and explanations:

After the fracture by tensile test, the samples are classified and are grooved in their center to

facilitate the crack propagation without shear.

The strong block is fixed on the Charpy machine. A specimen is placed between the two

clamps then it is tightened.

The system {mobile clamp / sample / bloc "in the air"} is plunged in a container in which

there is liquid nitrogen. After 25 minutes the sample is cooled at a sufficiently low

temperature to obtain brittle fracture, the removable clamp is slipped quickly on the massive

block by the two cylinders. Once installed, the hammer is launched and hits the block in the

air to break the specimen into two parts.

The design of the complete system (without the sample) is represented by the figure 12. The

cross represents the impact point with the Charpy hammer. It is not located at the center of the

rectangular surface but it shifted towards the left to bring more energy during the break and to

avoid the bending of the sample.

9

Figure 12: Schema of the complete device for the Charpy impact test machine

c) Fractography :

For general characterization of fracture surfaces an optical microscope is used. For detailed

microstructural analysis the device which is used is a scanning electron microscope. The

observations are carried out using a 30kV electron beam in secondary and backscattering

electron modes.

d) Topography:

The confocal microscope is a measuring microscope for topographical analysis of various

different applications. It allows characterizing both very smooth and rough surfaces. The size

of the surface to be analyzed is determined by the objective used. For big surfaces, “extended

topography” mode is used: it breaks up the entire surface on several small surfaces. This

mode enables to join several topographies to represent the total surface.

The final representation is done thanks to the stitching method (assembly of images). First

the total depth of analysis is defined. Then the definition is regulated, i.e. the number of plans

which will be photographed. The software of the confocal microscope saves data files with

the values of the topography’s points. The values can be imported to Matlab or Excel. Figure

13 shows an example of a surface from the topography data to Matlab representation.

Figure 13: Final Matlab representation

of a rough tensile fracture surface

10

Results:

� Tensile tests:

Obtained tensile test results are given in figure 14. It is seen that IF steel is much more

formable than the DP600 with a maximum elongation of 41% versus 25%. Whereas the

DP600 steel, due to the martensite content in its microstructure, has higher yield (σe) and

tensile (σp) strengths of σe = 348.75 MPa and σp = 620.67 MPa compared to σe = 130.17

MPa and σp = 294.57 MPa of IF steel.

Figure 14: Curves obtained by tensile tests on IF steel and DP600 steel samples

� Longitudinal Brittle Fracture of Tensile Test Specimens

To see whether it is possible to obtain complete brittle fracture in both of these steels,

undeformed specimens are fractured in the Charpy impact test setup explained earlier. The

shiny microstructures obtained (figure 15a and figure 15b) show that brittle fracture could be

obtained successfully.

Tensile test on DP600 and IF steels

-100,00

0,00

100,00

200,00

300,00

400,00

500,00

600,00

700,00

-5,00 0,00 5,00 10,00 15,00 20,00 25,00 30,00 35,00 40,00 45,00

Strain (%)

Str

ess (

MP

a)

IF steel (2,8 mm^2)

DP600 steel (4 mm 2̂)

On the left, Figure 15a presents an optical microscope photography of the fracture surface after

the Charpy fracture. On the right, Figure 15b shows a SEM fractography of the shiny surface

which is a cleavage surface.

11

� Microstructural Characterization of Fracture Surfaces :

As explained in the experimental methodology part, the brittle fractured cross sections of IF

and DP600 steels are examined using a scanning electron microscope.

The micro examination of IF and DP600 steels revealed that:

- In different parts of the specimens both transgranular and intergranular fracture

morphologies are observed. However, for IF steel, the mechanism of brittle fracture is

generally in transgranular manner (figures 16 and 17) away from the neck, in the

undeformed part of tensile specimen.

Figure 16: SEM image of transgranular crack propagations

in the undeformed part of an IF steel sample.

Figure 17: SEM image of transgranular crack propagations

in the undeformed part of a DP600 steel sample.

Nevertheless, near to the neck of the tensile test specimens intergranular crack

propagation among the severely deformed and elongated grains is observed to be more

favourable (figure 18 and 19).

12

Figure 18: SEM image of intergranular crack propagations

in the neck region of an IF steel sample.

Figure 19: SEM image of intergranular crack propagations

in the neck region of an IF steel sample.

- A number of microvoids are also observed on the grain boundaries that separate the

brittlely fractured grains (figure 20 and 21).

Figure 20: SEM image of voids on the grain boundaries

in an IF steel specimen.

13

Figure 21: SEM image of voids on the grain boundaries

in a DP600 steel specimen.

- The other half of these microvoids could also be observed in the opposite part of the

specimen (figure 22).

Figure 22: Two SEM fractographies of the exact opposite

brittle fracture surfaces of an IF steel samples.

Open like a “book”.

14

� Topographical Analysis of Longitudinal Brittle Fracture Surfaces:

The first idea was to make a topographical representation of each opposite brittle fracture

surfaces from the profilometry data to a Matlab representation and superposition.

But the profilometer has some limits. The size of the voids is around 1 and 2 micrometers

what causes problems of resolution. Moreover, the light is scattered by the rough and irregular

sides of the voids. Consequently, the laser light which allows drawing the surface gives

unreliable analysis in depth. Figure 23 is a SEM photography of the neck region of a DP600

steel sample after the tensile fracture. Figures 24 and 25 show two topographies of this

fractography with different enlargements. Black parts are dispersed on the topographical

surface. These last represent the lack of data for the topography.

Figure 23 Figure 24: Topography 20X of enlargement

Figure 25: Topography 50X of enlargement

Discussion:

An inter and transgranular crack propagation modes are observed during the longitudinal

brittle fracture. IF steel has a quasi total intergranular fracture in the neck and only

transgranular away from the neck region. DP600 steel, also, has inter and transgranular modes

of crack propagation in the neck and only transgranular in the undeformed part.

Two sides of voids are found on the grain boundaries. These cannot be due to the brittle

fracture. This may provide to be a nice way to observe void growth. Figure 26 shows a

smooth surface and some voids along the grain boundaries after an electropolishing. The

figure 27 shows voids after the brittle longitudinal fracture. This last reveals that voids follow

the grain boundaries too and presents the mechanisms of the brittle fracture and the damage

due to the tensile deformation.

15

Figure 26: SEM image of an IF specimen after

Mechanical and electrochemical

polishing preparation.

Figure 27: SEM image of an IF specimen after

a longitudinal brittle fracture preparation.

Conclusions:

• A new method to characterize ductile damage is tested which produced promising

results. Longitudinal brittle fracture, of IF and DP600 steel samples, is done to

characterize the ductile fracture due to tensile tests. Thanks to a SEM analysis, a

clear observation of voids caused by a deformation is realized and it proves that the

longitudinal brittle fracture is achieved.

• Inter and transgranular propagation modes are observed together in the brittle

fracture. Their evolution with the deformation in the material can be studied.

• A topographical representation of brittle fracture surfaces is limited by the scattering

of the light in deep voids. These last are very small compared to the roughness of the

surfaces.

16

Recommendations:

A hypothesis could be submitted: the deformation decreases the interaction force of grain

boundaries so intergranular crack propagation is favorised during brittle fracture giving the

possibility to observe the voids situated along the grain boundaries.

To check it, the Nano-Identation would be used to determine the evolution of the hardness

at the grain boundaries and the tomography to determine the volume of voids.

The future works would allow developing a model of the evolution of the voids in function

of the stress (and the strain) and checking if the crack propagation modes depend of the

deformation.

17

Acknowledgments

This project has been possible thanks to the Mechanical Engineering department of the

Technische Universiteit Eindhoven. I would like to thank Prof.dr.ir. M.G.D. Geers for

accepting me to work in his laboratory, dr.ir. J.P.M.Hoefnagels for his eye of supervisor and

ir. Cem Tasan for his time and his humanity.

18

Appendix A

1. Fractures:

a- Ductile fracture: [3]-[4]

Ductile fracture occurs in formable metals that goes through severe plastic deformation.

Generally the FCC materials are more ductile than others. During the tensile test of such

materials, fracture occurs in five steps:

Figure 28: Steps of a ductile fracture [5]

The observed microvoids can be generated from non-metallic inclusions. For example,

voids on the surface may be initiated by sulphide inclusions or are nucleated at precipitated

carbide.

Figure 29: SEM image presenting a void

due to an inclusion.

b- Brittle fracture: [3]-[4]-[6]

The brittle fracture occurs without significant plastic deformation. This type of fracture

appears generally in BCC or HCP metals. The crack starts through or along the grain

boundaries. The propagation of the crack is fast and the fracture is a cleavage fracture.

There are different conditions to obtain a brittle fracture: a triaxial deformation, the low

temperatures or a very fast stress.

19

Figure 30: SEM image of bainitic steel’s fracture

by cleavage [6]

The figure 30 shows a cleavage initiation facet by inclusion of another phase in the steel

(aimed by the white arrow).

c- Ductile-to-Brittle Transition: [3]-[4]-[7]-[8]

The D-to-B transition is defined by the temperature of transition Td (or DTBT) under

which material starts to behave in a brittle way. To determine Td, Charpy impact tests are

carried out at different temperatures and the fracture energy (J.cm-²) as a function of the

temperature is obtained.

The Td is determined when the curve increases a lot from the low energies (brittle) to the

high energies (ductile) as it’s shown on the figure 31:

Figure 31 [9]

For DP steels, the DTBT is situated around -100°C [8]. DTBT temperature is influenced by

several factors:

• Chemical components:

The main chemical components which affect the Td in steels are carbon and manganese. It

can increase of 25°F for each 0.1% in more of C and it can decrease of 10°F for each 0,1% in

more of Mn. Other components are presented in the next picture with their effects:

20

Components Effect on Td Comments

P increase of 13°F per 0,01%

to avoid Bessemer

process

N

difficult to evaluate his

effect harmful for the toughness

Ni

decrease the Td

good for toughness if

more than 2%

Mo

increase the Td

Almost the same effects

than C

O

Increase the Td, effect of

deoxidation

Td=5°F for 0,001%,

Td=650°F for 0,057%

Si increase the Td from 0,25% used in killed steel

Consequently, more there is C in the steel and more the steel is brittle. The Charpy test

curves take different forms with the different concentrations:

Figure 32: Curves evolution in function of the Carbon concentration. [4]

• The grain’s size:

ASTM is the number of grains by surface units. If the ASTM raises (consequently the

diameter of grains decreases), the Td decreases.

For example, the fall of the grain’s size from ASTM = 5 to ASTM = 10 in the middle steel

changes the Td from 70°F to -60°F.

• The microstructure:

Steel which is composed only by martensite has the best toughness whereas a pearlite

composition has the worst. A bainitic structure which has a good toughness is easy to work.

• Production process:

Rolling at high temperature and the des-oxidation by Al decrease the DTBT

21

2. Damages in DP and IF steels:

- Microstructure and failure of DP steel: [7]-[10]-[11]

This steel is composed of two parts, so different zones are observed on the fracture surfaces

after a tensile test:

Figure 33: SEM image, cleavage facets on the fracture surface [11]

Figure 34: Fractography showing voids on the surface fracture [11]

The figure 33 presents the cleavage zone due to the brittle part (martensite) of the material

and the figure 34 refers to the ductile (ferrite) part which is defined by the voids.

There are two mechanisms for the voids formation:

• Separation of the ferrite and the martensite phases

• Fracture of the martensite: Before the fracture, the martensite is elongated and a

neck is formed.

Figure 35: SEM image of a sample during a fracture

by tensile test. [11]

Cleavage

Voids

Void formation by

martensite fracture

22

Dual Phase steel is noted with the next manner: DPxxx, with x a number. More xxx is important and

more the martensite concentration rises in the steel.

- Microstructure and failure of IF steel:[12]-[13]-[14]

Analyse of an IF steel sample:

Figure 36: SEM images of fracture surfaces

of an IF steel sample. [12]

The fractography (figure 36) shows the fracture surface is partly ductile and partly brittle.

The voids refer to the ductile part. The size of the voids depends of the toughness of the

samples: when the toughness increases the average voids size decreases.

The lack of carbides at the grain boundaries causes intergranular brittle fractures. The

micro-cracks are nucleated preferentially at random boundaries. The fracture occurs in a

typical intergranular fracture mode when a high fraction of random boundaries exits and when

they are connected to each other.

The combined process of intergranular and transgranular fracture occurs in a ductile manner

when the fraction of random boundaries is low.

: Figure 37: Schematic representation of grain boundary structure-dependent

fracture process in polycrystal.

Path A: combined process of intergranular

and transgranular fractures.

Path B: typical intergranular fracture. [14]

Brittle Ductile

23

Appendix B

RESOLUTENESS OF THE MAXIMUM WEIGHT WHICH CAN BEAR THE STEEL SAMPLES

The goal of this proof is to calculate the limit mass that can bear our samples without plastic

deformation.

After the tensile test, the values of the limit elastic and plastic stresses of steel are known.

The lowest elastic stress will be taken to get just one bloc which will be used for all the

samples.

The results obtained are:

e

σ (IF steel) = 130.17 MPa et e

σ (DP 600 steel) = 342.41 MPa.

Consequently, the e

σ of the IF steel will be used for the calculation.

Representation of the bloc’s influence on our samples:

First relation : ( ) ( / ) *

:

ff w w wf

with

σ σ= =

Fbloc = m * g

fσ

w

0

wf

24

0 0

0 0

0

bloc

Calculation of the moment caused by the bloc:

* *

* * *

* *( / ) * *

*( / ) ² *

* ( / ) * ( ^ 3) / 3

(1)

Then : * . . (2) with m the mas

. . ² / 3

wfh

wfh

w

M w dA

M w dw dh

M dh w w wf f dw

M h f wf w dw

M h f w

M h f

f wf

M F l m g l

wf

σ

σ

σ

σ

σ

σ

=

⇔ =

⇔ =

⇔ =

⇔ =

⇒ ⇔

= =

=

∫∫

∫ ∫

∫ ∫

∫

s of the bloc

Equalizing (1) and (2) :

. . ( . . ²) / 3

Numerical mapping with h = 10^-3 m ; / ; g = 10 m.s-² ; l = 3 cm ; 4 mm.

After the calculation, the result is m = 2,3

( . . ²) /(3. . )m g l h f wf

f F A wf

m h f wf g lσσ

σ

= ⇔

= =

=

1 kg.

25

BIBLIOGRAPHY

[1] Lemaitre, J., A Course on Damage Mechanics, Springer-Verlag, Berlin, 1996

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[5] Wikipedia picture

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[9] Google images source

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