Life determination of riveted aircraft structure by holographic NDE.

In a project funded by the Federal Aviation Administration's (FAA) Aging Aircraft Program, a, Portable Holographic Inspection System (PHITS) has been further det eloped. The technique involves taking a double exposure white light reflection hologram af aircraft structures. Each exposure is taken at a slightly different load state, and the resulting interferogram shows the deformations that occur between the two load states. Results showed that the rivets in a simple lap joint, designed to simulate the longitudinal lap splice on a Boeing 737, behaved in two distinct and easily recognisable modes. The first mode occurred at low loads and was an indication that friction forces between the two sheets of the lap joint dominated the load transfer mechanism. Indications were that the second mode related to higher loads for which the friction forces played a much lesser role. The load at which the changeover begins to occur has been called the critical load (L(crit)). Preliminary experiments showed that structures with a high value of L(crit) had a fatigue life of order ten times that of a normally fastened splice. L(crit) can be readily determined in the field using the PHITS system. Research designed to establish the relationship between fatigue life and L(crit) is continuing, An understanding of that relationship could lead to a technique capable: of fatigue life determination in typical aircraft structures.