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81.
Fracture experiments on symmetric-tilt grain boundaries in Cu are interpreted using the Peierls-Nabarro continuum model of dislocation nucleation as a starting point. Good agreement is found only when the continuum model is modified according to the results of atomistic simulations. The same experiments are also reproduced by direct Molecular Dynamics simulations of fracture propagation and dislocation emission from a microcrack placed in the interface plane of the symmetric-tilt (221)(221) grain boundary in fcc Cu. Direction-dependent fracture response is observed, namely the microcrack advancing by brittle fracture along the [11
] direction and being blunted by dislocation emission along the opposite [
4] direction. Moreover, the simulations allow us to establish important differences with respect to the continuum-model predictions due to the shielding of the stress field at the crack-tip and to the presence of the residual stress at the interface. 相似文献
82.
Binary blends of unbranched polyethylene (PE) and 5-10% model ethylene-butene random copolymers are used to determine the effects of composition heterogeneity on phase separation in the melt, semicrystalline morphology, plane strain fracture toughness JC and tensile modulus and yield strength. Slowly cooled samples of melt-miscible blends are appreciably tougher (JC = 5.2 kJ/m2) than unblended PE (JC = 2.7 kJ/m2). A blend with the same average short chain branch concentration, but which is phase separated in the melt state, has JC= 3.3 kJ/m2; dispersed domains of amorphous polymer have little effect on toughness. Enhanced toughness is associated with nonuniform morphology formed on slow cooling “one phase” melts composed of chains with different amounts of branching. The relative number of chemically different chains, as opposed to absolute branch concentrations, seems most important. Tensile properties are relatively unaffected by blending at these levels. Results from these model blends are used to consider the properties of compositionally heterogeneous ethylene copolymers. © 1994 John Wiley & Sons, Inc. 相似文献
83.
84.
Robert S. Hoy 《Journal of Polymer Science.Polymer Physics》2011,49(14):979-984
In this Perspective, I describe recent work on systems in which the traditional distinctions between (i) unentangled versus well‐entangled systems and (ii) melts versus glasses seem least useful, and argue for the broader use in glassy polymer mechanics of two more dichotomies: systems which possess (iii) unary versus binary and (iv) cooperative versus noncooperative relaxation dynamics. I discuss the applicability of (iii–iv) to understanding the functional form of strain hardening. Results from molecular dynamics simulations show that the “dramatic” hardening observed in densely entangled systems is associated with a crossover from unary, noncooperative to binary, cooperative relaxation as strain increases; chains stretch between entanglement points, altering the character of local plasticity. Promising approaches for future research along these lines are discussed. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011 相似文献
85.
Antonella Cristiano Alba Marcellan Bert J. Keestra Paul Steeman Costantino Creton 《Journal of Polymer Science.Polymer Physics》2011,49(5):355-367
Fracture properties of model elastomeric networks of polyurethane have been investigated with a double‐edge notch geometry. The networks were synthesized from monodisperse end‐functionalized polypropylene glycol precursors and a trifunctional isocyanate. All reagents were carefully purified and nearly defect‐free ideal networks were prepared at a stoichiometry very close to the theoretical one. Three networks were prepared: an unentangled network of short chains (Mn = 4 kg mol?1), an entangled network of longer chains (Mn = 8 kg mol?1) and a bimodal network with 8 kg mol?1 and 1 kg mol?1 chains. The presence of entanglements was found to increase significantly the toughness of the rubber, in particular at room temperature, relative to the bimodal networks and to the short chains network. Fracture experiments were carried out at different strain rates and temperatures and showed for all three networks a marked decrease in fracture toughness with increasing temperature and decreasing strain rate which mirrored reasonably well the rate and temperature dependence of tan δ, the dissipative factor. However the proportionality factor between tan δ, and GIC was very material dependent and the shift factors obtained for the master curves of the viscoelastic properties could not be used to build fracture energy master curves. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010 相似文献
86.
On the problem of crack system with an elliptic hole 总被引:1,自引:0,他引:1
The problem of cracks with an elliptic hole in an infinite plane is investigated. By introducing the fictitious loads on the
hole edge and using the Muskhelishvili method, the problem is reduced to solving a system of mixed-type integral equations
in which some are Fredholm equations but others Cauchy-type singular ones. A numerical method is suggested and can be used
for the treatment of other similar equations. The numerical results for some typical examples are given, showing that the
method is really effective. 相似文献
87.
Intercalated nanocomposites of modified montmorillonite clays in a glassy epoxy were prepared by crosslinking with commercially available aliphatic diamine curing agents. These materials are shown to have improved Young's modulus but corresponding reductions in ultimate strength and strain to failure. The results were consistent with most particulate‐filled systems. The macroscopic compressive behavior was unchanged, although the failure mechanisms in compression varied from the unmodified samples. The fracture toughness of these materials was investigated and improvements in toughness values of 100% over unmodified resin were demonstrated. The fracture‐surface topology was examined using scanning electron and tapping‐mode atomic force microscopies and shown to be related to the clay morphology of the system. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1137–1146, 2001 相似文献
88.
《Composite Interfaces》2013,20(5):337-349
The lamination of composite elements such as sheets or fibres made from ceramic powders represents a cheap and easy way of making tough ceramics. The fabrication and failure behaviour of such layered structures is described. It is shown that crack growth along the interfaces is dominated by dynamic effects due to the storage of excess elastic energy and that effects of loading state, in the silicon carbide/graphite system at least, appear to be relatively unimportant. Crack deflection at interfaces is also discussed and it is shown that observations made in these systems are not consistent with existing theories. Various possibilities are investigated. 相似文献
89.
D.W. Wheeler 《哲学杂志》2013,93(3):285-310
This paper describes a study of the behaviour of diamond coatings when subjected to solid particle erosion from sand particles. The coatings were deposited by chemical vapour deposition (CVD) onto tungsten substrates and tested using a high velocity air–sand erosion test facility. The erosion tests were conducted using particle impact velocities of between 33 and 268 m/s. Examination of the eroded test specimens showed that the principal damage features were circumferential cracks and pin-holes. Comparison with Hertz impact theory revealed that the measured circumferential crack diameters were more than double the predicted Hertzian contact diameter. Moreover, a trend of increasing circumferential crack diameter with coating thickness, which is not predicted by Hertz, was found. Instead, the crack diameters showed good agreement with those predicted by the theory of stress wave reinforcement, which is more commonly associated with liquid impact damage of brittle materials. During impact, the bulk compression and shear waves are reflected at the rear surface of debonded regions of the coating to return to the front surface and reinforce the Rayleigh surface wave, which generates a tensile stress. Where this stress exceeds the local tensile strength of the coating, a ring of cracks surrounding the area of impact is created. The results from the present study therefore suggest that stress wave reflection is responsible for the formation of the cracks at locally debonded regions of the coating. This hypothesis was supported by images acquired using scanning acoustic microscopy, which showed that circumferential cracks and pin-holes were only found on areas of the coating that had become delaminated by multiple particle impacts during the erosion tests. 相似文献
90.