Crack propagation at a constant crack tip stress intensity factor

1980 ◽  
Vol 16 (4) ◽  
pp. R215-R218 ◽  
Author(s):  
E. Smith
Keyword(s):  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Crack Propagation ◽  
Crack Tip ◽  
Crack Tip Stress

Fatigue crack propagation experiment and numerical simulation of 42CrMo steel

2020 ◽  
Vol 234 (14) ◽  
pp. 2852-2862
Author(s):  
Jianwei Dong ◽  
Weichi Pei ◽  
Hongchao Ji ◽  
Haiyang Long ◽  
Xiaobin Fu ◽  
...  
Keyword(s):  
Numerical Simulation ◽  
Stress Intensity Factor ◽  
Fatigue Crack ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Crack Propagation ◽  
Fatigue Crack Propagation ◽  
Crack Tip ◽  
42Crmo Steel ◽  
Crack Tip Stress

42CrMo steel is widely used in ultrahigh-strength structures such as low-speed heavy-duty gears. Mastering the fatigue crack propagation law has important significance for predicting structural fatigue life. Firstly, the fatigue crack propagation experiment is used to obtain the upper and lower thresholds value of type I fatigue crack propagation of 42CrMo steel compact tensile specimen under the alternating load of stress ratio R = 0.1. The Paris formula describing the relationship between the fatigue crack propagation rate and the crack tip stress intensity factor between the upper and lower thresholds value is obtained. Scanning electron microscopy was used to observe the microscopic features of different stages of fatigue fracture. The results show that the twin boundary can provide a place for crack initiation; the defects in the material can promote the initiation and extension of fatigue cracks. The fatigue crack propagation of 42CrMo steel compact tensile specimens was numerically simulated by the finite element method. The relationship between the crack tip stress intensity factor and the crack length was obtained. The analysis results show that the crack tip stress intensity factor calculated by the plane finite element method differs slightly from the experimental results during the stable extension stage. After correction, the correlation coefficient between the numerical simulation correction value and the crack tip stress intensity factor value obtained by the experiment is 0.9926. Finally, the fatigue crack propagation rate corresponding to the crack tip stress intensity factor in the finite element results is calculated by the Paris formula and briefly analyzed. Compared with the experimental results, it shows that the numerical simulation is consistent with it, indicating the accuracy of the numerical simulation method, which can effectively predict the initiation and propagation of fatigue cracks in 42CrMo steel compact tensile specimens.

Crack-Tip Shielding Effect on Fatigue Crack Propagation in Porous Silicon Carbide

2011 ◽  
Vol 83 ◽  
pp. 28-34
Author(s):  
Keisuke Tanaka ◽  
Yasuki Kita
Keyword(s):  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Crack Propagation ◽  
Crack Extension ◽  
Crack Tip ◽  
Crack Propagation Rate ◽  
Opening Displacement ◽  
Crack Tip Shielding ◽  
Crack Tip Stress

A sharply notched specimen of porous silicon carbide with porosity of 37% was fatigued under four-point bending. The opening displacement of a fatigue crack was measured at several positions along cracks by using scanning electron microscopy. The crack propagation curve was divided into stages I, II, and III. The crack propagation rate first decreased with crack extension in stage I and became constant in stage II. In stage III, the crack propagation rate increased again. The range of crack opening displacement measured in SEM was lower than that calculated from the applied load range by FEM, suggesting that the anomalous variation of the crack propagation rate with crack extension was caused by crack-tip shielding due to crack face contact. The crack-tip stress intensity factor was estimated as a true crack driving force from the relation between the crack opening displacement and the applied load. The amount of crack-tip shielding increased very quickly with crack extension, reducing the crack-tip stress intensity factor in stage I. In stage II, the increasing applied stress intensity factor is balanced by the increase in the crack-tip shielding. The crack-tip stress intensity factor increases with crack extension in stage III.

scholarly journals Experimental Study on the Caustics of Moving Cracks and Elliptical Curvature under Impact Loading

2021 ◽  
Vol 2021 ◽  
pp. 1-12
Author(s):  
Haohao Luo ◽  
Renshu Yang ◽  
Yanbing Wang ◽  
Guoliang Yang ◽  
Chengxiao Li ◽  
...  
Keyword(s):  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Impact Loading ◽  
Crack Tip ◽  
Test System ◽  
Moving Crack ◽  
Linear Elastic ◽  
Elastic Fracture ◽  
Crack Tip Stress

A dynamic caustics test system was used, and different moving cracks were analysed to study the interaction between the crack growth rate, stress intensity factor, and curvature of the elliptical end of a moving crack under impact loading. Based on the linear elastic fracture mechanics theory, linearly fitting of the crack tip stress intensity factor and the elliptical curvature were employed to obtain the specific functional expressions. ABAQUS software was used to numerically simulate the moving crack fracture process passing through different elliptical curvatures. The crack tip stress intensity factor was calculated by the stress extrapolation method. The stress intensity factor obtained from the numerical calculation and the caustics test was consistent. The test and numerical simulation results showed that the direction of moving cracks entering and passing through the elliptical defects shows a certain regularity. As the ellipse curvature increased, the moving crack stress intensity factor passing through the ellipse gradually decreased, and the moving crack also passed easily through oval defects.

Crack Growth Sensitivity to the Magnitude and Frequency of Load Fluctuation in Stage 1b of High pH Stress Corrosion Cracking

CORROSION ◽  
10.5006/3711 ◽  
2021 ◽  
Author(s):  
Hamid Niazi ◽  
Greg Nelson ◽  
Lyndon Lamborn ◽  
Reg Eadie ◽  
Weixing Chen ◽  
...  
Keyword(s):  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Crack Initiation ◽  
Plastic Zone ◽  
Crack Propagation ◽  
Crack Growth ◽  
Crack Tip ◽  
Cyclic Plastic Zone ◽  
Secondary Cracks

Pipelines undergo sequential stages before failure caused by High pH Stress Corrosion Cracking (HpHSCC). These sequential stages are incubation stage, intergranular crack initiation (Stage 1a), crack evolution to provide the condition for mechanically driven crack growth (Stage 1b), sustainable mechanically driven crack propagation (Stage 2), and rapid crack propagation to failure (Stage 3). The crack propagation mechanisms in Stage 1b are composed of the nucleation and growth of secondary cracks on the free surface and crack coalescence of secondary cracks with one another and the primary crack. These mechanisms continue until the stress intensity factor (<i>K</i>) at the crack tip reaches a critical value, known as <i>K</i><sub>ISCC</sub>. This investigation took a novel approach to study Stage 1b in using pre-cracked Compact Tension (CT) specimens. Using pre-cracked specimens and maintaining <i>K</i> at less than <i>K</i><sub>ISCC</sub> provided an opportunity to study crack initiation on the surface of the specimen under plane stress conditions in the presence of a pre-existing crack. In the present work, the effects of cyclic loading characteristics on crack growth behavior during Stage 1b were studied. It was observed that the pre-existing cracks during Stage 1b led to the initiation of secondary cracks. The initiation of the secondary cracks at the crack tip depended on loading characteristics, <i>i.e</i>., the amplitude and frequency of load fluctuations. The secondary cracks at the crack tip can be classified into four categories based on their positions with respect to the primary crack. A high density of intergranular cracks formed in the cyclic plastic zone generated by low R-ratio cycles. The higher the frequency of the low <i>R</i>-ratio cycles, the higher the density of the intergranular cracks forming in the cyclic plastic zone. The crack growth rate increased with an increase in either the amplitude or the frequency of the load fluctuations. The minimum and maximum crack growth rates were 8×10<sup>-9</sup> mm/s and 4.2×10<sup>-7</sup> mm/s, respectively, with <i>R</i>-ratio varying between 0.2 and 0.9, frequency varying between 10<sup>-4</sup> Hz and 5×10<sup>-2</sup> Hz, and at a fixed stress intensity factor of 15 MPa.m<sup>0.5</sup>. It was found that avoiding rapid and large load fluctuations slowed down crack geometry evolution and delayed onset of Stage 2. The implication of these results for pipeline operators is that reducing internal pressure fluctuations by reducing the frequency and/or amplitude of the fluctuations can expand Stage 1 and increase the reliable lifetime of operating pipelines.

An Experimental Investigation of the Crack Tip Stress Intensity Factors in Plates Under Cylindrical Bending

1962 ◽  
Vol 84 (4) ◽  
pp. 542-546 ◽  
Author(s):  
Fazil Erdogan ◽  
Ozcan Tuncel ◽  
Paul C. Paris
Keyword(s):  
Stress Intensity Factor ◽  
Stress Intensity ◽  
Intensity Factor ◽  
Stress Intensity Factors ◽  
Crack Tip ◽  
Tensile Tests ◽  
Critical Value ◽  
Intensity Factors ◽  
Cylindrical Bending ◽  
Crack Tip Stress

This experimental study was undertaken to investigate the validity of the theory based on the crack tip stress intensity factors to explain the fracture of thin cracked plates subjected to static bending moments. Plexiglas sheets were used as specimens and the loading was pure cylindrical bending. The results indicate that there is in fact a critical value of the stress intensity factor at which the crack starts growing. It was found that, while in static tensile tests the crack growth was unstable, in the case of bending, the external load (here, the bending moment) which starts the crack growing is not sufficient for the complete fracture of the plate if it is maintained constant. That is, when the critical value of the stress intensity factor is reached, the crack starts growing on the tensile side of the plate whereupon the crack tip takes a triangular shape and the system again becomes stable. In order to make the crack grow further, a considerable increase in the load is required.

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