Cyclic-fatigue crack growth in composite and adhesively-bonded structures: The FAA slow crack growth approach to certification and the problem of similitude

In 2009 the US Federal Aviation Administration (FAA) introduced a slow crack-growth approach for certifying composite and adhesively-bonded structures. This approach requires that the growth of a delamination or disbond is slow, stable and predictable under cyclic-fatigue loads. To predict growth in aircraft structures requires a methodology for translating laboratory crack-growth data to full-scale structures. Whilst this need not be a fracture-mechanics based approach, the present paper focuses on fracture mechanics approaches since they have been widely adopted for this purpose for certifying aircraft structures. This approach uses the 'similitude hypothesis' combined with the concept of a crack-driving force (CDF) to link the results from laboratory tests to the cyclic-fatigue behaviour seen in full-scale aircraft tests. The present paper reveals that the range of the strain-energy release rates, Delta G, is not a valid crack-driving force. In contrast, in the present paper, a valid scheme is identified and proven to be appropriate. (C) 2016 Elsevier Ltd. All rights reserved.