A structural member of a mortar projectile is inspected. The structural member is a composite structure having a thermoplastic matrix with a filler of steel balls. The composite structure has a central cavity into which is placed a strobe light. Surrounding the composite structure is a detection arrangement having a detection medium of photographic film. When the strobe light is activated the film is exposed and developed to obtain an image indicative of the distribution of the steel balls in the matrix. The image is analyzed to determine if the structural member is acceptable.