Quasi-brittle Crack Research Articles

Rapid quasi-brittle tearing of a thermoelastic strip

The displacement-controlled rapid tearing of a thermoelastic strip is modeled as steady-state propagation of a quasi-brittle crack. The strip satisfies the fully-coupled equations of thermoelasticity, and the small-scale plastic effects are represented by a Dugdale inelastic zone which also serves as a heat flux source. An asymptotic analysis gives expressions for the zone length, COD and dynamic fracture toughness. These expressions show, upon comparison with non-thermal results, the importance of fully-coupled thermoelastic effects, and that all the problem characteristic lengths, which range over several orders of magnitude, are prominent features. Zone heat flux values, based on experimental results for near-crack temperature gradients, are then used for calculation purposes. The calculations show that, independent of a particular fracture criterion, thermal effects noticeably increase inelastic zone size and dynamic fracture toughness.

Features of mechanics of quasi-brittle crack propagation in concrete

Physical observations of crack propagation in concrete are discussed in the present paper. Based on these physical observations, different models proposed for mode I fracture of concrete are examined. In the past, verification of validity of fracture models has always been difficult. It is proposed that notch sensitivity and size effect be used as criteria to evaluate the validity of fracture models. In addition, a successful model should also satisfy a ‘portability’ condition, namely the model should be applicable to structures of an arbitrary geometry. Discussion on the effects of different loading conditions, i.e. mixed-mode loading conditions and different loading rates, on the fracture process in concrete is also presented.

Thermodynamics of the propagation process of quasibrittle cracks

Thermodynamics of the propagation process of quasibrittle cracks

Crack propagation and cleavage initiation in Fe-2.6%-Si single crystals under controlled plastic crack tip opening rate in various gaseous environments

Crack propagation and crack initiation measurements were carried out in Fe-2.6%-Si single crystals under a new type of control: The elastic strain rate is controlled before yielding. Beyond the yield point, however, the rate of plastic elongation of the specimens is non-linearily controlled. Since plasticity is exclusively concentrated onto the crack tip. the plastic crack tip opening rate is prescribed with reasonable accuracy in this type of experiment. In b.c.c. FeSi single crystals at temperatures below 100°C the crack tip opening rate determines whether the crack advance is ductile or brittle. The critical crack tip opening rate, at which the fracture mode changes, is strongly temperature dependent and yields an activation energy of 0.68 eV for the initiation of cleavage. This value is close to the activation energy reported in the literature for dislocation glide in FeSi. In a small range of the crack tip opening rate, below the critical rate, stable brittle fracture (‘quasi-brittle fracture’) is found. Evidence is reported that plasticity may be responsible in a yet unknown way for the stable quasi-brittle crack growth.

Kinetics of stress relaxation and conditions for quasibrittle crack propagation

Kinetics of stress relaxation and conditions for quasibrittle crack propagation

The “Superdislocation” Model of a Plastic Zone near a Wedge-Shaped Crack

A simple anisotropic dislocation theory of the plastic zone near a nonsymmetric wedge-shaped crack is developed. The zone is described by some superdislocations with multiple Burgers vectors. The main parameters of the equilibrium zone (the strength and spread of the relaxation shears) are determined. Simple expressions for a symmetrical quasi-brittle crack are derived both, in anisotropic and isotropic elasticity. Capabilities of the theory developed to describe the interaction of different shear-type planar defects are discussed. The critical pressure is calculated necessary for crack plastic closing as well as the parameters of cracks arising during deformation under pressure. [Russian Text Ignored.]

Effect of grain size and temperature on fatigue crack propagation in A533 B steel

Crack growth-rate tests were performed in a medium-strength pressure vessel steel, A533 B, which had been annealed to produce materials with widely varying prior austenite grain sizes. Testing was at two temperatures, normal room temperature and liquid nitrogen temperature, both on the lower shelf of the material's Charpy energy curve. Results show that both grain size and temperature influence quasibrittle crack growth, mainly through their effect on K Ic' . The threshold and power law region of the da/dN vs ΔK plot seem relatively uneffected while the upper limiting ΔK is shifted extensively. The data is presented and compared with that already published.