14 Rubber & Plastics News • January 12, 2004 www.rubbernews.com

Technical

Fractography aids study of tire tread separationsBy J.W. Daws

Exponent Failure Analysis Associates

Tire failure analysis usually is fo-cused on the mechanism of the separa-tion of the outer steel belt from the tirecasing and inner steel belt. The analystmust determine the point of origin of theseparation from the appearance of thefracture surfaces. Smith1 described themicroscopy of tire tread pieces gatheredfrom roadsides in Arizona, Californiaand New Mexico. However, it is oftenthe case in actual forensic investigationthat the tire tread pieces are not recov-ered, leaving the analyst to deal with ex-amining the inner steel belt’s exposedouter surface. This situation makes anunderstanding of the tire casing frac-ture surface important. In field treadseparation events, the fracture surfacesoften can be confounded by pre-tear pol-

ishing, post-separation skidding and im-pact damage.

Daws5 described an experiment in

inches) and then across the tread at onepoint along the outer steel belt wires.The cut tires were then run on a flat-track test machine at 112 kph (70 mph)to failure. Several general observationsresulted from creating tread belt separa-tions in the laboratory environment.From the physical perspective, the tirecasing never lost pressure during theseparation event. In general, the belt-leaving-belt separations occurred in lessthan 30 seconds from the start of thetest. The separated tread and outer steelbelt pieces always were hot to the touch,and in some cases, there was heat-relat-ed discoloration of the rubber. The treadseparation always generated two trian-gular flaps at the region of initiation.This flap pair consisted of one flap,formed by a triangular piece of treadand outer steel belt bounded by a wire ofthe outer steel belt, that opened in a di-rection opposite to the direction of rota-tion of the tire. The second of the flapswas adjacent to the first along the sameouter steel belt wire and opened in thetire’s direction of rotation. The first ofthese has been called a “leading edge

Executive summaryIn-service catastrophic radial tire failure is often a separation of the tread

and outer steel belt from the tire casing and inner steel belt. These separationsgenerally occur in the field at high temperature and high speed. Forensic analy-sis of the resulting failure surface appearance, or fractography, can be veryhelpful in determining the origin and progression of the tire tread separation.

Surfaces corresponding to the gradual cracking of the skim rubber along theedges of the belts, initiation of a belt-leaving-belt separation, high-speed tear-ing and termination of the separation are described. Examples of surfaces madein laboratory-created separations are compared to similar surfaces found onfield-failure tires. The fractography shown provides the tire failure analysis acatalog of comparative surfaces that are useful in determining the evolution offailure in a given tire.

which tire tread belt separations werecreated in a laboratory study. Severaldifferent characteristic surfaces wereidentified in the course of that study.Daws6 also described the mechanism bywhich these surfaces were created dur-ing a tread belt separation. The objec-tive of this technical note is to documentthe appearance of the outer skim sur-face of the inner steel belt resultingfrom a tread separation. Knowledge of

the different types of surface appear-ances will be useful to the failure ana-lyst in the diagnosis of specific cases.

General observationsand discussion

Tire tread belt separations were creat-ed by cutting the tire between the twosteel belts circumferentially aroundboth sides to a depth of about 50 mm (2

TECHNICAL NOTEBOOKEdited by Harold Herzlichh

Fig. 3. Edge cracks in field tread separation. Fig. 4. Initiation region in a laboratory study tire.

Fig. 2. Micrograph of edge cracks showing crack-arrest marks.Fig. 1. Edge cracks in laboratory study tire.

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flap” because its triangular end at thetire shoulder points in the direction oftire rotation. The second flap is normal-ly called a “trailing edge flap” for similarreasons.

Another observation from the ma-chine tests is that the leading edge flappropagates most of the way around thecircumference of the tire while the trail-ing edge flap experiences only a smallamount of tear propagation. This resultis reasonable since the leading edge flaptends to be torn open on contact with themachine frame (or the vehicle body, fora field tire) at each revolution of the tire,while the trailing edge flap is pushedclosed, and, in the case of the machinetest tires, both types of flaps were equal-ly likely. This is because the cuts used togenerate the initiation of flaps resultedin a situation wherein both flaps werecreated simultaneously. During the ini-tiation of the flaps, centrifugal force andstresses associated with the change ofcurvature of the tread while in contactwith the road surface cause the tearingof the skim rubber between the two steelbelts. When the flaps reach a certainsize, however, the leading edge flap alsois torn open by forces resulting fromcontact with the machine frame. Thetrailing edge flap is pushed closed bythose same forces. The tearing of theflaps from the tire casing normally be-gins when the flap widths approach thewidth of the attached tread.

Fracture surface observationsGeneral—Four main types of fracture

surfaces were observed in both laborato-ry-created and field-failure tires. Thefirst of these was found circumferential-ly on both edges of the uncut regions ofskim rubber between the two steel belts.The second type of crack surface ob-served was associated with the centrifu-gal force generation of the flaps at theinitiation of the belt-leaving-belt separa-tion. The third crack region observedwas associated with the rapid tearing ofthe tread and outer steel belt away fromthe inner steel belt. The fourth crack re-gion observed was associated with the fi-nal separation. Each of these is shownin the accompanying figures and de-scribed in the following sections.

Edge crack generation—Smith1 calledthe circumferential cracks “ring tears,”and associated them with small-scalecyclic deformations of the belt edges.These edge cracks propagate radiallyinto the skim rubber. Since the ends ofthe steel belt wires are recognized to bethe initiation points for these cracks,they develop simultaneously around thecircumference of the steel belt edge onboth sides of the tire (Huang and Yeoh3).

Crack growth rates depend upon the tiredesign and materials used, but initialradial growth rates in new tires are as-sumed to be very small, probably muchless than 0.1 nm/rev. As the cracks growin the radial direction, the crack growthrate is thought to evolve to a nearly con-stant level for the first 5-20 mm of depth(Lake4). As the crack depth progressesbeyond this level, the growth rate accel-erates. These edge cracks normally areobserved in belt-leaving-belt field sepa-rations, but they often are polishedsmooth as tire operation continues be-cause of the slow growth very near theedge of the steel belt.

Fig. 1 shows a close-up view of theedge cracks that were typical of thosegenerated on the laboratory tests. Fig. 1shows clearly, in the lower part of thephotograph, the knife cut region thatwas introduced in the skim rubber be-tween the two steel belts in order to fa-cilitate the tread separation. The edgecracks grew from the edge of the knifecut into the skim rubber between the

two steel belts. Because an artificialcrack of 50 mm depth had been intro-duced between the belts on both sides ofthe tire, these edge cracks developedand progressed very quickly in the pres-ence of high strain energy density. Edgecrack growth rates as high as 0.1mm/rev were observed. The rapid tearzone is situated above the edge crack re-gion and fills the top third of Fig. 1.

Fig. 2 shows a micrograph of the edgecracks. Crack-arrest marks, or beachmarks, clearly can be seen running per-pendicular to the direction of crackgrowth. These beach marks denote thecrack tip front at a particular time. Notethat the crack-arrest marks terminateat what appear to be ridges or radiallyoriented steps. These ridges separate re-gions where the crack growth from eachinitiation point occurs on a slightly dif-ferent path within the bulk of the skimrubber. In the case of field tires, theseedge cracks grow from the ends of thewire strands that make up the steel beltreinforcement because the steel wire isnot brass-plated at the cut ends. In thisexperiment, the edge cracks grew frompoints where the overlap of inner andouter belt wires created stress concen-trations in the skim rubber at the cutedge.

Fig. 3 shows the zone of edge crackingin a tire that experienced a field-servicetread separation. In this case, the edgecracks grow in fatigue from the edge of

the outer steel belt into the skim rubberbetween the two steel belts. Since theinitial fatigue crack growth rate is verylow in field tires, it is normal to havepolishing of the edge crack surfaces anda corresponding reduction in the sharp-ness of the radially oriented tear ridgesthat characterize these cracks. The rela-tive movement of the crack surfaces iscaused by the strains generated whenthe tread is in road contact during eachrotation.

The depth of these cracks in both thelaboratory samples and in field tiresvaries around the circumference of thetire. Circumferential variation in crackdepth on a given side of the tire iscaused by stress state variations in thetire resulting from variations in the rub-ber dimensions that occur naturally inthe tire building process and by the sta-tistical variation in crack initiation andearly crack development. As the cracksprogress more deeply into the skim rub-ber in the radial direction, the crackgrowth per revolution accelerates. Thenormal outcome of this characteristic isthe generation of one large thumbnail-shaped pocket between the two steelbelts. This large pocket is generally thelocation at which the tread separationcan initiate in field tires. The currentexperiment shows that the developmentof these pockets can be extremely rapidin field tires once the crack depth movesbeyond the approximate 5-20 mm

boundary of constant growth rate. Forcrack depths beyond 50 mm, the currentexperiment showed cracks propagatingat rates approaching 0.1 mm/rev, whichwould be on the order of 35 mm/km ofvehicle travel.

Flap initiation—The region at thepoint of initiation of the flap tips is tornduring operation by centrifugal and con-tact patch stresses. In this region, dis-tinct beach marks are observed. Thesemarks go from the separation line to thebelt edges at the shoulders of the tire.Relative to the tip of the flap, the beachmarks produce arcs, showing that theflap edge along the tire’s shoulder is be-ing pulled further during each tear thanis the flap edge along the separationline, which is consistent with the tirehaving curvature across the tread area.The tear propagation rate of thesecracks along the tire’s circumferencewas on the order of 10 mm/rev in thecurrent experiment. Fig. 4 shows one ofthe flap initiation zones from laboratorytest sample. The knife cut region can beseen clearly in the lower portion of thephotograph. The initial separation line,which follows the wire line of the outersteel belt, is highlighted for clarity. Thehorizontal arrow in the photographshows the direction of propagation of thecrack for the flap tip below the separa-tion line. The beach marks are well de-fined in one of the triangular regions un-

Technical

Fig. 5. Initial separation region in a tire with a field tread separation.

Fig. 6. Rapid tearing zone in laboratory study tire.

Fig. 7. Rapid tearing region on a tire with field tread separation.

See Study, page 16

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der the flaps in this example.Fig. 5 shows the initial separation

line for flap formation from a tire thathad experienced a field tread separa-tion. Below and to the right of the sepa-ration line is the thumbnail pocket thatwas developed by the edge cracks grow-ing and linking together under fatigueloading. In this particular case, the edgecrack pocket extended about 85 percentof the way across the tire’s tread areabefore the flaps formed and the treadand outer steel belt separated. As wouldbe expected, there was a significantamount of polishing at the tire shoulderbecause of the large interfacial move-ment that occurs in a pocket of this size,as well as other distortion of the cracksurfaces near the tire centerline. In fact,the flap tips in this case did not gener-ate the characteristic beach marks, be-cause once the tread and outer steel beltbroke at the tire shoulder, the flap al-ready was separated for nearly the fullwidth of the tire. As seen in the labora-tory studies, rapid tearing of the flapnormally begins at the point where theflap width is approximately equal to thetread width.

Rapid tearing—Once the flaps haveseparated across the entire tread width

at one location, the tear crack propaga-tion in the skim rubber between the twobelts occurs rapidly. When the leadingedge flap experiences contact with exter-nal surfaces, the tear propagates atrates on the order of 100 mm/rev orhigher. The skim rubber in this regionnormally will peel close to the innersteel belt on one half of the separationregion and close to the outer steel belton the other half. The resulting thick-thin aspect of the skim rubber remain-ing on the outer surface of the innersteel belt is very distinctive. Fig. 6shows this type of surface from one ofthe laboratory study tires. The dividingline between the thick and thin skim isa transition region that normally followsthe centerline of the tread. The width ofthe transition region was highly vari-able in the laboratory study, but tendsto be very narrow in field-failure tires.

The direction of propagation of thetear and the tire’s rotational directiondetermine whether the skim remainingon the outer surface of the inner beltwill be thick or thin on the serial and op-posite serial sides. As seen in Fig. 6, theregion of thin skim is the one showingwire lines from the inner steel belt inthe lower portion of the photograph,while the region of thick skim in the up-per portion of the photograph showswire lines from the outer steel belt. Thehorizontal arrow in the photographshows the direction of propagation of the

crack. In the laboratory study, the thickskim zone on the inner steel belt surfaceof the casing is on the opposite serialside of the tire for the leading edge flap.The trailing edge flap produces the thickskim on the serial side of the tire.

The locations of the thick and thinskim arise because of the compound cur-vature of the inflated tire’s surface.When the tread and outer steel belt arepulled off the inflated casing, the edge ofthe crack forms an arc in the direction ofpropagation of the tear. This arc has theappearance of a “C” with the open endtoward the direction of crack propaga-

tion (beach marks associated with therapid-tearing region also will be in theshape of a “C”). In the case of the lead-ing edge flap on a right side tire, therewill be thick skim rubber remaining onthe opposite serial side of the tire andthin skim rubber remaining on the seri-al side of the tire. There will be a her-ringbone pattern in the skim rubberthat corresponds to the steel wire beltangles. For the trailing edge flap, thepropagation direction is opposite to thatof the leading edge flap and thereforethe crack front arc and the thick andthin skim locations will be reversed. Ifthe tire is mounted on the vehicle’s leftside, the skim thicknesses will be re-versed.

Fig. 7 shows the rapid tear region ona tire that experienced a field tread sep-aration. This particular tire was fromthe left side of a vehicle, so the thick andthin skim regions are reversed from thatof the laboratory study tires. Note thatthe transition region is non-existent inthis particular tire. One would also ex-pect that, for tires built for right-handdrive countries, the skim patterns wouldbe reversed from those found in left-hand drive countries since the steel beltwire angles normally are reversed forthe inner and outer steel belts.

Termination/Separation—The treadand outer steel belt separate completelyfrom the tire casing when the leadingand trailing edge flaps meet. The pointis marked by a distinctive reversal ofthe skim thicknesses in the rapid tear-ing region. This characteristic patternfollows naturally from the patterns ob-served in the rapid tearing region previ-ously discussed. Fig. 8 shows the sepa-ration termination pattern from one ofthe laboratory study tires. The flappropagation directions are indicated onthe photograph. As previously noted, theskim patterns for the two flaps will beopposite. This is shown clearly in Fig. 8,where the pattern noted “A” is on the op-posite serial side for the leading edgeflap and on the serial side for the trail-ing edge flap.

Fig. 9 shows the separation region ona tire that experienced a field tread sep-aration. The distinctive pattern indicat-ing the separation termination point isfound in the middle of the matching pat-terns shown by “A” and “B.” In the caseof this tire, the leading edge flap origi-nated at the lower right extremity of thephotograph. The trailing edge flap expe-rienced very little growth as the leadingedge flap progressed completely aroundthe tire.

ConclusionsThe following conclusions are drawn

from the analysis of tire tread belt sepa-rations created on a laboratory machineand comparison of those fracture sur-faces with those found in field tires:

1. Edge cracks in the skim rubber be-tween the two steel belts of a radial tireleave characteristic radial tear ridges.

2. The depth of the edge cracks be-tween the two steel belts is variablearound the circumference of the tire.

3. The propagation rate of the edgecracks in the radial direction starts outat a very low level and becomes veryhigh as the final separation becomes im-minent. Propagation rates in the radialdirection at 50 mm of crack depth ashigh as 0.1 mm/rev were observed inthis experiment.

4. The tread separation initiation re-gion is characterized by a line along theouter steel belt wires bounding two tri-angular regions that were created by thegeneration of flaps. There often arebeach marks present in one or both ofthe triangular regions.

5. The leading edge flap normallypropagates circumferentially at a higherrate than the trailing edge flap becauseof contact with stationary surfaces.

6. The rapid tearing of the skim be-tween the two steel belts begins whenthe flaps attain a width close to the fulltread width.

7. The rapid tearing region is charac-terized by distinct patterns of thick andthin skim that depend on the rotation ofthe tire and the flap propagation direc-tion.

8. The separation termination point ischaracterized by a distinctive reversal ofthe skim thickness pattern in the rapidtear region.

References1. R.W. Smith, Rubber Chem. Technol., 70, 283(1997).2. Fay, et al., SAE Paper No. 1999-01-0447.3. Y.S. Huang and O.H. Yeoh, Rubber Chem. Tech-nol., 62, 709 (1989).4. G.J. Lake, Rubber Chem. Technol., 74, 509 (2001).5. J.W. Daws, “The Fractography of Tire Tread Sep-arations,” paper No. 67 presented at a meeting ofthe Rubber Division, American Chemical Society,April 28-30, 2003, San Francisco.6. J.W. Daws, “Failure Analysis of Tire Tread Sepa-rations,” Practical Failure Analysis, Vol. 3, No. 5,October 2003, pp. 73-80.

Fig. 8. Separation termination point from a laboratory study tire.

Fig. 9. Separation termination point for a tire with a field tread separation.

Technical

StudyContinued from page 15

The authorJohn W. Daws is a senior managing engineer in Exponent Failure Analysis Associ-

ates’ Vehicle Engineering practice and is based in Phoenix. He has more than 19years of technical and managerial experience in the tire indus-try. Daws specializes in the analysis of tire failure, having exam-ined tires from machine tests and field returns over a number ofyears.

He has experience in modeling vulcanization of rubber com-pounds using finite element analysis and kinetic modeling.

Daws has participated in the development of passenger tiresfor vehicle manufacturers as well as for the replacement market,and has extensive experience in the various types of testing per-formed in those environments.

He also has extensive machine development experience in thetire manufacturing arena and has spent considerable time relat-ing tire performance to manufacturing processes used in the in-dustry.

Prior to joining Exponent, Daws was employed by Michelin North America Inc.,where he was responsible for the determination of vulcanization effects on tire perfor-mance using finite element analysis, machine testing and field testing.

He can be reached by phone at 623-587-4161; fax at 623-581-8814; or e-mailjdaws@exponent.com.

Daws

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