Modelling large-strain deformation of thermo-rheologically complex materials : characterisation and validation of PMMA and iPP

In this study an attempt is made to describe the thermorheologically complex deformation behaviour of the glassy polymer PMMA and semi-crystalline polymer iPP, by using a constitutive modelling approach [1]. For both polymers, it is shown that this approach successfully captures the thermorheologically complex behaviour of PMMA and iPP. Moreover, the model is capable of predicting yield stress and creep lifetime of PMMA using only one parameter set

The influence of indenter-surface misalignment on the results of instrumented indentation tests

A quantitative study is presented on the influence of the sample misalignment on load-displacement curves measured in instrumented indentation. Three different tip geometries are considered: a Berkovich tip, a spherical tip and a flat-ended punch. A special alignment tool was developed that allowed us to perfectly align the sample surface perpendicular to the loading-axis of the tip, regardless of the tip geometry and the origin of the misalignment. Moreover, this tool enabled us to systematically vary the angle of misalignment and study its effect on the indentation results.It is shown that sample-misalignment angles smaller than 1.2¿ have no effect for the Berkovich and spherical tips, whereas flat-ended punch indentations are extremely sensitive to these small alignment errors. The strongest influence is observed in the linear elastic region, where the contact stiffness decreases markedly with increasing misalignment. In the plastic regime, the sensitivity to misalignment disappears.Finally, we present a simple method to correct the influence of sample misalignment on the load-displacement curves obtained in flat-ended punch indentation

Quantitative assessment and prediction of the contact area development during spherical tip indentation of glassy polymers.

This paper describes the development of the contact area during indentation of polycarbonate. The contact area was measured in situ using an instrumented indentation microscope and compared with numerical simulations using an elasto-plastic constitutive model. The parameters in the model were obtainedusing macroscopic tests. Indentations were performed on samples with different thermal histories and at different speeds. For all cases, the numerical modelcorrectly predicted the development of the contact area during indentation. For increasing strain rates, the contact area decreased at equal indentation depths. Annealing the samples resulted in a smaller contact area at equal indentation depth. Using only numerical simulation, it was also shown that pile-up around the indenter resulted from localization effects and was, thus, promoted by strainsofteningproperties of the indented material. Strain hardening, on the other hand, will tend to promote sink-in. Finally, we performed simulations of loadrelaxation during indentation. The results indicate that about 40% of the total observed relaxation may be assigned to plastic effects

Numerical simulation of flat-tip micro-indentation of glassy polymers: influence of loading speed and thermodynamic state

Flat-tip micro-indentation tests were performed on quenched and annealed polymer glasses at various loading speeds. The results were analyzed using an elasto-viscoplastic constitutive model that captures the intrinsic deformation characteristics of a polymer glass: a strain-rate dependent yield stress, strain softening and strain hardening. The advantage of this model is that changes in yield stress due to physical aging are captured in a single parameter. The two materials studied (polycarbonate (PC) and poly(methyl methacrylate) (PMMA)) were both selected for the specific rate-dependence of the yield stress that they display at room temperature. Within the range of strain rates experimentally covered, the yield stress of PC increases linearly with the logarithm of strain rate, whereas, for PMMA, a characteristic change in slope can be observed at higher strain rates. We demonstrate that, given the proper definition of the viscosity function, the flat-tip indentation response at different indentation speeds can be described accurately for both materials. Moreover, it is shown that the model captures the mechanical response on the microscopic scale (indentation) as well as on the macroscopic scale with the same parameter set. This offers promising possibilities of extracting mechanical properties of polymer glasses directly from indentation experiments

Numerical simulation of flat-tip micro-indentation of glassy polymers: influence of loading speed and thermodynamic state

Flat-tip micro-indentation tests were performed on quenched and annealed polymer glasses at various loading speeds. The results were analyzed using an elasto-viscoplastic constitutive model that captures the intrinsic deformation characteristics of a polymer glass: a strain-rate dependent yield stress, strain softening and strain hardening. The advantage of this model is that changes in yield stress due to physical aging are captured in a single parameter. The two materials studied (polycarbonate (PC) and poly(methyl methacrylate) (PMMA)) were both selected for the specific rate-dependence of the yield stress that they display at room temperature. Within the range of strain rates experimentally covered, the yield stress of PC increases linearly with the logarithm of strain rate, whereas, for PMMA, a characteristic change in slope can be observed at higher strain rates. We demonstrate that, given the proper definition of the viscosity function, the flat-tip indentation response at different indentation speeds can be described accurately for both materials. Moreover, it is shown that the model captures the mechanical response on the microscopic scale (indentation) as well as on the macroscopic scale with the same parameter set. This offers promising possibilities of extracting mechanical properties of polymer glasses directly from indentation experiments

Indentation: the experimenter's holy grail for small-scale polymer characterization?

Our recent work on extracting mechanical properties of polymers from indentation experiments shows that linear viscoelastic behavior, post-yield and crazing properties, and size effects can be characterized using a combined experimental-numerical approach. More research is needed in the field of full constitutive parameter extraction, thin film properties assessment, and size effects

Indentation: the experimenter's holy grail for small-scale polymer characterization?

Our recent work on extracting mechanical properties of polymers from indentation experiments shows that linear viscoelastic behavior, post-yield and crazing properties, and size effects can be characterized using a combined experimental-numerical approach. More research is needed in the field of full constitutive parameter extraction, thin film properties assessment, and size effects