Modelling indentation creep of polymers: a phenomenological approach

The room temperature nanoindentation creep behaviour of a range of amorphous and semicrystalline polymers has been investigated. It was found that experimental data during the first 20 s at constant load were well fitted by a logarithmic equation relating the fractional increase in penetration depth during creep to [ A / d (0)]ln( t /τ + 1) where A and τ are constants, d (0) is the initial penetration depth at the end of the loading period and t is the time at peak load. For samples of ultra-high molecular weight polyethylene and polypropylene, above their glass transition temperature at room temperature, it was found that the creep parameter A / d (0) increased at lower load and at higher loading rates when a Berkovich indenter was used. In contrast, samples of poly(methylmethacrylate) and poly(ethylene terephthalate), below their glass transition at the test temperature, showed no clear variation in A / d (0) with loading rate. The constant τ varied with loading rate (and hence loading time), following a power-law relationship. Fitting of creep data with the equation above enables prediction of the extent and rate of the creep occurring after any given combination of loading rate and maximum load with reasonable accuracy, particularly at higher load. Importance should be placed on the actual loading rate and time to reach peak load as this is shown to influence the initial creep kinetics dramatically.