Influence of impacts on static and low-cycle fatigue characteristics of composite specimens

Abstract

This paper describes the effect of impacts on the possible reduction of the structural characteristics and damage growth of graphite-epoxy specimens. The considered specimens are undamaged specimens and specimens impacted with two different energy levels. In particular, barely visible impact damage and visible impact damage are investigated. Static and low-cycle fatigue tests are performed under biaxial loading conditions, given by compression and shear, applied individually and in combination. Stiffness reduction, a small decrease of the collapse load and damage propagation can be observed with low-cycle fatigue tests cycling from zero to a load equal to 75% and 85% of the static collapse load. No changes in the structural characteristics are observed for cycling tests at lower loads. The results of the tests show that a large margin of safety exists for the investigated specimens, which can be the basis for saving additional weight in composite structures. Additionally, the obtained experimental data can guide the development of finite element models able to predict the effects of impact damage, and to increase the database necessary to develop damage tolerance criteria for structural design.

Keywords:

• impact
• composite material
• experimental tests
• biaxial loading
• low-cycle fatigue

Acknowledgements

The authors would like to thank Professor Tomasz Wierzbicki, Director of the Impact and Crashworthiness Laboratory at the Massachusetts Institute of Technology. This work has been partially supported by Regione Lombardia, through the project ‘COmposite materials Damage under Multi AXial loading - COD MAX’ (ID MAN-02 Rif. No. 16780).