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SynopsisSubmicrocracks, free radicals, and endgroups of scissioned molecules formed in polyethylene, polypropylene, and polycaprolactam under uniaxial tension have been investigated. Measurements were carried out by small-angle x-ray scattering, electron paramagnetic resonance, and infrared spectroscopy. The concentration of submicrocracks is almost the same as that of free radicals but is smaller than the concentration of scissioned macromolecules by approximately three orders of magnitude. The number of scissions per crack proved to be close to the number of macromolecules passing through the cross section of a submicrocrack calculated on the assumption of close packing. It is concluded that submicrocracks in stressed polymers are formed as a result of chain reactions of macromolecular decomposition initiated by the active end primary free radicals. 1509

A B S T R A C T Formation of incipient submicroscopic cracks in polymers under load have been studied by small-angle X-ray scattering. The main regularities of crack formation under different loading conditions have been analyzed. The connection between the submicrocrack concentration and the deformation of a stressed polymer has been shown. The main parameters of crack formation defining the strength properties of a polymer, i.e., the size of initial submicrocracks transverse to the axis of loading, which is determined by the structural heterogeneity of a material, and the concentration of submicrocracks before rupture, have been established. The analysis of the quantitative correlation between these parameters allows one to formulate the main statements regarding the micromechanics of polymer fracture. From comparing the volume and surface parameters of crack formation of a stressed polymer, the dominating role of the surface in the process of fracture has been demonstrated.

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We present the chronological series of acoustic emission (AE) signals detected in samples of Westerly granite loaded in different regimes, one of which included a feedback loop between the axial stress and AE activity. In both regimes, the fracturing system evolves without a characteristic energy scale and with the same scaling exponent independently of the applied feedback. The scaling properties of the waiting-time distribution are more sensitive to both the drive control and the stage of fracture process. The revealed period of a lack of time correlation during continuous loading is explained by the transition between the hierarchic levels in the heterogeneous material.