Alternative dynamic torsion test to evaluate the elastic modulus of polymers

Abstract

This work presents an alternative for the determination of the torsion modulus, G, of polymers. These materials may be subjected to shear stresses in some structural applications; thereby, the knowledge of G is of great interest. For this purpose, a mechanical system featuring a simplified torsion pendulum version and a rotational motion sensor (RMS) coupled to it was used to establish an angular position as a function of time. The applied technique is considered non-destructive and makes it possible to obtain G without the Poisson’s ratio through an equation derived from mechanical spectroscopy and material strength. The main goal is to present and validate the employment of this method for polymers. Therefore, circular cross-sectional samples of extruded polytetrafluoroethylene (PTFE) were subjected to torsional stresses, in which a physical and quantitative explanation is given for the frequency and G curves as a function of the prefixed rotational inertia (I), length (L), and diameter (d). Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were also made to ensure the reliability of data. It is possible to establish an L/d ratio, which explains why G converges to a single value when the sample dimensions are different from each other. It was found that G is approximately 350 MPa for an L/d ratio equivalent to 10.64. Such a value is within limits found in the literature, opening the possibility of assessing other polymers.

Mechanical properties assessment of a 3D printed composite under torsional and perpendicular stress

“…The samples were clamped with an effective length of L = 75.00 ± 0.05 mm following an aspect ratio (L/a) of 15.6. This ratio is enough to generate the relaxation curve as well as to avoid off-axis distortions and is considered standard for the measurement (Piedade et al , 2020). Also, an aspect ratio value greater than 10 (L/a > 10) is recommended to make the sample free of stress induced by clamping (Ward and Sweeney, 2004).…”

“…The complete view and description of the system can be seen in more detail in Refs (Pintão et al , 2017) and (Piedade et al , 2020). The method for measurement of G modulus consists of applying an 8° torsion on the sample extremity that is attached to the pendulum with the other extremity being attached to the static base of the system; this configuration makes the sample oscillate freely until its movement stops.…”

“…The measurement is non-destructive, and different from classical dynamic methods; the Poisson’s ratio is not required to compute G, which is an interesting feature when measuring composite materials. This method was validated for biomaterials (Pintão et al , 2017) and more recently for polymers (Piedade et al , 2020) and is now considered in the proposed comparative study in this work.…”

Torsion modulus with CaCO3 fillers in unsaturated polyester resin - mechanical spectroscopy

“…Authors [28,30] have validated Equation 7; now, it is applied to a composite material, which is a different situation than that of a homogeneous material (CP-Ti). Therefore, it is necessary to validate it for this case.…”