Quasi-static intergranular brittle fracture: Dynamic embrittlement

A mode of brittle fracture is described which is fundamentally different from the rapid transgranular cleavage or intergranular decohesion that is usually associated with that term. It involves stress-induced diffusion of surface-adsorbed embrittling elements along grain boundaries, and it occurs by slow, step-wise crack growth, the rate of which can, in principle, be calculated from the knowledge of the relevant intergranular diffusion coefficient, the stress profile at the crack tip and the dependence of the stress for grain-boundary decohesion on the concentration of the embrittling element. This mode of fracture is postulated to be possible in any high-strength alloy with a low-melting-point element adsorbed on the surface if the applied stress is high enough. Known examples include the brittle type of stress-relief cracking in steels, tin-induced cracking of Cu-Sn alloys, oxygen-induced cracking of iron-, copper-, and nickel-based alloys, and the group of phenomena known as liquid-metal embrittlement and solid-metal embrittlement.The paper is dedicated to Dr. Frantiek Kroupa in honour of his 70^th birthday.This work is supported by National Science Foundation Grant CMS 95-03980.     

Oscillatory instability in two-dimensional dynamic fracture

The stability of a rapid dynamic crack in a two-dimensional infinite strip is studied in the framework of linear elastic fracture mechanics supplemented with a modified principle of local symmetry. It is predicted that a single crack becomes unstable by a finite wavelength oscillatory mode at a velocity vc, 0.8cR相似文献   

Chemical states of oxygen segregated intergranular fracture surfaces of molybdenum

The Auger and electron energy loss spectra (EELS) of a grain boundary fracture plane of bicrystal molybdenum (32 wt.ppm oxygen) are compared with the spectra of pure and oxidized molybdenum. The Auger spectrum of the fracture surface contains molybdenum and oxygen peaks, and the Mo M_4,5NN line coincides with that of the pure metal. The interfacial Auger transition peak is observed on the low energy side of the Mo N_2,3VV Auger peak. Both AES and EELS spectra of the fracture plane are different from those of the oxidized molydenum. These results show that the segregated oxygen is bound to the grain boundary fracture plane as if it were adsorbed.     

Magic angles and cross-hatching instability in hydrogel fracture

The full 2D analysis of roughness profiles of fracture surfaces resulting from quasistatic crack propagation in gelatin gels reveals an original behavior characterized by (i) strong anisotropy with maximum roughness at V-independent symmetry-preserving angles and (ii) a subcritical instability leading, below a critical velocity, to a cross-hatched regime due to straight macrosteps drifting at the same magic angles and nucleated on crack-pinning network inhomogeneities. Step height values are determined by the width of the strain-hardened zone, governed by the elastic crack blunting characteristic of soft solids with breaking stresses much larger than low strain moduli.     

Deformation instability and fracture at liquid-helium temperature

The instability of plastic deformation during the extension and compression of aluminum and during the extension of titanium and stainless steel is studied, and the temperature-rate region for the existence of deformation instability is determined. The form of the jumps in the flow stresses and their relationship to the macroscopic localization of strain and to fracture of the sample are also investigated. The role of the localization of microstrain and of fracture in the macroscopic manifestations of strain inhomogeneity is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 647–651 (April 1997)     

Dynamic instability of collective myosin Ⅱ motors

Some kinds of muscles can oscillate spontaneously,which is related to the dynamic instability of the collective motors.Based on the two-state ratchet model and with consideration of the motor stiffness,the dynamics of collective myosin Ⅱmotors are studied.It is shown that when the motor stiffness is small,the velocity of the collective motors decreases monotonically with load increasing.When the motor stiffness becomes large,dynamic instability appears in the forcevelocity relationship of the collective-motor transport.For a large enough motor stiffness,the zero-velocity point lies in the unstable range of the force-velocity curve,and the motor system becomes unstable before the motion is stopped,so spontaneous oscillations can be generated if the system is elastically coupled to its environment via a spring.The oscillation frequency is related to the motor stiffness,motor binding rate,spring stiffness,and the width of the ATP excitation interval.For a medium motor stiffness,the zero-velocity point lies outside the unstable range of the force-velocity curve,and the motion will be stopped before the instability occurs.     

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Dynamic instability of self-induced bidirectional waveguides in photorefractive media

We report on the experimental observation of a dynamic instability in the interaction of counterpropagating self-trapped beams in a photorefractive strontium barium niobate crystal. While the interaction of copropagating spatial optical solitons exhibits only transient dynamics, resulting in a final steady state, the counterpropagating geometry supports a dynamic instability mediated by intrinsic feedback. Experimental observations are compared with and found to be in qualitative agreement with numerical simulations.     

Dynamic fracture model for acoustic emission

We consider the spin-glass phase of the Sherrington-Kirkpatrick model in the presence of a magnetic field. The series expansion of the Parisi function q(x) is computed at high orders in powers of τ = T _c - T and H. We find that none of the Parisi-Toulouse scaling hypotheses on the q(x) behavior strictly holds, although some of them are violated only at high orders. The series is resummed yielding results in the whole spin-glass phase which are compared with those from a numerical evaluation of the q(x). At the high order considered, the transition turns out to be third order on the Almeida-Thouless line, a result which is confirmed rigorously computing the expansion of the solution near the line at finite τ. The transition becomes smoother for infinitesimally small field while it is third order at strictly zero field. Received 3 March 2003 Published online 4 June 2003 RID="a" ID="a"e-mail: andrea.crisanti@phys.uniroma1.it RID="b" ID="b"e-mail: tommaso.rizzo@phys.uniroma1.it RID="c" ID="c"e-mail: temtam@helios.elte.hu     

Study of chemical states at intergranular fracture planes of iron-phosphorus alloys by auger and electron energy-loss spectroscopies

Auger and electron energy-loss spectroscopies (ELS) have been used to study chemical states at the fracture surfaces of iron-phosphorus alloys. The transgranular (TG) and intergranular (IG) fracture planes have similar Auger P LVV transition energies and similar Fe 3p loss energies. These indicate that phosphorus and iron atoms are dispersed atomically in layers segregated at IG planes. The energy separations between the inter-and intra-atomic Auger transition peaks in P LVV show that the TG and IG planes and Fe₃P have similar energy separations between the P 3p and Fe 3p levels. The layers segregated at IG planes are estimated to be equivalent to a monolayer, as indicated by the dependence of the loss energy of the valence-conduction-band transition on primary-electron kinetic energy from 200 to 500 eV. The phosphorus concentration in the segregated layers is saturated to ~ 20 at.% above a bulk phosphorus concentration of 2 at.%.     

Dynamic instability of strongly confined thin polymer films in spinodal dewetting

We present a general criterion for determining the dynamic instability of a thin polymer film that is strongly confined to a substrate. When such a polymer film is heated above its glass transition temperature, it dewets from the substrate by a spinodal dewetting scenario, in which the onset of the instability is governed by dispersion force and residual thermal stress. It turns out that the thermal stress plays an important role when there is a special interaction between a polymer and a substrate.     

Dynamic properties of combustion instability in a lean premixed gas-turbine combustor

We experimentally investigate the dynamic behavior of the combustion instability in a lean premixed gas-turbine combustor from the viewpoint of nonlinear dynamics. A nonlinear time series analysis in combination with a surrogate data method clearly reveals that as the equivalence ratio increases, the dynamic behavior of the combustion instability undergoes a significant transition from stochastic fluctuation to periodic oscillation through low-dimensional chaotic oscillation. We also show that a nonlinear forecasting method is useful for predicting the short-term dynamic behavior of the combustion instability in a lean premixed gas-turbine combustor, which has not been addressed in the fields of combustion science and physics.     

Dynamic instability of thin viscoelastic films under lateral stress

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