This dissertation focuses on investigating and developing techniques for measuring residual strength estimate of a damaged composites from purely nondestructive signals. The nondestructive evaluation (NDE) system relates an ultrasonic guided wave(s) (UGW) emanating from a structure and reasonably estimates the structures residual strength using the Average Stress Criterion.
The UGW signals are flexural and dependent upon the material properties and plate geometry, which change under impact or some other damaging mechanism and is thus destructively observable as a strength reduction. Correlating strength to signal requires three objectives to be accomplished: 1) Parameterize broadband mini-impactor excitation source, 2) develop algorithm for processing spatiotemporal data for phase velocity information, and 3) relate UGW degradation to physical strength all in the context of Average Stress Criteria.
Parameterization of the mini-impactor hammer, which is a broadband excitation source, was the starting point. Seventy-two design permutations of plate material, thickness, and mini-impactor define the experiment domain. Ten experiments per permutation results in seven hundred and twenty experiments filling the domain with temporally aligned raw data. Temporal and spectral analyses are conducted with the modus: “follow the energy” and found 75\% of the analyses agree in terms of significant and deployable permutations to real world applications.
Knowing future data will be spatiotemporal UGW, an algorithm (WAVSVD) uses singular value decomposition to reveal all spatial and temporal patterns, some which are muted in our physical experience. WAVSVD is also used to denoise data. WAVSVD captures more datapoints than traditional 2DFFT and captures the phase velocity and frequency attenuation because of damage. WAVSVD is the UGW processing backbone and helps identifies the max characteristic wavenumber at attenuation as the degrading metric to use in the residual strength estimate.
Impact residual strength can be modeled with Average Stress Criterion. The assumption being that the stress distribution from edge of damage to edge of plate is developed as an Average Stress Criterion mixture of the material degradation and cracking. The Average Stress Through-hole correlates residual strengths of impacts to those of holes, while the Average Stress Crack captures the characteristic shift that needs to occur to relate the damage size to an equivalent crack.
