Strain measurements along zero-strain trajectories as possible structural health monitoring method for debonding initiation and propagation in aircraft sandwich structures

The paper presents a strain-based structural health monitoring (SHM) method for damage detection and propagation assessment in composite sandwich structures. As case example, an idealized Airbus A-340 spoiler is considered. The damage assessment is realized by static strain measurements along so-called zero-strain trajectories (ZST) at the loaded structure. A zero-strain direction exists for every spatial strain state with major strain directions with opposite signs (tensile and compression). Connecting zero-strain directions at various points of a structure yields a ZST. This allows the monitoring of a ZST by, e.g., a distributed fiber optical sensor (FOS) or fiber Bragg grating sensors (FBG). However, for the undamaged pristine state and defined loading there would be no strain measured along these trajectories. A structural change, i.e., damage initiation and propagation would influence the strain state and therefore yield a strain value along the ZST of the pristine structure. The possible application of strain measurements along a ZST for debonding initiation and growth was demonstrated and its potential and issues for real online monitoring will be discussed, by means of an experimentally validated FE-model.