COMPARING THE BEHAVIOUR OF CARBON-EPOXY ANGLE PLY LAMINATES IN THREE AND FOUR-POINT BENDING

1 General Introduction Three and four-point bending tests can be used to determine the flexural properties of thin unidirectional or cross-ply laminates, but these tests are not recommended for angle-ply laminates because the... [ view full abstract ]

1 General Introduction

Three and four-point bending tests can be used to determine the flexural properties of thin unidirectional or cross-ply laminates, but these tests are not recommended for angle-ply laminates because the coupling between different modes of deformation such as bending and twisting leads to a larger apparent bending stiffness. [1, 2]. The physical constraints imposed by the test apparatus prevent the coupon from deforming freely, thus inducing internal loads and producing a more complex stress state than what occurs in cross-ply laminates or in isotropic materials [3]. This stress state, however, is more representative of what a laminate component may experience while in service. There is the potential for these bending tests to be useful in predicting the behavior of larger laminate components, by representing the stress state produced by the surrounding physical constraints of in-service structures. The current work compares the internal loading, internal stress state, deformation and bending stiffness for three and four point bending tests of carbon-epoxy angle-ply laminates relative to unconstrained coupons in pure bending. Different ply lay-ups representing laminates with a range of flexural stiffnesses are considered.

2 Methodology
A 4-ply (+/-α)s carbon-epoxy layup was simulated using the commercial software ANSYS. The three and four-point bending tests were simulated using ideal, simply supported constraints and applied displacements. The loading points and the supports were both prevented from rotating about the longitudinal axis of the coupon. The applied displacement was calibrated for each ply layup so that each coupon would produce the same vertical deflection when subjected to a pure moment (unconstrained and free to deform). Each ply was individually represented and local coordinate systems were used to define the ply angles.

3 Simulation Results
In pure bending, angle-ply laminates that were free to deform without constraint twisted about their longitudinal axis as seen in Figure 1. In three and four-point bending, this twist was prevented from occurring creating an internal torsional moment about the longitudinal axis of the coupon as presented in Figure 2. This internal torsional moment in the coupon is larger for the four-point bending set-up and was as large as 67% of the applied bending moment. Coupons with ply layups of 40˚ produced the largest internal moment. Increased normal stresses in the longitudinal direction (σx) as well as larger in-plane (τxy) and transverse (τxz) shear stresses were also predicted for the three and four-point bending when compared to pure bending.

An examination of the response of angle-ply laminates in three and four-point bending shows significant differences in the generated stress state. The physical constraints applied by the test set-up induce additional internal loads which must be accounted for when designing coupon tests to represent realistic in-service loading conditions.

4 References
D.Wowk, D. Thibaudeau, C. Marsden “Quantifying the Shear Coupling Effect in Point Bending Tests of Angle Ply Laminates”. ICCM 19, 19th International Conference on Composite Materials, Montreal Canada, e-proceedings, page 8249.
M. Grediac “Four-point bending tests on off-axis composites”. Composite Structures, Vol 24 pp 89-98, 1993.
N. J. Pagano, J. C. Halpin “Influence of End Constraint in the Testing of Anisotropic Bodies” Journal of Composite Materials, Vol 2, No. 1 pp 18, 1968.