# Stability and Failure of the Edge-Closed Honeycomb Sandwich Panels with Face/Core Debonding
## 3 Test Results and Discussion  
### Strain Distribution  
As the quantity and variety of specimens were too large, only strain measurement results of typical specimens of which experimental phenomena were easy and clear to be observed were selected for analysis in this section, and the trends of the strain-load curves of other samples were basically consistent. As shown in Figures 5-7, the strain values of the inner and outer skin remained the same only at the initial stage of loading, this was attributed to the beveled area closed to the edge of the specimen which led to the initial loading eccentricity. Thereafter, the strain values of the back-to-back resistance strain gages were different, which indicates that the structure was subjected to bending load due to the existence of eccentric loads. When the slope of the strain-load curves showed a reverse trend (vertical red dotted line in the figure), it indicated that the structure was buckled.  
Figure 5: Typical strain-load curves of the third non-defective compressive sample (CD0-3).  
Figure 6: Typical strain-load curves of the third compressive sample with defective diameter 40 mm (CD40-3).  
The strain-load curves of the non-defective specimens (CD0-3) are shown in Figure 5. When the load reached 65 kN, the slope of strain-load curves of outside skins in the beveled zone turned to a positive value (such as 8 in Figure 5). Subsequently, the slope of the curves at other positions also showed a reverse trend (such as 3, 4 in Figure 5). The beveled area was the weak area of the edge-closed honeycomb structure, so the beveled area was generally damaged before other locations, which indicated that the structure had been damaged before the overall buckling occurred.  
The strain-load curve of the test piece (CD40-3) with debonding defects is shown in Figure 6. When the load reached 60 kN, the slope of the strain-load curves of inner sheets in the debonding area turned to a positive value (such as 4, 17). The slope of the curve of the inner sheets around the honeycomb area changed from negative value to positive one (such as 3, 5) when the load continued to increase to 65 kN. Finally, the curve slope of the outer sheets in the beveled area showed a reverse trend (such as 14) when the load was 75 kN. Firstly, the sandwich panels with a defective size of 40 mm locally buckled in the debonding area, and then local debonding of the panel and the honeycomb core occurred in the beveled area near the loading end.  
The strain-load curves of the test specimens (CD80-3) with debonding defects are shown in Figure 7. When the load reached 35 kN, the curve slope of the inner sheets in the center of the debonding area showed a reverse trend (such as 4). When the load continued to 50 kN, the slope of the curve of the outside sheets in the beveled area changed to positive value (such as 8). Meanwhile, the strain curves in the debonding area changed randomly (such as 15, 17). Therefore, the sandwich structure with a defect size of 80 mm firstly suffered from local buckling in the defect area at a lower load, and subsequently damage occurred in the debonding area and beveled area.  
The strain-load curves of ages 4 and ages 11 in the center of the inner and outer sheets of the different type specimens are selected to drawn in Figure 8, and are contrasted to discuss. For edge-closed honeycomb sandwich panels, the phenomenon of strain-load curves' slope changing from negative to  
Figure 7: Typical strain-load curves of the third compressive sample with defective diameter 80 mm (CD80-3).  
positive occurred on the inner sheets (gages 4), therefore, the inner sheets with beveled section was more prone to buckling or damage. In addition, the larger the debonding size was, the more likely the curve slope was to have inflection points of positive and negative value under low load.