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Given the discussion above, the difficulty in interpreting stress data from high pressure experiments on polyphase materials becomes clear. Unless one knows if a material deforms in an IWL or LBF style, interpretation of stresses measured in individual phases becomes problematic. In high‐pressure experiments that use lattice strain to calculate stresses, the stresses are measured in the individual phases and there is not currently a technique that is used to independently measure the bulk strength of the material. Looking at the two deformation experiments on Brg and Fp that are shown in Figures 2.2 and 2.3, a large discrepancy is observed. Miyagi & Wenk (2016) find room‐temperature strength of Brg to be ~1.4 times that of Fp, while in contrast Girard et al. (2016) find Brg to be about four times stronger. Notable differences exist between these two experiments, primarily that the work of Girard et al. (2016) is performed at high temperature and under steady‐state conditions. Computations by Kraych et al. (2016) find the strength contrast to be ~4 at 30 GPa and room temperature, but find that this decreases at high temperatures. Although the calculated strength of Brg in this study is similar to those of experiments at room temperature and high temperature, the strength contrast trend is inconsistent with the two‐phase experiments. However, in the experiments this may be due to microstructure and the effect of IWL versus LBF‐style deformation. Miyagi & Wenk (2016) did not recover samples for microstructural analysis; however, the larger sample in Girard et al. (2016) is amenable to documentation of microstructure. Microstructural analysis from this experiment seems to indicate that although Brg appears to be interconnected, as would be expected for LBF behavior, strain seems to be largely partitioned into the softer Fp, which is expected for IWL behavior. Based on the description of Handy, an LBF should result in large differences in stresses measured in each phase whereas the stresses measured in each phase for IWL should be close. One possibility for this discrepancy is that the samples in Girard et al. (2016) are transitioning between LBF and IWL, and if this is the case this transition would occur at strains greater than 60–70%. Indeed, this study concluded that at high strains shear weakening and strain localization is expected to occur. In contrast to Girard et al. (2016), a study on 72% CaGeO₃ Pv + 28% MgO as an analog for Brg + Fp found a strength contrast of ~2 and found that the stresses in CaGeO₃ in the two‐phase experiments were higher than in single‐phase experiments. Based on this, it was concluded that a LBF behavior dominates (Y. Wang et al., 2013). Another analog study on NaMgF₃ Pv + NaCl analogs with a strength contrast of ~10 found systematically lower stresses in NaMgF₃ with the addition of 15‐70% NaCl (Kaercher et al., 2016). In contrast to NaMgF₃, stress levels remained ~ constant in NaCl independent of volume fraction. Deformation behavior of samples with volume fractions of NaCl > 50% were consistent with an IWL. Samples with less than 50% NaCl were not consistent with either an IWL or an LBF and it was concluded that the behavior of the samples were somewhere intermediate to IWL and LBF.

There are several reasons why high‐pressure experiments may not match stress levels expected for either IWL or LBF microstructures. IWL and LBF are bounds on stress and strain partitioning, so it is not surprising that experiments may deviate from these two end‐member behaviors. In Handy’s formulation, constant volume deformation is assumed, and this may not necessarily be the case for high pressure (and high temperature) experiments, particularly if pressure or temperature changes occur during deformation, such as in DAC experiments and to a lesser extent in large volume deformation. Although nonlinearity of deformation is accounted for, the behavior predicted by Handy (1990, 1994) assumes plastically isotropic phases. This is not strictly true for most minerals particularly those deforming via dislocations. Finally, given that the percolation threshold for a phase is ~30–40% by volume, there is a range of phase proportions where interconnectedness of both phases occurs. Although simultaneous interconnectedness of phases may not be stable to higher strains, it may be stable over relatively small strains experienced in experiments, and one would expect that this would result in stress and strain partitioning that is intermediate to IWL and LBF end members.