BEASY staff attended the 2016 AA&S Conference in Grapevine, TX, and met with many customers to discuss their applications and developments underway at BEASY. We had a lot of new interest at our booth this year in the areas of fatigue crack growth in residual stress fields and the modelling of corrosion related damage.
Dr Sharon Mellings from BEASY also jointly authored a paper with Keith Hitchman and Joy Ransom of FTI on some recent work on the prediction of Crack Propagation through Cold Expansion Residual Stress Fields.
Analytical Verification of Crack Propagation through Cold Expansion Residual Stress Fields
Sharon Mellings, John Baynham - CM BEASY Ltd, Keith Hitchman, Joy Ransom - FTI
Figure 1 High Load Transfer Specimen with Split Sleeve Cold Expansion (section view)
Split-Sleeve Cold Expansion (SsCx) of holes is a widely adopted method to enhance the fatigue life of such holes in metallic structures by the generation of deep residual stress fields, the efficacy of which has been well documented in numerous specimen, component, and full scale physical test programs.
While physical tests will always be the "gold-standard" for confirmation of the fatigue life improvement realized by using SsCx, such tests are costly and time consuming. Computational resources improve in efficiency and cost year-over-year, and have become a widely used tool in solving the unique analytical problems posed by fracture mechanics within a domain containing residual stresses. Typically, commercial mature non-linear finite element (FE) computational tools are utilised to determine the residual stress field, and those localised fields are then combined with the far field cyclic stress using superposition principles. More advanced fracture mechanics codes have developed a novel capability to incorporate this complex loading in a fracture analysis.
BEASY was used to perform fatigue crack growth simulations, applying boundary conditions for both the far field cyclic loading and the residual stress field. The automatic crack growth process uses optimised adaptive surface meshing routines around the crack and breakout edges which results in a reliable simulation that accumulates incremental growth vectors to predict the position of successive crack fronts. The software creates the surface of the new grown crack, automatically repeating the process until a defined growth criterion is satisfied.
The software provides a realistic crack growth simulation, including the crack path and associated number of load cycles required to advance the crack at each increment, incorporating the effects of the local residual stress field.
This presentation describes the use of BEASY to provide analytical estimates of crack propagation rates through the residual stress field surrounding an open hole that has been cold expanded using the SsCx process. The effect of the SsCx process in 2000-series aluminium was modelled by FTI using non-linear finite element analysis. The resultant residual stress field was then used to define appropriate crack-face tractions for a BEASY crack propagation analysis under constant amplitude loading.
Figure 2 Selected Test Case
The presentation discusses the following results:
* Residual stress field prediction, including likely crack initiation location
* Stress intensity predictions along the crack front at various crack lengths
* Predicted crack front shape throughout crack growth history
* Comparison of predicted crack growth rate and crack growth evolution with available physical test data
Click here to download the presentation