3.1. Complex eigenvalue analysis (CEA) results
Since friction is the main cause of instability, which causes the stiffness matrix in Eq. (3) to be asymmetric, complex eigenvalue
analysis has been undertaken to assess the brake stability as the friction coefficient values. It was observed that high values for this
parameter tend to facilitate two modes merging to form an unstable complex mode. In addition, an increase in the friction
coefficient leads to an increase in the unstable frequency. Figure 6 shows the results of a complex eigenvalue analysis with
variation of the friction coefficient (μ) between 0.2 and 0.6. As predicted in the complex eigenvalue analysis, as the friction
coefficient further increases, real parts of eigenvalues, the values that can be used to gauge the degree of instability of a complex
mode, increase further, as well, and more unstable modes may emerge. This is because the higher coefficient of friction causes the
variable frictional forces to be higher resulting in the tendency to excite greater number of unstable modes. In the past, a friction
coefficient of 0.35 was typical. However, brake compounds today possess coefficient of friction that is 0.45 or higher, which
increases the likelihood of squeal. This poses a greater challenge for brake designer to develop a quiet brake system.
In an earlier work (Nouby et al., 2009), an attempt was made using parametric study to reduce squeal at 12 kHz. Based on the
earlier study, a decision was taken to understand the effects of influencing variables of squeal at 6.2 kHz using DOE.