Metoda napovedovanja nastanka termičnih razpok na torni površini avtomobilskih zavornih diskov : doktorska disertacija

slovenian

doktorska dela

2013

Univerza v Mariboru, Fakulteta za strojništvo

In this doctoral thesis, the method of service life determination for brake discs due to thermal cracks on the rubbing surface is developed and presented.To the authorćs knowledge, this is the first published work that systematically discusses all the areas that influence the service life assessment at known loadings. The presented methodology will serve scientists and development engineers as a tool for optimizing the geometry and materials of the brake discs. ßervice life determination is a particularly multidisciplinary research field, as it combines the knowledge areas of testing, materials, mechanics, numerical methods, heat transfer and service life prediction theories. The beginning of the thesis focuses on the analysis of the results of previous research done in the discussed fields and discovered imperfections that are in the continuation handled in detail. ased on the requirements of the accurate temperature field determination, the numerical method for predicting convective cooling coefficient, which considers the real test conditions, was presented. It was discovered that the results of the presented method differentiate from the results of the presently published analytical and numerical methods that cannot consider the influence of the cooling air blowing during the experiment. In addition, the method for radiation emission coefficient of the brake disc during the operation prediction was presented. The results obtained with the method showed the deviation of 47 % regarding the presently published results. Detailed analysis of the temperature dependent physical material characteristics (such as thermal conductivity, specific heat and thermal expansion) of the lamellar grey cast EN-GJL-250 was made. It was ascertained that some of the results deviate from the presently published results up to 48%. emperature, stress and strain fields generated during the presented loading cycle were analysed with finite element method. For the accurate determination of the stress and strain fields the detailed analysis of the mechanical material properties of the EN-GJL-250 grey cast iron was made. The results of tensile and pressure tests at room temperatures and at increased temperatures are presented. The results are statistically treated with the Weibull probability theory. ased on the requirements of the computational models for service life assessment, a detailed analysis of the low-cycle fatigue parameters of the used grey cast was made. Low-cycle fatigue parameters were conducted at room temperatures and at increased temperatures. Strain-life curves were obtained and low-cycle fatigue parameters were determined. Based on the numerical results, service life was calculated using the presented hypothesis and compared with the experiment results. The experiment was conducted on a brake dynamometer on five brake discs that were tested according to the analysed testing procedure for fatigue crack resistance. t was established that the agreement of the experimental results and the results of the presented methodology was 94 %, which confirms the accuracy of the research presented in this doctoral thesis.

08.08.2023