Z. Marciniak et alii, Frattura ed Integrità Strutturale, 34 (2015) 1-10; DOI: 10.3221/IGF-ESIS.34.01 1 Focussed on Crack Paths Models of initiation fatigue crack paths proposed by Macha Z. Marciniak Opole University of Technology, Poland D. Rozumek Opole University of Technology, Poland A BSTRACT . Professor E. Macha devoted his academic life to solving the problems connected with random multiaxial fatigue in components of machines and structures. In his studies he formulated stress, strain and energy criteria related to critical plane concept. He also proposed several methods to determine critical plane position. In particular, he formulated and verified weight functions applied in order to determine critical plane position. The variance method constituted another significant contribution to the development of methods for determining critical plane position. Apart from these criteria, Macha was exploring energy approach in fatigue of materials and the development of fatigue cracks. He has also observed that strain characteristics multiplied by stress amplitude determined at specimen half-life are applied to estimate fatigue life using energy criteria. However, for cyclically instable materials, stress amplitude value may differ a lot; therefore he proposed the method to determine energy fatigue characteristics directly from experimental research. K EYWORDS . Multiaxial fatigue criteria; Energy; Crack growth; Variance method; Weight function. I NTRODUCTION redicting service life of different objects is a very important issue for modern engineering. Wrong service life estimation may result in accidents and disasters. Therefore, studies aimed to understand and control this phenomenon, started already in the 19 th century, are continued today. The multitude of problems connected with it suggests that scientists still have plenty of work ahead. Initially, the scope of studies was limited to uniaxial, constant- amplitude issues only. With increasing knowledge on the phenomenon, the interest in multiaxial fatigue (most frequent in engineering practice) was growing. At the same time, many stress assessment criteria were proposed. Another step in the development involved attempts to assess life for random loads. This problem was explored by Professor Macha as well [1- 4]. He started his work from proposing mathematical models to assess fatigue life for materials in the conditions of random complex stress state, where besides stress criteria he demonstrated the method for determining critical plane position using weight functions. Further studies were connected with strain and energy criteria, and methods used to P