Issue 33

M. Sakane et alii, Frattura ed Integrità Strutturale, 33 (2015) 319-334; DOI: 10.3221/IGF-ESIS.33.36 320 nonproportional parameters that introduce stress terms in combination with strain terms demonstrated that the additional hardening should be considered in estimating nonproportional LCF lives [1]. Some papers reported that the amount of the additional hardening is material dependent [2]. Certain FCC materials give large additional hardening resulting in a significant reduction in LCF lives but BCC materials little additional hardening resulting in apparent reduction in LCF lives [3]. The amount of additional hardening is material dependent even in FCC materials. The additional hardening is closely related with the microstructure development in the materials but the relationship is not well understood, whereas several studies have challenged this topic [2, 4-10]. The objective of this paper is to discuss the microstructure development in proportional and nonproportional loadings performed in the author’s laboratory [2-4,7]. This paper firstly discusses the material dependence of the additional hardening by selecting five materials with a FCC crystallographic structure. The materials selected were Type 304 stainless steel (304SS), pure Cu, pure Ni, pure Al and 6061 Al. Tension-torsion proportional and nonproportional loading tests were performed and stress responses were experimentally obtained. The stress responses are discussed in relation with the stacking fault energy of the materials. The slip morphologies and dislocation structures are also discussed in relation with the stacking fault energy of the materials. Next, microstructures of specimens fatigued under 14 proportional and nonproportional loading paths are discussed in relation with the stress response on 304SS at room and high temperatures. Microstructures discussed are dislocation, cell, stacking fault and twin. Microstructure map is proposed to identify the boundary of cell formation. M ATERIAL DEPENDENCE OF ADDITIONAL HARDENING ive FCC materials with different stacking fault energies were cyclically loaded using proportional and nonproportional strain paths. The five materials and their stacking fault energies are tabulated in Tab. 1. All these materials have a FCC crystallographic structure. 304SS has the lowest stacking fault energy of 20 erg/cm 2 , pure Cu and pure Ni have intermediate stacking fault energies of 40 erg/cm 2 and 80 erg/cm 2 , respectively, and the two aluminums have the highest stacking fault energies of 200 erg/cm 2 . Tension-torsion cyclic loading tests were performed using the hollow cylinder specimen of which the shape and dimensions are shown in Fig. 1 under the strain paths shown in Fig. 2, where  is the axial strain and  the shear starin. Tension loading was used as a proportional loading and alternating test between tension and torsion in every one cycle was used as a nonproportional test, Fig. 2 (a). Ten cycles with constant strain amplitude were repeated to obtain stable stress amplitude after the saturation of strain hardening. The stress amplitude at 10 th cycle in each block was recorded as representative one of the data. Mises strain amplitude of 0.05% was increased every 10 cycles, Fig. 2 (b). SUS304 Pure Cu Pure Ni Pure Al 6061 Al 20 40 80 200 200 Table 1 : Stacking fault energies  (erg/cm 2 ) of five FCC materials. Figure 1 : Shape and dimensions of specimen. Fig. 3 shows cyclic stress-strain curves of the five materials. 304SS gives large additional hardening, Cu and Ni show intermediate additional hardening and Al and 6061 Al no additional hardening. Since no difference is found in the elastic part of the stress-strain curves in the figure, only the plastic part has the influence on the additional hardening. To quantify the relationship between the plastic strain amplitude ( p  ) and stress amplitude ( σ ), the following equation is introduced. F