Micromechanical studies of fresh and weathered fibre cement composites. Part 2: Wet testing

This paper presents a study involving in situ SEM fracture studies of fibre cement composite materials tested in a wet environment. An in situ three point bend loading device coupled with ‘wet cell’ techniques have facilitated the simultaneous loading and observation of micro fracture processes in wet synthetic and cellulose fibre cement composite materials. The micromechanistic fracture behaviour is similar to earlier dry testing in that failure is a complex combination of microcracking, stress redistribution, fibre debonding, fibre pull-out and fibre failure. The onset of the first microcracking stage of this process has been identified as occurring at the limit of proportionality and which develops progressively to the ultimate strength as the tensile zone moves through the thickness of the composite. During ageing due to natural weathering it is believed that there is an increase in interfacial fibre bound which increases the propensity for fibre failure as opposed to fibre pull-out, as observed, and leads to greater strengths in aged products. The extra effect of localised moisture on performance is substantial additional localised microcracking when wet, as well as a small drop in strength. This may be associated with enhanced stress redistribution and more widespread microcrack initiation under wet conditions.