Mechanical Hysteresis as an Nde Tool for Evaluating Composite Honeycomb Damage

Abstract

Honeycomb composites are finding ever increasing use on aircraft structures, making nondestructive detection of defects contained within honeycomb structures all the more important. This paper focuses on a new detection technique which makes use of the mechanical hysteresis seen as loops in the force-displacement curves. It was observed from load test data that internal damage in honeycomb sandwiches causes the average slope of the force-displacement curves to decrease and the area contained within the hysteresis loop to increase. To satisfy the inspection speed and one-sided access requirements of NDE techniques, a dynamic loading approach was pursued where an accelerometer was used to tap the surface of the test sample. Much of the research focused on the deduction of the force-displacement curves from an acceleration curve. This greatly increased the speed of the technique as well as reduced it to a one-sided test, where only access to the outer surface of the structure is needed.

NDE Detection and Characterization of Damage in Honeycomb Sandwich Composites

“…These images determine quantitatively the degradation of stiffness due to flaws or damages and the enhancement of stiffness due to internal reinforcements, core splices and potted core. Structures used in the experiments included actual honeycomb sandwich panels and GFRP solid composites [7,8].…”

Development of a Portable Mechanical Hysteresis Measurement and Imaging System for Impact Characterization in Honeycomb Sandwich Structures

“…In previous work [2], we reported on a novel mechanical hysteresis measurement where the area enclosed in a load-displacement plot, Fig. 1, is correlated to the level of damage in honeycomb composite materials.…”

“…In the case of a crack in fiber reinforced composites, the level of frictional loss should increase dramatically due to fibers intertwining with each other when compressed and then having to untangle as the load is reduced and the structure returns to its original shape. As previously described in [2], the heart of a tap test-based mechanical hysteresis measurement is the instrumented tap probe, an accelerometer with hard metallic tup for tapping the part/sample surface. From just the acceleration data, both load (tapper mass x acceleration) and displacement (the double integration of acceleration with time) can be deduced.…”