Engineering Failure Analysis 17 (2010) 1155–1172

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Engineering Failure Analysis

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Thermal seizures in automotive drum brakes

O.P. Singh *, S. Mohan, K. Venkata Mangaraju, M. Jayamathy, R. BabuTVS Motor Company Ltd., Research & Development, P. Box no. 4, Harita, Hosur 635 109, Tamilnadu, India

a r t i c l e i n f o

Article history:Received 20 November 2009Received in revised form 1 February 2010Accepted 3 February 2010Available online 10 February 2010

Keywords:Metallurgical failure analysisWearThermal expansionThermal seizureFinite element analysis

1350-6307/$ - see front matter � 2010 Elsevier Ltddoi:10.1016/j.engfailanal.2010.02.001

* Corresponding author. Tel.: +91 4344 276780x3E-mail addresses: omprakash.singh@tvsmotor.co

a b s t r a c t

In automotive industry, drum brake system is used on two types of wheels: cast and spoke.Brake drum, brake panel and brake drum liner are important components of the brake sys-tem. Failures of these components observed during high-g braking on spoke wheels of amotorcycle are reported and systematically analyzed in this paper. The brake drum andthe panel were found to have seized during high speed brake applications. Excessive wearon the drum liner made of cast iron was also observed. Metallurgical analysis (chemicalanalysis, hardness test and microstructure analysis) of the liner revealed that excessivewear on the liner was not due to any change in material properties. Hence, further stepswere taken to investigate the problem. An experimental testing methodology was devel-oped to simulate these failures. For the same material, testing conditions, and design spec-ifications of cast and spoke wheels, no failure was observed in the cast wheels. This unusualfailure was further investigated using three-dimensional steady state finite element anal-ysis (FEA) of both cast and spoke wheels. The methodology adopted for determining thethermal and structural boundary conditions have been described in detail. Energy balancemethodology was employed to determine the heat flux values on the drum liner. The struc-tural boundary conditions are determined experimentally and validated with FEA. The pre-dicted temperature from FEA for cast and spoke wheels compares reasonably well with theexperiments. It was found that the failure of the brake system in the spoke wheels was dueto excessive thermal expansion of the brake panel and the drum beyond the specified limit.An optimum range of labyrinth clearance between the brake drum and the brake panel wasrecommended for the brake system of cast and spoke wheels.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Brakes are important safety components. Both disc and drum brake work on the same principle: friction and heat. Whenresistance or friction is applied to a turning wheel, the vehicle’s brake system causes the wheels to decelerate and finallystop. During this process heat is generated causing the brake temperature to rise. The factors, which determine the vehicledeceleration, are vehicle weight, braking force, coefficient of friction, and pressure distribution over the braking surface area[1–3]. Apart from these, another important factor is how efficiently the brake system converts the wheel motion into heatand subsequently, how quickly this heat is dissipated from the brake components. Disc brake components are fully exposedto the atmosphere and hence heat removal is efficient. On the other hand, drum brake components are fully enclosed insidethe brake assembly. This may result in comparatively higher temperature vis-à-vis disc brake system under same brakingconditions. High temperature of the drum brake shoe may cause brake fade and eventually lose effectiveness. Fading isthe result of too much heat build-up within the drum [4]. Hence, the drum brakes can only operate as long they can absorbheat generated by kinetic energy lost due to decelerating the wheels. Once the brake components are themselves becomesaturated with heat, they lose the ability to stop a vehicle.

. All rights reserved.

502/+91 4344270502; fax: +91 4344 276649..in, om.prakashh.singh@gmail.com (O.P. Singh).

1156 O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172

At high temperatures, other components of the brake system undergo higher thermal expansion. Since the geometry ofthe drum brake is more complicated than the disc brake, maintaining the component dimensions at high temperature withinthe desirable limit is very critical. Thermal expansion of a component beyond a certain limit may interfere with the otherneighboring components. This may result in the thermal seizure or locking of the drum brake. This, in turn, can result in fur-ther rise in temperature of the system and consequently, it can trigger the failure of other components. The failure underinvestigation is the rear drum brake of a spoke wheel in the two-wheeler vehicle observed during high-g panic braking. Var-ious components of the brake, namely drum brake liner, brake drum, brake panel, brake shoe were observed to have failed.Fig. 1 shows the computer-aided design (CAD) model of the spoke and the cast wheel. Rim, arm and brake drum are cast assingle component in cast alloy wheel whereas in spoke wheel the brake drum and the rim are connected with thin spokes.Hereafter, we will state alloy wheel instead of cast alloy wheel. The brake panel from one side and the drum from the otherside house the brake shoes and drum brake liner. Heat generated due to brake application is dissipated through the panel,drum, spoke (or arm) and the rim. One intriguing part of this failure is that no failure was observed in alloy wheels for thesame design specifications and under the same testing conditions. We investigate this unusual failure in this paper. We havepresented a systematic analysis methodology to find the root cause of the failure in the following sections.

The paper is organized as follows. In Section 2, we describe the failure. Metallurgical analysis of the drum liner is dis-cussed in Section 3. In Section 4 we describe the experimental methodology developed for failure investigation. Finite ele-ment modeling for both alloy and spoke wheel are presented in Section 5. Results from thermo-mechanical analysis usingFEA is discussed and compared with experiments. We conclude the results in Section 6.

2. Failure description: visual inspection

Figs. 2 and 3 show the failed drum brake sample of the brake panel assembly. High pitch noise and brake locking (‘sei-zure’) was experienced when this failure was encountered. Following points are observed on the failed component:

(a) Depth of color change (from dark brown to light brown) is more near seized area (Fig. 2).(b) Wear in the brake drum liner (Fig. 2).(c) Permanent scratch marks (Figs. 2 and 3a).(d) Initial silver gray color has changed to brown color (Figs. 2 and 3a).(e) Variable wall thickness on brake panel, it is less in the scratch marks area (Fig. 3b).(f) Brake panel material peel off (Fig. 3)

Brake panel

Brake drum

Rim

Spoke

(a)

(b)

Brake shoe

Drum liner

Arm

Rim

Brake drum

Fig. 1. Computer aided design (CAD) model of the brake system in (a) spoke wheel and (b) cast alloy wheel. The braking force is applied through the brakepanel, which actuates the cam and pushes the brake shoes against the inner surface of the drum, which rotates together with the axle. The brake panel in (b)is not shown to reveal the inner brake components.

Color changed to brown

Material peel off and scratch marks

Drum liner wear

Seizure zone

Fig. 2. Picture of the failed rear brake drum. The circled zone was locked (or seized) with the brake panel. Change in the color of the brake drum can be seen.

O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172 1157

Color changed to dark brown

Brake Shoe

Material peel off

Brake panel with shoe

Scratches & depth marks

(a)

(b)

Fig. 3. (a) Left panel shows CAD model showing the brake panel with brake shoe and corresponding failed part is shown in the right panel and (b) seizedbrake panel with shoe.

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Root cause of the above multiple failures are not known. Liner wear can be due to high heat generation between the inter-face of brake shoe and the drum liner or due to inferior quality of the material. Brake shoe material investigation revealed thepresence of cast iron wear particles from the drum liner. Further, why alloy wheel shows no failure where as spoke wheelfailed is another intriguing aspect. We investigate the liner material characteristics first in the following section.

3. Metallurgical analysis of drum brake liner

As mentioned earlier, excessive wear on the spoke wheel drum brake liner was observed. It is made up of grey cast iron.Many reasons can be attributed to this failure mode; the prominent reasons could be,

(i) improper chemical composition,(ii) improper hardness value of the material,

(iii) excessive free ferrite in the brake drum near working region,(iv) clearances between the brake liner and the brake drum and(v) initial surface finish of the brake liner and the brake drum.

Failure mode (iv) results from too small clearance between the brake shoe and the drum liner. Too small clearance willdrag, expand with increased heat, and the seizure between the drum and the brake lining may occur. Furthermore, if theclearance is not equal around the circumference of the drum brake, rear-end of a vehicle, it may fishtail (oscillate from sideto side) as the brake assembly locks-up. On the other hand, an excessive large clearance between the brake drum and liningwill cause a low pedal and delay in braking. During brake testing we made sure that failure does not occur due to improperclearances. Further, failure mode (v) was also ruled-out as the surface finish of both components was within desired limit.Hence, we investigated the failure mode (i–iii) in detail.

3.1. Chemical analysis

Slight variations in the chemical composition will alter the mechanical properties of cast iron. Carbon and silicon are typ-ical alloying elements in cast iron, which changes the ferrite–pearlite ratio. A sample taken from the drum brake was ana-lyzed for the chemical compositions (see Table 1). The percentage composition of these materials is under desired limit eventhough phosphorus, sulphur and chromium have compositions near maximum limit.

3.2. Hardness investigation

Wear rate of cast iron depends upon the hardness of the material [5]. Two spoke wheel samples termed as samples 1 and2 are taken for hardness measurement. The cross sections of both the samples are examined using a brinell hardness testingmachine. A 10 kgf force with a 1 mm ball diameter was used for the core hardness measurement of the drum brake liner. Thehardness values are shown in Table 2. The brinell hardness number of samples 1 and 2 are 202 HB and 207 HB, respectively,which are within the prescribed limit (185–235 HB).

3.3. Metallographic analysis

The common microstructure of an automotive grey cast iron is a matrix of pearlite with graphite flakes dispersedthroughout. Due to variations in foundry practices (e.g. change in cooling rates), changes in the nucleation and growth ofgraphite flakes can occur which may lead to changes in the desired properties [6,7]. Hence it is important to study the micro-structure to identify whether the material meets the required specification.

Fig. 4 shows the microstructure of as-polished samples 1 and 2 drum brake liner obtained using an optical microscope.The structure consists of branched and interconnected graphite flakes type-A (ASTM designation A247) in matrix, which ispearlite and ferrite. The average size of the flakes are 87 lm (sample 1) and 100 lm (sample 2). Type-A flake graphite (ran-dom orientation) is preferred in automotive (e.g. clutch and brakes applications [8], cylinders of internal combustion engines[9–11]) as it is superior in wear resistance. At low loading conditions the graphite flakes feeds the contact surface and formsa graphite film in the contact surface, which results in excellent wear resistance of grey cast iron [12–14]. Further, type-Aflake graphite adds to the thermal conductivity that helps in reducing thermal distortion [15,16]. Type-D and E are also per-missible on the outer surface of the liner to a depth of 1.0 mm (non-functional area). Fig. 5 shows the matrix structure ofsamples 1 and 2 after etching with 2% nital (2 cm3 HNO3 in 98 cm3 alcohol) at room temperature. Nital helps in revealing

Table 1Chemical composition of the drum brake liner (wt.%).

Carbon Silicon Manganese Phosphorus Sulphur Chromium

3.5 1.7 0.7 0.13 0.11 0.14

Table 2Hardness and microstructure parameters.

Parameter Sample 1 Sample 2

Microstructure Graphite flakes in pearlitic matrix Graphite flakes in pearlitic matrixCore hardness (HB) 202 207Average flake length (lm) 87 100Free ferrite (%) 8 4.2

Fig. 4. Unetched microstructure of gray cast iron of the drum brake cast iron insert from (a) sample 1 and (b) sample 2. 100�.

O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172 1159

the ferrite grain boundaries and reveal phases and constituents such as pearlite. The matrix is predominantly pearlitic andshows patches of ferrite (light brown).

The amount of free ferrite in the working region is critical for the proper functioning of the drum liner. The volume frac-tion or area percentage of ferrite in matrix was estimated by imparting red color to ferrite phase and blue color to pearlitephase using solarizing (thresholding) as demonstrated in Fig. 6. ASTM E 562 standard test method was used. Various mea-sured parameters are listed in Table 2. The free ferrite percentage was 8% in sample 1 and 4.2% in sample 2. These values arewithin the prescribed limit of maximum 10%.

3.4. Discussion

The metallurgical analysis of the drum brake liner described above suggests that brake liner material meets the specifi-cation standard. Hence, the failure (excessive wear) of the liner may be due to other factors, which needs further investiga-tion. The reason for the unusual failure mode will be discussed later.

Fig. 5. Etched microstructure of gray cast iron of the rear wheel hub insert from (a) sample 1 and (b) sample 2 revealing presence of graphite flakes inpearlitic matrix in both the samples. Etchant: nital 2%. 100�.

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4. Experimentation and testing of spoke and alloy wheel brakes

4.1. Experimental procedure

Excessive metal wear on the brake drum (Fig. 2) and the brake panel (Fig. 3) was observed in spoke wheels but not in alloywheels. Firsthand analysis suggests that rubbing between the brake drum and brake panel due to thermal expansion is theprimary cause of this failure. Since the failure was observed in panic braking condition, systematic experimental procedurehas to be devised which would give maximum temperature rise at the liner. Recently, Qi and Day [17] conducted a statisticaldesign of experiments approach to find the effect of various parameters on the temperature rise at the disc/pad interface.They reported that the number of braking applications has the strongest effect on the interface temperature in comparisonwith other factors, i.e. friction loads, sliding speeds and friction material composition. In the experimental procedure de-scribed below, the number of braking application has been considered as the primary parameter.

Four different vehicles are selected for the purpose of investigating the above failure. Henceforth, we will call them ref-erence vehicle 1 (abbreviated as Ref. vehicle 1), Ref. vehicle 2, Ref. vehicle 3 and Ref. vehicle 4. Ref. vehicles 1 and 2 are fittedwith brake drum with alloy wheels while reference vehicles 3 and 4 have drum brake with spoke wheels (samples 1 and 2).

Fig. 7 shows a braking cycle used for vehicle brake testing. A braking cycle includes vehicle deceleration (application ofbrakes), acceleration, then running the vehicle at constant speed and again applying brake to stop the vehicle. Initially, ref-erence vehicles were run at a constant speed of 50 km/h. The maximum possible braking (on the verge of wheel skidding butno skidding allowed) is applied to stop the vehicle within minimum time. Maximum heat generation rate inside the drumbrake would be possible this way. This process ensures that test duration is minimized with maximum heat generation in-side the brake drum. Table 3 describes the test conditions.

Temperature measurement at an exterior surface (see Fig. 13, circled point) is conducted on each of the reference vehicle’sdrum, panel and rim after every 10 braking cycle. The circled point was selected away from the liner zone because the rise in

Fig. 6. Area percentage of pearlite and ferrite phase in the working region of gray cast iron insert in (a) sample 1 and (b) sample 2.

O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172 1161

temperature would not be rapid and it would give fairly consistent temperature readings due to less rapid cooling when thevehicle is stopped. Non-contact infrared temperature sensor is used to measure the temperature. Experiments are carriedout on a test track with only rider as payload. Fig. 8 shows the variation of temperature after every 10 braking cycle for dif-ferent components of the drum brake. It is to be noted that at the end of 60th brake application temperature asymptotes to aconstant value. Wheel hub reaches the highest temperature followed by the brake panel and the wheel rim. The rise in brakepanel temperature is low initially compared to the wheel hub. The reason is that the brake panel is connected to the brakeshoe through a small pin and the brake cam (Fig. 10c) whereas the wheel hub communicates with a larger brake shoe areavia the brake drum. This effectively results in smaller thermal resistance between the brake shoe and the wheel hub com-pared to that between the brake panel and the pin. This difference in the thermal resistances has manifested in the form ofhighest temperature of the brake drum.

Heat conduction through the brake shoe to the brake panel dominates only after the brake drum temperature shows signof saturation. It is interesting to note that even after brake drum temperature reaches a saturation value, the brake paneltemperature keeps rising. The rate of temperature rise decreases with time. Notice that Ref. vehicle 3 experiences the highestrise in temperature at any point of time.

It is also to be noted that the rise in temperature of the wheel hub fitted with spoke rims (Ref. vehicle 3 and 4) was ob-served to be faster when compared to that of alloy wheels (Ref. vehicle 1 and 2) under similar braking conditions. Rise intemperature of the brake drum and the brake panel would change the labyrinth clearance (see Fig. 9). Labyrinth clearancedesign is an important design aspect of the drum brakes. Providing the proper labyrinth clearance is critical for primarily tworeasons. First, high clearance would allow water and foreign particles (dirt and grit, etc.) to enter inside the brake assembly.Water and other corrosive salts entry can cause corrosion and reduce the braking effectiveness [18,19]. This is one of the

Veh

icle

s

peed

Time

nth

braking acceleration steady speed

(n+1)th braking

Fig. 7. Experimental procedure followed for the drum brake testing. Time between the start of nth and start of (n + 1)th braking constitute a cycle.Temperature of the brake drum components is measured after every 10 braking cycle.

Fig. 8. Temperature variations of various drum brake components (wheel hub, brake panel and wheel rim) on four different reference vehicles. Wheel hubhas the highest temperature rise. Temperature values asymptotes to a constant value after 7th braking cycle (after 60th brake application).

1162 O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172

main reason for providing a complex passage (see Fig. 9b) to avoid entry of water and foreign materials inside the brakedrum. Further discussion on this is provided in the finite element analysis section.

4.2. Observations and discussion

Failures described earlier (see Figs. 2 and 3) are observed during this systematic on-road testing. Scratches and depthmarks all along the periphery of the brake panel and drum brake are observed. The initial axial clearance between the brakepanel and the wheel hub was 0.50 mm in both alloy and spoke wheels. Failures were observed in the vehicles fitted withspoke wheels (Ref. vehicle 3 and 4) and not in vehicles with alloy wheels (Ref. vehicle 1 and 2). Apart from thermal expan-sion, improper alignment of brake drum can also reduce the labyrinth clearance locally along the periphery of brake drumand panel. Since the failure has occurred in the spoke wheels only, we discounted the effect of the misalignment as a cause offailure. In the next section, we present the thermo-mechanical study using finite element modeling to find out the thermalexpansion of wheel hub and brake panel at high temperatures.

Brake Shoe

Brake Panel

Brake drum

Spokes

Rim

Brake Panel

Brake drum

(b)

(a)

Labyrinth

Fig. 9. (a) CAD model showing a cut section of the brake drum and the panel and (b) schematic diagram showing the labyrinth clearances between thebrake drum and the panel. The arrow marks indicates how water or any foreign materials follow the path to enter inside the brake shoe area.

O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172 1163

5. Finite element model

FEA has become an important and reliable tool to analyze and predict wide range of failure mechanisms in different fields.Using this tool fatigue failures have been investigated in wheels by many researchers [20–26]. Since, the temperature variesover each braking cycle, contribution of fatigue element to the failure [31] cannot be ruled-out altogether. However, the con-tribution due to the thermal fatigue in the present failure would be very difficult to distinguish from the overload fracture. Inthe finite element analysis presented below, fatigue is not modeled. Fig. 10 shows the finite element model of the wheel hubassembly for alloy and spoke wheel. Few assumptions are made in the numerical model. First, brake shoes are not includedin the model. This simplification of the numerical model is justified in the light of failure observed above. Failures are ob-served on the brake panel and the drum. Hence, it is necessary to investigate the thermal expansion of brake drum andthe panel. It is expected that modeling the details of the brake components would give a more accurate estimation of thethermal expansion of the wheel hub. However, the efforts, in terms of modeling and computational time required, wouldbe enormously high. Second, steady state analysis is performed instead of transient analysis. Steady state analysis is justifiedon the basis of Fig. 8. It is clear from the figure that temperature of the brake drum slowly reaches a peak value and then itasymptotes to a constant value with time (i.e. with successive brake applications). It is this value of high temperature thatcauses the wheel hub to expand beyond certain limit. Hence, estimating the maximum thermal expansion under steady statecondition is justified.

Third, spokes are not modeled (see Fig. 10b) whereas arms are modeled on the alloy wheel (see Fig. 10a). The basis for thisare the observations made from Fig. 8. The temperature of wheel rim of reference vehicles 3 and 4 (fitted with spokes) isabout 40 �C only even after 60 brake applications. This suggests that spokes do not conduct heat efficiently due to small

Adiabatic

(a)

Heat flux

(b)

Convection heat transfer

(c)

Fig. 10. Finite element model of (a) alloy wheel, (b) spoke wheel and (c) shows the CAD model describing how the heat flux values are applied in thesimulation model.

Table 3Brake test conditions.

Brake test Condition

Vehicle mass with rider (M) 170 kgTest speed (V) 50 km/hStopping time (t) 4 sBrake applied Rear onlyBrake input Maximum before wheel lockTest site High speed test trackNo. of braking cycle 7No. of times brakes applied in each cycle 10 times for each temperature measurement (equal distance-100 m) between successive brakingTemperature reading Infrared thermo meter

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contact area at the wheel hub. This increases the contact resistance between the wheel hub and spokes. Due to this highthermal resistance spokes acts as insulators when compared to alloy wheels. In following two sub-sections, we describedthe methodology used for determining the thermal and structural boundary conditions. Sequential analysis is performed.First, temperature distribution is obtained from the thermal analysis. This temperature distribution is applied as a body forcein the linear structural analysis to get the temperature induced thermal expansion. The commercially available softwareANSYS 10.0 is used. Element Solid70 is used for the thermal analysis and element Solid45 for the structural analysis. Elementsize of 2 mm is used for meshing.

5.1. Thermal boundary conditions: heat transfer coefficient and heat flux

Heat transfer coefficients (HTC) on the cooling surfaces and heat flux value inside the brake liner are needed to determinethe temperature distribution of the brake assembly. Measurements of these two quantities are very difficult due to contin-uous rotation and complex geometry of the wheel. We followed the energy balance methodology for determining HTC andheat flux. We have,

Table 4Heat transfer coefficients for the brake components calculated at mean air temperature of 100�.

Brake component V (m/s) Re = VD/m Nu h (W/m2 K)

Drum 36.84 1.21 � 105 245 101Arm 59.38 5.02 � 104 137 221Rim 65.88 1.59 � 105 295 167

O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172 1165

FfdSdt|ffl{zffl}

Work against friction

¼ mcpdTdt|fflfflfflffl{zfflfflfflffl}

Internal energy

þ hAðT � T0Þ|fflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflffl}Convection heat loss

þ ArT4|fflffl{zfflffl}Radiation heat loss

ð1Þ

where Ff is the braking force (N); S, the braking distance; m, the mass of the brake drum (kg); c, the specific heat capac-ity (J/kg K); T, the temperature of brake drum (K); A, the area of cooling surface of the brake drum (m2); h, the coeffi-cient of heat transfer between the brake drum and air (W/m2 K); T0, the temperature of surrounding air (K); t, thebraking time (s) and r (Stefan–Boltzmann constant) = 5.7 � 10�8 (W/m2 K4). Since the heat transfer by convection ismore dominant at low temperature of the drum brake, radiation effect can be neglected [27–29]. Sheridan et al. [28]found that in almost all braking condition about 90% of the heat dissipation is by convection and hence radiation effectcan be neglected. Noyes and Vickers [29] developed a thermal model based on the numerical finite difference methodand concluded on the energy flows at the end of 10 stops from 96.6 km/hr that 60% of the energy is dissipated by con-vection, 34% remains as stored thermal energy and the remainder is lost by radiation and conduction to the lining.Clearly, radiation effect is negligible.

As the temperature of the drum brakes reaches steady state after successive brake applications, the rate of change of tem-perature dT/dt becomes negligible (see Fig. 8). Hence, change in internal energy of the system can be neglected. Above equa-tion becomes,

FfdSdt|ffl{zffl}

Work against friction

� hAðT � T0Þ|fflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflffl}Convection heat loss

ð2Þ

Flows over the brake surfaces are complex and turbulent in nature. Most of the correlations for heat transfer coefficient avail-able in literature are suitable for a particular flow over a surface. Churchill–Bernstein [30] suggested a correlation for esti-mating the surface averaged Nusselt number at various velocities for wide range of flow conditions. The correlation is of thefollowing form:

NuD ¼ 0:3þ 0:6Re1=2Pr1=3

½1þ ð0:4=PrÞ2=3�1=4 1þ ReD

282;000

� �5=8" #4=5

; RePr � 0:2 ð3Þ

where NuD is the surface averaged Nusselt number with characteristic length of diameter D, ReD is the Reynolds number.Average heat transfer coefficients are calculated using h = NuDk/D, where k is the thermal conductivity of air. The valuesof NuD and h are calculated for drum, arm and the rim from the above correlation. These are listed in Table 4. Note that theseh values are approximate as flows over various surfaces of brakes are non-uniform contrary to the assumption.

In Eq. (2) dS/dt can be calculated using equation of motion,

S ¼ udrumt þ 12

at2 ð4aÞ

dSdt¼ vðtÞdrum ¼ udrum þ at ð4bÞ

a ¼ VðtÞtire

t¼ Vvehicle

tð4cÞ

where udrum is the initial velocity of the drum, a is the vehicle deceleration, t is the time. Since angular velocity of tire anddrum remains constant at an instant, we have

VðtÞtire

rtire¼ xðtÞ ¼ vðtÞdrum

rdrumð4dÞ

Using Eqs. (4c) and (4d) can be used to find dS/dt.Work done against friction or the heat available for dissipation in a vehicle traveling on a flat surface can be calculated by

(see Fig. 11),

E ¼ linear KEþ rotational KE� ðengine system lossþ velocity drag lossÞ ð5Þ

Fnet = ma Fdrag=1/2Cd AρV2

R1 R2 Ffront = μ1R1 Frear = μ2R2

Vair Vvehicle= Vair

E1rot=1/2I1ω1

2

Elin=1/2MV2

E2rot=1/2I2ω2

2

W=Mg

Fig. 11. CAD model showing the energy balance of a running two-wheeler motorcycle on straight road with a rider on it.

1166 O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172

The energy E is the available energy for dissipation into heat (Edissipate). In mathematical form,

Table 5Parame

Facto

DragLossProjeMomMomAnguDrum

Edissipate ¼ Elin þ E1rot þ E2

rot � ðkEtot þ EdragÞ ð6aÞ

Edissipate ¼ Ff S ¼ 12

MV2 þ 12

I1x21 þ

12

I2x22

� �� ðkEtot þ FdragSÞ ð6bÞ

Fdrag ¼12

CdApqV2 ð6cÞ

Ff ¼ l1F front þ l2Frear ð6dÞ

where Elin linear kinetic energy of the vehicle, Erot is the kinetic energy due to rotation of wheels, Edrag is the energy lost dueto velocity drag, Etot is the vehicles total energy. Potential energy of the vehicle (EP:E ¼W � vertical drop) should be added tothe above equation if the road test is conducted on an inclined surface. M is the total mass of the system (vehicle and therider), I1 and I2 are moment of inertia of front and rear wheel, x, the angular velocity, Fdrag is the velocity drag, Cd, the dragcoefficient, Ap is the projected area of the vehicle, k is the loss factor expressed in percentage of total energy lost due to en-gine internal frictional forces in clutch engaged condition. Notice that in the absence of frictional force, no heat will be gen-erated in the brakes due to brake application. Engine frictional loss can vary from engine to engine depending upon designand lubrication characteristics. Angular velocity of both the wheel is equal since wheel radius is equal. Using Eq. (6b), heatflux can be determined from the following formula,

Q flux ¼Edissipate

Adrum liner � stopping timeðtÞ ð7Þ

Notice that smaller the stopping time larger will be the heat flux value. Using the parameter values given in Tables 3 and 5,we have

Q flux; spoke ¼ 244:95 kW=m2

Q flux; alloy ¼ 244:33 kW=m2

ters used for heat flux calculation.

rs Value

coefficient, Cd 0.4factor (k) 0.35cted area, Ap (m2) 0.6ent of inertia (spoke wheel) (kgm2) 0.329ent of inertia (alloy wheel) (kgm2) 0.317lar velocity (x) (rad/s) 46

brake liner area (m2) 0.01055

Table 6Physical properties of cast iron and aluminum used in thermo-mechanical simulations.

Properties Aluminum Cast iron

Thermal conductivity (W/m2 K) 92 47.8Thermal expansion (m/mK) 2.21e�5 1.25e�5Density (kg/m3) 2790 7800Young’s modulus (MPa) 73,000 90,000Poisson’s ratio 0.33 0.18

O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172 1167

Fig. 10c shows the heat generation and dissipation area on the brake liner. At any instant heat is generated in the area cov-ered by angle (a + b). Essentially, this is the area of brake shoe liner. Drum brake liner area is covered by angle (a + b + c + d).Here we assume that total heat is dissipated though the brake drums liner area. This assumption is quite reasonable since thebrake shoe continuously rubs against the drum brake liner during the brake application. This assumption was also consid-ered by Rowson [32] and found reasonably good agreement with the bulk surface temperature rise. Further, since we areinterested in the overall steady state thermal expansions of the brake drum, any effect of local variations can be neglected.

The above heat flux and heat transfer coefficients are used to solve the three-dimensional conduction equation given by:

1a@T@t¼ @

2T@x2 þ

@2T@y2 þ

@2T@z2 þ

_Gkm

ð8Þ

where a = km/qCp is the thermal diffusivity of the material, km is the thermal conductivity, _G is the heat generation per unitvolume. Above equation is known as Fourier–Biot equation. Since the brake liner is made up of grey cast iron and the drumfrom aluminum, different properties are specified as listed in Table 6. For the problem under investigation transient and vol-umetric heat generation can be neglected. Heat flux boundary conditions are specified at the drum brake liner and heattransfer coefficients on the exterior surfaces cooled by air. Eq. (8) is solved for unknown temperature using the followingheat transfer boundary conditions,

Q flux ¼ �km@T@nðW=m2Þ ð10Þ

at drum brake liner where n = x, y and z and

�km@T@n

����surface

¼ hðT � T0Þ ð11Þ

at exterior surfaces. Since Qflux and h are known, temperature variations in the drum brake can be determined.

5.2. Structural boundary conditions and validation

Thermal expansion behavior of drum brake components is complex due to the presence of various components. To deter-mine the thermal expansion in Ansys, some region of the drum brake has to be fixed i.e. displacement equal to zero has to bespecified to avoid rigid body motion. This is not known apriori. Though in actual condition, every component undergoes somedegree of thermal expansion depending upon their temperatures. To understand the relative thermal behavior of differentsections of the drum brake, an experiment is conducted in which the whole of the drum brake was heated to high temper-ature of 175 �C. Two drum brakes (spoke wheel) are placed in an oven for 30 min. Diameters at various locations (see Fig. 12)are measured within one minute after taking out from the oven using a digital vernier caliper.

Fig. 12 shows the line diagram of diameters (numbered 1–9) that are measured before and after heating. Table 7 showsthe non-dimensionalized diameters before and after heating and the thermal expansion values. Values for diameters 1, 3 and9 are not reported due to error in measurements. Diameters 7 and 8 show lower thermal expansion (about 50% less) than thediameters on drum liner side (2, 4 and 5). Under actual operating conditions where temperatures are much higher on drumliner areas, thermal expansion on diameters 7, 8 and 9 are expected to be much smaller than diameters 2, 4 and 5. Hence, thenodes on the diameters 7 were taken for the structural boundary conditions.

This boundary condition is validated with FEA. In the FE model, whole of the wheel hub was set at uniform temperature at157 �C (this is the drop in temperature from 175 �C while measuring dimensions in the heated condition) and the structuralboundary condition is specified as mentioned above. FEA results shows thermal expansion of 0.31 mm and it is 0.33 from themeasurement at diameter 2, an error of 6%, which is reasonable. This experimental method was adopted, as it was very dif-ficult to measure these expansions during actual running conditions.

5.3. Results and discussions

Fig. 13 shows the temperature distributions in alloy and spoke wheel from FEA. The encircled dot shown in Fig. 13a is thetemperature measurement point in the experiment described above. The mean value of temperature at this location from theexperiment is 140 �C and from FEA is 168 �C, (difference of 28 �C and 20% error). Similarly, for spoke wheel the respective

Fig. 12. Line diagram showing the brake drum diameters indicated by numbers. Thermal expansions at these diameters are measured in uniform heatedcondition.

Table 7Thermal expansion measurement at various locations on two reference vehicles under uniform temperature. Values are non-dimensionalized by the diameternumber 2 (=110 mm).

Ref. diameter Ref. vehicle 1 Ref. vehicle 2

# Before heating After heating Difference Before heating After heating Difference

2 1.122 1.125 0.003 1.153 1.155 0.0024 1.002 1.004 0.00255 1.001 1.003 0.0025 0.411 0.412 0.001 0.411 0.412 0.001366 1.438 1.440 0.00255 1.490 1.492 0.001737 0.424 0.425 0.00064 0.407 0.408 0.000828 0.397 0.398 0.001 0.371 0.372 0.00064

1168 O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172

values from experiments and FEA are 188 �C and 208 �C (difference of 20 �C and �10.60% error). The contribution to the errormay be from both experiments and FEA. First, in experiments, temperatures are measured when the vehicle was completelystopped and delay in measurement could have resulted in lower reading of temperature. Second, the average heat transfercoefficients are determined from the correlation, which is approximate. Third, other brake components are not included inthe FEA, which acts as heat sink during braking. And fourth, heat flux calculation is based on the assumption that heat gen-eration is constant. However, heat is not supplied continuously but decreases linearly with time with brake application [34].Nevertheless, considering the simplification of the problem, the FEA model predicts reasonably acceptable result.

We notice from Fig. 13 that the maximum temperature is located in the drum liner of both alloy and spoke wheel and thelatter shows the highest value. It is 292 �C in alloy wheel and 376 �C in spoke wheel. This difference is due to more coolingsurfaces of the alloy wheel compared to the spoke wheel. However, one feature is common in both the wheels. High tem-perature gradient exists in both the cases. Radial thermal expansion in the cylindrical coordinate system of the alloy wheelis demonstrated in Fig. 14a. Thermal expansion is high on the arms and low on the rim located between the arms. The meanradial, axial and total thermal expansion at the circled zone is 0.250 mm, 0. 274 mm and 0.370 mm, respectively. The cor-responding values for the spoke wheel (figure not shown) are 0.382 mm, 0.358 mm and 0.522 mm, respectively. In otherwords, total thermal expansion of spoke wheel is about 40% higher than the alloy wheel. Clearly, spoke wheel undergoesmore thermal expansion than alloy wheel under same condition. Fig. 14b shows the vector plot of total thermal expansionin the spoke wheel at the cross section passing through its center. The circled area (this is the seizure zone observed in thefailures) shows highest thermal expansion and the direction of expansion is outward (towards brake panel). The same fea-ture is observed in the alloy wheel as well (figure not shown). This would effectively reduce the labyrinth clearances be-tween the drum and the brake panel (see Fig. 9).

Thermal analysis of the brake panels of alloy and spoke wheel is performed independently using the mean temperatureobtained from the experiments. In structural analysis, center of the panel was constraint in axial direction. Fig. 15a shows thetotal thermal expansion of the brake panel in alloy wheel. The arrow marked in figure shows the measurement points forthermal expansion (plotted in Fig. 15b). These points are selected as the failure due to seizure was observed at these loca-tions (see Fig. 3a). Similarly, the corresponding thermal expansion points were taken from the alloy and spoke brake drum.These values are shown in Fig. 15b. Following points are to be noted. First, thermal expansion of the brake panel and drum inthe case of spoke wheel are of same order of magnitude between the location points 1–10. The thermal expansion values are

Fig. 13. The predicted temperature distribution in (a) alloy wheel and (b) spoke wheel. Notice that the spoke wheel shows higher temperature in the brakeliner than the alloy wheel.

O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172 1169

beyond 0.50 mm of specified limit in these locations. This is likely to interfere with each other in the operating condition athigh temperatures. On the other hand, the corresponding thermal expansions of alloy wheel components are comparativelyless than the spoke wheel. Second, the interference between the brake drum and panel of alloy wheel are not expected tooccur as these values are within the specified limit.

On the basis of above observations (both experiments and FEA), it appears that spoke wheel failure was primarily dueexcessive thermal expansion beyond the specified limit. The change in color (see Figs. 2 and 3) observed in the brake com-ponents was due to seizure between the brake drum and the panel, which resulted in high temperature rise. This, in turn,results in high working temperatures of the drum brake liner. The excessive material wear on the drum brake liner seems to

Fig. 14. (a) Contours of predicted radial thermal expansion in alloy wheel and (b) vector plot of total thermal expansion in the spoke wheel shown at aplane passing through the drum axis.

1170 O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172

have occurred due to higher brake operating temperature. High temperature rise is accompanied by high wear [33–35].Thermal expansions in alloy wheel are within the limit and hence, no failure was observed. This may be due to better coolingin alloy wheels than spoke wheels. Apart from providing structural rigidity, arms in alloy wheels also acts as a medium tocarry heat from the heat generation zone and thus, effectively reducing the maximum temperature inside the brake drum(see Fig. 13).

In practice, the labyrinth clearance should be provided on these bases: thermal expansion, water entry, wheel misalign-ment and the type of wheel hub (alloy or spoke). In certain cases, wheel hub misalignment can also cause wheel hub to rubagainst the brake panel. This problem may aggravate under severe thermal expansion. As the results suggest, higherlabyrinth clearance should be provided in spoke wheels than alloy wheels. To address water entry and other foreign particleentry problems, generally the path is made complex (see Fig. 9b).

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0 10 20 30Location

Ther

mal

exp

ansi

on (m

m)

Spoke wheelAlloy wheel

Brake panel (alloy)Brake panel (spoke)

Interference zone(b)

(a) Total thermal expansion (mm)

Fig. 15. (a) Contours of total thermal expansion on the brake panel on alloy wheel. Location 1 corresponds to the dot marks on arrow. Equal data pointswere taken along this arrow and (b) variation of thermal expansion with location points for alloy and spoke wheel.

O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172 1171

In the modified design, based on the simulation input, the initial clearance of 0.8 mm was provided between the brakepanel and the brake drum with spoke wheel. The vehicle was tested on the test track under similar braking conditions.No failure was observed. Hence it is concluded that a minimum clearance of 0.8 is reasonable to avoid the failures mentionedearlier.

It is also observed that clearance value more than 1.2 mm between the brake panel and the wheel hub results in waterentry problem. For high clearances, the geometric profile i.e. water entry path between these components would need opti-mization to avoid this problem. Hence, a clearance value between 0.8 to 1.2 mm would be optimum to avoid thermal relatedproblems.

6. Summary and conclusions

The failures observed during high-g braking in the brake drum, the drum liner and the brake panel in a vehicle’s brakesystem with spoke wheels has been investigated experimentally (laboratory and on-road testing) and numerically usingthree-dimensional finite element modeling. Seizure failure between the brake drum and the panel was observed in caseof spoke wheel but not in the aluminum alloy cast wheel. High wear in the drum brake liner was also observed on the brakesystem with spoke wheels. From the detailed analysis we conclude the following,

(1) Metallurgical investigation of the cast iron liner insert in the brake drum reveals that the liner meets the requiredmaterial characteristics (chemical composition, hardness and metallographic analysis). The cause of high wear isdue to other external factors.

1172 O.P. Singh et al. / Engineering Failure Analysis 17 (2010) 1155–1172

(2) Results from experiments and FEM reveals that steady state temperature attained in spoke wheels are fairly high com-pared to alloy wheels under same conditions.

(3) Thermal expansion of the brake drum and the panel in spoke wheel are higher (about 40%) compared to alloy wheel.Since the initial designed labyrinth clearance was less than that could be accommodated by thermal expansion, thisled to the seizure between the two components. Seizure was accompanied by further rise in temperature.

(4) Redesigned parts with appropriate labyrinth clearances shows no failures either in drum brake or the liner. The opti-mum range of labyrinth clearances to avoid seizure and water entry into the brake system would be between 0.8 to1.2 mm.

(5) High wear on the brake liner is attributed to the higher temperature generated due to seizure between the brake drumand the panel.

Acknowledgments

We gratefully acknowledge Mrs. Ashwini Singh for English text correction of the manuscript and Mr. Durga Shankar fortechnical suggestions.

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