Strain rate dependence in plasticized and un-plasticized PVC

An experimental and analytical investigation has been made into the mechanical behaviour of two poly (vinyl chloride) (PVC) polymers – an un-plasticized PVC and a diisononyl phthalate (DINP)-plasticized PVC. Measurements of the compressive stress-strain behaviour of the PVCs at strain rates ranging from 10−3 to 103s−1 and temperatures from − 60 to 100∘C are presented. Dynamic Mechanical Analysis was also performed in order to understand the material transitions observed in compression testing as the strain rate is increased. This investigation develops a better understanding of the interplay between the temperature dependence and rate dependence of polymers, with a focus on locating the temperature and rate-dependent material transitions that occur during high rate testing

Probabilistic estimation of the constitutive parameters of polymers

The Mulliken-Boyce constitutive model predicts the dynamic response of crystalline polymers as a function of strain rate and temperature. This paper describes the Mulliken-Boyce model-based estimation of the constitutive parameters in a Bayesian probabilistic framework. Experimental data from dynamic mechanical analysis and dynamic compression of PVC samples over a wide range of strain rates are analyzed. Both experimental uncertainty and natural variations in the material properties are simultaneously considered as independent and joint distributions; the posterior probability distributions are shown and compared with prior estimates of the material constitutive parameters. Additionally, particular statistical distributions are shown to be effective at capturing the rate and temperature dependence of internal phase transitions in DMA data

Finite element modelling of the mechanism of deformation and failure in metallic thin-walled hollow spheres under dynamic compression

Recent interest in lightweight metallic hollow sphere foams for aerospace applications requires a better physical understanding of dynamic properties of single spheres. Finite element modelling supported by high rate experiments was developed to investigate the underlying deformation and failure mechanisms of electrodeposited nickel thin-walled hollow spheres. Parametric simulation was performed to further explore the effect of sphere geometry (wall thickness to diameter ratio) and loading rate. It was found that decreasing the ratio of wall thickness to diameter tends to transit the side wall failure mode from bending to buckling. For a thin-walled sphere (the thickness to diameter ratio less than a critical value), the macroscopic dynamic behaviour is primarily dominated by the two deformation and failure mechanisms: (1) buckling failures of wall materials and (2) self-contacts of wall surfaces and wall-anvil contacts. At higher impact velocity (greater than a critical velocity), inertia effect due to dynamic localisation of wall crushing arises and significantly influences the deformation/failure mode of the sphere, resulting in an increased initial crushing strength and asymmetric deformation. Finally, the behaviour of hollow spheres was correlated to explore the power law behaviour of bulk foams with respect to the relative density; it was found that metallic thin-walled hollow sphere foams can be better approximated as open-cell rather than closed-cell foams. © 2012 Elsevier Ltd. All rights reserved

Strain rate dependency of laser sintered polyamide 12

Parts processed by Additive Manufacturing can now be found across a wide range of applications, such as those in the aerospace and automotive industry in which the mechanical response must be optimised. Many of these applications are subjected to high rate or impact loading, yet it is believed that there is no prior research on the strain rate dependence in these materials. This research investigates the effect of strain rate and laser energy density on laser sintered polyamide 12. In the study presented here, parts produced using four different laser sintered energy densities were exposed to uniaxial compression tests at strain rates ranging from 10−3 to 10+3 s−1 at room temperature, and the dependence on these parameters is presented

Strain rate dependency of laser sintered polyamide 12

Parts processed by Additive Manufacturing can now be found across a wide range of applications, such as those in the aerospace and automotive industry in which the mechanical response must be optimised. Many of these applications are subjected to high rate or impact loading, yet it is believed that there is no prior research on the strain rate dependence in these materials. This research investigates the effect of strain rate and laser energy density on laser sintered polyamide 12. In the study presented here, parts produced using four different laser sintered energy densities were exposed to uniaxial compression tests at strain rates ranging from 10−3 to 10+3 s−1 at room temperature, and the dependence on these parameters is presented

Application of Linear Viscoelastic Continuum Damage Theory to the Low and High Strain Rate Response of Thermoplastic Polyurethane

Abstract Background Understanding the mechanical response of elastomers to applied deformation at different strain rates and temperatures is crucial in industrial design and manufacture; however, this response is often difficult to measure, especially at high strain rates (e.g. &gt; 100 s− 1), and more predictive methods to obtain constitutive relationships are required. Objective The objective of the research described in this paper is to develop such methods. Method The paper outlines a novel approach combining quasi-static monotonic tests in tension and compression, quasi-static cyclic tests in tension, and high strain rate tests in compression, with dynamic mechanical analysis and time-temperature superposition. A generalized viscoelastic model incorporating continuum damage is calibrated. Results The results show that a model calibrated using data from quasi-static compression and dynamic mechanical analysis can be used to adequately predict the compressive high strain rate response: hence, this paper provides an important step in the development of a methodology that avoids the requirement to obtain constitutive data from high strain rate experiments. In addition, data from FE models of the dynamic mechanical analysis experiments are provided, along with a discussion of data obtained from tensile and cyclic loading. Conclusions The paper demonstrates the effectiveness of ‘indirect’ predictive methods to obtain information about high rate behaviour of low modulus materials. </jats:sec

Improved materials characterisation through the application of geometry reconstruction to quasi-static and high-strain-rate tension tests

This paper presents novel materials characterisation techniques for expanding the range of data obtained from tensile tests at quasi-static and high strain rates. Through the application of photography and a geometry reconstruction technique, we obtain data for Steel and Zirconium, with an emphasis on the new opportunities afforded by these techniques. The paper extends the state of the art in tensile characterisation, improving the range of data that can be obtained, and is supported by a number of validation measurements. In particular, calculations of cross-sectional area, shape and ellipticity are presented. These calculations can be performed as functions of both time and axial position. Therefore, it is possible to calculate mean true stress-strain relationships in the material, without the corrections that are required when such relationships are calculated simply using load and displacement data from the ends of the specimen. Steel and Zirconium were selected for their distinct degrees of anisotropy, giving a robust assessment of the capabilities of the techniques. In the future, such measurements will allow researchers to more closely measure, understand, and model, mechanical properties of materials over a wide range of strain rates. © 2012 Elsevier Ltd. All rights reserved

Dynamic behaviour of silks: Nature’s precision nanocomposites

Silk is often cited as a material worth imitating, due to its high strength and toughness. In order to produce a synthetic analogue, or enhanced natural version, the microstructural basis of these properties must be understood. Current understanding is that silk deforms through the detachment of nano-scale crystallites, in the manner of a damaged composite. This picture forms the basis for constitutive models, but validation data is limited to low strain-rates. Here we present a programme of research in which high-rate behaviour is studied through ballistic impact experiments. These have been applied to the silk of the Bombyx mori moth, as harvested from cocoons, and to the major ampullate thread of the golden orb weaver spider Nephila edulis. Longitudinal wave-speeds, and air drag coefficients, have been calculated for selected cases. Differences between the response of various silks and a similar synthetic fibre, nylon, are discussed, and future plans are presented