International Journal of Fatigue 23 (2001) S207–S214 www.elsevier.com/locate/ijfatigue Effects of grain size and mechanical pretreatment on strain localization in FCC polycrystals K. Obergfell a , P. Peralta b,* , R. Martinez b , J.R. Michael c , L. Llanes d , C. Laird e a Institut fu ¨r Werkstoffkunde, Universita ¨t Karlsruhe (TH), I. 76131 Karlsruhe, Germany b Department of Mechanical and Aerospace Engineering, Arizona State University, P.O. Box 876106, Tempe, AZ 85287-6106, USA c Sandia National Laboratory, Mail Stop 1405, Albuquerque, NM 87185, USA d Departamento de Ciencia de Materiales e Ingenierı ´a Metalu ´ rgica, Universidad Polite ´cnica de Catalun ˜a, Barcelona 08028, Spain e Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA Abstract Polycrystalline copper samples where the grains in the cross-section had crystallographic axes parallel to the load related to ‘single slip’ directions and three grain sizes, were ramp loaded and then step-tested to obtain their Cyclic Stress–Strain Curves (CSSCs). Plateaux were found for medium grain size at about 89 MPa, whereas large grained samples showed plateaux at 72 MPa, which correlated with the measured Taylor and Sachs factors, respectively. No plateaux were found when grains were smaller than 200 μm or ramp-loading stresses were below 87 MPa. Comparisons are made with nickel polycrystals and it is found that the plateaux in copper are narrower than those reported in nickel. The differences are attributed to a more homogeneous dislocation structure in nickel, due to lower elastic interactions across grain boundaries and easier cross-slip behavior as compared to copper.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Cyclic stress–strain curve; Load histories; Dislocations; Grain size; Strain localization 1. Introduction Strain localization, often in the form of Persistent Slip Bands (PSBs), is the main cause of fatigue crack nucleation in smooth specimens of many metallic materials. The factors than enhance strain localization in FCC polycrystals, to the point that a plateau appears in their CSSCs, are not completely understood. The results available in the literature regarding this issue show a fair amount of variability. Wang and Laird [1] reported the presence of a plateau in polycrystalline copper that had been ramp loaded, i.e. the amplitude of the cyclic load was gradually increased to a desired value in a given number of cycles. This enhances strain localization. The plateau was reported at a stress of 98 MPa, which is con- sistent with the Taylor model [2]. Llanes [3] did not find a plateau in copper with different grain sizes even though ramp loading was used as a mechanical pretreatment in many of his tests. He concluded that the presence of * Corresponding author. Tel.: +1-480-965-2849. E-mail address: pperalta@asu.edu (P. Peralta). 0142-1123/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII:S0142-1123(01)00136-0 a sharp 111–100 texture in the studied samples was responsible for the absence of the plateau. Peralta et al. [4] did not find a plateau in copper either, although the texture of their samples had components along directions inside the standard stereographic triangle. On the other hand, convincing plateaux have been reported in nickel polycrystals with different grain sizes and random tex- tures using just conventional testing [5,6]. The plateau stresses reported by Morrison [5] showed a small vari- ation with grain size and texture. Contradictory results have also been reported for other FCC materials [7,8]. Peralta and coworkers [9] later reported reproducible plateaux in ramp loaded copper polycrystals with coarse grains and ‘single slip’ textures after extended cycling during their step tests. The influence of grain size and the level of stress reached during ramp loading are yet to be investigated. Therefore, experiments were carried out to study the effects of these variables on strain local- ization in copper polycrystals with single slip textures.