Keywords: composite structures, damage mechanics, impact simulation, sandwich panels.

The paper describes recent progress on materials modelling and numerical simulation of foreign object damage (FOD) on fibre reinforced composite aircraft structures. To reduce certification and development costs, computational methods are being developed in the CEC research project CRAHVI [1] to predict structural integrity of composite structures under impact from birds, hailstones, tyre rubber and runway stone debris. Key issues are the development of suitable constitutive laws for modelling composites in-ply and delamination failures, determination of composites parameters from high rate materials tests, and the efficient implementation of new materials models into FE codes. Materials models are also required for deformable and fragmenting projectiles, such as birds or hailstones, whilst objects such as metal pieces or stones may often be modelled as rigid projectiles when impacting composite structures.

The work is based on the application of explicit FE codes to simulate composite shell and panel structures under high velocity impact. During impact loading of composite structures failure may occur by delamination, which is important in lower energy impacts and in failure initiation, and by in-plane ply failure which controls ultimate fracture and penetration in the structure. The approach adopted is to use continuum damage mechanics (CDM) for composites as a framework within which in-ply and delamination failure may be modelled [2]. The composite laminate is modelled by layered shell elements or stacked shells with a tied interface which may fail by delamination. The shells are composed of composite plies which are modelled as a homogeneous orthotropic elastic or elastic-plastic damaging material whose properties are degraded on loading by microcracking prior to ultimate failure. A CDM formulation is used in which ply degradation parameters are internal state variables which are governed by damage evolution equations.

The composites ply damage and interply delamination models have been implemented in a commercial explicit FE code [3]. The CDM ply failure model for fabric reinforced plies uses three scalar damage parameters representing modulus reductions under different loading conditions due to microdamage in the ply, and the delamination model requires two interface fracture energy parameters. Damage evolution equations supported by a materials test programme are introduced which relate the damage parameters to strain energy release rates in the ply.

Code validation studies are presented in this paper in which numerical simulations are compared with high velocity impact test data on idealised composite panels and shells. The structures investigated are cylindrical composite shells impacted by a steel ball projectile in gas gun tests, and sandwich panels with composite skins and honeycomb cores impacted by concrete projectiles to represent stone damage. Observed failure modes, structural damage and impact loads are well predicted by the FE simulation methods being developed.

1

CRAHVI: Crashworthiness of Aircraft for High velocity Impact, CEC Project: G4RD-CT-2000-00395, 2001-2004.

2

Ladevèze, P. Inelastic strains and damage, Chapt. 4 in Damage mechanics of composite materials, 1994; Ed. Talreja, R., Composite Materials Series 9, Elsevier, Amsterdam..

3

Johnson, A.F., Pickett, A.K., Rozycki, P. Computational methods for predicting impact damage in composite structures. Composites Science and Technoloy, 61, 2183 - 2192, 2001. doi:10.1016/S0266-3538(01)00111-7

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