Fracture micromechanism, material

The single edge specimens were loaded under three-point bending at cyclic frequency of 10 Hz, stress ratio of 0.1. The FCGR tests were carried out in the laboratory air at the temperature of 20 °C and 700 °C. Several control tests of the composites were performed at the temperature of 800 °C. Optical and scanning electronic microscopy was used for analysis of microstructure and fracture micromechanisms of the materials. 

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

Other researchers have substantially advanced the state of the art of fracture mechanics applied to composite materials. Tetelman [6-15] and Corten [6-16] discuss fracture mechanics from the point of view of micromechanics. Sih and Chen [6-17] treat the mixed-mode fracture problem for noncollinear crack propagation. Waddoups, Eisenmann, and Kaminski [6-18] and Konish, Swedlow, and Cruse [6-19] extend the concepts of fracture mechanics to laminates. Impact resistance of unidirectional composites is discussed by Chamis, Hanson, and Serafini [6-20]. They use strain energy and fracture strength concepts along with micromechanics to assess impact resistance in longitudinal, transverse, and shear modes. 

Herbert T. Corten, Micromechanics and Fracture Behavior of Composites, in Modem Composite Materials, Lawrence J. Broutman and Richard H. Krock (Editors), Addison-Wesley, New York, 1967, pp. 27-105. 

Wells J.K. and Beaumont P.W.R. (1982). Construction and use of toughness maps in a fracture analysis of the micromechanisms of composite failure. In Composite Materials Testing and Design. ASTM STP 787 (I.M. Daniel ed.), ASTM, Philadelphia, PA, pp, 147-162. 

Thermosetting epoxy polymers are widely employed in structural engineering applications and thus a knowledge of the mechanics and mechanisms of the fracture of such materials is of vital importance. The present Chapter discusses the fracture of epoxy polymers, concentrating on the use of a continuum fracture mechanics approach for elucidating the micromechanisms of crack growth and identifying pertinent failure criteria. 

A. Saxena, Mechanics and Mechanisms of Creep Crack Growth, in Fracture Mechanics Microstructure and Micromechanisms, eds. S. V. Nair, J. K. Tien, R. C. Bates, and O. Buck, ASM Materials Science Seminar, ASM International, OH, 1987, pp. 283-334. 

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. 

Bohse, J., Krietsch, T., Chen, J., Brunner, A.J., (2000) Acoustic Emission Analysis and Micromechanical Interpretation of Mode I Fracture Toughness Tests on Composite Materials , Proceedings ESIS Conference on Fracture of Polymers, Composites and Adhesives, ESIS Publication 27, pp. 15-26, Elsevier, Oxford. 

FCP resistance for the SINs increases with PU content up to 50% and is better in the prepolymer material than in the one-shot material, since the former always has larger values of percent energy absorption. With respect to micromechanisms of failure, the generation of discontinuous growth bands associated with shear yielding is involved in the SINs from the one-shot procedure. On the other hand, the fracture surfaces of the SINs from the prepolymer procedure show extensive stresswhitening phenomenon which is associated with the cavitation around PU domains and localized shear deformation. 

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. 

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 fracture 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. 

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