Thickness effect in composite laminates in static and fatigue loading

Thick Laminates (above 6mm) are increasingly present in large composites structures such as wind turbine blades. Designs are based on static and fatigue coupon tests performed on 1-4mm thin laminates. However, a thickness effect has been observed in limited available experimental data. For this reason standard experimental data cannot automatically be transferred to thicker laminates.

Different factors are suspected to be involved in the decrease of static and dynamic performance of thick laminates. These include the effect of self-heating, a mechanical scaling effect and the manufacturing process influence.

Self-heating during fatigue is related to the material energy loss factor. During dynamic loading a certain percentage of mechanical energy is dissipated into heat, leading to a rise in material temperature. When the temperature approaches the maximum service temperature of the material, a reduction in fatigue life can be observed. The work proposes an FE method to forecast self-heating, which is validated by using empirical data.

Scaling effects and coupon geometry influence the results of thickness scaled coupon tests. The thickness effect was studied with the help of compression and tension tests on thickness scaled coupons. In order to reduce the test effects of the scaled coupon tests the coupon geometry and clamping system are designed for optimal load introduction.

The manufacturing process and curing cycles are reported as one of the leading causes to explain possible scaling effects. Through-thickness lamina properties were studied using the sub-laminates technique. In this way, it was possible to relate the in-plane lamina properties with the manufacturing properties conditions. A relation between the mechanical properties and the process conditions is proposed.

In the case of static and fatigue properties, the sub-laminates tests report a large variation in resin related properties which is dependent on the manufacturing process. Scaled tests are studied from this point of view; the scaling effect is related to the manufacturing process, and the assumption of uniform strength fields is considered not valid for thick laminates in comparison with thin laminates.