The influence of two different trisamide-based additives on the morphological and mechanical behavior of isotactic polypropylene (PP) is investigated. Morphological investigations by wide-angle X-ray scattering (WAXS) show that only 0.04 wt.-% of one of the additives is enough to induce 80% beta modification in the crystalline part of. the polymer. The materials are investigated with quasistatic (tensile testing) as well as short-time dynamic (Charpy impact strength) methods and for the first time fatigue crack growth measurements are performed on alpha- and beta-nucleated PP. In all cases a much higher toughness of the material containing the beta-nucleating agent (beta NA) can be observed. The crack growth propagation rate is one order of magnitude smaller with the use of the beta-nucleating agent compared to the alpha polymer. Scanning electron microscopy (SEM) pictures are used to support the correlation of the mechanical behavior with the morphological changes. (C) 2014 Elsevier Ltd. All rights reserved.