Stable damage growth in composite sandwich panels under static compression

Abstract

In this paper, damage growth characteristics and failure mechanisms were studied experimentally for two types of graphite composite sandwich panels with titanium cores : [+45/0/-45/90]s IM7/PETI-5 and [+45745/0/90]s IM7/5260. Results from static compression tests showed that stable damage growth prevailed in all specimens before final catastrophic failure occurred. For both face sheet materials, damage propagated stably in a self-similar manner from the notch tip to the specimen edge until it reached the critical damage length. The master curve which related the instantaneous damage length to the applied load was established for each face sheet material using the least square method. The master curve was shown to be independent of the original notch configurations. However, the master curve was found to be different for different face sheet materials, suggesting that the rate of damage growth was highly dependent upon the properties of face sheet materials. The failure mechanisms of the composite sandwich panel were also investigated using the scanning electronic microscope (SEM). The SEM results showed that failure initiated via kinking in the 0°-ply cross-section, which was followed by matrix cracking, delamination, and splitting in the off-axis plies. Based on this study, it is concluded that failure characteristics of notched composite sandwich panels are significantly affected by the presence of a honeycomb core. For sandwich panels with little or no core stiffness, local buckling near the notch tip can easily occur since there is no constraint against the outof-plane deformation. For sandwich panels with a stiffer core, shear failure through the thickness is the dominant failure mode as the core can stabilize the face sheets against local buckling. As a result, the residual strength, the critical damage length, and the composite fracture toughness are all affected by the core stiffness in the out-of-the-plane direction. Another interesting phenomenon found was that damage growth from the notch tip was stable rather than catastrophic. This is an important observation since it may be feasible to develop a sandwich structure that is capable of arresting a propagating crack.