Damage precursor detection for structures subjected to rotational base vibration

• A novel approach to detect damage precursors through the nonlinear dynamic response of a structure subject to varying rotation base excitation.
• The damage precursor detection is verified using nano-indentation, which revealed a compliance effect at the surface of areas of high stress prior to crack initiation.
• Dynamic nonlinearities include: stiffening due to gyroscopic motion and high-response amplitude at the fundamental mode, softening due to inertial forces and gyroscopic loads, and localized microscopic material damage and micro-plasticity.
• Dynamic analytical model tracks changes in the nonlinear structural stiffening as a function of the structural response and the loading cycles.

This paper presents a nonlinear dynamic methodology for monitoring precursors of fatigue damage in metallic structures under variable rotational base excitation. The methodology accounts for important nonlinearities due to the complex loading generated by variable rotation and structural degradation. The sources of the nonlinearities include: structural stiffening due to gyroscopic motion and high-response amplitude at the fundamental mode, softening due to inertial forces and gyroscopic loads, and localized microscopic material damage and micro-plasticity. The loading intensity and number of vibration cycles increase the influence of these effects. The change in the dynamic response due to fatigue damage accumulation is experimentally investigated by exciting a cantilever beam at variable rotational base motions. The observed fatigue evolution in the material microstructure at regions of large stresses (and the resulting progressive structural softening) is tracked by quantifying the growth in the tip response, the change in the fundamental natural frequency of the beam and the skewedness of the stepped-sine response curve. Previous understanding of the structural dynamic behavior is necessary to ascertain the damage precursor location and evolution. Nanoindentation studies near the beam clamped boundary are conducted to confirm the gradual progression in the local mechanical properties as a function of loading cycles, and microstructural studies are conducted to obtain qualitative preliminary insights into the microstructure evolution. This study demonstrates that careful monitoring of the nonlinearities in the structural dynamic response can be a sensitive parameter for detection of damage precursors.