The case study focuses on the thermal fatigue failure of a gray cast iron brake disc used in firefighting vehicles, detailing the impact of temperature on braking performance and associated failures. Various tests, including microscopy and finite element analysis, were employed to investigate the crack development and stress distribution within the brake disc under operational conditions. Recommendations for mitigating thermal cracking are provided, emphasizing the importance of material selection and design modifications to enhance brake disc durability.

1. Veermata Jijabai Technological Institute, Mumbai.
MECHANICAL ENGINEERING DEPARTMENT
Case Study On
BY
PANKAJ U. ARVIKAR
M.TECH (AUTOMOBILE)
172100015
1

2.  The braking system represents one of the most fundamental safety critical
components in modern passenger cars.
 Braking performance of a vehicle can significantly be affected by the
temperature rise in the brake components.
 The frictional heat generated at the interface of the disc and the pads can
cause a high temperature.
 The temperature may exceed the critical value for a given material, which
could bring undesirable effects, such as brake fade, local scoring, thermo
elastic instability, premature wear, brake fluid vaporization, bearing failure,
thermal cracks, and thermally excited vibration.
 In practice, most brake discs are made from Gray cast iron because of it
high thermal conductivity, high thermal diffusivity and low cost.
2

3. 3
• High Compressive stress
• High Temperature (about 800 C during hard braking)
• High Wear
• High Speed Impacts
• Fatigue (due to cyclic stress)
• Corrosion
• High coefficient of friction
• High Hardness
• High compressive strength
• High wear resistance
• Low density
• Easy to manufacture
• Thermally stable (expansion)
• Good thermal conductivity
• High melting temperature
• High corrosion resistance
• Wear resistance
• Less costly

4. 4
Materials Properties
Cast iron, ductile
• Moderate density
• Good thermal conductivity
Cast iron, gray
• Thermally stable
• High coefficient of friction
• Corrosion resistance
• Low cost
Carbon ceramics
• Low density
• Thermally stable
• High thermal conductivity
• High fracture toughness
Bronze • High thermal conductivity
Copper
• High thermal conductivity
• Low thermal expansion
High carbon steel
• High Fracture Toughness
Low Alloy steel
Low carbon steel
• High Fracture toughness
• Moderate density
Tungsten alloys
• Thermally stable
• High fracture toughness
Nickel based alloys
• Heat absorb capacity
• Corrosion resistance
Stainless steel
• High fracture toughness
• Corrosion resistance

5. 5
 Cheapest
 Lightest
 High Thermal Conductivity
 Less thermal expansion
 High coefficient of friction
 Constant wear rate
 High strength
 Easy to manufacture
 High wear resistance
 High durability to withstand heat and pressure
 Corrosion resistance

6. 6
• Cheapest process
• Allows complex shape
• Poor surface finish
• High labour intensity
Sand Casting
• Good surface finish
• Good Accuracy
• Moderate cost
• Less labour intensity
Evaporative pattern
sand casting
• Good accuracy
• Good surface finish
Ceramic shell
evaporative mold
casting
Investment casting
Laser beam machining
• Better surface finish
• High accuracy
• High cost
• High accuracy
• Does not require any finishing
• High cost

7. 7
 Good surface finish
 Moderate accuracy
 Require less machining and polishing
 Casting can be easily removed
 High manufacturing rate
 Less labour intensity
Sub-processes
Casting
Shot
blasting
(Remove residual
stresses)
Turning
Mill
Balancing
(Flattened from
both sides)
Ground
finishing
(Smoothen
edges)
Surface
hardening
Anti-
corrosive
coating
Quality
assurance
Packaging
Final
inspection

8. 8
Material Gray cast iron
Approx. weight 4 kg
Raw Material cost (Rs. 50/kg)
Rs. 200
Machining cost
Rs. 175
Transportation cost
Rs. 150
Inventory cost
Rs. 100
Casting cost
Rs. 200
Labour cost
Rs. 150
Quality cost
Rs. 125
Miscellaneous (electricity, invoice
etc)
Rs. 200
Total cost/ piece = Rs. 1300

9.  Causes for thermal fatigue failure :-
1. High speed with heavy braking loads
2. Elevated temperature gradients
3. Continued environmental exposure
 In this study, the failure analysis of a brake disc, used in a fire fighting
vehicles, was carried out.
 Detailed studies including visual examination, optical microscopy, and
scanning electron microscopy, X-ray energy dispersive spectroscopy
(EDS), chemical analysis, metallography and hardness measurements
were performed to determine the cause(s) of failure.
 In addition, finite element analysis (FEA) to determine the
temperature profile and to estimate the Von Mises stresses distribution
that arise during the braking. 9

10.  Brake Disc in this case was designed to withstand local scoring, thermo
elastic instability, premature wear, brake fluid vaporization, bearing failure,
thermal cracks.
 The table shows the basic characteristics of brake disc.
10
Item Value
Inner disc diameter, mm 66
Outer disc diameter, mm 262
Disc thickness (TH), mm 23
Disc height (H), mm 51
Vehicle mass (m), kg 1385
Initial Speed (v0), kmph 28
Deceleration (a), m/s2 8
Effective rotor radius (Rrotor), mm 100.5
Rate distribution of the braking forces ɸ, % 20
Factor of charge distribution of the disc εp 0.5
Surface disc swept by the pad Ad, mm2 35993

11. Fig. Schematic Diagram of Disc Brake Assembly
11

12. Fig. General View of Brake Disc
a) Lock Bolts Side b) Vehicle Side
12

13. Cracks are seen to run radially from interior to the exterior of the
disc.
Crack
Fig. Pictures of Failed brake disc
13

14. Fig. Close-up view of the fracture surface of the
mechanically opened crack
 The crack was propagated in
semi elliptical shape from the
external surface to the internal
one of the friction surface.
 Three zones (see A, B and C
zones in Fig) exhibiting definite
and different signs of
morphologies and heating were
observed.
• zone A - shows smooth surface due to oxidation and was brown-blue tinted,
indicating temp. reached to 400–500 C.
• zone B - coarse grainy morphology, orange-yellow tinted, suggesting lower
temperatures.
• zone C - shows a gray bright color with coarse grains.
14

15. Fig. SEM micrographs of A, B and C
zones
Fig. SEM micrograph of the A zone
showing the intergranular pattern
15

16. Fig. SEM micrograph of the B zone
showing the lamellar pattern
Fig. SEM micrograph of the C zone
showing the cleavage fracture.
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17.  To verify if the brake material met the specification, a chemical analysis was
carried out.
 Table presents the results of such analysis indicating the brake disc was
manufactured from a lamellar cast gray iron, in accordance with the requirement.
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18.  To put in evidence any sort of thermal effect due to the pressing action of
the brake pads against the rims, the hardness measurements were carried out
in a radial direction of the hub and of both the friction surfaces.
 In Table are given the results obtained together with the derived UTS.
18

19. Fig. Optical micrograph showing A
type graphite flakes. As polished.
100X. Rim
Fig. Optical micrograph showing
graphite flakes dispersed in a pearlitic
matrix. 320X. Hub.
19

20. Fig. Optical micrograph showing
micro crack surrounding graphite
flakes. 80X. Rim.
The following table also summarized all the metallographic evidences observed
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21.  The simulation was carried out as thermal analysis in transitory regime to
determine the temperature profile into the disc and linear-elastic mechanical
one to estimate the Von-Mises stress distribution.
 A gray cast iron type twenty-five was chosen as material in accordance with
the hardness values.
 Dealing with the thermal analysis, an evaluation of the vehicle dynamics
was used to find the heat flux equation.
The conditions are as follows:
Brake speed - 70 km/h
Brake duration - 1 s
Hydraulic pressure exerted during braking actions - 5 MPa
Atmospheric temperature - 20 C.
 In addition, an external forced convection of the areas out of the pads
contact and internal ones for the air cooled holes were used.
 In using these conditions, the heat flux is approximately 7 W/mm2. 21

22. Fig. Trend of the temperatures versus time 22

23. Fig. Temperatures distribution at
the end of braking action
Fig. Von Mises compression
stress distribution
23

24.  The morphological evidences together with the FEA are in agreements in
identifying the areas of the cracks development.
 In particular, the stress is localized in radial direction on the friction surfaces.
 The intergranular pattern, confirmed by metallographic examination, and the
lamellar one observed on the fracture surface also revealed the cracks
propagated by fatigue from the external friction surfaces to the internal ones.
 No defects were found at the edge of the rim that could have contributed to
cracks origin.
 The fracture surface color at the edge of the rim indicated a high temperature
reached during the brake application.
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25.  During braking, the exterior of the friction surface is subjected to periodic
tensile and compressive circumferential stress which leads to fatigue crack
initiation and propagation.
 The temperature, Von Mises stress, and the total deformations of the disc
increased because the thermal stresses are additional to mechanical stress
which caused the crack propagation and fracture of the bowl and wears off the
disc.
 In axial direction, the temperature always presents a large fluctuation in the
frictional contact surfaces. So it can be explained that thermal fatigue is
generally easy to occur in the surface layer.
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26.  Also a brake model with long distance and low pressure could reduce the
residual stress caused by a severe emergency braking, and could contribute
to the delay restraint of the initiation and propagation of thermal fatigue cracks
to some extent.
• Increase the yield and fatigue strength of the disc material.
• Decrease the braking temperature.
• Re-design the hub-rotor unit to eliminate constraint stresses.
 There are three ways to eliminate thermal cracking in brake disc(rotor):
26

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discs. Engineering Failure Analysis 16 (2009). doi:10.1016/j.engfailanal.2008.01.009. P. 152–163
2) Thomas J. Mackin, Steven C. Noe. Thermal cracking in disc brakes. Engineering Failure Analysis
9 (2009). P. 63–76
3) A.Belhocine, M. Bouchetara. Temperature and Thermal Stresses of Vehicles Gray Cast Brake.
Journal of Applied Research and Technology. P. 674-682.
4) Yang, Z., Han, J., LI, W., Li, Z., Pan, L., Shi, X. Analyzing the mechanisms of fatigue crack
initiation and propagation in CRH EMU brake discs. Engineering Failure Analysis (2013). doi:
http://dx.doi.org/10.1016/j.engfailanal.2013.07.004.
5) Qifei Jian, Yan Shui. Numerical and experimental analysis of transient temperature field of
ventilated disc brake under the condition of hard braking. International Journal of Thermal
Sciences 122 (2017). P. 115-123.
6) M. Boniardi F. DErrico, C. Tagliabue, G. Gotti, G. Perricone. Failure analysis of a motorcycle brake
disc. Engineering Failure Analysis 13 (2006). P. 933–945.
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