*4.2. Identifying Variables*

A further investigation of variables was conducted for the oscillatory tests. These tests calculated the yield stress and crossover stress of one particular sample but assessed the influence of plate size, plate roughness, overfilling the gap, and pre-shearing the sample. The results are summarized in Figure 6.

**Figure 6.** Comparison of (**a**) yield stress and (**b**) crossover stress of LiC2 worked for 60 strokes using an overfilled sample compared to a pre-sheared sample compared to a standard 5% trim.

The results based on plate properties indicate that, as expected, the smaller plate measured a higher yield stress and crossover stress than the larger plates. In addition, it appears that the surface roughness of the plate does not cause a significant difference in the results. This is also expected, as one of the advantages of oscillatory rheometry is the minimal sensitivity to plate roughness.

A pre-shear of 5 s<sup>−</sup><sup>1</sup> for 5 s prior to measuring the critical stresses was done to intensify the effects of sample manipulation during testing and assess the sensitivity of the test to initial conditions. This investigation revealed that pre-shear had a significant effect on calculating yield stress but had a small effect on calculating crossover stress. Due to the relatively large standard deviation observed when using pre-shear, it was not used for subsequent tests.

Overfilling of the sample with a 100% trim was found to significantly affect results for both yield stress and crossover stress. In addition to overestimating both stresses, a higher standard deviation was noticed for results obtained using an overfilled gap. Therefore, care was taken to cleanly trim every sample at 5% above the measurement gap.

#### *4.3. Rheometer Penetration Test Results*

Here, the results of all four tests considered (cone penetration, rheometer penetration, yield stress, and crossover stress) will be compared. The first set of results presented involves looking more closely at the rheometer penetration test. Because different normal forces must be used to test different greases, here, each normal force has an associated set of data. For simplicity, only one set of data will be presented; however, this set is particularly representative of all data collected. Figures 7 and 8 compare the rheometer penetration test done using a 5 N normal force to other consistency tests.
