'REDUCTION OF RESIDUAL STRESSES IN CAST BRAKE ROTORS

A Materials Science & Engineering Capstone Senior Design Project
Sponsored by ThyssenKrupp Waupaca

Trevor Gibson (Project Leader)

Carol Deming (Engineer)

Lance Taylor (Engineer)

Nick Weinberg (Engineer)

Prof. Paul Sanders,
Materials Science and Engineering, Michigan Tech

Greg Miskinis,
Director of Research and Process Development, ThyssenKrupp Waupaca'

<https://www.asminternational.org/documents/10192/18931279/2012_2nd-michigantech.pdf/b1bf1fbd-e84e-4411-b431-da34a9c1d80a>

'Residual stress is an elastic stress that develops due to solid state
cooling variations such as changes in section thickness. Residual
stresses can be either tensile or compressive and transform from plastic
to elastic at approximately half the melting temperature of the
material. These stresses are present within a part when no external load
or thermal gradient is present. Residual stress is a common problem with
brake rotors. With significant section thickness variation and tight
tolerances even the slightest distortion in a rotor may cause brake
judder. The lateral runout specification on a passenger vehicle rotor is
typically about 40 micrometers. Lateral runout is the measurement of
sideways motion allowed in the disc.

Different types of residual stresses can be observed during the cooling
of brake rotors. The hat reaches half the melting temperature at which
point the stress accommodation gradually changes from plastic to elastic
before the plate section that is still in the plastic regime. This
causes a build-up of residual stresses at the intersection of the plate
and hat. The stress transformations and cooling rate variances of the
rotor’s hat and plate sections can be seen in Figure 1. Residual
stresses can lead to distortion during heat treatment when the part is
heated above the stress relief temperature. The resulting WARPING or
twisting of the rotor surface can lead to vibrations while braking due
to an increase in lateral runout.'

So... ...you're wrong, Arlen.

And no one but you is surprised.

You need more?

'The Effect of Residual Stress on the Distortion of Gray Iron Brake Disks

M. W. Shin, G. H. Jang, J. K. Kim, H. Y. Kim & Ho Jang

Journal of Materials Engineering and Performance'

<https://link.springer.com/article/10.1007/s11665-012-0397-7>

'It is known that disk WARPING or uneven disk thicknesses induce
pulsation during brake applications. While it is known that the system
robustness is important for reducing the amplification of the vibration,
the major source of brake-induced vibrations is the fluctuation of the
friction torque produced at the sliding interface as a result of the
dimensional variation of the disk. When the disk temperature is
increased by friction heat during braking, the heat often causes
dimensional instability of the disk, permanently modifying the runout or
disk thickness variation (DTV) of a disk and producing brake judder. In
particular, the residual stress, which is developed in a cast as a
result of different local cooling rates, is known to be one of the
important factors for reducing a disk’s propensity for juddering, and
stress relief by heat treatment is known to be an effective method for
ensuring the dimensional stability of a disk at high temperatures.'

Still not enough?

OK!

'Introduction to Gray Cast Iron Brake Rotor Metallurgy

Mark Ihm TRW Automotive'

<https://pics.tdiclub.com/data/500/tutorial-ihm_1_.pdf>

'Casting Cooling in the Sand Mold

• The time the casting spends in the sand mold is typically 20 to 60
minutes. This time is often referred to as the “shake-out” time.

• The time the casting spends in the sand provides slow uniform cooling
of the castings.

– A long shake-out time can help minimize residual stresses that can
lead to rotor warpage in service.

....

Stress Relieving Heat-Treatment of Gray Cast Iron Rotors

• Stress relieving of gray cast iron rotors can be performed to minimize
rotor warpage that can occur under extreme service conditions.

• Stress relieving of semi-finished rotors is done in Europe for many
rotors designed for high performance vehicles.'

But you know more than everybody, right!