Notch Plastic Zone Research Articles

高負荷応力下における切欠き底小寸法疲労き裂の進展挙動

The growth and closure behavior of a small crack initiated at a notch root was investigated for SUS 304 and SM41A steels under the situation of enhanced notch plasticity. An acceleration of growth rate as well as an increase in crack-opening range were observed for a small crack which was embedded in the notch-plastic zone. All of the growth data for the small crack at the notch root were successfully correlated with those for long cracks by the effective stress intensity factor range, ΔKeff. ΔKeff was found to be a controlling parameter for crack growth up to higher nominal stress range, as far as the crack closure was observed and the notch-root plastic region was surrounded by the elastic region. A primary controlling factor for the closure of the small crack at the notch root was found to be notch-root plasticity. This was supported by the analytical study by using the finite element method.

FATIGUE AT NOTCHES SUBJECTED TO REVERSED TORSION AND STATIC AXIAL LOADS

AbstractReversed torsion with and without a superimposed end load has been applied to 1% Cr‐Mo‐V steel specimens containing sharp notches. Crack propagation was monitored by a sensitive d.c. potential drop system that measured crack depths between 25 μm and 0.6 mm from the root of the notch.Stress intensity factors do not satisfactorily correlate all the crack growth data but a strain intensity factor which is a function of material properties and notch plastic zone size shows a significant improvement and provides a single upper bound solution for both ambient and elevated temperatures. This solution permits designers to make safe lifetime assessments.At room temperature cracks initially propagate by mode II along the surface, and mode III radially but at low stresses crack growth is continued by mode I propagation. At higher stresses a transition to mode I cracking is avoided. Elevated temperature causes a brittle layer to form and in this case cracks initially propagate by mode I which then translates to mode III cracking at high stresses.Mode III thresholds are significantly higher than mode I thresholds but a constrained shear strain zone, as found at the root of notches subjected to torsion, permits the initiation and generation of a mode III crack. The application of an axial load enhances the mode III crack propagation rate since this increases the effective crack tip intensification factor.

Fatigue crack growth in a fillet welded joint

Existing theories for the growth of cracks at weld toes have proved difficult to verify because of a lack of experimental proof at short crack depths and slow growth rates. Arbitrary initial defect sizes have been employed in life calculations coupled with approximate two-dimensional stress analyses. In this study, the fatigue performance of a stress relieved fillet weld is determined by both theory and experiment. Crack growth results for shallow (less than 1 mm depth) elliptical cracks at weld toes are used to test an elastic expression for stress intensity using a correction factor from a three-dimensional stress analysis. No evidence of higher than expected growth rates, observed by others for very short cracks and cracks in notch plastic zones, is apparent. Integration of a growth law that includes the threshold stress intensity factor provides fatigue life predictions for various stress ratios and from experimentally measured defect depths. Needle peening the weld toe improves the fatigue life by retarding crack growth up to 1 mm below the weld toe.

PREDICTION OF FATIGUE LIFETIME OF NOTCHED MEMBERS

Abstract— A simple model for the estimation of the total fatigue life of notched members is presented. The number of cycles to failure is estimated as the summation of the cyclic life spent in: (1) the initiation of a dominant fatigue crack at the notch root; (2) the very early growth of this crack within the notch plastic zone; (3) the subsequent fatigue crack growth in the elastic stress‐strain field of the notch; and (4) the elastic stress field of the bulk material. Theoretical and experimental results are compared.

ELASTIC-PLASTIC FRACTURE MECHANICS FOR INITIATION AND PROPAGATION OF NOTCH FATIGUE CRACKS

Initiation and propagation are considered to be controlled by the extent of total plastic shear deformation φ. Crack initiation and crack propagation occur when φ, exceeds a critical threshold value which can be equated to threshold conditions determined from linear elastic fracture mechanics analyses. When a crack is in a plastically deformed zone φt=φp+φe. where φp is the component of φt due to notch bulk plasticity and φe, is the component of φt due to a linear elastic fracture mechanics (LEFM) analysis of the crack tip plasticity field. When cracks initiate at notch roots φt > φth. As the crack propagates in the notch plastic zone the rate of decrease of vp will be different from the rate of increase of φe and it is possible for φt to decrease to a level below φth thereby creating a non-propagating crack.

Estimation Of Local Cyclic Stress Ratio Due To Confined Notch Plasticity

Estimation Of Local Cyclic Stress Ratio Due To Confined Notch Plasticity