Deformation micromechanics studies

Basic mechanical behaviours, such as plastic deformation, deformation micromechanisms, and fracture, are successively presented. The characteristics of the studied polymers are gathered in Table 7. 

As we have done so often before, we look to Ashby for a revealing systemization of the data around which the micromechanical study of fracture is built. In fig. 11.20 a summary of the various fracture mechanisms is presented. One idea that emerges is that often at low temperatures, the sample undergoes virtually no plastic deformation before fracture, while at high temperatures, the material is seen to deform enormously. In addition, examination of the fracture surfaces reveals that different mechanisms are at play in the different results. 

Details of the deformation micromechanisms of polymers can be investigated using different techniques. One of the most common approaches is to prepare specimens for TEM studies from the deformed sample taken from locations close to the fracture surface using an ultramicrotome, and this can be followed by the usual chemical and physical treatments. The thin films may also be stretched... 

Micromechanical theories of deformation must be based on physical evidence of shock-induced deformation mechanisms. One of the chapters in this book deals with the difficult problem of recovering specimens from shocked materials to perform material properties studies. At present, shock-recovery methods provide the only proven teclfniques for post-shock examination of deformation mechanisms. The recovery techniques are yielding important information about microscopic deformations that occur on the short time scales (typically 10 -10 s) of the compression process. 

Nevertheless, as response data have accumulated and the nature of the porous deformation problems has crystallized, it has become apparent that the study of such solids has forced overt attention to issues such as lack of thermodynamic equilibrium, heterogeneous deformation, anisotrophic deformation, and inhomogeneous composition—all processes that are present in micromechanical effects in solid density samples but are submerged due to continuum approaches to mechanical deformation models. 

Along this line, the analysis of the plastic deformation, the micromechanisms of deformation, and the fracture behaviour of the series of aryl-aliphatic copolyamides studied in [1] (Sect. 6) is quite suitable. 

To study the influence of these structural parameters on the micromechanical mechanisms of toughening, several techniques of electron microscopy were used. Electron microscopic techniques allow investigations not only of detailed morphology but also of the micromechanical processes of deformation and fracture (3, 9-11). 

Figure 3. Electron microscopic techniques used to study micromechanical processes in polymers (a) investigation of fracture surfaces by SEM (b) investigation by TEM of ultrathin sections prepared from deformed and selectively stained bulk material and (c) deformation of samples of different thicknesses (bulk, semithin, and ultrathin)9 using special tensile stages with SEM, HVEM, and TEM. The technique in (c) shows the possibility of conducting in situ deformation tests in the electron microscope.

Selected RPs as a particular structural component, micromechanics analyses, together with classic mechanics theory, should provide a means for predicting optimum fiber orientations and material thicknesses for specific load conditions. In addition to the analytical, predictive type of micromechanics research, there is also a significant amount of experimental micromechanics research that has been done, i.e., determination of stress concentration at fiber ends and crossovers, investigations of deformation and firacture modes, and crack propagation studies. Such work helps the analyst in establishing realistic assumptions of material behavior and in comparing observed mechanical behavior with predicted behavior. 

As the performance of the composite is profoxmdly dominated by the micromechanical deformation process, its knowledge and control are critical for the improvement of composite properties. The effect of particle characteristics and interfacial adhesion on the micromechanical deformation processes in PP-wood composites was investigated by Renner et al. [7]. They proposed a failure map as well as the practical results and considered the influence of matrix characteristics on deformation and failure in PP-natural fiber composites in other research [24]. Hietala et al. [78] studied the effect of chemical pre-treatment and moisture content of wood chips on the wood particle aspect ratio during the processing and mechanical properties of WPCs. The use of pretreated wood chips enhanced the flexural properties of the wood chip-PP composites. Moreover, the use of undried wood chips compared to dried one can improve and reduce the flexural strength and flexural modulus, respectively. On the other hand, they concluded that the use of pretreated and undried wood chips lead to the highest aspect ratio after compounding. The effect of composition and the incorporation... 

It is clear that this present study introduces a new method of following the micromechanics of fibre-reinforced composites. The ability to measure the strain at a point in an individual fibre should lead to a significant increase in our understanding of the deformation of these important materials and of fibre/matrix interactions. 

Stribeck N (2009) Deformation behavior of nanocomposites studied by X-ray scattering Instrumentation and methodology, in Nano-and Micromechanics of Polymer Blends and Com-posites (Eds. Karger-Kocsis J and Fakirov S), Hanser Publisher, Mtinchen, Germany, Vol. 1, Gh. 8, pp. 269-300. 

Attempts have been made to manufacture particles on the nanometer scale for applications such as controlled release and intravenous delivery systems. A comparison evaluating the processability and solid dosage performance of spray-dried nanoparticles and microparticles was conducted (41). In this study, nanoparticle suspensions were prepared by wet comminution in the presence of stabilizers, converted into dried particles using a spray-drying process and subsequently compressed. Compacts prepared from microparticles and nanoparticles were found to differ in their internal structure and micromechanical deformations. 

A hybrid atomistie/eontinuum mechanics method is established in the Feng et al. [70] study the deformation and fracture behaviors of CNTs in composites. The unit eell eontaining a CNT embedded in a matrix is divided in three regions, whieh are simulated by the atomic-potential method, the continumn method based on the modified Cauchy-Bom rule, and the classical continuum mechanics, respectively. The effect of CNT interaction is taken into account via the Mori-Tanaka effective field method of micromechanics. This method not only can predict the formation of Stone-Wales (5-7-7-5) defects, but also simulate the subsequent deformation and fracture process of CNTs. It is found that the critical strain of defect nucleation in a CNT is sensitive to its chiral angle but not to its diameter. The critical strain of Stone-Wales defect formation of zigzag CNTs is nearly twice that of armchair CNTs. Due to the constraint effect of matrix, the CNTs embedded in a composite are easier to fracture in comparison with those not embedded. With the increase in the Young s modulus of the matrix, the critical breaking strain of CNTs decreases. 

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