Theo A. Tervoort

Last Name

Tervoort

First Name

Theo A.

ORCID

0000-0002-4962-9841

Organisational unit

09482 - Vermant, Jan / Vermant, Jan

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Publications 1 - 10 of 71

Chemical Aspects of Direct Coagulation Casting of Alumina Suspensions
Tervoort, Elena; Tervoort, Theo A.; Gauckler, Ludwig J. (2004)
Journal of the American Ceramic Society
Fatigue-Crack Propagation of High-Density Polyethylene Homopolymers: Influence of Molecular Weight Distribution and Temperature
Cerpentier, Robin R.J.; van Vliet, Tim; Pastukhov, Leonid V.; et al. (2021)
Macromolecules
The present study focuses on the influence of the molecular weight distribution (MWD) on the crack-growth kinetics of fatigue-crack propagation in high-density polyethylene (HDPE) homopolymers. Compact-tension specimens of HDPE homopolymer grades, with polydispersities ranging from 2 to 45 and weight-averaged molar mass ranging from 49 to 450 kg/mol, are tested in cyclic loading at temperatures ranging between 23 and 92 °C. Through a variation of sample thickness, linear elastic-fracture mechanics is shown to apply for the chosen geometry (compact tension). It was found that the crack-propagation kinetics obey the Paris–Erdogan law, for which the Paris–Erdogan exponent m is (highly) similar for all grades tested (m = 3.9), implying that the Paris–Erdogan prefactor A is the governing parameter for the crack-growth kinetics. Relatively poor correlations are observed when the prefactor A is plotted as a function of both the tie-molecule fraction derived from the theoretical model by Huang and Brown, J. Mater. Sci.1988,23, 3648, and the average number of effective physical cross-links per chain as derived by Tervoort et al., Macromolecules2002,35, 8467. A far better correlation is observed between prefactor A and the weight-average molecular weight (Mw), which improved further when Mw is corrected for the width of the MWD, taking into account the z-average molecular weight Mz, through the ratio Mz/Mw. A power-law correlation of prefactor A with Mw and the width-corrected Mw reveals slopes of −3.4 and −3.3, respectively. Because a molecular slip within the fibrils would require chain transport through the crystalline blocks, the temperature dependence of the fatigue-crack-growth kinetics is investigated to identify the underlying molecular processes. This investigation reveals the existence of a high-temperature and a low-temperature deformation process, both of which can be related to chain-slip mechanisms through their respective activation energies (125 and 50 kJ/mole), as their activation energies are considerably lower than that required for chain scission (430 kJ/mol). This, combined with the power-law exponent of −3.4, would suggest a possible connection between the underlying failure mechanisms of craze fibrils and reptation-like dynamics. Furthermore, experiments at elevated temperatures on a selection of homopolymer grades suggest that the MWD has no influence on the temperature dependence of fatigue-crack propagation for HDPE homopolymers.
Elasticity, Plasticity and Fracture Toughness at Ambient and Cryogenic Temperatures of Epoxy Systems used for the Impregnation of High-Field Superconducting Magnets
Brem, André; Gold, Barbara J.; Auchmann, Bernhard; et al. (2021)
Cryogenics
This study evaluates the thermal expansion coefficient, as well as the elastic-, plastic- and fracture behaviour, at ambient and cryogenic (liquid nitrogen) temperatures, of four different epoxy systems that are used in high-field superconducting magnets, with the emphasis on rate-dependent plasticity as measured by uniaxial compression testing. As expected, both the elastic and plastic behaviour of the epoxy systems at room temperature depend strongly on their distance to the glass transition temperature, but become similar at cryogenic temperature. The rate dependency of the yield stress at room temperature of the four epoxy systems was similar and is well described by the Eyring model. At cryogenic temperatures the rate dependency disappears and the yield stress of all four epoxy systems approach a similar athermal value. The fracture toughness remained equal or even increased upon cooling to cryogenic temperatures for each of the four epoxies. However, the fracture toughness values of the four epoxies tested were quite different from each other, suggesting that fracture toughness not only depends on the van der Waals interactions between the segments but is also determined by other molecular characteristics, such as the network structure.
No yield stress required: Stress-activated flow in simple yield-stress fluids
Pagani, Gabriele; Hofmann, Martin; Govaert, Leon E.; et al. (2024)
Journal of Rheology
An elastoviscoplastic constitutive equation is proposed to describe both the elastic and rate-dependent plastic deformation behavior of Carbopol® dispersions, commonly used to study yield-stress fluids. The model, a variant of the nonlinear Maxwell model with stress-dependent relaxation time, eliminates the need for a separate Herschel-Bulkley yield stress. The stress dependence of the viscosity was determined experimentally by evaluating the steady-state flow stress at a constant applied shear rate and by measuring the steady-state creep rate at constant applied shear stress. Experimentally, the viscosity’s stress-dependence was confirmed to follow the Ree-Eyring model. Furthermore, it is shown that the Carbopol® dispersions used here obey time-stress superposition, indicating that all relaxation times experience the same stress dependence. This was demonstrated by building a compliance mastercurve using horizontal shifting on a logarithmic time axis of creep curves measured at different stress levels and by constructing mastercurves of the storage- and loss-modulus curves determined independently by orthogonal superposition measurements at different applied constant shear stresses. Overall, the key feature of the proposed constitutive equation is its incorporation of a nonlinear stress-activated change in relaxation time, which enables a smooth transition from elastic to viscous behavior during start-up flow experiments. This approach bypasses the need for a distinct Herschel-Bulkley yield stress as a separate material characteristic. Additionally, the model successfully replicates the observed steady-state flow stress in transient-flow scenarios and the steady-state flow rate in creep experiments, underlining its effectiveness in capturing the material’s dynamic response. Finally, the one-dimensional description is readily extended to a full three-dimensional finite-strain elastoviscoplastic constitutive equation.
Modeling energy storage and structural evolution during finite viscoplastic deformation of glassy polymers
Xiao, Rui; Ghazaryan, Gagik; Tervoort, Theo A.; et al. (2017)
Physical Review E
Finite anisotropic elasticity and material frame indifference from a nonequilibrium thermodynamics perspective
Hütter, Markus; Tervoort, Theo A. (2008)
Journal of Non-Newtonian Fluid Mechanics
A closed set of Eulerian evolution equations for nonisothermal finite isotropic and anisotropic elastic behavior is derived using nonequilibrium thermodynamics. In particular, it is shown that to describe the state of elastic deformation, the deformation gradient F is preferred as internal variable to the more well-known left and right Cauchy-Green strain tensors. With the energy and entropy functions being frame invariant, the stress tensor expression as obtained in terms of F satisfies the principle of material frame indifference. As an alternative to the F-formulation, it is also shown how elasticity can be described in terms of the mass density and the isochoric deformation gradient, over(F, ̃) = F / root(det F, 3), instead.
Melting-Induced Evolution of Morphology, Entanglement Density, and Ultradrawability of Solution-Crystallized Ultrahigh-Molecular-Weight Polyethylene
Christakopoulos, Fotis; Bersenev, Egor; Grigorian, Souren; et al. (2021)
Macromolecules
The melting-induced change in the density of physical network junctions, which are formed by chain entanglements and network junctions due to anchoring of chain segments to crystals, is studied by H-1 NMR T-2 relaxometry for solution- and melt-crystallized ultrahigh-molecular-weight poly- ethylene (UHMWPE), sc-UH, and mc-UH, respectively. The NMR results are complemented by real-time synchrotron wide- and small-angle X-ray scattering (WAXS and SAXS) analyses to extract the sizes of the crystalline lamellae and intercrystalline domains. Below the melting temperature, the network of physical junctions is denser in the amorphous phase of mc-UH than the one in sc-UH owing to a lower entanglement density and a smaller number of physical junctions from polymer crystals in sc-UH. However, the difference in the total density of physical junctions between mc-UH and sc-UH films decreases with decreasing crystallinity during melting. At the end of the melting trajectory, at vanishing crystallinity, the volume-average entanglement density, as characterized by the NMR method, is approximately the same in sc- and mc-UH. This indicates that the entanglement density in sc-UH films increases during melting owing to the fast buildup of local chain entanglements. These entanglements are formed by segments of the same chain, neighboring chains, or both due to a displacement of chain fragments upon lamellar thickening and due to the so-called "chain explosion" that occurs locally in the amorphous domains. The increase in the entanglement density in sc-UH is additionally confirmed by the solid-state drawability of sc-UH films that were annealed in the melting region but below the end of melting. The maximum draw ratio decreases and the drawing stress increases with the increasing annealing temperature. © 2021 American Chemical Society
Non-linear, rate-dependent strain-hardening behavior of polymer glasses
Wendlandt, Michael; Tervoort, Theo A.; Suter, Ulrich W. (2005)
Polymer
Collagen nerve conduits releasing the neurotrophic factors GDNF and NGF
Madduri, Srinivas; Feldman, Kirill; Tervoort, Theo A.; et al. (2010)
Journal of Controlled Release
Additive Manufacturing of Polyolefins
Christakopoulos, Fotios; van Heugten, Paul M.H.; Tervoort, Theo A. (2022)
Polymers
Polyolefins are semi-crystalline thermoplastic polymers known for their good mechanical properties, low production cost, and chemical resistance. They are amongst the most commonly used plastics, and many polyolefin grades are regarded as engineering polymers. The two main additive manufacturing techniques that can be used to fabricate 3D-printed parts are fused filament fabrication and selective laser sintering. Polyolefins, like polypropylene and polyethylene, can, in principle, be processed with both these techniques. However, the semi-crystalline nature of polyolefins adds complexity to the use of additive manufacturing methods compared to amorphous polymers. First, the crystallization process results in severe shrinkage upon cooling, while the processing temperature and cooling rate affect the mechanical properties and mesoscopic structure of the fabricated parts. In addition, for ultra-high-molecular weight polyolefins, limited chain diffusion is a major obstacle to achieving proper adhesion between adjunct layers. Finally, polyolefins are typically apolar polymers, which reduces the adhesion of the 3D-printed part to the substrate. Notwithstanding these difficulties, it is clear that the successful processing of polyolefins via additive manufacturing techniques would enable the fabrication of high-end engineering products with enormous design flexibility. In addition, additive manufacturing could be utilized for the increased recycling of plastics. This manuscript reviews the work that has been conducted in developing experimental protocols for the additive manufacturing of polyolefins, presenting a comparison between the different approaches with a focus on the use of polyethylene and polypropylene grades. This review is concluded with an outlook for future research to overcome the current challenges that impede the addition of polyolefins to the standard palette of materials processed through additive manufacturing.

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Theo A. Tervoort

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