Fracture Simulation newsletter
An interesting paper was presented at the recent Aircraft Structural Integrity Conference held in Jacksonville in November 2017 by Mark Ryan of Lockheed Martin.
Fracture Mechanics and Risk Methods Used to Analyze the F-16 Wing Carry Through Bulkhead (WCTB) Upper End Pad Radius
The presentation describes a study of cracking found in a bulkhead structure. BEASY simulations were used to predict SIFs and crack shape evolution.
Models were typically created from existing F-16 ABAQUS finite element models using the BEASY Abaqus Interface.
For further information please contact us
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
BEASY is pleased to announce the release of a new version of the BEASY Fatigue & Crack Growth Software.
The primary benefits of this new release are that it provides a further enhancement of the software's automatic crack growth capabilities and major improvements in the over simulation times.
For typical models, the remeshing time has been reduced by between 40% and 80%.
This release incorporates a significant improvement to the adaptive meshing routines. As a result cracks are inserted into models more quickly, even in areas of complex geometry. The crack surface mesh quality is also improved with a continuous quadrilateral mesh now created along the entire crack front. This further improves the accuracy of the SIF solution and provides greater stability when running fatigue crack growth simulations.
Other significant enhancements include:
Improved BEASY remeshing:
- Reductions in the remeshing times
- Improved meshing for re-entrant corners
- Improved meshing for partial crack growth
- Mesh trimming of surface breaking cracks
- Improved meshing for cracks crossing zones
J-Integral ring quantity improved
Enhanced crack library
New flight block based load spectrum file
Improved interfacing with FE models
Creation of a crack face loading routine
Reduced solver time
Extension of FE model creation and residual stress simulation to 2D
Improvements to model creation from PATRAN
New structure for zone interface cracks
Additional ABAQUS support features
BEASY staff attended the 2015 AA&S Conference in Baltimore in April and met with many customers to discuss their applications and update them about projects 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, fracture behaviour of composite plates as well as the modelling of corrosion related damage. There was also considerable interest in the BEASY Corrosion Manager software which is used to predict and simulate galvanic corrosion in structures such as aircraft.
Of particular interest was the impact of residual stresses on the crack path and the rate of growth of cracks. The effect on the crack path can be clearly seen in the above figure where the lines show the predicted crack fronts as the crack grows from a hole with, and without, cold working. The red lines show the predicted crack fronts without cold working and the green dotted lines show the predicted crack fronts with cold working. The study clearly indicates the need to include residual stresses in crack growth calculations as otherwise unexpected failure modes can occur, and excessively conservative designs can result.
Please contact us for background papers on some of the topics presented at AA&S 2015, quoting the name(s) of the papers below:
- Analysis of Fatigue Crack Growth for CFRP-Strengthened Steel Plates with Longitudinal Weld Attachments
- Analysis of CFRP reinforced steel plates
- Analysis of fatigue crack growth for welded connections under bending
- Calculation of bending fatigue life of thin-rim spur gears
- Analysis of fretting fatigue life of dovetail assemblies based on fracture mechanics method
Another area of considerable interest at the AA&S conference was the use of computer simulation to assess the risks associated with corrosion. During the product development process, various design configurations are often exposed to actual environmental conditions for an extended period of time to evaluate corrosion damage in the structural assembly. However, these testing methods require anywhere from several months to several years of exposure time in order to complete. Computer modelling has the potential to significantly shorten, and reduce, the cost of testing by providing a corrosion simulation option that can be used to supplement these long term experimental tests.
In the model shown a computer simulation is used to predict the risk and extent of corrosion damage on a test structure.
Dr Sharon Mellings from BEASY recently attended the ICAF conference in Helsinki and presented a paper describing some recent work on the simulation of corrosion and fracture damage.
Airframe structures regularly operate in environments that can create conditions which allow high levels of corrosion damage, and this damage can lead to stress concentrations within the structure and potential development of cracks. The generation of electrical fields that are necessary for corrosion can occur even when only a thin film of electrolyte is present on the surface of the structure.
Computation of this electrical field can be used to identify areas in the airframe structure that are most susceptible to corrosion damage and which, after possible fatigue crack initiation, may lead to structural failure. Corrosion simulation, by taking account of the properties of the electrolyte and the structural materials, can determine the rate of material loss from the structure.
In this conceptual development, material is removed from the modelled surface (corresponding to corrosion occurring over a given exposure time) the stress concentrations can be evaluated and, if required, cracks can be introduced into the identified problem areas, to identify vulnerability to fatigue failure.
The geometry change caused by corrosion mass loss can be used to perform stress analysis of the structure, to determine the stress concentration in the component at the corresponding time in the life of the aircraft. If it is then assumed that cracks initiate at the peak stress locations, the subsequent crack growth can be simulated. This crack growth takes into account the corrosion damage and will inherently include local stress concentration due to the damaged surface. In the crack growth simulation, the full crack path and direction can be determined together with the fatigue life.
For a copy of the paper, or for more information about corrosion and fracture simulation, please contact us
BEASY will be attending the Aircraft Airworthiness & Sustainability Conference where we will be presenting our corrosion and fracture mechanics simulation solutions for aerospace structures. The conference, will take place between March 30th and April 2nd in Baltimore, Maryland.
We welcome you to come and visit us on booth 206 to find out more about the capabilities of BEASY simulation products, or how BEASY fracture modelling can provide the solutions you need. Alternatively to make an appointment to meet Tom Curtin at the conference, or to obtain further information, please contact us