Seyed Hamid Reza Sanei

On the origin of indentation size effects and depth dependent mechanical properties of elastic polymers

Abstract Indentation size effects have been observed in both polymers and metals but, unlike in metals, the origin of size effects in polymers is not well understood. To clarify the role of second order gradients of displacements, a model polymer is examined with spherical and Berkovich tips at probing depths between 5 and 25 μm. Applying different theories to determine the elastic modulus, it is found that with a pyramidal tip, the elastic modulus increases with decreasing indentation depth, while tests with the spherical tip yielded essentially constant values for the elastic modulus independent of indentation depth. The differences between these tips are attributed to second order displacement gradients, as they remain essentially constant with a spherical tip while they increase in magnitude with decreasing indentation depth applying a Berkovich tip.

Characterization, synthetic generation, and statistical equivalence of composite microstructures

Mechanical behavior and reliability of composites are driven significantly by microstructural variability. Such variability can be present in the form of both morphological and constituent property variability. To understand the effect of this variability on macroscopic mechanical behavior, many statistically equivalent microstructures must be evaluated. This requires the ability to generate such microstructures. In this work, morphological variability was quantified by image analysis of actual microstructures. To reproduce this variability, a methodology was developed in which random microstructures are generated and subsequently adjusted to simultaneously match both short- and long-range statistics of actual microstructures. Synthetic microstructures were generated at a length scale of 70 µm, corresponding to the length scale at which fiber volume fractions of adjacent microstructures are uncorrelated. The utility of this methodology was also demonstrated for larger microstructures containing defects such as alignment fibers, voids and resin seams.

Generation of Synthetic FRP Microstructures Based on Experimentally Observed Microstructures

One of the key defects in composite materials is the large variability in mechanical properties. To capture the variability of strength in FRPs, random microstructures have to be analyzed. Developing a realistic model for generation of random microstructures required first imaging a carbon reinforced epoxy and then quantifying prominent microstructural features. Microstructures were synthetically generated including experimentally observed microstructural features such as elliptical fibers, alignment fibers, voids, and resin seams. Material periodicity of microstructures was considered to facilitate the application of displacement periodic boundary condition for later finite element analysis.

Multiscale Stochastic Analysis of FRP based on variability in fiber volume fraction, epoxy stiffness and strength

For accurate prediction of composite failure, microstructural variability must be considered. The distribution of epoxy stiffness and hardness were determined by nanoindentation and used in stochastic finite element modeling. Another key microstructural feature, fiber volume fraction variability, was determined by image processing of an SEM image. Stochastic failure analysis was implemented on a micromechanics model of hexagonal fiber packing to predict the initiation of failure under multiaxial loadings. Three failure criteria were employed for characterization of failure: maximum stress, von Mises, and Christensen. Failure envelopes were developed for stochastic and average models. The results revealed that the variability in epoxy strength influence the failure behavior significantly, whereas, stiffness variability has minimal effect.

Length Scale Dependent Deformation in Polymers

Length scale dependent deformation of polymers has been observed in different experiments including micro-beam bending and indentation tests. Here the length scale dependent deformation of polydimethylsiloxane is examined in indentation testing at length scales from microns down to hundreds of nanometers. Strong indentation size effects have been observed in these experiments which are rationalized with rotation gradients that can be related to Frank elasticity type molecular energies known from liquid crystal polymers. To support this notion additional experiments have been conducted where Berkovich and spherical indenter tips results have been compared with each other.© 2012 ASME

INDENTER TIP DEPENDENCE IN THE DETERMINATION OF ELASTIC MODULUS IN POLYMERS

The two most common outputs of nanoindentation experiment are hardness and elastic modulus. Length scale dependent deformation in polymers has however been observed in different experiments such as microbeam bending, torsional thin wires and indentation testing which may affect the mechanical testing. Unlike in metals where the size dependency is attributed to necessary geometry dislocations, the origin of length scale dependent deformation in polymers is not well understood. In this study, elastic modulus of polydimethylsiloxane (PDMS) is determined using both Berkovich and spherical tips. Observing different trends for elastic modulus upon the change of indentation depth using these two different tips brings up the question which tip should be used to get the real mechanical properties of PDMS which is discussed here. Surface roughness, surface effects and the imperfection of the Berkovich indenter tip are negligible at the studied length scale.Copyright

ON THE TIME DEPENDENCE OF SIZE EFFECT IN POLYDIMETHYLSILOXANE

Nanoindentation tests at the nano-micrometer scales are conducted to investigate the depth and time dependent deformation mechanisms of polydimethylsiloxane (PDMS). Astonishing indentation size effects observed in these experiments are analyzed with an existing theoretical hardness model, and the effects of loading time on the hardness and indentation stiffness of PDMS are studied. The change in the indentation recovery with respect to indentation depth and loading time are analyzed. Furthermore, it is shown that the stiffness of PDMS obtained at the maximum applied force can be efficiently applied to validate the applied theoretical hardness model with the experimental results.Copyright