The fracture surface of a wet N5 composite is shown in Fig.14a. The evidence of fiber-matrix debonding and fiber pull-out can be observed in this figure in the form of several holes and pulled out fibers. This is in contrast to failure features for the dry N5 composite shown in fig.14b,where there appears to be no indications of fiber debonding or fiber pullout . The fiber are elongated and fractured in this figure, similar the features observed in previous studies on tensile surface of CNF/epoxy composites. The debonding effect in N1 composites results in a decrease of flexural strength of wet composites, shown in fig.9.However, as the CNF content increases, the difference between the flexural strength of wet and dry composites reduces,such that the wet N5 composites show a higher strength than the dry N5 composites. Therefor, addition of sufficiently high content of CNFs can help in retaining the strength in composites after environmental exposure. Similar effect are also noted in the syntactic foam composites containing 2 and 5 wt% CNFs.