Numerical Simulation Method Of Fatigue Crack In Rib-to-Deck Welded Joints Of Orthotropic Steel Bridge Deck

The rib-to-deck welded joint is a typical fatigue detail in orthotropic steel bridge deck.Because the deck is under frequent vehicle loads during the service period,and the initial defect is easy to be produced during welding,fatigue cracks are easy to appear at weld root of this position.To understand the fatigue performance of the orthotropic steel bridge deck better,researching the fatigue propagation law in this position and accurately assessing the fatigue life are of great significance.This thesis did some research on numerical simulation of fatigue crack in rib-to-deck welded joints and the mechanism of the fatigue cracking.The main research work and conclusions are listed as follows:(1)A new method was developed for simulating elliptical and semi elliptical fatigue crack propagation.This method are based on the principle of the Linear Elastic Fracture Mechanics,the Interaction Integral,and the Paris formula.This method can be realized by using ANSYS to program command file which is similar to using a programming language.During the process of the method,the 1/4 point singular element and the Interaction Integral method in ANSYS are needed to calculate the stress intensity factor range.(2)The fatigue crack at weld root of the rib-to-deck welded joint was simplified to a single semi elliptical crack.The above method was used to carry out numerical simulation of fatigue crack propagation.By changing the thickness of the deck and the form of the U-rib,setting the initial crack at weld root and weld toe of the deck plate respectively,it can be found that,in the orthotropic steel bridge deck fatigue experiment model of Qingshan Yangtze River Highway Bridge,the weld root is more prone to crack than weld toe of the deck plate,and from the point view of fatigue crack propagation simulation,Upset U-rib has no improvement on fatigue performance,while increasing the deck thickness can effectively improve the fatigue performance of the structure.(3)Through the laws that the numerical simulation shows,the fatigue crack propagation laws of the concerned areas can be researched.It is showed that,in the longitudinal direction,the negative impact of the initial defect location within a certain distance is almost the same.The effect of initial crack shape on the propagation rate and fatigue life is mainly reflected in the initial stage of crack propagation.The initial cracks in different shapes will become the same stable shape after a period of propagation.After the shape has maintained for another period of time,the crack will gradually become very flat.(4)By studying the change law of the stress intensity factor range of the deepest point of crack,it can be found that when the depth of crack is more than about half of the thickness of the deck,the stress intensity factor range of the deepest point will be decreased.This means that the crack propagation rate in the depth direction will slow down.If the stress intensity factor range becomes less than the threshold value,the crack propagation in the depth direction will tend to stop,and the crack will not appear on the deck surface.(5)The stress intensity factor range of the deepest point and end point in length direction of the crack was researched.The relationship between the stress intensity factor range and the crack shape was also researched.It can be found that,when the crack depth is the same,the deepest point of longer crack has larger stress intensity factor range.This means that more flat crack tend to propagate in the depth direction.On the contrary,less flat crack tend to propagate in the length direction.