In a project funded by the Federal Aviation Administration's (FAA) Aging Aircraft Program, a Portable Holographic Inspection System (PHITS) has been further developed. The technique involves taking a double exposure white light reflection hologram of 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 recognizable 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. Preliminary experiments showed that structures with a high value of critical load had a fatigue life of order ten times that of a normally fastened splice. Critical load can be readily determined in the field using the PHITS system. Research designed to establish the relationship between fatigue life and critical load is continuing. An understanding of that relationship could lead to a technique capable of fatigue life determination in typical aircraft structures.
As the application of advanced composite materials and adhesively bonded components becomes increasingly numerous in aircraft structures, so is the number of aircraft containing such structures that can be classified in the aging aircraft category. The effect of environmental and in- service aging of such structures is not well known or understood, neither have NDE techniques been able to satisfactorily qualify and quantify the loss of structural integrity due to the aging process. This paper will present the latest developments in the practical use of a field portable holographic interferometric testing system. The system results, known as holographic interferograms, provide a better understanding of how a structure is behaving under realistic loads in the presence of defects, damage and material property aging. The system has been applied to a variety of defects in composite and adhesive bondlines, as well as artificial environmental aging of these materials. The holographic interferograms produced form these investigations will be briefly reviewed and their impact on structural integrity of the component discussed.
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