Structural integrity assessment in the transportation sector

Abstract Pressures for increased transportation efficiency often lead to structural weight reduction campaigns; some designers reduce weight by selecting high strength materials and increasing allowable stresses. However, material strength improvements are not generally accompanied by better fracture toughness or better resistance to fatigue crack growth. Pressures for increased transportation economy sometimes cause operators to extend vehicle life rather than to incur major capital costs for new equipment. However, aging structures may experience widespread cracking, a situation that has not been fully considered even for some structures that were designed in accordance with a damage tolerance philosophy. Design for high performance or extension of service life thus have the potential to put the integrity of the affected structure at risk. One deals with such situations either by reducing fracture mechanics principles to practice or by assessing a service failure after the fact. Both reactions require methods of engineering calculation that are well defined, easy to apply, and reasonably conservative. These criteria often conflict with the standards of accuracy demanded for basic research in the mechanics of fracture. A sacrifice of such standards can be worthwhile, however, provided that the engineering method captures the service environment's principal effects on crack growth and that the method is ultimately based on good research results. How the results of fracture research are reduced to practice depends on material properties, attachment details, the mission of the structure, and its service environment. Three examples are presented to illustrate how the foregoing factors affect the reduction of research to practice. The damage tolerance concept originally developed for airframes is first discussed. The concept can be generalized, but its application implicity relies on the methods used to design, manufacture, and inspect airframes. The other examples cover structures which differ from airframes in one or more basic respects.