Multiaxial and variable amplitude fatigue of concrete

Concrete structures are often exposed to cyclic loads induced by traffic or machine vibrations. To assess fatigue lifes, SN curves serve, which are usually validated by experiments. During testing, reference specimens are exposed to uniaxial and constant amplitude fatigue. However, under real operating conditions loads occur multiaxially and with alternating amplitude and frequency. Thus, the progressively increasing material damage has to be mathematically captured. In this regard, Palmgren-Miner's linear accumulation rule is well established, although it neglects distinct sequence effects. In the article, the SN concept is first extended to multiaxial stress states by introducing an equivalent stress under the notion of isotropic damage. Then, a modified damage accumulation procedure is developed taking into account effects of loading and unloading on residual numbers of cycles to failure in multistage loading processes. The new approach is verified to experimental data taken from the literature and elaborated to a design chart. An example demonstrates the practical application.