Application of In-situ Computed Tomography for Validation of Open Hole Fatigue Model Predictions

Abstract

The objective of the High Fidelity Database and Validation Protocols for Structural Failure Mode Characterization project sponsored by ONR (N00014-14-C-0128) is to provide the required material characterization data and associated guidelines to effectively benchmark computational analysis methods. The development of fatigue damage within composite specimens was characterized using in-situ computed tomography to capture damage initiation and propagation on a layer-by-layer basis. Specimens (24-ply, quasi-isotropic) were tested in 50,000 cycle blocks of OHTT (R=0.1) up until 250,000 cycles from 20-90% of ultimate static strength. DIC and insitu CT scanning were performed to capture residual stiffness and internal damage state associated with increasing cycle number. The interaction of in-plane ply damage and delamination modes was observed to most severely develop for fatigue loadings above 60% UTS at 50,000 cycles. Once reaching runnout at 250,000 cycles, the specimens were subsequently tested in quasi-static tension to ultimate (two-piece) failure and the residual stiffness and strength were recorded. Notably, it was determined that a correlation exists between increasing damage state and increasing residual strength. From inspection, the 0o embedded plies exhibited large splits at the hole edges that increased in length as the loading cycles increased. Two specimens were additionally tested to ultimate two-piece failure in fatigue at 90% of ultimate load to estimate a percent of life for the specimens tested using in-situ CT. The number of cycles to failure were 727,495 and 842,298, which indicates that the earlystage matrix-dominated fatigue damage in the laminates was captured.