Alberto D’Amore

On the viscoelastic Poisson's ratio in amorphous polymers

Poisson’s ratio is defined as the ratio of the lateral contraction to the elongation in the infinitesimal uniaxial extension of a homogeneous isotropic body. In a viscoelastic material, Poisson’s ratio is a function of time (or frequency). In this paper, the time-dependence of the Poisson’s ratio is analytically evaluated from the bulk and shear responses using the relations between the viscoelastic functions in the Laplace domain. It has been found that, in the region of α-relaxation, Poisson’s ratio may be a nonmonotonic function of time, with a weak minimum at short times, when the shear response is broader than bulk response such that the ratio τG/τK is much larger than 1, or a monotonically increasing function of time if the shear and bulk responses share similar timescales and relaxation time distributions. The latter case is verified using experimental data from the literature for a cross-linked polymer, whereas the former case is verified for two linear polymers.

The relative placement of linear viscoelastic functions in amorphous glassy polymers

A procedure is described and analyzed for the determination of the relative placement of linear viscoelastic functions and their relative relaxation∕retardation spectra for a commercial polycarbonate (LEXAN GE). The complete set of viscoelastic functions in creep and relaxation was obtained from two simple experimental data, namely the linear viscoelastic response in shear and the pressure volume temperature (PVT) behavior. The dimensionless bulk compliance was extracted from PVT data showing that it coincides with the memory function appearing in the Kovacs, Aklonis, Hutchinson, Ramos phenomenological theory [Kovacs, A. J., J. J. Aklonis, J. M. Hutchinson, and A. R. Ramos, J. Polym. Sci., Polym. Phys. Ed. 17, 1097 (1979)]. Our results are compared with the relevant literature data obtained on different polymers and show that polycarbonate fulfills simultaneously the responses features concerning the relative placement of the bulk and shear moduli [Kono, R. J., Phys. Soc. Jpn. 15, 4 (1960)], the shapes of...

Isobaric PVT Behavior of Poly(Carbonate) (PC)

The scaling law for relaxation times, τ(V,T) = ℑ(TVγ), recently proposed by Casalini and Roland, is utilized in the framework of KAHR (Kovacs Aklonis Hutchinson and Ramos) phenomenological theory. With this approach it is shown that the Pressure, Volume, Temperature (PVT) data obtained on Poly(carbonate)(PC) can be reliably predicted, in the region of the alpha relaxation, by using only two fitting parameters, namely: the relaxation time in the reference condition, τg, and the fractional exponent, β that describes the dispersion of the alpha relaxation.

Finite element calculation of residual stress in dental restorative material

A finite element methodology for residual stresses calculation in dental restorative materials is proposed. The material under concern is a multifunctional methacrylate-based composite for dental restorations, activated by visible light. Reaction kinetics, curing shrinkage, and viscoelastic relaxation functions were required as input data on a structural finite element solver. Post cure effects were considered in order to quantify the residual stresses coming out from natural contraction with respect to those debited to the chemical shrinkage. The analysis showed for a given test case that residual stresses frozen in the dental restoration at uniform temperature of 37°C are of the same order of magnitude of the strength of the dental composite material per se.

Modeling the Residual Stresses in Reactive Resins‐Based Materials: a Case Study of Photo‐Sensitive Composites for Dental Applications

Residual stresses in reactive resins‐based composites are associated to the net volumetric contraction (shrinkage) arising during the cross‐linking reactions. Depending on the restoration geometry (the ratio of the free surface area to the volume of the cavity) the frozen‐in stresses can be as high as the strength of the dental composites. This is the main reason why the effectiveness and then the durability of restorations with composites remains quite lower than those realized with metal alloys based materials. In this paper we first explore the possibility to circumvent the mathematical complexity arising from the determination of residual stresses in reactive systems three‐dimensionally constrained. Then, the results of our modeling approach are applied to a series of commercially available composites showing that almost all samples develop residual stresses such that the restoration undergoes failure as soon as it is realized.

Thermal residual stresses in amorphous thermoplastic polymers

An attempt to calculate the internal stresses in a cylindrically shaped polycarbonate (LEXAN‐GE) component, subjected to an arbitrary cooling rate, will be described. The differential volume relaxation arising as a result of the different thermal history suffered by each body point was considered as the primary source of stresses build up [1–3]. A numerical routine was developed accounting for the simultaneous stress and structural relaxation processes and implemented within an Ansys® environment. The volume relaxation kinetics was modeled by coupling the KAHR (Kovacs, Aklonis, Hutchinson, Ramos) phenomenological theory [4] with the linear viscoelastic theory [5–7]. The numerical algorithm translates the specific volume theoretical predictions at each body point as applied non‐mechanical loads acting on the component. The viscoelastic functions were obtained from two simple experimental data, namely the linear viscoelastic response in shear and the PVT (pressure volume temperature) behavior. The dimension...

Rheology and mechanics of polyether(ether)ketone – Polyetherimide blends for composites in aeronautics

In the present work rheological and mechanical properties of PEEK-PEI blends were investigated. Besides the pure components, blends with PEI concentration ranging from 10% to 90% in mass were considered. Oscillatory experiments in controlled atmosphere were conducted at different frequencies and temperatures. The frequency responses at different temperatures allowed using the TTS principle to reconstruct the master curves. All systems showed a shear thinning behavior and a flux index increasing with the percentage of PEI. The zero-shear viscosity was computed with the implementation of the Cross model and showed a decreasing behavior with the percentage of PEI. The relaxation time estimated from the crossover value of storage and loss moduli didn’t change significantly with blend composition, suggesting the non-sensibility of the elasticity of the system. Lastly, tensile tests were executed to investigate the dependence of Young modulus in the different blends.

Toughening the Macro Defect Free (MDF) cements

High performance polymer based‐cements (i.e. cements having a water to cement ratio, w/a, close to 0.1) almost totally free of defect, have been developed in recent years claiming a great interest for their very high tensile strength compared with traditional cements. These materials can be produced utilizing very simple unit operations and further formed in complex shapes with high surface quality. Indeed these materials have the disadvantage of being too much brittle and for this main reason they are rarely utilized in practice. Toughening the Macro Defect Free Cements (MDF) implies the strict control of the structure. The structure is governed by the process‐induced paste modifications. In this paper we show that toughening can be achieved with elastomeric particles having well defined size distribution and physico‐chemical characteristics in relation with both the binder polymer and the cement chemistry.

Self-learning health monitoring algorithm in composite structures

The paper describes a system that it is able of monitoring the health state of a composite structure in real time. The hardware of the system consists of a wire of strain sensors connected to a control unit. The software of the system elaborates the strain data and in real time is able to detect the presence of an eventual damage of the structures monitored with the strain sensors. The algorithm requires as input only the strains of the monitored structured measured on real time, i.e. those strains coming from the deformations of the composite structure due to the working loads. The health monitoring system does not require any additional device to interrogate the structure as often used in the literature, instead it is based on a self-learning procedure. The strain data acquired when the structure is healthy are used to set up the correlations between the strain in different positions of structure by means of neural network. Once the correlations between the strains in different position have been set up...

Timescales and properties of PSA (pressure sensitive adhesives)

The adhesive performances of a PSA are usually defined by three properties: tack, shear resistance, and peel strength. All these properties can qualitatively be correlated to the linear viscoelastic spectrum of PSA. In many cases, selected frequency intervals can be isolated within the linear viscoelastic spectrum, representing “windows” associated to the in-service performances of the adhesive Indeed, the “fine tuning” of linear viscoelastic properties was considered by PSA manufacturers of being a tool to tailor the individual products in order to meet the user’s requirements in different applications, a case that in not reflected in our work.