Method of Crack Formation Analysis Based on Mechanoluminecence

doi:10.4236/msa.2012.310108

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Materials Sciences and Applications, 2012, 3, 739-744

http://dx.doi.org/10.4236/msa.2012.310108 Published Online October 2012 (http://www.SciRP.org/journal/msa)

Method of Crack Formation Analysis Based on

Mechanoluminecence

Sergej Aman, Alexander Aman, Juergen Tomas

Mechanical Process Engineering, Otto-von-Guericke University Magdeburg, Magdeburg, Germany.

Email: [email protected]

Received July 24th, 2012; revised August 21st, 2012; accepted September 23rd, 2012

ABSTRACT

This paper presents the method of monitoring of cracks formation based on analysis of light impulses that appear during

the cracks formation. The light impulses appear simultaneously in the ultraviolet (UV) and near-infrared (NIR) wave-

length ranges. However, during the plastic deformation, where the building of micro cracks with narrow width is domi-

nant, the light appears frequently in the UV-wavelength range. With the cracks growth the width become wide-ranging

and the spectrum of light shifts to the NIR-waveleng th range. This spectral shift was used to estimate the stage of cracks

formation as degree of damage. The fracture of tested material can be predic t ed based on estima te d degree of da mage.

Keywords: Crack; Light Impulses; Fracture; Spectral Shift

1. Introduction

The fracture of materials by repeated loading can be di-

vided in two stages: crack initiation and crack propaga-

tion [1]. This arrangement assumes that an intensive

generation of single micro cracks takes place during the

first stage—crack initiation. By repeated stressing the

crack initiation takes a major part of the total stressing

time [1-5]. The second stage—crack propagation occurs

when the concentration of small single micro cracks at-

tains a critical degree and micro cracks coalesce together

to form larger cracks. In this phase the cracks width be-

come wide-ranging. The future intensive stressing leads

to the formation of a macroscopic crack and to fragmen-

tation. The most important and difficult problem in frac-

ture mechanics seems to be the use of crack initiation

data in predicting of the remaining useful life time of

machinery components. This problem is of great impor-

tance for safety and reliability of machinery and plants.

A number of models of fatigue damage evolution have

been developed to predict the remaining useful life time

[1-3]. However, it is difficult to predict exactly remain-

ing useful life based only on fracture models due to the

inherent stochastic nature of cracks.

Gupta et al. [4,5] have shown that the small defects in-

side the material surface can be detected with ultrasonic

signals. This method performs the monitoring of micro

cracks during the early stages of fatigue damage on po-

lycrystalline alloys.

There are a number of methods of fracture analyses

based on fracto-emission during the cracks formation

[6-9]. The fracto-emission included the emission of elec-

trons, ions, ground state neutral particles, free radicals,

electromagnetic radiation in the frequency range from

tenths of Hz up to X-ray frequencies [5-9].

In this paper, the light emission which appears during

the loading of specimens (mechanoluminescence) will be

used to estimate of degree of cracks formation. Mecha-

noluminescence (ML) is a widely spread phenomenon

that is often described in the literature [6,7,10]. Nearly

50% of all inorganic salts and organic molecular solids

exhibit mechanoluminescence [7]. Accordin g to Chandra

[7], there is to distinguish between plastic and fracture

ML. The first one occurs during the plastic deformation

due to generation of microcracks and the second one oc-

curs during the fragmentation .

The fundamental scientific aspect is the subject of

common investigation of ML. However, a practical ap-

plication of ML in process engineering and in visualiza-

tion of cracks development seems to be successful. In

mechanical process engineering, the ML is used to con-

trol grinding processes [11,12]. A direct monitoring of

crack initiation during plastic deformation is useful to

predict material fracture [13].

The generally accepted mechanism ML is based on the

micro discharges occurring during the crack opening.

During the initiation and propagation of cracks the elec-

tric charges at the fresh created crack surfaces were pro-

duced. If the charge density i.e. electrical field in crack

opening reaches a critical value, the series of micro dis-

Method of Crack Formation Analysis Based on Mechanoluminecence

740

charges take place.

The ambient gas molecules penetrate into crack open-

ing. Thus takes place the collisions of gas molecules with

accelerated electrons emitted from crack surface. By the

return to ground state of molecules, excess energy will be

emitted as light. In this case, the ML-spectrum is consis-

tent with the emission spectrum of the ambient gas [6].

During a micro-gas discharge, a large number of photons

are usually emitted within a few nanoseconds [10].

In opposite to material specific micro processes, the

gas discharges appears directly during the initiation and

propagation of crack. That means that a high time resolu-

tion can be achieved by use of the measurement tech-

nology based on micro gas discharges. In terms of gas

discharge the spectral distribution of the light emissions

depends on the properties of ambient gas only. In this

context, the properties of tested material do not signify-

cantly affect the measurement design and analysis of the

data. Therefore, this method will be not material specific.

Thus, the focus of this paper is to test the characteristic

features of the micro gas discharges regarding to moni-

toring of cracks formation.

Sugar, sylvinite and quartz exhibits a strong ML. The

ML in those materials is caused by micro discharges of

nitrogen penetrated from ambient atmosphere in the

cracks opening [6,7]. In this paper the test measurements

were carried out by use of sugar, sylvinite and quartz

glass specimens.

2. Description

One of the characteristic features of the micro gas dis-

charges is the intensive light emission. For example,

about hundred photons can be radiated during the few

nanoseconds from crack tip where a gas discharge takes

place [14]. Based on these properties of micro gas dis-

charge a Multi Pixel Photon Counter (MPPC) was pre-

ferred for our investigation [10]. The MPPC is a type of

photon-counting device made up of multiple APD (ava-

lanche photodiode) pixels operated in Geiger mode [15].

This allows the counting of single photons or the detec-

tion of multi-photon pulses with excellent time resolutio n

of about 400 ps.

The applied MPPC (part number S10985-100C), see

Figure 1, consists of four parallel channels (5). Each

channel contains of 256 avalanche photodiodes switched

together. This has the consequence that the individual

signals (impulses) of all APD pixels are added and the

amplitude of resulting impulse is proportional to the

number of all photons reached the surface of MPPC. The

large work surface of the channels (4 × 4 mm) allows the

use of different combination of color filters (4) In that

way a simple spectral analysis of the light pulses (3) in

the nanosecond range is possible.

The test breakage of the specimen (1) takes place in a

loading cell (2) between two cutter (3), see Figure 2. A

first cutter is fixed stationary on a bench (4) and the other

cutter is mounted on a moveable bolt (5). As result the

cleavage of samples occurs between two sharp edges of

cutter plates. The load force F and the stressing rate can

be varied by use of piezoaktor (6). After the loading the

specimen can be removed from the chamber and damage

spurs that appear by contact with cutter plats can by ana-

lyzed.

The light was collected by lens (7) and transmitted to

the MPPC (8) by means of a broad band transmission

optical fiber waveguide (9). The light filter (10) is

mounted at the surface of MPPC. The MPPCs exhibits a

wide spectral response from 320 nm to 900 nm. This

broad spectral response of MPPCs was used to detect

photons in two wavelength ranges—UV/VIS/NIR and

NIR. The intensity in the UV/VIS/NIR was measured

without any filter (WF), i.e. in the wavelength range

Figure 1. Design and operating mode of the MPPC-sensor

(1: MPPC-sensor, 2: specimen, 3: light impulses, 4: filter, 5:

channels).

Figure 2. Experimental set-up (1: specimen; 2: loading cell;

3: gutters; 6: window with lens; 7: MPPC; 8: optical fiber

waveguide; 9: filter).

Method of Crack Formation Analysis Based on Mechanoluminecence 741

from 320 nm to 900 nm. The NIR intensity was meas-

ured by means of the Schott glass filter RG 665 that

transmitted light with waveleng ths larger than 665 nm.

To improve the resolution by detection of multiple

photon impulses, every MPPC channel was loaded by a

resistor of 5 ohm instead of the 50 ohm recommended by

manufacturer. Therefore, the amplitude of the signal was

reduced in 10 times. Due to this, the amplitude of a sin-

gle photon impulse was below the average noise level

and only the intensive multiple photon pulses caused by

gas dischar ge were detected.

3. Results and Discussion

The cleavage of sugar specimens was performed at at-

mosphere pressure between two cutters in a loading cell,

as shown in Figure 2. Specimens with a size of (2 × 4 ×

3) mm3 made from commercial DIAMANT tea sugar

were used to test the waveform of light emission. A typ-

ical waveform of light emission during the cleavage of

sugar is represented at the Figure 3. The characteristic

time of light emission is abou t of 600 µs that correspon d s

to the stressing time. The amplitude of impulses is sto-

chastically distributed during the emission time. How-

ever the impulse amplitude becomes higher with stress-

ing time and then abruptly reduces at the end of stressing.

The loss of the specimen integrity o ccurs in this time.

It was shown that the light intensity shifts from ultra-

violet (UV) to NIR wavelength range during the devel-

opment of micro-cracks [14]. With the crack expansion

the NIR component of ML-emission becomes larger.

According to this, the crack development can be speci-

fied by means of relationship (ratio) between the UV/VIS

and NIR components of spectra. In our work we will use

the relationship between light intensity without any filter

(Iwaf) and light intensity with RG 665 filter (IRG). To cha-

racterize the degree of crack development, i.e. degree

00.2 0.4 0.6 0.8

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

without filter

RG665

Time t inms

(

)

Intensity (a.u)

Time t in ms

Figure 3. The light emission during the cleavage of sugar

crystals.

of fracture, a deformation-breakage parameter (DB-pa-

rameter) was introduced. It can be by calculated from

light intensities Iwaf and IRG.

The intensities Iwaf and IRG were integrated along the

emissions time to obtain the cumulative intensities Jwaf

and JRG. Figure 4 shows the temporal behavior of Jwaf

and JRG of signals represented in Figure 3.

The cumulative intensities were normalized by its

maximum values. Finally the DB-parameter was found

as difference between two normalized cumulative inten-

sities JRG,n and Jwaf,n:

RG nwafn

DB JJ





 (1)

Figure 5 represents the change of DB-parameter dur-

ing the cleavage of sugar crystals. By the crack initiation

i.e. during the plastic deformation the number of gener-

ated micro crack with narrow opening increases and con-

tribution of UV/VIS-intensity (owing from narrow cracks

openings) to joint intensity increases. As consequence,

the DB-parameter becomes negative. When the concen-

tration of small single micro cracks achieves any critical

degree, the micro cracks coalesce together to form large

cracks. In this phase, the crack openings become wide-

ranging and the DB-parameter increases. Finally, during

the specimen fragmentation DB-parameter becomes posi-

tive.

DB-parameter, represented on Figure 5 reaches a

threshold value of –0.017 at the end of crack initiation

phase. This threshold value of DB-parameters is specific

for sugar specimens with dimensions (2 × 4 × 3) mm3. If

the DB-parameters becomes lower –0.017 then the tran-

sition from cracks initiation phase to fragmentation of

specimen occurs. In this context the critical value of

DB-parameter can be interpreted as a fracture threshold.

In case of ductile sugar samples the fragmentation will be

00.2 0.4 0.6 0.8

35 Jwaf

JRG



Ti me

tin ms

cumulative intj. in a.u.

Time t in ms

Figure 4. The temporal behavior of cumulative intensities

Jwaf and JRG during the cleavage of sugar crystals.

Method of Crack Formation Analysis Based on Mechanoluminecence

742

00.1 0.20.3 0.40.50.6 0.7

-0.03

-0.02

-0.01

0.01 DB-parameter

t i

Time t in ms

DB-parameter i n a.u.

Figure 5. The temporal behavior of DB-parameters during

the cleavage of sugar crystals.

attained only by sufficiently large number of small

cracks generated during the soft inelastic deformation.

This sufficiently large number of small cracks performs

the smooth change of DB-parameter during the crack

initiation phase.

Figure 6 illustrates the change of the DB-parameter

from second sugar specimen. To check the state of the

damage, the stressing was stopped by the loading force

of about 7 N. The loading was repeated 8 times with

stressing rate 0.2 mm/s before the fragmentation occurs.

The photo shown in Figure 6 was taking directly before

fracture. One can see the formed bulge of the formerly

flat specimen surface. The behavior of DB-parameters by

the stressing of two different sugar specimens (Figures 5

and 6) is similar. However, the threshold value of DB-

parameters of second specimen is –0.019. The difference

in the threshold values of DB-parameter of two speci-

mens can be caused due uncertainty in positioning the

sample after taking photos.

It should be emphasized that the drastic change in ma-

terial behaviour is a characteristic feature of fracture [15].

Indeed, the DB-parameter varies significantly at the end

of crack initiation phase. At first, the DB-parameter

shows an intensive plastic deformation after attainment

the threshold value. This intensive plastic deformation

occurs prior to fragmentation. The following fragmenta-

tion of specimen is marked by rapid increasing of

DB-parameters. It is difficult to detect those phenomena

(fracture and plastic deformation) by means of measure-

ment of material resistance i.e. by use of typical force-

displacement curve. Both phenomena are overlapping

each other. In this case, the change of the material resis-

tance will be caused by both—rapid deformation and

fragmentation. Moreover, the time resolution of com-

Deformation Fragmentation

D B = - 0,027

Time t in ms

00,15 0,3 0,450,6 0,750,9 1,15

Deformation Breaka ge

-0.019

Figure 6. The change of the DB-parameter represents the

shear stressing of sugar specimen. The stressing was stop-

ped before fracture to take the photograph of deformation.

One can see the formed bulge of the formerly flat surface of

specimen.

mercial sensors applied to such type of measurements is

too low to detect those rapid phenomena.

The similar change of DB-parameter was observed by

stressing of quartz glass (Figure 7) and sylvinite (Figure

8) specimens. The stressing of every sample was re-

peated with stressing rate of 0.2 mm/s and stop force of

15 N. At the start of loading, the applied force was not

large enough to produce th e fragmentation. However, the

stressing was repeated before the fragmentation occurs.

After each loading the tested specimen was removed

from the cutting cell and microscopically examined.

Special attention was paid to the repositioning of the

sample to its original site in the cutting cell.

The exact repositioning of sample was necessary to

obtain reproducible data.

With reference to Figure 7, one can see that the clea-

vage of quartz specimens takes place only after a soft

deformation phase, accompanied by the generation of

micro cracks in the contact zone. The irregular change of

DB-parameter, reflects a stiff elastic-plastic (brittle) be-

havior of quartz. Compared with stressing of soft plastic

(ductile) sugar, see Figure 6, the number of cracks is

reduced. However, the crack size, i.e. the amplitude of

up- and downturns of DB-parameter, increases. In oppo-

site to sugar, the quartz glass shows a sequence of small

fractures occurring after the cleavage of specimen.

The typical threshold value of DB-parameter by the

crack initiation in the quartz specimens is about –7. The

Method of Crack Formation Analysis Based on Mechanoluminecence 743

00.05 0.1 0.150.2 0.25 0.3 0.35 0.40.45

Time t in ms

Deformation Fragmentation

DB = -7

Figure 7. The change of the DB-parameter during the shear

stressing of quartz glass specimen. The left photo represents

a narrow contact spur of the cutting plate on the sample

surface. The right photo represents a breakage surface with

a characteristic morphology of brittle fracture.

00.2 0.4 0.6 0.8 11.2 1.4

Time t in ms

Frag ment-

ation1

Deformation

DB = -0,036

Fragmentation

Figure 8. The change of the DB-parameter during the

stressing of sylvinite specimen. The left photo represents a

contact spur associated with plastic deform ation during the

cracks initiation. The right photo represents a breakage

surface with cr ac ks.

brittle behavior of quartz is verified by photo of breakage

surface (right photo Figure 7). The sharp edge of the

cutter produce a narrow contact spur on the sample sur-

face with a width of about 100 microns, see left photo.

Only small deformation was observed before fracture. A

fracture surface with characteristic pattern of stiff elas-

tic-plastic (brittle) fracture is presented on the right photo.

One can see relative smooth surface with striations and

large cracks.

Figure 8 shows the elastic-plastic behavior of sylvinite.

The soft plastic contribution is reflected by the smooth

change of DB-parameter during the phase of cracks ini-

tiation. However, from time to time the plastic deforma-

tion is accompanied by generation intermediate cracks

marked by fluctuations of DB-parameter. In these phases

the brittle behavior of specimen is dominant.

The photo links verified this mixed behavior. Indeed,

the surface with significant plastic deformation of speci-

men is presented. However, the large striations owning

from contact zone is a specific feature of brittle fracture.

The right photo represents a smooth breakage surface

with large cracks.

4. Conclusions

The cracks formation was characterized by use of the

light impulses emitted during the micro gas discharges in

cracks opening. The periodical stressing of sugar, quartz

and sylvinite samples shows the existence of fragmenta-

tion thresholds. When the concentration of micro cracks

overcomes a fragmentation threshold the crack openings

become wide-ranging and fragmentation occurs. By

means of an introduced deformation-breakage parameter

this transition can be predicted based on the crack initia-

tion data.

The developed measurement technique can be suc-

cessful applied to cracks formation analyses. Often the

plastic deformation and generation of cracks are occur-

ring simultaneously. However, it is possible to distin-

guish between those processes by means of the deforma-

tion-breakage parameter. The monitoring of plastic de-

formation and generation of cracks can be accomplished

separately with high temporal resolution.

Finally, it should be mentioned that the fracture analy-

sis based on light emission in real situations is often

connected with difficulties. The main problem is the

change of material transparence caused by formation of

new cracks. To reduce the influence of specimen trans-

parence on results of the fracture analyses, the DB-pa-

rameter was introduced. Only the normalized cumulative

intensities were used to calculate the DB-parameter. Due

to this the results of crack formation analysis will not be

significantly affected by change of specimen transpar-

ence.

5. Acknowledgements

This work is carried out in the framework of the DFG

priority program: Partikel im Kontakt—Mikromechanik,

Method of Crack Formation Analysis Based on Mechanoluminecence

744

Mikroprozessdynamik und Partikelkollektive (SPP 1486).

Project: AM 336/1-2.

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