Out-of-plane tensile test methods

Interlaminar or out-of-plane testing for laminated composites has not received as much attention in test method development as in-plane testing, yet many structures fail from out-of-plane loads. There are two basic approaches for out-of plane testing: direct out-of-plane loading by bonding a composite laminate between two fixture blocks, and indirect out-of-plane loading using a curved beam. Both concepts are being considered for possible standardisation by ISO and ASTM.

The direct out-of-plane load is an adaptation of ASTM C297, C633 and D2095. The method uses either square or circular loading blocks which are pulled out-of-plane often using a universal joint to aid alignment. The strength is determined simply by dividing maximum load prior to failure by the specimen cross-sectional area. In many instances the failure will be close to the bonded attachment. A reduced cross-sectional area in the gauge length can help force the failure to within the gauge length.

The curved beam approach to out-of-plane tensile strength (not modulus) takes advantage of the out-of-plane tensile loading induced in the bend of a curved laminate beam subjected to an opening moment. Load is applied using either a four point bend test fixture or in a tensile test machine with suitable loading hinges attached to the beam.