The Effect of Residual Stress on the Distortion of Gray Iron Brake Disks
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JMEPEG (2013) 22:1129–1135 DOI: 10.1007/s11665-012-0397-7
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, and Ho Jang (Submitted April 16, 2012; in revised form July 31, 2012; published online October 2, 2012)
Thermal distortion of gray iron brake disks due to residual stress and its effect on brake vibrations were studied. The residual stress of heat- and non-heat-treated gray iron disks was measured using neutron scattering. Dynamometer tests were performed to measure the friction force oscillation caused by the disk runout during brake applications. High-temperature tensile tests were carried out to find out possible plastic deformation due to residual stress during brake applications. The results showed that the average residual stress of the heat-treated disk (47.6 MPa) was lower than that of the non-heat-treated disk (99.6 MPa). Dynamometer tests at high temperatures (up to 600 °C) indicated that the residual stress pronounced the runout: the increase in disk runout after the tests for the non-heat-treated sample was more than twice that for the heat-treated sample. This difference correlated well with the neutron scattering results and the dimensional changes after a separate vacuum heat treatment. The high-temperature tensile tests showed severe reductions in yield strength at 600 °C, suggesting that disks produced with no stress relaxation could be deformed during severe braking.
Keywords
automotive, cast irons, heat treating, tribology
1. Introduction Gray iron has been used to produce brake disks (or drums) since the early stages of vehicle development. This is because gray iron has good material properties for brake disks, such as high thermal conductivity, good machinability, wear resistance, good castability, excellent damping capacity, and low cost (Ref 1, 2). On the other hand, it also has several undesirable properties for brake disks, such as relatively high specific gravity, dimensional instability at high temperatures due to residual stress, and inherent casting defects. While other materials such as aluminum-based metal matrix composites and ceramic-based carbon fiber composites have been developed as alternatives for brake disks, most vehicles still rely on the tribological properties of gray iron for brake disks, and much effort has been devoted to improving the shortcomings of gray iron disks, such as their excessive wear and corrosion, which are known to be root causes of brake judder (Ref 3-5). In particular, brake judder has been an important issue in vehicle comfort in recent years, and various methodologies for reducing this low-frequency vibration have been used (Ref 6-8). 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 M.W. Shin, G.H. Jang, and Ho Jang, Department of Materials Science and Engineering, Korea University, 1, 5-ga, Anam-dong, Sungbuk-gu, Seoul 136-713, Republic of Korea; and J.K. Ki
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