Application of the Small Punch Creep-Recovery Test (SPCRT) for the Estimation of Large-Amplitude Viscoelastic Properties of Polymers

The Small Punch Creep-Recovery Test (SPCRT) is a novel miniature test used to estimate the viscoelastic properties of polymers and biomaterials. The current investigation related to the SPCRT is limited to Finite Element Method (FEM) simulations and experimental tests on PVC. The aim of this investigation was focused on: (i) extending the experimental tests to other polymers with dissimilar viscoelastic properties; (ii) deepening the influence of non-linear viscoelastic properties in the estimation capabilities of the SPCRT; and (iii) developing a numerical methodology to estimate and take into account the viscoelastic recovery produced during the unloading step of compressive creep-recovery tests (CCRT) and SPCRTs. The experimental tests (CCRTs and SPCRTs) were done on polyethylene PE 500, polyoxymethylene POM C, nylon PA 6, and polytetrafluoroethylene (PTFE), with a range of creep loads, in the case of CCRTs, in the whole elastic regime and the surroundings of the yield strength of each material. The experimental results confirmed that the SPCRT was an accurate and reliable testing method for linear viscoelastic polymers. For a non-linear viscoelastic behavior, SPCRT estimated the viscoelastic properties obtained from CCRTs for creep loads near the yield strength of the polymer, which corresponded with large-amplitude viscoelastic properties in dynamic creep testing. In order to consider the viscoelastic recovery generated in the unloading step of CCRTs and SPCRTs, a Maxwell-Wiechert model with two branches was used, simulating the different steps of the experimental tests, and solving numerically the differential equation of the Maxwell-Wiechert model with the Runge-Kutta-Fehlberg (RKF) numerical method. The coefficients of the elements of the Maxwell-Wiechert model were estimated approaching the straining curve of the recovery step of the simulation with the same curve registered on each experimental test. Experimental CCRTs with different unloading times demonstrated that the use of this procedure derived in no influence of the unloading step time in the viscoelastic properties estimation.