Direct imaging of moisture effects during slow dynamic nonlinearity

Transient hysteretic dynamic behaviors, commonly known as slow dynamics, have been observed to exist in many materials, but a physical cause for the phenomena remains elusive. These dynamic behaviors obfuscate an array of physical measurements associated with oil exploration, structural health assessment, and seismic analysis. In this work, a modified resonant ultrasound spectroscopy (RUS) measurement configuration is coupled with an environmental scanning electron microscope (ESEM) to study the behavior of a porous solid material at the micro-scale before and after mechanical resonance excitation. The experiment was carried out in two steps: a validation of the modified RUS configuration was performed, and then ESEM images were collected before and after mechanical vibration excitation was applied to the sample. The validation results demonstrate that RUS vibration responses displayed nonlinear transient hysteretic behavior consistent with slow dynamics within the region of vibration energy studied. The ESEM images of the material in different water-based moisture conditions reveal that dynamically enabled moisture transport occurs within a pore with a size of tens of microns. The results show that pore moisture migrates out of, and back in to, the pores in response to mechanical vibration, which is coincident with observed transient hysteretic softening and recovery behaviors associated with slow dynamics.