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Failure analysis of a motorcycle brake disc M. Boniardi a , F. DÕErrico a, * , C. Tagliabue a , G. Gotti b , G. Perricone b,1 a Department of Mechanics, Politecnico di Milano, Via La Masa 34, 20156 Milano, Italy b Brembo S.p.A., Viale Europa 2, 24040 Stezzano (BG), Italy Received 7 April 2005; accepted 30 April 2005 Available online 13 September 2005 Abstract A certain number of motorbike disc brakes, made of stainless steel, shown the presence of small cracks only after a few thousands miles. These cracks were mainly located nearby the holes placed on flange to ventilate and refresh pads. According to results, the deterioration can be led back to thermal cyclic strain (related with the heating–cooling cycles developed during the brake action) superimposed to the mechanical strain caused by braking torque. This work analyses the aforesaid disc brakes investigating both the main causes and the evolution of its deterioration in order to find out possible solutions. The short lifespan of such discs has to be ascribed to the rapid decay of the mechanical properties of the manufacturing material. Material decay is liable for starting cracks. Several actions could be chosen to face this problem. The choice of a particular chemical composition, which will be demonstrated to be unfit for the purpose, produced an extreme tempering of the steel as a direct result of its protracted exposure to high temperatures (a situation which can be considered usual referring to disc brakes). In this work, we present the effect of choosing a different kind of steel, characterized by a greater resistance to the tempering processes. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Fatigue; Thermal fatigue; Metallurgical examinations; Brake failure 1. Introduction A motorbike brake disc is composed of two main parts, a flange and a bridge (Fig. 1). The former faces pressure from pads and therefore is subjected to higher temperatures during braking; the latter works in cooler conditions and has the function of transferring brake torque to the wheel hub. 1350-6307/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2005.04.012 * Corresponding author. Tel.: +39 02 2399 8215; fax: +39 02 2399 8202. E-mail addresses: [email protected] (F. DÕErrico), [email protected] (G. Perricone). 1 Tel.: +39 035 605 700; fax: +39 035 605 605. Engineering Failure Analysis 13 (2006) 933–945 www.elsevier.com/locate/engfailanal
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Failure Analysis of a Motorcycle Brake Disc

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Page 1: Failure Analysis of a Motorcycle Brake Disc

Engineering Failure Analysis 13 (2006) 933–945

www.elsevier.com/locate/engfailanal

Failure analysis of a motorcycle brake disc

M. Boniardi a, F. D�Errico a,*, C. Tagliabue a, G. Gotti b, G. Perricone b,1

a Department of Mechanics, Politecnico di Milano, Via La Masa 34, 20156 Milano, Italyb Brembo S.p.A., Viale Europa 2, 24040 Stezzano (BG), Italy

Received 7 April 2005; accepted 30 April 2005Available online 13 September 2005

Abstract

A certain number of motorbike disc brakes, made of stainless steel, shown the presence of small cracks only after afew thousands miles. These cracks were mainly located nearby the holes placed on flange to ventilate and refresh pads.

According to results, the deterioration can be led back to thermal cyclic strain (related with the heating–coolingcycles developed during the brake action) superimposed to the mechanical strain caused by braking torque.

This work analyses the aforesaid disc brakes investigating both the main causes and the evolution of its deteriorationin order to find out possible solutions. The short lifespan of such discs has to be ascribed to the rapid decay of themechanical properties of the manufacturing material. Material decay is liable for starting cracks.

Several actions could be chosen to face this problem. The choice of a particular chemical composition, which will bedemonstrated to be unfit for the purpose, produced an extreme tempering of the steel as a direct result of its protractedexposure to high temperatures (a situation which can be considered usual referring to disc brakes).

In this work, we present the effect of choosing a different kind of steel, characterized by a greater resistance to thetempering processes.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Fatigue; Thermal fatigue; Metallurgical examinations; Brake failure

1. Introduction

A motorbike brake disc is composed of two main parts, a flange and a bridge (Fig. 1). The former facespressure from pads and therefore is subjected to higher temperatures during braking; the latter works incooler conditions and has the function of transferring brake torque to the wheel hub.

1350-6307/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2005.04.012

* Corresponding author. Tel.: +39 02 2399 8215; fax: +39 02 2399 8202.E-mail addresses: [email protected] (F. D�Errico), [email protected] (G. Perricone).

1 Tel.: +39 035 605 700; fax: +39 035 605 605.

Page 2: Failure Analysis of a Motorcycle Brake Disc

Fig. 1. Scheme of a motorcycle brake disc.

934 M. Boniardi et al. / Engineering Failure Analysis 13 (2006) 933–945

The degree of reliability of the braking action is directly proportional to the firmness of the entire brak-ing system. This means that the friction coefficient between the disc and the braking pad must be kept asmuch as possible constant, under any condition.

Hence, the rotor is designed to withstand both the maximum possible deceleration (emergency braking)and a series of braking cycles. The main problem to deal with in braking cycles is the control of the fadingphenomena. High frequency braking cycles lead a raising of the disc temperature. Therefore, after a periodof temperature growth, the disc will reach its balance between the heat generated by braking action and thedissipated heat. At the balance a steady state temperature is reached. If the steady state temperature is toohigh, several problems happen among which a decay of friction coefficient and a not constant behaviourbetween following brakings.

To face this problem, designers focus on disc performances operating with high temperatures (500–600 �C or more). Discs are then checked on a test bench which simulates extreme braking conditions, suchas an alpine downhill or a sport race.

Other issues are wear and corrosion. To prevent problems with these phenomena, disc brakes are usuallymade of martensitic stainless steel. Because of their narrow thickness (usually less than 10 mm) discs aregenerally obtained by milling of flat hot rolled sections. These sections are milled in the annealed (normal-ized) condition. Then, are subsequently quenched and finished off.

2. The case history

In our analysis, we worked on two brake discs (280 mm diameter, 8 mm thickness) made of martensiticstainless steel. The discs were produced using two similar types of steel coming from two different suppliers(from now on, named A and B). They could both be compared to the AISI 410 standards, except for somechemical modifications aimed at gaining the hardening response. In fact, in brake applications, a surfacehardness of 32–36 HRC (in the quenched condition) is required.

The B type discs show the presence of small cracks (detailed in Fig. 2) after a few thousands miles of use,whereas in the A type this deterioration is not noticeable [1]. These cracks were clearly visible even with thenaked eye. They triggered off from the ventilation holes, spreading for some millimeters in the radialdirection.

A deeper analysis was necessary to investigate and understand the dissimilar behaviour of the two kindsof discs.

Page 3: Failure Analysis of a Motorcycle Brake Disc

Fig. 2. Detail of a crack placed on a type B disc.

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To exclude from the analysis the potential problems ascribable to the applied milling process, some pro-totypes were obtained by laser cutting with regards to the cutting method, the behaviour did not show anyvariation, since the B type discs failed independently from the cutting process.

The analysis includes both the unused samples of the A and B type, and some used and cracked B typesamples, which from now on will be named BU.

3. Observations and results

3.1. Visual and chemical analyses

Every BU sample showed cross cracks arising from the more external holes. As depicted in Fig. 3, crackswere aligned with holes fastening the disc to wheel hub and they are the only type of cracks occurred.

The uniform colour of the braking surface demonstrated at least a 500–550 �C working temperature.Quantometric analyses were carried out on the two rotors samples (A and B) to determine their chemical

compositions. Results are reported in Table 1. It can be observed that the A and B steels were two partic-ular kind of martensitic stainless steel, similar to the AISI 410, except for some chemical differences notdeclared by manufacturers.

The main difference was the greater content of molybdenum and vanadium of the A type steel, and thehigher manganese percentage of B steel. Usually, the least expensive means of increasing hardenability at agiven carbon content is by increasing the manganese content [2].

3.2. Selection and removal of samples

The A, B and BU discs were sectioned by band saw, paying attention to have a constant lubrication andcooling. Some metallographic samples were obtained, polished and chemically etched (glyceregia, 3 partsglycerol, 3 parts HCl, 1 part HNO3) to reveal their microstructure.

Page 4: Failure Analysis of a Motorcycle Brake Disc

Fig. 3. BU rotor characterized by two cross cracks highlighted by the circles.

Table 1Comparison of chemical composition of type A and B discs

C Si Mn P S Cr Ni Mo Al Cu Ti Nb V B Sn

Disc A 0.078 0.38 0.86 0.031 0.003 12.30 0.124 0.100 tr. 0.068 0.003 tr. 0.100 tr. 0.003Disc B 0.066 0.36 1.51 0.019 0.004 12.16 0.084 0.026 tr. 0.066 0.002 tr. 0.038 tr. 0.001

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The BU-1 and BU-2 samples were extracted from the BU disc nearby the holes involved by the twocracks (Fig. 4).

The BU-1 sample was mechanically opened, paying attention not to damage the crack surfaces. Subse-quently the fracture surface was investigated with a scanning electron microscope (SEM).

3.3. Metallography and microstructures

Fig. 5 shows an example of the microstructures revealed on the A, B and BU disc surface, nearby theflange centre. From the metallographies reported in Fig. 5(a) and (b), it can be inferred that the microstruc-ture was martensite not tempered. This microstructure is peculiar of a quenching heat treatment used forgiving hardness to the flange surface. A quite different martensitic structure can be noted in Fig. 5(c): itappears less refined and with a certain amount of widespread carbides.

The area close to disc spokes (spokes are the building blocks of the brake bridge), was analyzed too andthe microstructures are shown in Fig. 6.

For the A sample, a ferritic microstructure was observed, as shown in Fig. 6(a). It shows a grain refining(5 lm mean length) by recrystallization due to the hot rolling. The microstructures of the B and the BU

Page 5: Failure Analysis of a Motorcycle Brake Disc

Fig. 4. BU disk sectioned; the BU-1 and BU-2 samples chosen for the metallographic and fractographic researches are highlighted.

M. Boniardi et al. / Engineering Failure Analysis 13 (2006) 933–945 937

spokes, reported in Fig. 6(b) and (c), respectively, were ferritic too. They revealed the same recrystallizationphenomenon but with a grain average dimension between 20 and 30 lm. The recrystallization phenomenonis better visible at lower magnification. A more accurate hot rolling could explain the difference between theA and B sample grain size. Cracks happened however faraway from spokes where microstructures aremartensitic.

Therefore, going from flange to spokes, substantial changes in microstructures were noticeable. Suchchanges suggest that only the flange was subjected to quench hardening: probably an induction hardening.In fact, the flange had a martensitic structure, while the bridge had a ferritic and normalized one. Finally,Fig. 7 shows a metallography performed on the BU-2 sample which highlights the crack features: the pat-tern is essentially transgranular and flat, as expected in fatigue crack propagation.

3.4. Hardness tests and results

Hardness was measured on all the three kinds of discs. The measure was carried out both on the entireflange and spokes area. Lower hardness values were measured on the spokes, due to the softer ferritic struc-tures: this is in line with the previous metallography analyses.

Figs. 8 and 9 report the hardness values and the relative measurement positions for the brand new A andB discs. Flange areas of A and B types exhibited similar values, proving that both the discs types weresubjected to analogous heat hardening treatments. Instead, the analyzed BU sample shown a considerabledecrease of hardness values in several positions, as summarized in Fig. 10.

To check that the hardness decay registered in the centre of the flange was not due to a local over-heat-ing, the hardness measurements on the BU disc was extended in the circumferential direction too. Thisanalysis confirmed that the phenomenon was not local, but rather symptomatic.

3.5. Fractographic analysis

Fig. 11 reports a microfractography of the BU-1 sample fracture surface. It is possible to distinguishclearly into two different zones. The former is planar, in which the crack generates a fatigue pattern (thedirection of which is indicated by the pointer in the illustration). The latter nail-shaped was caused bythe final mechanical opening of the crack exerted in the laboratory.

Page 6: Failure Analysis of a Motorcycle Brake Disc

Fig. 5. Microstructure of A (a) and B (b) an BU (c) samples revealed in correspondence of the center of the flange (positions 3).

938 M. Boniardi et al. / Engineering Failure Analysis 13 (2006) 933–945

Page 7: Failure Analysis of a Motorcycle Brake Disc

Fig. 6. Microstructures of A (a), B (b) and BU (c) flange discs.

M. Boniardi et al. / Engineering Failure Analysis 13 (2006) 933–945 939

Page 8: Failure Analysis of a Motorcycle Brake Disc

Fig. 7. Microstructure nearby the crack in the BU-2 sample.

Fig. 8. Hardness measurements of the A-1 and A-2 samples.

940 M. Boniardi et al. / Engineering Failure Analysis 13 (2006) 933–945

Some remarks can be inferred from the examination of the surface: oxides can be noticed in the A zone,strengthening the hypothesis that identifies the heat fatigue as the prior damage mechanism.

Fig. 12 shows a magnification of two details, A and B, taken from Fig. 11: here are highlighted the crackstopping lines. Between them there is an average distance of about 40 lm.

3.6. Test of resistance to tempering

In order to investigate the residual stress field, X-ray diffractometric analyses were performed on thethree disc flange surfaces. The goal was to be sure that the failure was not influenced by prior tensile stres-ses. A not detrimental compressive state of stresses was revealed. The average value was �270 MPa for thedisc A, �277 MPa for the disc B, �540 MPa for the disc BU.

Page 9: Failure Analysis of a Motorcycle Brake Disc

Fig. 9. Hardness measurements of the B-1 and B-2 samples.

Fig. 10. Hardness measurements of the samples and of the BU-2.

M. Boniardi et al. / Engineering Failure Analysis 13 (2006) 933–945 941

The resistance to tempering of the A and B discs was analyzed by means of experimental tests carried outin a laboratory furnace. In order to do that, some samples were cut from the hardened flange of the twodiscs.

The tests consisted of putting two samples of A and B materials into the furnace, exposing them to threeconstant temperature values (450�, 500� and 600�) with variable time intervals (0, 5, 6, 12, 24 h). After everytest, the samples were pulled out from the furnace to allow air cooling and then their hardness wasmeasured. Fig. 13 shows the obtained results. The study of the diagram shown that, if exposed to450 �C, neither the first nor the second type of discs, reveal tempering phenomena. But after 12 h ofexposure to 500 �C, the B sample was tempered, whereas the A sample managed to keep almost the sameoriginal hardness.

Page 10: Failure Analysis of a Motorcycle Brake Disc

Fig. 11. BU-1 sample crack surface: (a) comprehensive view of the three area A, B, C (crash breaking caused by the mechanicalopening of the crack); (b) detail of area A, in which can be clearly noticed the presence of widespread heat oxidation phenomena; (c)detail of area B, involved in the final phase of the crack propagation.

942 M. Boniardi et al. / Engineering Failure Analysis 13 (2006) 933–945

4. Analysis of results and discussion

During braking, the disc surface is subjected to strains which are sum of two components. The first com-ponent is a mechanical strain, due to brake torque and pad friction. The second one is induced by the ther-mal strains produced by unavoidable temperature gradients present on the disc.

A schematic representation of the mechanical loading of a disc is reported in Fig. 14. When the disc ismoving during a brake, the compressive circumferential stresses reach their highest values in the small areaapproaching pads. Highest values are located in the centre of the flange due to the particular pressure dis-tribution. As soon as flange passes across pads, the material is loaded in tension. Increasing the distancefrom pads, mechanical stresses rapidly decrease.

Therefore in braking, at each disc rotation, areas around holes are subjected to alternative (tension tocompression) cyclic stresses. Moreover, in traction situation, the stress intensification factor on the hole

Page 11: Failure Analysis of a Motorcycle Brake Disc

Fig. 12. Magnification of the B and C areas from Fig. 11 (the crack arrest lines are underlined).

M. Boniardi et al. / Engineering Failure Analysis 13 (2006) 933–945 943

is about 3 times the nominal stress in flange [3]. As already said above, another phenomenon is superim-posed to enable the crack formation under fatigue-in-temperature condition.

In fact, thermal strains develop because heating by friction are not uniform. Smaller or greater thermalgradients depend on many factors like the disc geometry, braking pressure, caliper type, stopping intervalbetween two consecutive brakes, etc.

This means that holes are also subjected to heating–cooling cycles at each rotation. This phenomenonproduces thermal differential strains of contiguous material portions. Therefore at each rotation, the por-tions of material nearby the holes are subjected to thermal-fatigue phenomena.

It sounds clear that different boundary conditions may anticipate crack start and shorten crack propa-gation period. For example, this may occur as a consequence of an increase in torque brake (similar to anemergency brake), an increase in braking frequency (as it happens in ABS conditions) or for materialfatigue limit decay. Decay in the fatigue limit may be caused when high temperatures are developed or whenthe material itself exhibits poor resistance to the operating conditions. This last case can be well related tothe evidences obtained on the tests carried out on the A and B discs.

In fact, the comparison between the microstructures observed on the braking flanges revealed a greaterconcentration of carbides for the B type disc, according to its higher sensitivity to tempering.

Page 12: Failure Analysis of a Motorcycle Brake Disc

Fig. 13. Hardness diagram, considering time and exposure temperature.

Fig. 14. Scheme of the mechanical loading exerted during the braking action.

944 M. Boniardi et al. / Engineering Failure Analysis 13 (2006) 933–945

So, the protracted high temperature exposure caused the tempering of the B type disc, while the A typedid not show any kind of damage linked to the temperature exposure. Tempered steel loose its mechanicalproperty given by the heat treatment because the microstructure was changed. It is useful to underline thatall the kinds of fully hardened steels are potentially inclined to tempering phenomena when heated, buteach of these may exhibit different behaviours at the same time-temperature value [4].

Among the martensitic steels, the AISI 410 type is the most used. Unfortunately, together with the 403and the 416 types, is the one which reveals the fastest decay of mechanical properties associated with theraising of the temperature; whereas the 422 type retains a good mechanical behaviour up to 650�, a resis-tance which is mainly due to the addition of molybdenum, vanadium and tungsten [2].

Page 13: Failure Analysis of a Motorcycle Brake Disc

M. Boniardi et al. / Engineering Failure Analysis 13 (2006) 933–945 945

According to these elements, the choice of the most appropriate material for the construction of brakediscs must take into account surely an elevated hardness, necessary to avoid premature wear damages, butalso it must keep this hardness constant during high temperature working cycles. So, it is required the use ofmaterials with a good resistance to tempering, at least in the conditions in which they will be likely to work.

5. Conclusions

The lifespan increase of a motorcycle brake disc depends strictly on the geometry (position of holes,shape of spokes, etc.), the material properties at high temperatures and operating conditions.

In particular, operating under stable loading conditions and disc geometry, the inadequate material candrastically reduce the disc lifespan.

With regard to the work presented herein it is possible to conclude that:

� The cracked disc was made of steel similar to the AISI 410 type, apart from some chemical differencesand was subjected to the full heat tempering treatment on the flange.� The cracks were a consequence of the excessive tempering of the martensite, due to the high temperatures

working conditions, and proved by the precipitation of carbides and hardness decay. Consequently thesteel was subjected to a worsening of its mechanical properties.

The inadequacy of the B type martensitic steel, with such strongly marked tendency to tempering is evi-dent. According to this fact all the other components of the same material should be replaced to avoid thesame problem.

The A type steel, which is another variation of the AISI 410, can be chosen as a suitable substitute. Thereason why it must be considered an improvement derives from its greater amount of alligants such asvanadium and molybdenum, which enable the material to strengthen its resistance to tempering.

References

[1] Sasada M, Okubo K, Fujii T, Kameda N. Effects of hole layout, braking torque and frictional heat on crack initiation from smallholes in one-piece brake discs. Seoul 2000 FISITA World Automotive Congress # F2000G331; 2000.

[2] ASM Handbooks. Properties and selection: irons, steels, and high performance alloys. vol. 1. Materials Park, Ohio: ASMInternational; 1990.

[3] Pilkey WD. Peterson�s stress concentration factors. 2nd ed. Wiley; 1997.[4] Thelning KE. Steel and its heat treatment. 2nd ed. London: Butterworths; 1984.