Pantazopulos_Damage Assessment Using Fractography as Failure Surface

FEATURE

Damage Assessment Using Fractography as Failure SurfaceEvaluation: Applications in Industrial Metalworking Machinery

George A. Pantazopoulos

Submitted: 9 July 2011 / Published online: 13 August 2011

� ASM International 2011

Abstract Fractography is a powerful analytic tool for the

evaluation of failure surface topography and root-cause

analyses. Fractography, embracing both light and electron

optics methods, is utilized in modern failure analysis and is

recognized by the engineering community as a unique

process for industrial problem solving, evaluating

machinery/component failures, and providing solutions for

performance improvements. In the present study, the role

of fractography is highlighted through characteristic fail-

ures of industrial machinery components. Low-power

stereomicroscopy and Scanning Electron Microscopy

(SEM) micro-fractography are the principal analytic tools

that were used in the context of the present research.

Keywords Macrofractography � Microfractography �Metalworking machinery � Fatigue � Overload

Introduction and Background Information

Fractography etymologically originates from Latin fractus

meaning fracture and the Greek graphy (Greek: cqa9ux)

meaning writing, and by extending this term to the

meaning describing, it, therefore, means a description of

the fracture; see also Ref. [1]. The historic evolution of

fractography and its contribution to the determination of

failure modes were addressed in Ref. [2]. Apart from

industrial applications, fractographic techniques are fre-

quently used in forensic engineering and safety sectors

aiming toward accident prevention. Fractographic assess-

ment used in the field of materials and manufacturing

engineering, has made significant advances, since it con-

tributes mainly to the progresses in the following areas; see

also Ref. [3]:

• Basic understanding of physics and micromechanics of

fracture.

• Material’s improvement in terms of resistance to

deformation and damage.

• Analyses of crack evolution and failure history in root-

cause investigation.

Numerous practical applications of fractography in the

industrial field were also presented in Refs. [4, 5]. Frac-

tographic interpretation has been acknowledged for its

significant impact to the society for many decades. Frac-

tography is often used in combination with microstructure-

characterization techniques (light and scanning electron

microscopy) in the majority of material failure analysis

research projects [6–11]. Low-power stereomicroscopy

(magnifications up to approximately 925), under vertical

or oblique light illumination is the first approach in fracture

evaluation, called macrofractography. Sequentially,

microfractographic investigation is carried out preceded by

Scanning Electron Microscopy (SEM) employing second-

ary and/or backscattered electron-imaging modes for

topographic and compositional evaluations. Energy Dis-

persive Spectrometry (EDS) detectors incorporated in SEM

provide additional capability for semi-quantitative chemi-

cal elemental analysis of selected areas. Various

characteristic failure events, including mainly fatigue and

overload (brittle and ductile) fractures, are illustrated in the

present article, highlighting the value of the fractographic

evaluation in the determination of the failure mode(s) and

root-cause analysis (see Table 1).

G. A. Pantazopoulos (&)

ELKEME Hellenic Research Centre for Metals S.A.,

252 Piraeus Street, 17778 Athens, Greece

e-mail: [email protected]

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J Fail. Anal. and Preven. (2011) 11:588–594

DOI 10.1007/s11668-011-9503-7

Analytical Techniques

Macrofractographic evaluation was performed using a high-

resolution digital camera and a stereomicroscope. High-

magnification fractographic observations were carried out on

ultrasonically cleaned fracture surfaces using a SFEG Scan-

ning Electron Microscope equipped with a secondary electron

detector for topographic evaluation and a backscattered

electron detector for compositional imaging.

Investigation

Macroscopic Evidence of Fatigue: Spalling Fracture of

Back-up Rolls

Visual examination images of steel back-up rolls and

fragments are illustrated in Figs. 1 and 2a. Rolls were

damaged during operation in a reversing cold rolling mill.

Linear/elongated-type spall and irregular ‘‘fan-shaped’’

Table 1 Characteristic failure cases investigated by fractographic evaluation

Failure case

number Component Machine

Principal failure

mode

Principal fractographic

characteristic(s)

1 Back-up roll Cold rolling mill Surface fatigue Linear spall—beach marks

2 Clamping block Drawing machine Impact overload Flat fracture—mixed mode

(transgranular faceted—fine dimpled)

3 Steel stud Caster Fatigue Flat fracture—microscopic fatigue striations

Fig. 1 Spalling failure of a back-up roll in a reversible cold rolling mill: (a) overall view, (b, c) closer views of the fracture surface, and (d)

details of the fracture surface showing the linear shape of the spall and the presence of beach marks manifesting the direction of the fatigue crack.

‘‘Fan-shaped’’ marks emanating from the fatigue area depict the fast fracture zone. The arrows show the direction of the fatigue crack

propagation

J Fail. Anal. and Preven. (2011) 11:588–594 589

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marks originating from fatigue boundary are typical fea-

tures of spalling fracture (Fig. 1d). Low-power stereomi-

croscopy (macro-fractography) indicated the occurrence of

fatigue crack-propagation (FCP) and final (overload) frac-

ture zones; see Fig. 2b–e. The final fracture zone is a

brittle, overload fan-shaped area comprising chevron rays

(shear steps) emanating from the fatigue crack zone

(Fig. 2d, e). Fatigue striations are either poorly visible or

rarely seen on the fatigue zone because of the high roll

hardness. The susceptibility of striation formation is

Fig. 2 (a) Roll fragments coming from spalling failure; (b, c) optical stereomicrographs showing the fatigue crack-propagation (FCP) zone

containing characteristic circular beach marks; and (d, e) ‘‘fan-shaped’’ final fast fracture zone that originated from the FCP zone boundary

590 J Fail. Anal. and Preven. (2011) 11:588–594

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reduced significantly for hardness levels [50 HRC. Brittle

overload occurs usually via intergranular fracture mode

leading to fast fracture and final failure; see also Ref. [6].

Characteristic macroscopic features of fatigue and their

interpretations are presented in detail in Ref. [12]. Spalling

failures are surface fatigue fractures that are usually gen-

erated by the presence of firecracks on the roll surface.

Firecracks are thermal-fatigue cracks running perpendicu-

lar to the outer roll surface and formed in the bruised area

of the roll surface as a result of non-uniform stress con-

ditions; see also Ref. [13]. Radial- and circumferential-

crack propagation under cyclic loadings led to detachment

of an outer shell of the roll causing fragments, severe

damage of the roll and rolling production shutdown. A

detailed study on metallographic and fractographic inves-

tigation of rolls is reported in Ref. [6].

Preventive actions concerning the rolling mill operation

are the following:

• Review of lubrication/cooling conditions aiming to min-

imize friction damage and succeed in achieving efficient

heat exchange, reducing temperature alterations.

• Avoid roll surface damage, such as bruises, roll marks,

thermal shock, and scratches.

• Monitoring of proper roll condition using NDT (Non-

Destructive-Testing, such as ultrasonic, penetrant test-

ing, etc.) and performing surface grinding aiming to

eliminate surface cracks.

• Review of pass-schedule conditions aiming to reduce

rolling stresses and extend roll life.

Microstructural Influence on Fracture Mode: Failure of

Clamping Block in a Tube-Drawing Machine Due to

Dynamic Loading Conditions

Clamping blocks are load members used in metal tube-

drawing machines and are often subjected to dynamic loads

during start-up of the production line. In the present

investigation, a fractured clamping component was sub-

mitted for failure analysis. Visual inspection and low-

power stereomicroscopy showed a flat, fracture surface

appearance consistent to the absence of macroscopic duc-

tility. Furthermore, SEM microfractographic investigation

revealed an irregular and fairly complex fracture pattern

(Fig. 3). A mixed-mode fracture mechanism has taken

place; crack propagation followed the lowest strain energy

path passing through the carbide particles and causing

characteristic transgranular facets; a limited strain has been

accommodated in the tempered martensite matrix that

exhibited a micro-plasticity failure mode with the devel-

opment of fine dimples (see Fig. 4a). Fine-dimpled area

corresponds to limited macroscopic plasticity, since dimple

size is related to total elongation and reduction of area.

Similar fine-dimpled areas in steels possessing tempered

martensite microstructure have been reported in Ref. [7].

At high micro-void nucleation rates, limited void growth is

achieved, stimulating localized necking and accelerating

final fracture. Transgranular carbide facets and some

intergranular features are shown at higher magnification

(Fig. 4b).

SEM micrographs on transverse cross sections revealed

a microstructure consisting of coarse primary chromium

carbide islands (precipitated during solidification), unevenly

distributed in a tempered martensite matrix (Fig. 5a).

Carbides possess high hardness and wear resistance, but

they are extremely sensitive under dynamic- and thermal-

shock conditions. Carbide clusters, secondary cleavage as

well as intergranular cracks assisting carbide de-bonding,

and pull-out from the matrix are also evident (Fig. 5b). The

material used for the manufacturing of the load member

corresponds to a high-chromium/high-carbon tool steel,

and its surface hardness reaches up to 62 HRC. The pres-

ence of non-metallic inclusions deteriorates the compo-

nent’s performance, facilitating premature failure.

Fig. 3 (a, b) SEM micrographs (SE imaging) showing the microscopically irregular—rough fracture surface topography of the clamping block


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The failure of the load member was attributed to

dynamic-shock loading during production cycle start-up.

Careful review of the machine’s operating conditions in

combination with the selection of the appropriate steel grade

and metallurgical condition, possessing ‘‘clean’’ (potential

steelmaking processes: Electroslag Remelting, ESR; and

Vacuum Induction Melting, VIM) and fine microstructure

are suggested as necessary failure-preventive actions.

Torsional-overload failures of knuckle joints attributed to

abnormal machine’s operation were also reported in Ref. [8].

Microscopic Evidence of Fatigue: Failure of Steel Stud

Acting as Supporting Member of a Caster

In industrial metalworking operations, stress cycling is the

predominant loading mode resulting in fatigue failures of

machine components; see also Refs. [9–11]. A foundation

stud supporting the base of a casting machine, fractured

after a short period in service, leading to unexpected pro-

duction interruption and machine downtime. Typical

fatigue fracture is evident showing characteristic ratchet

marks as a result of multiple crack initiation due to stress

concentration (Fig. 6a). Crack initiation took place at the

thread root which acted as typical stress raiser in studs and

bolts. Ratchet marks were formed when cracks nucleated at

different positions and joined together creating shear steps

on the fracture surface. Beach marks which indicate suc-

cessive positions of the fatigue crack-front were also

present (Fig. 6b). The lack of macroscopic plastic defor-

mation and the extended fatigue zone suggested the

occurrence of high cycle fatigue under relatively low stress

amplitude. The existence of fine fatigue striations

Fig. 5 (a) SEM micrograph (BSE imaging) taken on transverse to fracture sections showing the microstructure of the tool steel component

consisting of primary chromium carbides dispersed in a tempered martensite matrix; and (b) detail of (a). Transgranular fracture and

intergranular de-cohesion of carbide particles (nital 2% immersion etching)

Fig. 4 SEM micrographs (SE imagings) at higher magnification showing the various fracture modes: (a) transgranular cleavage facets adjacent

to fine-dimpled areas manifesting localized plasticity; and (b) transgranular cleavage and intergranular fracture operated as dominant damage

mechanism of carbide islands; isolated fine-dimpled areas are also discerned


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constituted an additional fingerprint of high cycle fatigue

(Fig. 7a). Counting and recognizing striation patterns is a

diligent process requiring high level of technical expertise

and education, and should be performed by qualified sci-

entific and technical personnel; false patterns, resembling

fatigue striations, are very often found on fracture surfaces

and could provoke misleading conclusions. Aspects of

fatigue striations, under various loading conditions are

reported in Ref. [14]. Fast fracture area of the stud is

characterized by the presence of elongated non-equiaxed

dimples as an evidence of final ductile tear under the

presence of shear stresses (Fig. 7b).

The steel stud has failed because of the occurrence of

high cycle/low-stress amplitude fatigue mechanism. The

selection of higher strength steel grade and the review of

installation conditions (such as torque levels, clearances,

misalignments, etc.) are suggested as failure-preventive

actions aiming to prolong the fasteners’ service life.

Fig. 6 Optical stereomicrographs showing (a) details of the thread circumference depicting the presence of ratchet marks (ridges) suggesting the

occurrence of multiple-origin fatigue mechanism, and (b) the smooth fracture surface of steel stud

Fig. 7 SEM micrographs (SE imaging) showing (a) the presence of fine fatigue striations, and (b) the evidence of dimpled ductile overload

taking place at the fast fracture zone. Elongated, non-equiaxed dimples resulted from the complex stress state including the application of shear

stresses


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Conclusions

Fractographic evaluation provides vital information of the

micromechanics of fracture processes, crack evolution, and

microstructural influence on damage progress and accu-

mulation. The findings of the fractographic examination

could, many times, be stand alone, highlighting the failure

mode(s) and assisting in root-cause analysis and failure

prevention. Competent and qualified personnel are neces-

sary to implement fractographic procedures and their

interpretation, since misleading features coming from

analysis may contribute to incorrect decision making,

provoking potential economic losses or safety risks.

References

1. ASM Handbook, vol. 12: Fractography. ASM International,

Materials Park, OH (1992)

2. Lynch, S.P., Moutsos, S.: A brief history of fractography. J. Fail.

Anal. Preven. 6(6), 54–69 (2006)

3. Hull, D.: Fractography, Observing, Measuring and Interpreting

Fracture Surface Topography. Cambridge University Press,

Cambridge (1999)

4. Wulpi, D.J.: Understanding How Components Fail, 2nd edn.

ASM International, Materials Park, OH (2000)

5. Sachs, M.W.: Practical Plant Failure Analysis. CRC Press,

Boca Raton, FL (2009)

6. Pantazopoulos, G., Vazdirvanidis, A.: Fractographic and metal-

lographic study of spalling failure of steel straightener rolls.

J. Fail. Anal. Preven. 8(6), 509–514 (2008)

7. Psyllaki, P., Papadimitriou, K., Pantazopoulos, G.: Failure modes

of liquid nitrocarburized and heat treated tool steel under

monotonic loading conditions. J. Fail. Anal. Preven. 6(6), 13–18

(2006)

8. Pantazopoulos, G., Sampani, A., Tsagaridis, E.: Torsional fail-

ure of a knuckle joint of a universal steel coupling system

during operation—a case study. Eng. Fail. Anal. 14(1), 73–84

(2007)

9. Pantazopoulos, G., Zormalia, S., Vazdirvanidis, A.: Investigation

of fatigue failure of roll shafts in a tube manufacturing line.

J. Fail. Anal. Preven. 10(5), 358–362 (2010)

10. Pantazopoulos, G., Vazdirvanidis, A., Toulfatzis, A., Rikos, A.:

Fatigue failure of steel links operating as chain links in a heavy

duty draw bench. Eng. Fail. Anal. 16(7), 2440–2449 (2009)

11. Pantazopoulos, G., Zormalia, S.: Analysis of the failure mecha-

nism of a gripping tool steel failure operated in an industrial tube

draw bench. Eng. Fail. Anal. 18(6), 1595–1604 (2011)

12. Sachs, N.W.: Understanding the surface features of fatigue

fractures: how they describe the failure cause and the failure

history. J. Fail. Anal. Preven. 5(2), 11–15 (2005)

13. Rolls for Metalworking Industries. Iron and Steel Society (ISS),

Warrendale, PA (2002)

14. DeVries, P.H., Ruth, K.T., Dennies, D.P.: Counting on fatigue:

striations and their measure. J. Fail. Anal. Preven. 10(2), 120–137

(2010)


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