Micromechanical experiments

Micromechanical experiments made so far can be roughly divided into two parts (i) design of special techniques to measure and evaluate separately different contributions in the net force, such as adhesion, friction, deformation, and (ii) imaging of various heterogeneous surfaces such as blends, composites and microphase separated structures by conventional SFM s to collect statistical information and understand the origin of the mechanical contrast. Many of the micromechanical experiments were discussed elsewhere [58, 67, 68, 381, 412-414]. Here we will focus on recent advances in analytical applications of the active probe SFM. 

The micromechanical experiments in a SEM, characterize the damage evolution on preselected areas of film on substrate systems. 

The large resistance of the membrane to area dilation has been characterized in micromechanical experiments. The changes in surface area that can be produced in the membrane are small, and so they can be characterized in terms of a simple Hookean elastic relationship between the isotropic force resultant N and the fractional change in surface area a = A/Ao — 1 ... 

The shock-induced micromechanical response of <100>-loaded single crystal copper is investigated [18] for values of (WohL) from 0 to 10. The latter value results in W 10 Wg at y = 0.01. No distinction is made between total and mobile dislocation densities. These calculations show that rapid dislocation multiplication behind the elastic shock front results in a decrease in longitudinal stress, which is communicated to the shock front by nonlinear elastic effects [pc,/po > V, (7.20)]. While this is an important result, later recovery experiments by Vorthman and Duvall [19] show that shock compression does not result in a significant increase in residual dislocation density in LiF. Hence, the micromechanical interpretation of precursor decay provided by Herrmann et al. [18] remains unresolved with existing recovery experiments. 

An important aspect of micromechanical evolution under conditions of shock-wave compression is the influence of shock-wave amplitude and pulse duration on residual strength. These effects are usually determined by shock-recovery experiments, a subject treated elsewhere in this book. Nevertheless, there are aspects of this subject that fit naturally into concepts associated with micromechanical constitutive behavior as discussed in this chapter. A brief discussion of shock-amplitude and pulse-duration hardening is presented here. 

Underlying all continuum and mesoscale descriptions of shock-wave compression of solids is the microscale. Physical processes on the microscale control observed dynamic material behavior in subtle ways sometimes in ways that do not fit nicely with simple preconceived macroscale ideas. The repeated cycle of experiment and theory slowly reveals the micromechanical nature of the shock-compression process. 

Our reason for stressing the concept of representative volume element is that it seems to provide a valuable dividing boundary between continuum theories and molecular or microscopic theories. For scales larger than the RVE we can use continuum mechanics (classical and large strain elasticity, linear and non-linear viscoelasticity) and derive from experiment useful and reproducible properties of the material as a whole and of the RVE in particular. Below the scale of the RVE we must consider the micromechanics if we can - which may still be analysable by continuum theories but which eventually must be studied by the consideration of the forces and displacements of polymer chains and their interactions. 

In order to calculate, for a considered polymer under defined experimental conditions (constant strain rate or temperature), the occurrence of the micromechanism transition, one has to plot the temperature or strain rate dependence of the various critical stresses. Hereafter, we will assume that experiments are performed as a function of temperature at constant strain rate. 

E. E. Glickman and V. I. Igoshev, Micromechanism of Solid Metal Induced Embrittlement Comparison of the Model with the Experiment, Surface Physics, Chemistry, Mechanics, No. 128-136 (1989) (in Russian). 

H. D. Epinosa and R. J. Clifton, Place Impact Experiments for Investigating Inelastic Deformation and Damage of Advanced Materials, in Experiments in Micromechanics of Failure Resistant Materials, AMD Vol. 130, ed. K.-S. Kim, ASME, New York, 1991, pp. 37-56. 

As usual with optical interferometry, craze length, craze thickness, crack velocity, and fracture toughness are obtained from the experiment, and local material properties are obtained from the preceding results and the use of some models of crack-tip craze micromechanics. 

Treating cells with CD to disassemble their actin cytoskeleton has been described many times in the literature. When cells were studied by scanning force microscopy (SFM) after CD exposure, a significant reduction of membrane stiffness was reported for various cell types [41,42]. Since the acoustic impedance also decreases, it seems reasonable to propose that the QCM may serve as a micromechanical probe to study membrane stiffness. Further experiments will be presented below that support this point of view. 

Recent experiments by Dr. Bar-Cohen el al. ha e shown that ultrasonic oblique insonification can be used to characterize thermal damage to composites [156]. Using an inversion technique based on a micromechanical model, the reflected ultrasonic signals arc analyzed to determine the overall laminate stiffness constant before and after loading. Another technique developed by the NASA to encompass the limitation of pulse-echo ultrasonic and photomicroscopic methods is diffuse-field acoustoultrasonic coupled vibration damping [157]. Both NASA techniques are complementary and arc used to assess microstructural damage accumulation in ceramic matrix composites. 

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