Runout, and consequent juddering and pulsation through the brake pedal, is a multi-million dollar per year warranty problem for car
manufacturers. There is some suspicion that the runout can be caused by relaxation of residual casting stresses when the disc is
overheated during severe-braking episodes. We report here neutron-diffraction measurements of the levels and distribution of residual
strains in a used cast iron brake disc rotor. The difficulties of measuring stresses in grey cast iron are outlined and three-dimensional
residual-strain distributions are presented and their possible effects discussed.
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