The Arrest and Retardation of Fatigue Cracks Using Thin-Film Healing Materials

Damage tolerance concepts have been applied to aerospace vehicles for decades to improve safety.  Here, the remaining service life is calculated using fracture-mechanics concepts based on service loads, material, component geometry, and crack size as input, and is used to establish safe inspection intervals.  Damage tolerance methodology has proven very successful for many applications, but is not yet practical for all aerospace components, especially those difficult to inspect or subjected to high-cycle fatigue loading.  To make damage tolerance a more-viable life management tool, a novel solution to this issue is provided by the authors, where fatigue cracks are “healed” by the introduction of a foreign material into the crack mouth, thus slowing or even stopping cracks.  This healing material is applied to a structural material (titanium and aluminum alloys) as a thin-film coating that is activated by heat, allowing the healing material to flow into the crack.  Upon solidification, the coating reduces the crack driving force by adhering the crack surfaces together and by leaving voluminous material in the crack wake (an artificial form of crack closure).  Experimental results have shown that crack arrest is possible at low values of crack driving force (DK).  At higher values of DK, crack growth is only delayed, but when crack growth resumes it does so at a greatly reduced crack growth rate.  The demonstrated crack healing processes can be exploited to insure safety of components that are difficult to inspect, or undergo high-cycle fatigue loading and will be applied to repair concepts for the extension of vehicle life.