A robust procedure for damage identification in a lattice spacecraft structural element by mean of Strain field pattern recognition techniques

A high stiffness and low weight lattice structure for launcher applications made with high modulus carbon fiber was manufactured by EADS CASA Space by using a new cost efficient fiber placement technology. The structure consisted of a composite lattice of intertwined, unidirectional carbon fiber bars. Several Fiber Bragg Gratings (FBGs) were bonded along these bars in order to measure strain during different tests performed on the structure. A robust procedure for defect detection based on Principal Component Analysis (PCA) and strain field pattern recognition techniques was used in order to identify different defects induced in the structure during static testing conducted until fracture. A test campaign of smaller, iso-grid structures was conducted with the aim of studying the sensitivity to detect small defects in the lattice structure. A PCA model was built for the healthy structure. Subsequently, different known damage conditions were projected into the PCA model (baseline). From this projection, various damage indices and detection thresholds were calculated. The results showed that even small damages located far away from the sensors could be detected by this technique.