Coarsening grain and intercrystalline network ferrite depress crack growth resistance of the materials strongly and result in unstable propagation of crack very easily.

From studying on the rotating shaft crack variations from relative stable status to unstable propagation, a real time method to monitor fatigue propagation of the transverse crack in a rotating shaft is presented, in which relative stable status is monitored by real time vibration measurement and unstable propagation of the crack is monitored by unrealtime dc-potential method.

2)Let an angle between the direction of 1 and the normal direction of the crack plane be the twist angle of crack plane, when 1 reaches a critical value, the crack appears in unstable propagation, and the initial direction, twist angle of crack plane and the corresponding fracture equation can be deduced.

By observation of the fracture surfaces and special metallography sections of C-Mn steel multipass MMA weld metals and simulated weld specimens which were fractured at -60℃ by Charpy V tests, the initiation of crack,metallographical characteristics of the metal near the crack, and the critical event of unstable propagation of crack were investigated into.

In peak-aged condition,a decreasing in matrix deformation ability resulted from dead pinning of dislocations by fine carbide particles precipitated from ferrite and,a increasing in matrix elastic strain energy conducted by coherence or partial coherence correlate with embrittlement controlled by crack unstable propagation in impact fracture process.

In this paper, the incremental path-independent integral is studied for unstable propagation of creep crack problem. Based on the experiment of the creep crack growth under high stress state, the relationship between the integral rate and crack growth rate under the unsteady growth process is worked out. At the same time, some parameters of the crack-tip fields, such as the stress, the plastic strain, the creep strain and the triaxiality stress etc.

For samples with two artificial pre-existing flaws the third phase is split into two parts and failure of the sample occurs only after both the unstable propagation of external wing cracks and coalescence of the internal cracks in the bridge.

Studies of the unstable propagation and arrest of brittle fractures were conducted on four steels: plain carbon steel, 3 pct Si steel, A-517, and 4340.

The specific surface energy and kinetics of the crack undergoing unstable fracture depend both on the maximum crack growth rate in the material and on the initial rate υ0 of its unstable propagation.

Unstable propagation starts with a rate which is lower than the maximum crack growth rate in the material.

By using the energy balance equation for propagating cracks, we estimate the specific surface energy and crack growth rate under the conditions of unstable propagation.

This paper describes the experimental investigation of the fast (unstable) propagation of crack in DCB (double-cantilever beams)specimens under impulsive loading of the order of 10~(-3) sec. (mechanical shook, 10~(-2)-10~(-4) sec). A new eddy-current method is developed. Differing from several methods usually used and also from that using eddy-current probeservosystem to "lock on" the crack tip, this method avoids the disadvantages of other methods, and has recorded the whole extension-time history...

This paper describes the experimental investigation of the fast (unstable) propagation of crack in DCB (double-cantilever beams)specimens under impulsive loading of the order of 10~(-3) sec. (mechanical shook, 10~(-2)-10~(-4) sec). A new eddy-current method is developed. Differing from several methods usually used and also from that using eddy-current probeservosystem to "lock on" the crack tip, this method avoids the disadvantages of other methods, and has recorded the whole extension-time history of the fast propagation, especially the initial portion just after the onset of extension of which little is known today. From the oscillograph of the acceleration and the displacement responses, onset of extension can be accurately determined, as well as the small displacements of the loading points on the specimen. From this the K_(1C)~(lmp) is easily calculated. from the experimental results, it can be concluded that, for materials which have the K_(IC)~(imp) values increasing with the loading speed, the extension or propagation of sharp crack under impulsive load is fully equivalent to a blunt crack under static load with the grip fixed, i. e., calculations can be made using the concept of equivalent K_Q values.

The paper is written with the aim of trying to investigate the mechanism of rock fracture by way of acoustic emission.Experiments of acoustic emission have been carried out by statically penetrating the bit into rock samples which,we adopted,are the marbles.Four types of indenters are applied : the flat-ended cylinderical punch,the spherical cap,the bullet-shaped cap and the wedge bit.Besides,the experiments of acoustic emission for rock fracture by the rolling disc-cutter are particularly interested to perform.The...

The paper is written with the aim of trying to investigate the mechanism of rock fracture by way of acoustic emission.Experiments of acoustic emission have been carried out by statically penetrating the bit into rock samples which,we adopted,are the marbles.Four types of indenters are applied : the flat-ended cylinderical punch,the spherical cap,the bullet-shaped cap and the wedge bit.Besides,the experiments of acoustic emission for rock fracture by the rolling disc-cutter are particularly interested to perform.The total number of acoustic emission events,the acoustic emission rate(AER) and their distributions have been measured by a monitoring system (Fig.1) with the frequency band of 100-150 KHZ.As a whole,the rock fracture procedure may be marked off as three stages.(a)With the loading increasing,there tend to be a lower level activity of acoustic emission characterizing the interior microcracking of rock sample and so the AER are less of accounting.Only the "initial surface removal takes place in closs vicinity to the pressure area by testing indenter can a smaller peak value of AER arises fortuitously.(b)In case the peak value of penetrating force occures,the load acting on rock sample decreases as the horizontal cracks are initiating nearly to the inner surface of rock sample.At the same time the AER rapidly changes with abruptly rising and falling.(c)Finally,the surface removals are obviously observed and the AER vanishes into nothing even to reload on the increase again.It is caused by the rapid propagation of horizontal cracks.These phenomena have been observed for all the marble specimens tested by various indenters as above.It is probable that the rapid variations described may be attributed to the unstable propagation of horizontal cracks.

In this paper, a method for finding the stress intensity factors K_Ⅰ and K_Ⅱ for sheet with finite width including slanting crack under uniaxial tensile loading is presented. It is. based on linear elastic fracture mechanics and by using functions of complex variables. And by the criterion of maximum shear strain (namely, (ε_θ) max is correlated with K_R and (d~2ε_θ)/(dθ~2)<0), an angle of crack propagation and an equivalent stress intensity factor K_(Ieq) are obtained. Thereby, the critical stress σ_C and crack...

In this paper, a method for finding the stress intensity factors K_Ⅰ and K_Ⅱ for sheet with finite width including slanting crack under uniaxial tensile loading is presented. It is. based on linear elastic fracture mechanics and by using functions of complex variables. And by the criterion of maximum shear strain (namely, (ε_θ) max is correlated with K_R and (d~2ε_θ)/(dθ~2)<0), an angle of crack propagation and an equivalent stress intensity factor K_(Ieq) are obtained. Thereby, the critical stress σ_C and crack tolerance a_C can be obtained when unstable propagation of crack occurs by using energy criterion.