Stable crack mechanism

Besides, in the same figure the points represent the values for PASF, calculated aceording to the Eq. (5.9) and the plots of Fig. 8.1 slope. As one can see, the dimensions and good correspondence is observed, from which the conclusion can be made, that PASF film samples fiucture by stable crack mechanism is related to ductile fracture mechanism. These results demonstrate clearly, that the value for polymer samples can be predicted, proceeding from the two indicated above factors polymer structure and its fracture mechanism [13]. 

Eracture mechanics concepts can also be appHed to fatigue crack growth under a constant static load, but in this case the material behavior is nonlinear and time-dependent (29,30). Slow, stable crack growth data can be presented in terms of the crack growth rate per unit of time against the appHed R or J, if the nonlinearity is not too great. Eor extensive nonlinearity a viscoelastic analysis can become very complex (11) and a number of schemes based on the time rate of change of/have been proposed (31,32). 

Fig. 17 Contact mechanics analysis of Herztian cracks within brittle materials.a Schematic description of a Hertzian cone crack induced under normal indentation by a rigid sphere, b Reduced plot of JC-field as function of cone crack length and for increasing loads pf < p// < pm during sphere-on-flat normal indentation of brittle materials. Arrowed segments denote stage of stable ring crack extension from Cf to cc (initiation), then unstable to ci at P = P,n (cone-crack pop-in) (From [67]). Branches (1) and (3) correspond to unstable crack propagation (dK/dc > 0), branches (2) and (4) to stable crack propagation (dK/dc < 0)...

Crack Stability. At low test speeds, stable crack growth with an extended stress-whitened plastic zone and crack blunting occur by the same mechanisms as those involved in the kinetics of the plastic zone, namely, rubber cavitation followed by shear deformation of the matrix. The ability of the matrix to shear is controlled by its relaxation behavior, which therefore determines its plasticity and the deformation imposed on rubbery particles distant from the notch. 

Another method to produce more stable cracking catalysts is the introduction of polyvalent cations into, e.g., Na-Y, in particular La or rare earth (RE) cations [435,436]. Hirschler [437] andPlank [438] independently suggested that formation of OH groups via introduction of polyvalent cations is due to the following chemistry (which is nowadays generally referred to as the Hirschler-Plank mechanism), illustrated here with two selected examples using chlorides of two- or three-valent cations, M" and M ", respectively ... 

Manhanty D.K., Savent G.Y. 1992. Experimental investigations on stable crack growth in three-point bend specimens under mode I and mixed mode (I and II) loading[J. Engineering Fracture Mechanics, (43) 41-53. 

Testing of Plastics - Instrumented Charpy impact test Procedure for determining the crack resistance behaviour using the instrumented impact test Part I Determination of characteristic fracture mechanics parameters for resistance against unstable crack propagation Part II Determination of characteristic fracture mechanics parameters for resistance against stable crack propagation... 

A critical research gap in corrosion science is the absence of the corrosion equivalent for the stress intensity factor (K) that has been the mainstay of structural mechanics for the past several decades. The stress intensity factor was developed to predict the behavior of pre-existing flaws in structural materials and the eventual life of a component under conditions in which the flaw develops into stable cracks. The power of K is in the concept of similitude well-defined cracks and crack tips that are different in size or shape but possess the same K (as determined by geometry, loading, and the theories of linear-elastic fracture mechanics) will experience the same mechanical driving force for crack growth. Thus, similitude allows small, well-defined samples to be tested in the laboratory to determine the conditions of crack growth and fracture and the results to be quantitatively extended to more complicated real-world structures containing cracks. Virtually... 

J. A. Salem, L. J. Ghosn, and M. G. Jenkins, A Strain Gage Technique to Measure Stable Crack Extension in Ceramies, in Post Conference Proceedings of the 1997 SEM Spring Conference on Experimental Mechanics (Society for Experimental Mechanics, Bethel, Connecticut, 1997) pp. 1 8. 

Fracture mechanics tests were conducted on an Instron-type displacement controlling test machine (Sanwa Testing Machine Co., Toyohashi, Japan) under a crosshead speed of 0.05 mm/min. All the fracture tests were made in three-point flexural modes. A stable crack extension was observed for all the specimens with the relative notch depth ao/Ws 0.4. However, the crack extension was always unstable for oo/W < 0.4. The apparent fracture toughness, Kq, at the onset of crack initiation for the critical load Pc was determined by... 

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