Relationship between the residual and total strain from creep-recovery tests of polypropylene/multiwall carbon nanotube composites

An assessment of accumulated irreversible strains in polymer composites is a crucial element for controlling dimensional stability of structural components and their remnant life. The residual strains as functions of total creep strains are analyzed by example of creep-recovery data of polypropylene (PP)/multiwall carbon nanotube (MWCNT) composites. To cover wide range of strains, creep test regimes with different stresses, loading time, and number of cycles were applied. Totally, data of 62 single creep-recovery tests for 7 material compositions were used for analysis. A general empirical relationship between the residual and total creep strain is established and finely described by a power law. The residual strain increases with increasing stress and time of loading and decreases with growing amount of MWCNT. The total creep strain, which is implicitly related to stress, time, and sample specificity, determines the contribution of irreversible deformation. This fact overcomes data variability within one series of samples. Similar empirical relationships are obtained for 25 polymer composites from literature reinforced with different types and amount of fillers and tested under different temperatures. The empirical relationship can be used for an express assessment of residual strains accumulated in a long term by performing just a few short-term control tests.