| Description |
Standardized tests exist for measuring the Mode I and Mode II interlaminar fracture toughness of unidirectional composite laminates; however, no such standardized tests exist for sandwich composites. Research has determined that the Mode I facesheet-core fracture toughness of a sandwich composite can be measured using the Single Cantilever Beam (SCB) test method. Unfortunately, a significant concern was recently uncovered and relates to the testing of sandwich composites with thin facesheets, which are extensively used in aerospace structures. This concern will be addressed within this study and will allow for the standardization of the SCB test method. A total of four data reduction processes were investigated. The facesheets of these structures tend to experience large load-point deflections. In order to reduce the excessive deflection, past researchers bonded secondary structures, commonly known as doubler(s), onto the facesheet. The doublers typically caused the disbond to grow towards the symmetry line of the sandwich composite. This change in the through-the-thickness depth was typically accompanied by a change to the fracture toughness. Two of the four data reduction processes explored in this study will eliminate any need for doublers. The remaining two data reduction processes will be used to illustrate this change in throughthe- thickness and its impact on the apparent fracture toughness. Furthermore, a limit on their use will be proposed with respect to minimum facesheet thickness. To further investigate the doubler's impact on the through-the-thickness disbond iv depth, the Climbing Drum Peel (CDP) test method was also explored in this study. The CDP test was used to impose varying amounts of curvature upon the facesheet near the disbond tip without regard to the facesheet thickness. Results from experimental testing were then compared to numerical results to determine if facesheet curvature near the disbond tip tends to cause the disbond to change location within the sandwich composite. Also, the fracture toughness values from both CDP and SCB were compared to determine if the former is a viable test method. Lastly, the as-cured variations of each fracture surface were quantified and examined in an attempt to reduce fracture toughness measurement scatter. |
