Uniaxial, shear, and poisson relaxation and their conversion to bulk relaxation: Studies on poly(methyl methacrylate)

Abstract

Multiple viscoelastic properties were determined for poly(methyl methacrylate) in the context of examining experimental limitations on deriving other properties from these measurements, specifically time dependent bulk modulus behavior. By means of master curves for uniaxial extension on plate and cylinder specimens, as well as shear (torsion) and Poisson behavior from measurements in the temperature range from −40°C to 125°C, it is established that excessive precision is needed for reliable interconversion of some properties to others. While normal inverse relations (modulus‐compliance) are readily obtained from one set of measurements, bulk (relaxation or creep) properties cannot be derived reliably from other functions and must be determined directly.

Time and temperature effects on Poisson’s ratio of poly(butylene terephthalate)

“…They evidenced an upper plateau at 0.49 for temperatures approaching the glass transition region, while the simultaneously measured creep compliance still displays an increasing trend. Similar results were obtained on amorphous poly (methylmetacrylate) by the group of Knauss [10]. In both the above-cited works the steepest part of the Poisson's ratio isothermal curve is located at temperatures lower than T g .…”

“…Finally, the data obtained from the Poisson's ratio measured in CDR experiments, shows an intermediate behavior, exhibiting values close to those measured for the moduli functions for temperature up to 40°C, while a deviation is exhibited for higher temperatures, with a slope similar to that shown by the # REL set of data. Previous literature data have always shown that the shift factors required for the Poisson's ratio master curve construction are the same as those obtained for other viscoelastic functions [10,17,20]. The difference found between master curves distribution and between the related shift factors are effects not fully understood, and that could be tentatively ascribed to an interference due to the contact extensometer.…”

“…Nevertheless, it was shown that only in few cases the assumption of a constant Poisson's ratio can be reasonably considered valid [2,4], and that neglecting its time and temperature dependence may lead to predicting models markedly deviating from the experimental observations [5][6][7]. More detailed information on the viscoelastic behaviour of Poisson's ratio is therefore important not only in principle [8][9][10], but also for practical reasons. In fact, it would allow us to interconvert material functions [1], to refine computational methods [2], to reduce the number of required experimental parameters to fit analytical models [8], to better estimate the effects of the so called Poisson's strains, originating from misfits in Poisson's ratio of joined materials [1].…”

“…An accurate measurement of the transverse strains is of crucial importance and various techniques have been employed, such as optical methods (i.e. Moirè interferometry [10,[17][18] and video extensometry [13,16]), bonded strain gages [19][20] [22]). In a work on epoxy resins we have also reported Poisson's ratio as increasing with time, temperature and strain, and decreasing with strain rate [14].…”

“…Direct measurements of Poisson's ratio has been carried out on polymeric materials by different types of experiments, in which the axial deformation was applied through a constant rate loading ramp [12][13][14][15], under a constant load (creep) [13,[16][17], under constant strain (stress relaxation) [10,14,18], or under sinusoidal loading-unloading (dynamicmechanical analysis) [19][20][21]. An accurate measurement of the transverse strains is of crucial importance and various techniques have been employed, such as optical methods (i.e.…”