Microscale Testing and Characterization Techniques for Benchmarking Crystal. . .
117
Fig. 10 Progression of surface strain in microsample of René 88 at different levels of global strain
and low elastic modulus of 72 GPa (less than 95% of grains within the sample).
The high Schmid factor points to the importance of resolved shear stresses on
primary slip systems; however, it is important to note that this is not the grain with
the highest Schmid factor, and that simply looking at individual grain orientations
is not sufficient. Clearly, other factors such as load sharing or shedding amongst
neighboring grains play an important role in strain localization [29, 110, 111].
Closer examination of Fig. 11 indicates that the region of the sample to the left
of the strain concentration is composed of a grain that exhibits both a low Schmid
factor (0.3) and a high elastic modulus (210 GPa), the opposite characteristics from
the grain in which strain is concentrated. In addition, the twins in this grain also
exhibit an elastic mismatch with the parent grain. Studies investigating the fatigue
behavior of René 88DT, for example, Stinville et al. [29, 51, 112] and Alam et al.
[110], have elucidated that crack nucleation occurs in twins in which there is a high
Schmid factor and a mismatch in elastic modulus between the twin and the parent
grain. These characteristics have been shown to result in a stress concentration and
to facilitate the formation of fatigue cracks. Similarly, the mismatches of elastic
modulus illustrated in Fig. 11c appear to result in a stress concentration. Modern
CPFEM models may be expected to capture and quantify these stress concentrations
and predict the onset and propagation of local intragranular plasticity in favorably
oriented grains in appropriate neighborhoods.
117
Fig. 10 Progression of surface strain in microsample of René 88 at different levels of global strain
and low elastic modulus of 72 GPa (less than 95% of grains within the sample).
The high Schmid factor points to the importance of resolved shear stresses on
primary slip systems; however, it is important to note that this is not the grain with
the highest Schmid factor, and that simply looking at individual grain orientations
is not sufficient. Clearly, other factors such as load sharing or shedding amongst
neighboring grains play an important role in strain localization [29, 110, 111].
Closer examination of Fig. 11 indicates that the region of the sample to the left
of the strain concentration is composed of a grain that exhibits both a low Schmid
factor (0.3) and a high elastic modulus (210 GPa), the opposite characteristics from
the grain in which strain is concentrated. In addition, the twins in this grain also
exhibit an elastic mismatch with the parent grain. Studies investigating the fatigue
behavior of René 88DT, for example, Stinville et al. [29, 51, 112] and Alam et al.
[110], have elucidated that crack nucleation occurs in twins in which there is a high
Schmid factor and a mismatch in elastic modulus between the twin and the parent
grain. These characteristics have been shown to result in a stress concentration and
to facilitate the formation of fatigue cracks. Similarly, the mismatches of elastic
modulus illustrated in Fig. 11c appear to result in a stress concentration. Modern
CPFEM models may be expected to capture and quantify these stress concentrations
and predict the onset and propagation of local intragranular plasticity in favorably
oriented grains in appropriate neighborhoods.