Fracture Load Assessment in Composite Structures
- Paper number
IAC-06-C2.1.05
- Author
Dr. R. Ramesh Kumar, Indian Space Research Organisation (ISRO), VSSC, India
- Coauthor
Dr. Dileep P.N., India
- Year
2006
- Abstract
Space structures made out of composite are more susceptible to damage as it is large in size with small skin thickness due its high strength and stiffness. Damages in composite are classified under interlaminar fracture which is delamination or separation between the layers and intralaminar fracture that is a crack in the thickness direction. The well- known MCCI approach based on finite element method is used to assess the critical load carrying capability of the structure with such defects. Thus, for a given load the strain energy release rate (G) is obtained and the value is compared with material properties on critical values of G to determine the critical failure load. For a composite material, the formidable task to follow the MCCI approach with an intralaminar fracture is to determine first analytically the unique on set of crack propagation direction from as many number of multiple crack initiation directions due to damage as the number of fibre orientations in a multilayered composites and with the inter laminar fracture, the delamination toughness ratio of mode I and mode II that varies with the type loading and fibre orientations. The well- known method to evaluate the delamination fracture toughness is mixed mode bending test and for intralaminar fracture toughness the compact tension or centre cracked specimens
Fracture toughness is an intrinsic property of a material and for a multilayered composite laminate, it varies depending up on the fibre orientation of each layer, material property like moduli etc. For a multilayered composite laminate, the strength properties can be as assessed based on unidirectional laminate properties following classical laminate theory. However, as on today there is no analytical method available to predict the toughness value of a laminate based on toughness values associated with the unidirectional laminate which will be useful for the assessment of fracture load of composite structure with any laminate configurations (layers with different fibre angels).
In the present study, the failure load of compressively loaded carbon-epoxy conical structure with delaminations observed between splice plate and basic composite skin is assessed using MCCI approach. The delamination fracture toughness under mixed for the high strength and high modulus carbon epoxy reported in literature is used [1-2]. For the intra laminar fracture study, an analytical solution is derived to assess the failure load of [0, ± 45, 90]s carbon composite laminate based on the toughness values of its sub laminates of [0], [90] and [± 45,]s laminates. Verified with test data for three other laminate configurations. A good agreement is established between analysis and test for failure load assessment for both inter laminar and intra laminar modes of fracture. References 1. Ramesh Kumar R., Praveen K.S and. Rao G.V, “Assessment of delamination fracture load of stringer stiffened composite panel”, AIAA Journal, Vol.41(3), Technical Notes, pp 551-554, 2003. 2. Dileep P.N. and Ramesh Kumar R., “A simple method for the evaluation of fracture toughness of a multi layered laminate based on the failure stress of sub –laminates”, International Journal of Fracture, Vol. 131(1), pp L3-L6, January, 2005.
- Abstract document