Investigating the damage development in threaded fasteners by simulating crack-propagation using numerical methods
Doctoral thesis
Accepted version
Permanent lenke
https://hdl.handle.net/11250/3131819Utgivelsesdato
2023Metadata
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Sammendrag
This project aims to use crack-propagation modelling to investigate a real case in the industry. In particular, the investigation is directed towards the development of a fatigue crack in a connecting rod big end bolt using numerical methods, where both the complex loading from the connecting rod operating environment and the residual stress and hardening from the manufacturing process for the bolt (cold-rolling) are included. Several models based on the Finite Element Method (FEM) were developed to obtain the stress field of the bolt in its pre-cracked state. The operating load was obtained by simulating the connecting rod with engine loads applied, then using the solution to drive the boundary conditions of a sub-model containing the bolt with its threaded connection. The cold-rolling process was simulated to obtain the residual stresses and these were mapped onto the connecting rod bolt sub-model.
Preliminary analyses of the bolt in a pre-cracked state highlighted the first loaded thread as the zone most susceptible to fatigue, particularly in the section of thread positioned away from the connecting rod big-end bearing. This corresponded with the observed initiation site: running the model with residual stress and hardening did not change this conclusion.
Once the initial state and load conditions of the bolt were obtained, the crackpropagation was simulated using the Boundary Element Method (BEM). A computationally efficient, weakly coupled FEM-BEM approach was used to simulate fatigue crack propagation for the connecting rod bolt including the residual stress field from cold-rolling and operational loads corresponding to those from engine operation. Two models, one including the residual stress and one without, had propagating fatigue cracks simulated until fracture occurred. This allowed for an investigation and lifetime estimation of the fracture, and the comparison with a model lacking the residual stress allowed for an investigation of the role played by these stresses.
It was found that the residual stress field had a significant influence on the direction of the crack-growth, which lead to a distinct form to the fracture surface. This fracture geometry corresponded well with the fracture surface observed in a real specimen. The estimated fatigue lifetime correlated with what was observed in reality and the model served as validation for the case study of this thesis.
The benefits of cold-rolling threads are demonstrated to be reduced due to the high mean stress in the first loaded thread on a bolted join with a realistic level of pre-load, and this may be further reduced by surface damage. In order to achieve a more thorough understanding of fatigue in cold-rolled threaded fasteners, experiments at high load ratios emphasising stress intensity range thresholds would be needed.
Består av
S.S. Haugland, R.J. Grant, K.E. Frøysa, R. Nordrik, "Finite Element Analysis of Thread Loads in a Connecting Rod Bolt", NAFEMS Nordic regional conference, 2020S.S. Haugland, R.J. Grant, K.E. Frøysa, R. Nordrik, "Fatigue crack growth of a bolt loaded in tension using the boundary element method", NAFEMS World congress, 2021
S.S. Haugland, R.J. Grant, "A parametric study of threaded fasteners using two-dimensional conceptual models", TBA
Based on Chapter 5 (planned): Working title "3D FEA of cold rolling process for a high-strength threaded fastener"
Based on Chapter 7.2 (planned): Working title "Modelling of fatigue crack growth in a threaded fastener under the influence of residual stress"