Investigation of the Compressive Behavior of a Freeze-Cast Cu Foam Using Acoustic Emission Measurement

Herein, the compressive behavior of Cu foams with lamellar structures processed by the freeze-casting method is interrogated in directions parallel and perpendicular to the direction of freezing. Deformation is monitored using simultaneous acoustic emission (AE) measurements and video recordings of changes at the sample surface. Significant differences are observed between the stress-strain curves acquired during compression in the two directions. For parallel loading, relatively high peak stress is detected at a strain of about 3%, followed by a plateau, and, at high enough loadings, the Cu foam hardens. The AE results suggest that the decrease in stress from the peak value to the plateau value arises from fractures in the thick lamellae lying nearly parallel to the freezing direction. For perpendicular loading, the energy of the AE events is lower because, in this case, the thinner struts that connect the thick lamellae bend and break more easily. For this case of perpendicular loading, the peak stress is missing from the stress-strain curve. Further deformation yields a gradual increase in the energy of the AE signals, suggesting that the thicker lamellae also break during densification.