Abstract
Due to stress concentration as well as welding residual stress, fracture due to vehicle fatigue loads is easy to occur in the weld and its adjacent position of long-span bridge, especially at the toe of weld between the U-rib and orthotropic steel bridge deck. In order to investigate the fatigue crack propagation mechanism of the toe of weld in long-span bridge, a multi-scale finite element model including the whole bridge, local components, the welding details and cracks was established firstly. And then, based on birth and death element technology, the welding heat and structural coupling process simulation was carried out in order to investigate the effect of welding residual stress on fatigue performance. Finally, based on the extended finite element method, the static analysis and the dynamic crack propagation analysis for the semi-elliptical crack in the established multi-scale finite element model were conducted. The welding process shows that the peak value of residual tensile stress in weld zone between the U-rib and top deck could reach the material yield strength, while the regions far away from the weld are in the state of compression. A 32ton standard fatigue vehicle specified in British Standard BS5400 was applied for the fatigue crack static analysis. The results show that when the vehicle is acting on different lanes, the fatigue crack initiating from the toe of weld between U-rib and top deck is mainly under compression in most cases. Only in several cases the fatigue crack is in the state of tension, however, the tensile stress required by the fatigue crack propagation has not be reached. Under the 50ton vehicle, the fatigue stress only at the toe of weld of U-rib can meet the requirements of the fatigue crack propagation. The dynamic analysis on fatigue crack reveals that the crack in the toe of weld of U-rib is in tensile stress state, and the fatigue crack is the mixed mode crack of Mode I, II and III. When the residual stress field was introduced, the stress state at the region where fatigue crack initiates will change and the angle of fatigue crack propagation will increase. The crack at the toes of weld of top deck under the 32ton and 50ton fatigue vehicles is mainly under compression. When the residual stress field was introduced, the crack will be subjected to be a tensile state. It will become mixed mode crack of Mode I, II and III and be dominated by Mode I.
Highlights
As one of the most fatigue-sensitive parts of an orthotropic steel bridge deck, the weld between the U-rib and the top deck is prone to fatigue cracking under the actions of the stress concentration, welding residual stress, and vehicle load
One method is based on the stress-life curve (S-N curve) evaluation method, while the other method is based on the fracture mechanics method
It has been proven that the evaluation method based on fracture mechanics has inherent advantages in predicting the fatigue life of steel bridges [4], which can overcome the deficiency of the evaluation based on the S-N curve method
Summary
In the XFEM, the element grid is divided into three types, as shown in Figure 1: conventional finite elements, elements penetrated by cracks and elements with crack tips. For an element penetrated by a crack and a crack tip element, because their displacement functions are discontinuous, their stiffness matrices are discontinuous When the maximum energy release rate is greater than its threshold, fatigue cracks initiate and propagate. Closure technology (VCCT) [17] is used to calculate the energy release rate ΔGi of the ith element, and the cyclic number increment ΔNi of the ith element can be obtained by the integral form of the Paris formula. Closure technology (VCCT) [17] is used to calculate the energy release rate ΔGi of the ith element, and the cyclic number increment ΔNi of the ith element can be obtained by the integral form of the Paris formula. e element with the smallest increment of the cycle number begins to crack and is assumed to be the kth element. e propagation length Δak and the increment of the cycle number ΔNk of the kth element are recorded and the crack length aj is updated to be equal to (aj−1 + Δak) and the cycle number Nj is equal to (Nj−1 + ΔNk). e cycle is entered until the maximum number of cycles jmax set by the program is reached
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