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The stress-strain behavior of microcracked polycrystalline materials (such as ceramics or rocks) underconditions of tensile, displacement-controlled, loading is discussed. Micromechanical explanation andmodeling of the basic features, such as non-linearity and hysteresis in stress-strain curves, is developed,with stable microcrack propagation and “roughness” of intergranular cracks playing critical roles. Ex-periments involving complex loading histories were done on large- and medium grain sizeb-eucryptiteceramic. The model is shown to reproduce the basic features of the observed stress-strain curves.
Microstructure-property connections for porous ceramics: The possibilities offered by micromechanics
(2016)
Microstructure of porous ceramics is highly “irregular”: it comprises pores and microcracks of diverse shapes and orientations.
This makes their quantitative modeling challenging, and one
often resorts to empirical relations containing Fitting Parameters and having somewhat uncertain range of applicability. We review the substantial progress made in modeling of “irregular” microstructures that does not seem to have been sufficiently utilized in the context of ceramics. We discuss the possibilities offered by micromechanics in developing microstructure–property relations for porous microcracked ceramics. After an overview of relevant micromechanics topics, we focus on several issues of specific interest for ceramics: nonlinear stress–strain behavior, effective elastic properties, and thermally induced microcracking. We discuss extraction of microscale Parameters (such as strength of the intergranular cohesion, density of cracks and pores, etc.) from macroscopic data and identify the extent of uncertainty in this process. We also argue that there is no quantitative correlation between fracturing process and the loss of elastic stiffness.
A specially designed scanning force microscope (SFM, atomic force microscope – AFM) was incorporated into the chamber of a commercial scanning electron microscope (SEM) to investigate vibrating SFM cantilevers at their resonance. Inherently, the spatial resolution of electron microscopy is higher than that of optical methods. In this paper we present vibration modes (eigenmodes) of two different SFM cantilevers. Their nonlinear behavior is also explored in order to depict their 2nd harmonics (twice the fundamental frequency). Imaging of the local vibration is performed by measuring the frequency- and phase-selective responses of the SE signal at different X and Y positions of the scanned electron beam.
The ultrasonic transmission between two air-coupled polypropylene (PP) ferroelectret (FE) transducers in dependence on the amplitude of the high-voltage exciting pulse revealed a strongly nonlinear electromechanical response of the FE transmitter. This phenomenon is described by a linear increase of the inverse electromechanical transducer constant t33(1) of the PP FE film with an increase of the exciting electrical pulse amplitude. Enlargement of t33(1) by a factor of 4 was achieved by application of 3500 V exciting pulses. The electrostriction contribution to t33(1) can be attributed to the electrostatic force between electrodes and the Maxwell stress effect. The nonlinear electromechanical properties of the PP FE result in a strong increase of its air-coupled ultrasonic (ACUS) figure of merit (FOM) under the high-voltage excitation, which exceeds results of the PP FE technological optimization. The FOM increase can be related to the increase of PP FE coupling factor and/or to the decrease of its acoustic impedance. A significant enhancement of the ACUS system transmission (12 dB) and signal-to-noise ratio (32 dB) was demonstrated by the increase of excitation voltage up to 3500 V. The nonlinear electromechanical properties of the PP FEs seem to be very important for their future applications.