Feature Article (a) Facesheet side (c) Core side (b) Facesheet side (d) Core side Figure 8. Representative images of fracture surfaces of samples cured under partial vacuum at different locations. Figure 9. Load vs Extension in edgewise compression test at 177°C (350°F ) of sandwich samples cured under full vacuum. facesheets is high, compared to the strength of the fracture interface. The sandwich falls apart rapidly as the crack in the interface propagates along the length of the sandwich specimen. When the interface strength is high compared to the bending strength of the facesheets, crack propagation is more stable. The crack arrests after a short propagation distance and other failure modes such as facesheet bending, delamination and core crushing begins to dominate. SAMPE Journal, Volume 52, No. 3, May/June 2016 Adhesive Bond Fracture Toughness Test A sample load/displacement curve is shown in Figure 11. All tests were conducted at room temperature. The load increases until it reaches a critical value and crack begins to propagate. As the crack propagates, load reduces. Critical strain energy release rate was calculated from three loading/unloading cycles for each specimen. 51