Manufacturing Error Detection in Plate and Cylindrical Composite Structures

Due to their superior weight to strength ratio of composites to common metallic structures, composite technology is widely used in aerospace industry. Assessment of damage in composites has gained interest after a large number accidents caused by unanticipated damages in the composite structures. Many different structural health monitoring applications were developed over the years due to the fact that composite materials may inherit damage from within, not always visible from surface. The most common types of errors encountered in the industry are due to misaligned fibers, a mix-up in ply order, and delaminations: all presenting changes in the vibro-acoustical performance of the composite structure. This paper discusses the change in the dynamic properties of a composite structure contains a manufacturing error such as a ply lay-up error, a ply angle error, and delamination. Both plate and cylindrical structure types were considered for the stated error types. Due to the dynamic methods used, the errors were considered to happen globally in the structure, targeting either the entirety of the structure or a large volume of it. Effect of symmetric errors, unsymmetrical and unbalanced errors, and mid-plane errors were considered in the case of ply orientations, and dynamic stiffness matrix was used to identify the error. Identification of the structure’s layup properties and manufacturing error identification is employed. From the measured modal properties of the structure, a back-tracking strategy was used to generate the ply lay-up of the composite structure given logical constraints such as the number of existing plies, assumption of bundled same angle layers induced with the same error type, et cetera. Accelerance, local damping, and mode shape match characteristics were studied for delamination. The studied delaminations were considered to be large enough to cause a phase difference locally around their environment. Due to this phase difference, a change in mode shape and a local damping increase in the structure occurs. Prepreg plates of a single carbon fiber system and filament wound hybrid cylinders consisting of glass and carbon fibers were manufactured for testing. Modal tests on plates and cylindrical composite structures were performed and compared with the analysis. In prepreg plates, natural frequencies measured from tests and obtained from finite element analysis were in agreement with an error up to 2%. For the considered cylindrical structures, it was shown that a maximum of 5% discrepancy occurred in the measured and calculated (via finite element analysis) first 5 natural frequencies of the structures containing ply orientation errors. The source of error was attributed to the change of local material properties in the structure. For filament wound hybrid composite cylinders, the thickness and volume fraction was not constant all along the cylinder. Therefore, an uncertainty was introduced due to the usage of the hybrid composite material, as the thickness of the layers was largely fluctuating on the regions where multiple materials were in use. A multiple error detection algorithm was generated to target a combination of all manufacturing errors present in the structure. A good match between the finite element model and experiment was shown in natural frequencies and mode shapes.