# Interface Direct Shear Tests on JEZ-1 Mars Regolith Simulant
## 4 Results
### Interface Direct Shear Response  
The interface direct shear characteristics are influenced by different factors such as sample density, shearing velocity, temperature, or interface roughness [11; 15; 19; 32; 33]. This paper investigated the mechanical response of the JEZ-1 simulant-steel interface under two roughness conditions: smooth and rough. The interface shearing results in terms of shear stress versus horizontal displacement are given in Figure 8. Under both smooth and rough steel surfaces, the mobilized shear stresses increase significantly with increasing normal stresses. Larger peak and residual shear stresses are mobilized with increasing surface roughness from smooth to rough at each normal stress (Figure 8). The peak and residual shear stresses are reached at larger horizontal displacement in the case of a rough surface rather than the smooth one.  
Figure 9 compares the vertical strain as a function of normal stress and horizontal displacement during interface shearing. In the smooth interface test, the normal strain curves show contraction then slight dilation (Figure 9a). While for the rough interface, the curves experience contraction then larger dilation (Figure 9b).  
All the tests on JEZ-1 simulant-steel interface for the two considered roughness are summarized in the Mohr plane in Figure 10 and compared with the results of soil-soil. The peak and residual adhesions (i.e., \(c_{\text{peak}}\) and \(c_{\text{cv}}\)), as well as peak and residual interface friction angles (i.e., \(\delta_{\text{peak}}\) and \(\delta_{\text{cv}}\)) are obtained from the linear envelopes in Figure 10. The \(c_{\text{peak}}\) values of the smooth and rough interface are 1.5 kPa and 4.3 kPa, and the \(c_{\text{cv}}\) values are 1.5 kPa and 2.8 kPa, respectively. The \(c_{\text{peak}}\) of the rough interface is somewhat higher than the one of internal \(c_{\text{peak}}\), which is likely due to the contact between the JEZ-1 grains  
Figure 8: Shear stress as a function of horizontal displacement from interface direct shear test on JEZ-1-steel plate: (**a**) smooth and (**b**) rough surfaces.  
Figure 9: Vertical strain as a function of horizontal displacement from interface direct shear test on JEZ-1-steel plate: (**a**) smooth and (**b**) rough surfaces.  
and the steel grooves that is caused by the sample preparation, however, the difference is only 1.2 kPa. All the interface \(c_{\text{cv}}\) values are lower than the cohesion of JEZ-1 (3.2 kPa). The cohesion and adhesion of JEZ-1 in this paper are lower than 5 kPa, in accordance with the in-situ data from wheel trenches or wheel scuffs [8; 9].  
The peak interface friction angles (\(\delta_{\text{peak}}\)) are 18.9\({}^{\circ}\) and 36.2\({}^{\circ}\), for the smooth and rough interfaces (Figure 10a). Figure 10b indicates that the residual interface friction angles of the smooth and rough interfaces, \(\delta_{\text{cv}}\), are 16.7\({}^{\circ}\) and 33.2\({}^{\circ}\), respectively. All \(\delta_{\text{peak}}\) and \(\delta_{\text{cv}}\) values are lower than the \(\varphi_{\text{peak}}\) and \(\varphi_{\text{cv}}\) of JEZ-1 (i.e., 40.6\({}^{\circ}\) and 39.7\({}^{\circ}\)), indicating that the shearing occurs in the interface zone rather than in the soil.  
The peak and residual interface friction angles versus normalized interface roughness are plotted in Figure 11, though the data points are limited. The results indicate that the interface friction angles get close to the internal friction angles when shearing against rougher steel plate (Figure 11), in agreement with Earth soils-structure interface results [34; 35].