Issue 37

M. Margetin et alii, Frattura ed Integrità Strutturale, 37 (2016) 146-152; DOI: 10.3221/IGF-ESIS.37.20 147 mobile working machines [2], components of both are subjected to combined loading dependent on external conditions. Such a loading causes a stresses in the critical point, and this stress state is nearly always multiaxual. In the process of life time estimation in multiaxial stress state, it is not sufficient to transform this state into uniaxial stress state according to static strength hypotheses, as they especially don't consider the cyclical properties of materials and the different effects of normal and shear stresses on the fatigue life time. Therefore, it's necessary to use a mathematical model that is both able to reduce the multiaxial stress state to uniaxial stress state and that respects the mentioned problems at the same time. The methodology of transformation into uniaxial stress state then needs to be able to include also the change in the direction of damage, hence to respect the directional characteristic of the fatigue process. Nearly a century passed since first attempts to tackle the problem of multiaxial fatigue have been made, and as for the situation today, there are plenty of criteria that consider component's multiaxial stress state. According to the methodology of assessment of loading process in the critical point, these criteria can be divided into stress-based criteria [3,4,5,6], strain-based criteria [7,8,9] and criteria based on fracture mechanics [10,11,12]. This text presents a new stress-based criterion that transforms the multiaxial stress state of a cyclic loading into an equivalent uniaxial stress. This criterion is based on the critical plane approach. After presentation in the text, the criterion is subsequently verified using proportional tension/compression and torsion loading in an experiment. M ULTIAXIAL FATIGUE CRITERION oday's most used stress-based criteria that transform multiaxial stress state into equivalent stress amplitude in critical plane are in the form the following linear or non-linear combination:       c e a f bσ dτ f N (1) Findley [3], McDiarmid [4] and Matake [5] have derived criteria for the calculation of the equivalent amplitude of shear stress as a linear combination of amplitude of shear stress and normal stress in the critical plane in the following form      eq f τ σ kτ f N (2) On the other hand, Carpintieri with Spagnoli [6] and Papuga with Ruzicka [13] have derived criteria for the calculation of the equivalent amplitude of normal stress in the critical plane as a non-linear combination in the following form      eq 1 2 f σ k σ k τ f N (3) The difference between the respective criteria is in the definition of the critical plane and in the form of material parameters k that consider the effect of normal and shear stresses. The resultant amplitude of the equivalent stress is then compared with the adequate fatigue life time curve in order to determine the finite fatigue life time or with fatigue limit in order to determine the infinite fatigue life time. The results achieved by presented hypotheses more or less correlate with the experimental results, however, there are some commonly known and well documented problems: Parameters weighting the effect of normal and shear stress in hypotheses are independent on the loading level (i.e. number of cycles to failure), which is not true universally [14,15]. Material parameters are based on conventional values (yield stress, fatigue limit) that are strongly dependent on the methodology of determination and sometimes their existence itself is questionable (fatigue limit being the example - there's no agreement on whether there is an actual amplitude of stress that isn't damaging). Neither one of the criteria provides correct results for both boundary loading conditions (pure torsion and pure tension/compression loading). T