Pak Sing Leung

Application of Morlet wavelet in the stress-time history editing of horizontal axis wind turbine blades

This article presents a novel method in extracting fatigue damage from the stress-time history of the horizontal axis wind turbine blades. A novel method based on wavelet transform (WT) is proposed to shorten the length of the stress-time history. Based on Morlet wavelet (Morl), WT is taken to the stress-time history to generate a plot of accumulative power spectral density (AccPSD) distribution with time. The variation of fatigue damage with AccPSD levels is studied to determine the cutoff level which is controlled by the differences in the global signal statistics (mean stress, root-mean-square and kurtosis). WT uses the cutoff level as a criterion in identifying fatigue damage parts at time locations of the stress-time history. Then, the fatigue damage parts are extracted from the original stress-time history and concatenated to form the edited stress-time history. In addition, Time Correlated Fatigue Damage (TCFD), which is an existing used by commercial software, is used to study the effectiveness of WT. The comparison of the differences in fatigue damage and stress-time history length produced by TCFD and WT is performed. It is found that the differences in fatigue damage of TCFD and Morl are +0.076% and +0.013%. The lengths of the edited stress-time history provided by TCFD and Morl are 89.8% and 82.3% of the original length, respectively. The results suggest that not only does WT improve accuracy of fatigue damage retained in the edited stress-time history, but also it provides the length of the edited stress-time history shorter than TCFD does. Also, the results suggest that WT is a useful method for performing accelerated fatigue tests. Finally, WT is suggested as an alternative technique in fatigue durability study, especially for the field of wind turbine engineering.

Coated Rotor and Slotted Pad in a Good Interactions

Coating is very important in some applications particularly disc brakes that are most commonly used in the passenger cars. Slotted friction material also has a great effect on the behaviour of the disc brake friction. Mixing between the coated rotor and the slotted material is of importance to improve the frictional behaviour of the disc brake. Slotted material used in this work is divided into two parts; each part has leading and trailing edge to give the advantages of using two pads in a single braking action. Simulating the kinetic energy of a passenger car to measure the friction force between the disc and pad at different operating conditions have been designed in a test rig. The effect of thermal loading at the sliding interface between the different friction material and the rotor of Floating Calliper Disc Brake (FCDB) has been studied in this work. Initial disc surface temperature was measured and controlled through a high quality technique by using a thermal camera that was fixed at suitable place close to the rotor. The signal from the transducer was amplified and displayed on a digital storage oscilloscope and then fed to a computer for the final analysis. The results showed the different values of coefficient of friction depending on the different operating conditions. The maximum coefficient of friction was 0.58 that was recorded at initial disc surface temperature of 150°C while the braking pressure was 20 bar. The minimum value for the friction coefficient 0.30 was also recorded at initial disc surface temperature of 50°C at rotating speed 500 rpm while the braking pressure was 25 bar.