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

Abstract

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.

Evaluation of the Viscoplastic Strain of High-Density Polyethylene/Multiwall Carbon Nanotube Composites Using the Reaction Rate Relation

“…2b). These facts have been discussed in more detail in [25,26]. All the materials investigated exhibited nonlinear viscoelastic-viscoplastic behavior.…”

“…The main advantage of the method is the fact that it does not require performing time-consuming creep recovery tests. Moreover, the ε vp -ε relation can be constructed based on only few test data, whereupon ε vp can be predicted for any stress, loading time or action of external factors [25]. Needless to say, a physical method for the registration of irreversible changes in the material structure should be added to clear up the accumulation kinetics of viscoplastic strains.…”

“…The latter ones, being implicitly related to stresses and time, determine the contribution of irreversible strains whatever their origin (e.g., viscoplasticity or damage). A power-law relation between the residual and total creep strains was found to be valid for a variety of polymer composites tested under different creep conditions [25].…”

“…Previously, based on an analysis of creep and creep recovery data for polypropylene (PP) and a high-density polyethylene (HDPE) filled with multiwall carbon nanotubes (MWCNTs), it has been reported that there is a unique empirical relationship between the residual and total creep strains [25,26]. The latter ones, being implicitly related to stresses and time, determine the contribution of irreversible strains whatever their origin (e.g., viscoplasticity or damage).…”

Modelling of Environmental Ageing of Polymers and Polymer Composites—Durability Prediction Methods

“…According to Eyring’s thermal activation flow theory, the strain rate (or the characteristic time) is given by the relationship where A 1 (s − 1 ) is a material constant and is the coefficient linked to the activation volume; σ is the applied stress. Eyring’s activated flow theory is widely used to assess plasticity-controlled failure in thermoplastic polymers and composites [ 31 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 ]. A detailed discussion is given in Section 2.3 .…”