Identification of the through-thickness rigidities of a thick laminated composite tube

In this paper, a novel experimental procedure is carried out for providing all the through-thickness rigidities of a thick glass-epoxy laminated ring cut from a tube, using only one diametral compression test. The procedure requires first to measure the strain field over a predefined area of the lateral surface of the ring coupon, and then to process it with the Virtual Fields Method. The region of interest where noise effects are minimized is first determined with simulated data. Then, the practical feasibility of the procedure is demonstrated with experimental data. The displacement fields are measured using the grid method. In order to filter the measurement noise at best, the strain fields are derived by polynomial fitting of the displacement fields. The validity of this approach is checked on simulated data. However, the experimentally identified stiffness values appear very scattered from one test to another. This is due to the non-uniform distribution of the load through the thickness, which is a well-known problem when testing thick composites. It has been shown that an optical system providing directly the images of both sides of the specimen can address this issue. Finally, such a system is set-up and successfully applied for identifying all the through-thickness rigidities of the ring.