Discrete Dislocation Model of a Fatigue Crack Under Shear Loading—Part 1

1969 ◽  
Vol 36 (4) ◽  
pp. 723-730 ◽  
Author(s):  
D. O. Swenson
Keyword(s):  
Fatigue Crack ◽  
Crack Propagation ◽  
Shear Loading ◽  
Dislocation Model ◽  
Shear Stresses ◽  
Mean Stress ◽  
Stress Range ◽  
Equilibrium Equations ◽  
Stage 1 ◽  
Tip Displacement

Fatigue crack propagation in most engineering materials has an initial stage of crack propagation, commonly designated Stage 1. Stage 1 cracks initiate and propagate from slip lines on the crystallographic planes of both polycrystal and single crystal alloys. A Stage 1 crack is usually thought to propagate solely by local shear stresses. A computer model of a crack and its plastic zones is developed. The positions of a series of dislocations are calculated in order that their combined stresses satisfy equilibrium equations and crack boundary conditions. The range of the crack tip displacement per loading cycle, which appears to be the best model for incremental crack growth, is independent of the mean stress value for mean stress greater than one-half the maximum stress and the stress range small. For all mean stress levels a minimum stress range is predicted by this program below which the cyclic tip displacement is zero.

Mean Stress Effect on the Shear Fatigue Crack Model—Part 2

1969 ◽  
Vol 36 (4) ◽  
pp. 731-735 ◽  
Author(s):  
D. O. Swenson
Keyword(s):  
Crack Growth ◽  
Shear Crack ◽  
Crack Tip ◽  
Shear Loading ◽  
Stress Effect ◽  
Mean Stress ◽  
Stress Range ◽  
Cyclic Shear ◽  
The Mean ◽  
Tip Displacement

An expanded version of Swenson’s shear crack fatigue model is developed to include the effect of the mean stress on cyclic shear crack growth. Five results are concluded for this model. 1 As the mean shear stress is increased for a specified stress range, the cyclic shear crack tip displacement decreases. 2 The shear displacement at the center of the crack appears to be independent of the mean stress and remains constant for a given stress range. 3 An inherent blunting process for crack growth predicts retardation of tip displacement and crack growth for small values of stress range and all mean stress values. 4 The overall plactic zone length for this model does not change during cyclic shear loading although plastic deformation is occurring. 5 The model’s crack tip displacement per cycle is a linear function of the half crack length for prescribed stress range, mean stress, and material properties.

scholarly journals Identification and Prediction of Mixed-Mode Fatigue Crack Path in High Strength Low Alloy Steel

Proceedings ◽  
2018 ◽  
Vol 2 (8) ◽  
pp. 504
Author(s):  
Jie Zhang ◽  
Cedric Kiekens ◽  
Stijn Hertelé ◽  
Wim De Waele
Keyword(s):  
Fatigue Crack ◽  
Digital Image Correlation ◽  
Crack Propagation ◽  
Digital Image ◽  
Mixed Mode ◽  
Standardized Test ◽  
Crack Tip ◽  
Shear Loading ◽  
Image Correlation ◽  
Crack Trajectory

The trajectory of fatigue crack growth is influenced by many parameters and can be irregular due to changes in stress distribution or in material properties as the crack progresses. Images of the surface of a standardized test specimen can be used to visualize the crack trajectory in a non-destructive way. Accurately identifying the location of the crack tip, however, is challenging and requires devoted image postprocessing. In this respect, digital image correlation allows to obtain full field displacement and strain fields by analysing changes of digital images of the same sample at different stages of loading. This information can be used for the purpose of crack tip tracking. This paper presents a combined experimental-numerical study of detection and prediction of fatigue crack propagation path by means of digital image correlation (DIC) and the extended finite element method (X-FEM). Experimental validation and analyses are carried out on a modified C(T) specimen in which a curved crack trajectory is triggered by introducing mixed-mode (tension + shear) loading. The developed tools are used for validating an automated framework for crack propagation prediction.

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