Finitely Scoped, High Reliability Fatigue Crack Growth Measurements

Classical procedures for characterizing fatigue crack growth behavior often suffer from uncertainties that make it difficult to plan for productive use of test instrument time, and that can result ultimately in too noisy measurements. An enhanced procedure has been implemented that is based on a fixed test time budget, and that establishes operating conditions that produce crack growth rates down to minimum measurable rates. The procedure features (1) a haversine pulse deformation test cycle followed by a rest period, (2) a strain peak that ramps linearly over time, (3) minimum and maximum limits on the strain peak chosen to avoid unproductive test time, and (4) a stress–strain probe cycle for purposes of observing strain energy density. A set of replicates of a carbon black filled, natural rubber bushing compound has been characterized via both procedures, and a statistical analysis is made to compare both. The new procedure significantly improves the quality of crack growth rate curve measurements.