Browsing by Subject "Moisture"

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Effect of moisture on mixed-mode TSR on a glass/epoxy interface
(2017-12) Ferreira Vieira de Mattos, Daniel; Liechti, K. M.; Huang, Rui; Rodin, Gregory J; Ravi-Chandar, Krishnaswa; Bonnecaze, Roger T
The understanding of interfacial failure in adhesively bonded structures is important for several sectors including transportation and infrastructure. This problem has motivated studies for several decades. Adhesives are polymeric and, as such, present time, temperature, strain rate and moisture dependence. The effect of moisture on interfacial adhesion and fracture is still an open problem and demands a deep multi-disciplinary study considering nonlinear viscoelasticity, fracture mechanics, diffusion, chemistry and surface science. This is justified through the mechanisms through which moisture can affect interfacial adhesion. The presence of moisture can degrade the interface integrity. The absorbed moisture also modifies the mechanical properties of the bulk adhesive as well as its interactions with substrates, which introduces changes in the response of the adhesively bonded structure as it is subjected to an external load. An additional complication for interfacial cracks constrained to grow along the interface is that crack growth is governed by the tensile and shear stresses at the interface as well as the interfacial interactions (adhesion energy, strength and range) embodied in traction separation relations and giving rise to the term mixed-mode fracture. This research investigates the effect of moisture on interfacial fracture for different mode-mixes. The content is developed in four parts. First, the adhesive is experimentally characterized via the following tests: mechanical loading, water diffusion, thermal and hygral expansion. These results introduce the second part: a nonlinear viscoelastic model calibrated considering all those measured properties. This model captures the effect of time, temperature, strain rate and moisture on the mechanical behavior of the adhesive. The third part deals with the fracture behavior of a glass/epoxy interface over a range of mode-mixes and moisture conditions. This is complemented by analyses including optical profile measurements of the fractured surfaces and extraction of traction and separation relations and toughness. Finally, a significant emphasis was placed on the numerical analysis which was required for each of the three components outlined above.
Spatial patterns of bedrock weathering at the hillslope scale inferred via drilling and multi-scale geophysical methods
(2018-08-17) Lee, Shawn S.; Rempe, Daniella M.; Paine, Jeffrey G.; Tisato, Nicola
The critical zone (CZ) comprises the near-surface region of the Earth where biological, physical and chemical weathering processes form porous soil and weathered bedrock from initially fresh bedrock. The structure of the CZ plays an integral role in near-surface processes including water cycling, however this subsurface structure is unmapped relative to surface topography, outside of isolated boreholes, road cuts, and landslides. Here, we characterize this structure and its variability within the Coastal Belt of the Franciscan in the Northern California Coast Ranges to test the hypothesis that there exists a relationship between weathering profile structure and topography. We compare spatially detailed data from boreholes from the Eel River Critical Zone Observatory (ERCZO) with surface seismic refraction and electrical resistivity surveys to develop the most probable seismic velocities associated with interfaces between (1) saprolite and weathered bedrock and (2) weathered and fresh bedrock. P-wave velocities calculated via regression ranged from 350-610 m/s for saprolite and soil, 610-1910 m/s for weathered bedrock, and > 1910 m/s for fresh bedrock. Extending these results to three hillslopes lacking borehole data reveals a common weathering profile structure, whereby weathering depth increases upslope across hillslopes and weathering thickness is roughly parallel to the ground surface along ridgelines. The maximum thickness of weathering, at the topographic divide, appears to scale with hillslope length. Significant seismic anisotropy, reflecting dominant bedding or fracture orientation, is evident in a subset of transects. Electrical resistivity tomograms reveal a consistent, shallow 2-8 m resistive layer associated with the weathered bedrock vadose zone underlain, in many cases, by a conductive zone which we propose reflects a seasonal groundwater system within weathered bedrock. Our seismic and electrical imaging therefore reveals a consistent structure across different hillslopes. In this tectonically active environment, we find that weathering profiles penetrate deeply beneath hillslopes in a broadly systematic pattern. This suggests consistency in weathering process across the landscape that operates at the hillslope scale where channels remain fresh and ridgetops show deep weathering.