Abstract: A method and apparatus for the nonlinear detection of imminent failure in a complex structural element is disclosed. The method and apparatus include sensing stress and strain displacement-related data for said structural element, analyzing the sensed stress and strain displacement-related data as a function of one or more principle components, generating a crack-growth rate function relating said data for each principle component over a load/unload cycle, determining a total crack-growth rate function based on at least one crack-growth rate function, deriving from said total crack-growth rate function at least one indicator function, monitoring trends in said at least one indicator function, and providing an indication when said monitoring detects an end-stage trend in said at least one indicator function.