evaluation of prevailing longitudinal forces in the couplings of electric trains

Railway administrations used to have no requirements to fatigue strength endurance of electric trains’ couplings. however, mass decommissioning of ice1 trains in germany due to cracked couplings and two coupling breakages of electric train in russia happened. forces transferred through couplings are low-levelled which are not compared with couplings’ yield stress. consequently, abovementioned problems arise from fatigue damage. need for detection of forces in electric trains’ couplings, development of fatigue strength requirements and test methodologies became obvious. in 2022 vniizht assessed forces into couplings of different electric trains under low and high temperatures; mileage was 9 600 km. maximal recorded forces under compression and tension were +117/ 128 kn – far less than yield stress of electric train couplings (+1500/-1000 kn). it verifies assumed fatigue damage. after data processing, statistical distribution of peak-to-peak amplitudes of forces in couplings per run unit and per service life of electric train (40 years) with account of their average annual run (130 and 170 ths. km for electric trains with constructional speed 120 and 160 km/h accordingly) was derived. impact of forces of different levels to fatigue strength was calculated regarding to fatigue curve, and value of peak-to-peak amplitudes was set to 4 (normalized couplings) and 5 (quenched and tempered couplings). estimated required resource values for couplings under multi-cycle load during service life of electric train and average annual run was 7,38 * 1015 kn^4 (normalized couplings); 1,43*1018 kn^5 (quenched and tempered). values and measures of this parameter do not have transparent physical meaning, but they set specific requirements to couplings’ endurance that can be checked during tests. endurance under cyclic loading depends not only on number of amplitudes of forces of each level, but also on asymmetry coefficient of load cycle r = pmin/pmax. in terms of endurance, most rigid is symmetrical cycle with forces of different signs (r = -1), which predominantly corresponds with longitudinal forces transferred through electric trains’ couplings in operation. developed methodology contains specific numbers of load cycles depending on force amplitude and asymmetry coefficient of load cycle r. for equal fatigue action of loads with different coefficient r values, load numbers should be increased by value of correction factor kr. for mostly used r values following kr values are determined: k -1 = 1,00 with r = -1; k0 = 1,70 with r = 0; k0,1 = 1,80 with r = 0,1; k0,2 = 1,95 with r = 0,2