A NUMERICAL AND ECPERIMENTAL INVESTIGATION OF OPEN HOLE COMPRESSION TESTING

Abstract

3D composites due to high delamination resistance are replacing the laminated composites in many applications. Messier-Dowty’s landing gear strut for Boeing 787, JSF inlet duct and Snecma motor’s aero-engine fan blade are some of the high profile aerospace applications of 3D woven composites. Holes are unavoidable in aerospace applications and can significantly affect the structural strength of any material and 3D composites are no exception. Currently ASTM D6484, an open-hole compression standard test method developed for 2D composites is being used for measuring the notched compressive strength of 3D woven composites. Since this test method was developed for 2D composites it is arguable whether the standard should be applied as it is for 3D composites. Thus, the aim of this project is to understand the failure stress state of OHC specimens by using FE and experimental techniques. To achieve this objective, ABAQUS 6.10 a commercial FE package is used to simulate the standard openhole compression test so that the stress state in the specimen near the point of failure can be studied in detail. FE Models were built from three different types of elements i.e. the conventional shell, continuum shell and solid elements. These models are validated by simulating the OHC testing of 2D continuous fiber-reinforced carbon/epoxy composites for which the experimental results are taken from the literature. It was found that solid elements provided better results and with these elements it was also straight forward to implement the fully orthotropic material definition required for modeling the behavior of 3D composites. Thus these were then used to study the failure modes of 2D and 3D GFRP woven composites. Experimental work on 2D and 3D woven GFRP OHC composite specimens was performed partially at NCCEF (National Composites Certification and Evaluation Facility), University of Manchester, UK and HITEC University, Pakistan. Detailed numerical and experimental study has shown that the damage modes developed in 2D and 3D GFRP woven composites were analogous. Similarly, magnitudes of longitudinal, transverse and in-plane stresses were observed in both composites. On the contrary, out of plane stresses were found significantly higher in 3D woven composites as compared to 2D woven composites due to 3rd directional reinforcement i.e. the z-yarns, however this increase in out of plane stress is not so significant as to cause change in dominant damage mode.. Conclusively, ASTM D6484 an open-hole compression standard test method was found reliable testing technique for 3D woven composites.