Flexural fatigue failures and lives of Eco-Core sandwich beams

Abstract

Eco-Core is a class of syntactic foam made from small volume of high char yield binder and large volume of a class of flyash for fire resistance application. Very little or no flexural fatigue data of this class of core material is reported in the open literature. This paper presents a flexural fatigue response of Eco-Core in a glass/vinyl ester composite face sheet sandwich beam. A four-point loaded flexural test specimen was designed and tested in static and fatigue loadings to cause tension failure in the core. The fatigue test was conducted at maximum cyclic stress (σmax) ranged from 0.7σct to 0.9σct, where σct is the static flexural strength of the core. The sinusoidal loading frequency of 2Hz with the stress ratio of 0.1 was used. Flexural fatigue failure modes of Eco-Core sandwich beam were classified: damage onset (single tension crack), damage progression (multiple tension cracks) and ultimate failure (a combination of tension and shear). These failures were characterized by 1%, 5% and 7% changes in compliance that corresponds to N1%, N5% and N7% lives. The fatigue stress-life (S–N) relationship was found to follow the well-known power law equation, σmax/σct=AoNα. The constants Ao and α were established for all three types of failures. The endurance limit was established based on 1million cycles limit and it was found to be 0.65σct, 0.70σct and 0.71σct, respectively for the three modes of failure. Flexural fatigue and static failure modes of Eco-Core sandwich beams were similar.