CASE HISTORY—PEER-REVIEWED

Failure Analysis of Motor Tire Bead Wires During Torsion Test

Souvik Das • Prashant Koli • Jitendra Mathur • Arthita Dey •

Tanmay Bhattacharyya • Sandip Bhattacharyya

Submitted: 30 January 2013 / Published online: 10 October 2013

� ASM International 2013

Abstract Torsion testing is used to determine the quality

of steel wire used for motor tire beads in pneumatic tires.

These steel wires must have good-tensile strength so that

the tire bead can support the finished tire safely, and yet

retain adequate ductility to deform easily around the

forming wheel. The present paper highlights premature

failure of bead wire which failed during torsion test. Tor-

sion property is one of the important parameters of tire

bead as it monitors both the metallurgical soundness and

surface quality of a drawn wire. From the analysis, it has

been concluded that probable reason for premature failure

is due to strain aging (dynamic and static) caused by

interstitial atoms which bounds the mobile dislocations

resulting in increases yield strength and decreases bead

formability. Moreover, the microstructural study indicates

that failed specimen has misaligned and broken lamella of

pearlite with globular cementite which creates the array of

voids. These voids hinder the rotation of pearlite during

torsion test thus leading to brittle fracture.

Keywords Motor tire bead � Torsion � Interstitial atoms �Globular cementite

Introduction

High carbon steel wires are vital material for variety of

applications such as bead wire, spring, and steel cord for

reinforcing pneumatic tires. The bead is that part of the tire

that contacts the rim on the wheel. Wires used for tire bead

applications are drawn from patented wire to achieve

specific properties required for critical applications. The

bead seats tightly against the two rims on the wheel to

ensure that a tubeless tire holds air without leakage. These

steel wires must have good-tensile strength so that the tire

bead can support the finished tire safely, and yet retain

adequate ductility to deform easily around the forming

wheel. Bead wire ductility, i.e., the ability of a high

strength steel wire to deform plastically without fracturing,

is determined by torsion value. Torsion and bend both are

important as these two aspects determined the material

quality and surely safety of the passengers in a running car.

Torsion property monitors both the metallurgical and sur-

face quality of a wire used for critical applications. The

performance of tire on road improves with the higher tor-

sion values offering improved passenger safety. It has been

reported earlier that the torsion property of high carbon

steel wire depends on optimum combination of strength

and ductility. A comprehensive work on torsion failure of

carbon steel wire was reported in 1969 [1]. Under torsion

stressing, principal planes are oriented at 45� to the lon-

gitudinal wire axis and maximum torsion shear stress

(equal to maximum principal stress in torsion) occurs in

planes parallel to and perpendicular to the longitudinal

axis. Therefore, ductile torsion failure, under torsion shear

stresses, appears as a flat break perpendicular to the wire

axis. Brittle failure, i.e., no shear stresses, occurs on a

principal plane having maximum tensile stress at 45� to the

wire axis. However, ductile torsion failure may also initiate

on a plane parallel to the wire axis giving a helix fracture

due to twisting torque of the torsion test. This is a ductile

longitudinal delamination failure [2]. During wire drawing

process, some amount of cementite suspension could take

place in high carbon steel wire depending on the

S. Das (&) � P. Koli � J. Mathur � A. Dey � T. Bhattacharyya �S. Bhattacharyya

Metallurgical Laboratories, R&D and Scientific Services, Tata

Steel Limited, Jamshedpur 831001, India

e-mail: souvik.das@tatasteel.com; souvikmet@gmail.com

123

J Fail. Anal. and Preven. (2013) 13:684–688

DOI 10.1007/s11668-013-9750-x

orientation of pearlite [3–5]. As the cementite is deformed,

it becomes thinner and array of voids will generated.

Extent of cementite dissolution during heavy cold working

will influence the materials mechanical strength and duc-

tility during subsequent strain aging [6–8]. Torsional

ductility is strongly dependent on mechanical properties,

and has been shown to be a function of strain aging [9, 10].

Strain aging is the process whereby interstitial solute atoms

(carbon, nitrogen) migrate toward mobile dislocations in a

deformed metallic structure and hold them by forming

interstitial atmospheres around the dislocations [11].

Therefore, it is important to prevent the decline of ductility

due to strain aging caused by carbon and nitrogen. In the

present paper, failure analysis of some of the tire bead

wires which failed during torsion test are carried out

through detailed metallurgical analysis in order to find out

the root cause of the failure.

Experimental Procedure

Few pieces of wires that had failed during different stages of

torsion test and also wire sample which have passed the

torsion test were collected from wire drawing mill. The

drawn wires were further subjected to stress relieving

annealing followed by coating. For torsion test Gauge

length of wire test was 20.3 cm (8 in.) conforming to

ASTM-D4975 [12]. The number of revolutions of a specific

length of wire twisted on its own axis in one direction until

rupture was counted. The samples were cleaned with ace-

tone to remove dirt for visual examination prior to

metallographic sample preparation. Specimens were pre-

pared from the fractured end of each failed wire samples for

fractography under Field Emission Gun Scanning Electron

Microscope (FEG-SEM) to identity mode of failure. The

analyses were performed at 15 keV accelerating voltage

and 510-8 A probe current. For microstructural analysis,

samples were individually mounted in electrically conduc-

tive copper-containing resin and polished by conventional

metallographic techniques. The polished samples were

etched with 3% nital solution (3 ml HNO3 in 97 ml ethyl

alcohol) for analysis of microstructure and studied under

FEG-SEM. Hardness testing was carried out on the wire

sample in different location in Micro hardness tester. Dur-

ing testing an applied load of 50 gf was used, and several

indentations were made to determine the average HV.

Chemical analysis of the wire samples was carried out in

LECO to determine the exact element concentration.

Table 1 Particulars of failed samples

Spec Fracture type

Chemical composition (wt.%)No. of twist during

torsion testC S P Si Ni

1 Helix fracture 0.78 0.011 0.011 0.175 0.013 10

2 0.79 0.008 0.012 0.169 0.011 19

3 0.79 0.011 0.013 0.172 0.013 32

4 0.81 0.008 0.012 0.170 0.012 34

5 Flat break 0.78 0.008 0.011 0.169 0.011 43

Fig. 1 (a)–(b) Delamination was observed in the surface after tension for sample #1

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Result and Discussion

Visual Observation

In case of sample #1 to 4 delamination of surface was

observed and the crack extends along the sample length,

imparting it a typical helical aspect (Fig. 1a, b). In case of

sample #5 no delamination was observed in the surface

after torsion test and fracture surface was flat in nature.

Materials

High-tensile strength bead wire 1.83 mm in diameter was

obtained by cold-drawing 5.5 mm steel rod in a multi-pass

Fig. 2 (a) shows brittle fracture which occurs as the helix type during torsion test of sample #2. (b) and (c) show brittle cleavage fracture

morphology with characteristic river patterns

Fig. 3 (a) Ductile torsion failure appears as a flat break perpendicular to the wire axis of sample #5. (b) Elongated dimples are shown in the

microstructure, which occur from micro-void emergence in places of high local plastic deformation

686 J Fail. Anal. and Preven. (2013) 13:684–688

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tandem wire drawing machine. Chemical composition (wt.%)

of motor tire beads (MTB) wires are provided in Table 1.

Fractography

Fractography was carried out for both passed and failed

sample (in torsion test). Brittle cleavage fracture morphol-

ogy with characteristic of river patterns was observed in

sample #2 (Fig. 2a, c). A twister structure was found inside

these river patterns (Fig. 2c). This shows cold-deformed,

fibrous material twisted upward in a counter-clock-wise

direction, and macro-shear dimples were found in the

deformed structure. Flat ductile fracture was observed in the

sample #5 which passed in torsion test (Fig. 3a, b).

Relation Between Nitrogen Concentration and Number

of Twist

Relationship between nitrogen concentration and number

of twists to failure is shown in Fig. 4. Variation of nitrogen

concentration, shown at each data point, was greater in

bead wires that failed at lower number of twists. Number of

twists to failure increased as nitrogen concentration

decreased. When a steel wire undergoes torsion testing,

shear strain increases with number of twists [1], and the

combination of twisting moment and resulting shear

stresses will twist the deformed fibers, and interstitial atom

will precipitate on and pin the dislocations. Consequently,

higher interstitial free nitrogen concentration will decrease

number of twist to failure and resulted in premature failure.

Microstructural Analysis

Microstructure of the wires of samples #1 and 5 are carried

out in FEG-SEM after etching with 3% nital. The size of

cementite particles in sample #1 (coarse pearlite) is larger

than sample #5 (fine pearlite). The microstructure of the

failed samples shows misoriented pearlite lamellae with

globular cementite and inside of the colony also become

bent, waved (Fig. 5a) whereas from the microstructure of

sample #5, it is observed that the pearlite colonies and

shear bands aligned along the direction of drawing axis

(Fig. 5b). Previous investigation by authors [13] has

reported that the formation of globular cementite during

drawing occurs by the densification of cementite through

wrinkling or buckling, and also by the carbon diffusion

accelerated due to deformation. Considering these points a

void formation around cementite particles would be

endorsed to the intense stress concentration at particles

whose size is much larger than the thickness of a cementite

lamella. Accordingly, the increase of the cementite particle

size gives rise to the stress concentration at higher level

resulting in the formation of larger void which resulted in

crack propagation for delamination during torsion.Fig. 4 Relation between nitrogen concentrations to number of twist

to failure

Fig. 5 (a) Microstructure of premature torsion sample #1. (b) Microstructure of wire sample #5

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Hardness Measurement

Micro-hardness measurement (HV) was done on both the

sample which have failed (sample #1) and passed the tor-

sion test (sample #5). The hardness value in sample #1 was

found to be non-uniform. There was variation in hardness

from one edge to the other edge of the wire samples

(Fig. 6). The hardness variation in the wire #1 leads to the

inference that there was improper thermal treatment pro-

cess. Sometimes due to temperature rise during the final

stage of drawing variation of mechanical properties has

been observed [14]. The temperature rise during defor-

mation encourages delamination in the wire by increasing

the strength of the samples, which has a detrimental

influence on the torsional properties and durability of the

wire.

Conclusions

Mainly the torsion failure occurred in the MTB wires due

to two reasons.

• Firstly the interstitial atoms collect around mobile

dislocations of the drawn wires, thereby pinning them

and making them immobile. Thus when a steel wire

undergoes torsion testing, shear strain increases with

the total angle of twisting or number of twists and the

combination of twisting moment and resulting shear

stresses will twist the deformed fibers, and intestinal

atom will precipitate on and pin the dislocations and

resulted in premature failure during torsion test.

Dislocation immobilization caused by interstitial nitro-

gen atmospheres can be minimized by proper thermal

treatment during the final manufacturing process which

will improved torsion ductility.

• Secondly the microstructural study shows that the

premature torsion failure specimen has misaligned and

broken lamella with globular cementite which can

create array of voids. The array of those voids would

act as one of the origins for delamination or offer the

preferential site for delamination during torsion, lead-

ing to brittle fracture.

• Moreover extra care should be taken to control wire

temperature after every pass during drawing operations,

if possible isothermal pass schedule should be imple-

mented which will increase the ductility of the final

wires. Ductile bead wire is essential for structural

integrity of a tire and subsequent passenger safety thus

much care is needed during the production of these

types of wires.

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