Emission following fracture

An early discovery we made involving adhesive failure and its effect on charged particle emission concerned the fracture of composites (a more detailed discussion of this work can be found in References 13, 15, and 20). Starting with the constituents of a composite, namely, the filaments and neat resin, we found that the electron emission following their fracture appeared as shown in Fig. 4. The most important point to observe is that the emission is very rapid, decaying with time constants on the order of 10 - 100 microseconds. Early identical emission curves are observed for the PIE from the same materials. 

J. T. Dickinson, L. C. Jensen, and M. K. Park, Time correlations of electron and positive ion emission accompanying and following fracture of a filled elastomer, Appl. Phys. Lett. 41, 443 (1982). 

J. T. Dickinson, M. J. Dresser, and L. C. Jensen, Time correlation of ion and electron emission from surfaces following fracture, in Desorption Induced by Electronic Transitions (DIET II) (W. Brenig and D. Menzel, eds.). Springer-Verlag, Berlin (1985). 

Significant neutral emission is also observed during and after fracture. When analyzing the time dependence of the observed NE, one must take into account both the time dependence of the emission itself and the time-of-flight of the neutral species (associated with Boltzmann velocity distributions) from the source to the mass spectrometer. We expect NE intensities to be much higher than the PIE intensities of the same molecular species due to the high probability of reneutralization as ions leave the surface. NE will be discussed more fully in the following section. 

Very recently, in our group, enhanced self-reinforced PP composites based on commercial PP fabrics were obtained by the film stacking method followed by compression molding introducing different contents of micron-sized quartz particles in the matrix films (unpublished results). Simultaneous improvements of composite tensile strength, ductility and fracture toughness were observed from the addition of quartz to the polymer matrix (Table 14.1). Enhanced degree of consohdation was obtained for the composites with quartz as evidenced from the improved mechanical properties and by SEM observations. This was also confirmed by acoustic emission analysis in situ in tensile tests. 

For a wide range of materials the emission of electrons (EE), positive ions (PIE), neutral species (NE), and photons (phE) has been observed accompanying fracture. We refer collectively to these emissions as fracto-emission. In this paper we review our work on fracto-emission where the fracture event involves interfacial or adhesive failure. The interfaces to be discussed include the following brittle materials/epoxy, glass/elastomers, and brittle materials/pressure sensitive adhesives. 

Chakrapani and Pugh [118] inferred that the mechanism for crack growth was repeated cycles of H diffusion to the crack tip region followed by brittle fracture. This was supported by discontinuous acoustic emission signals. It was also suggested that the role of H in the brittle fracture could be in the formation of brittle hydrides or to produce decohesion. They inferred that since SCC fractures tend to occur on 3140 planes, these may correspond to the habit or cleavage planes of a hydride. Fracture surfaces for SCC and pure-HE systems were different the latter tended to be flatter and without the pleated/stepped structure. It was speculated that this could be related to H fugacity and H entry kinetics, and the fact that, for SCC conditions, dissolution occurs at the crack tip. 

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