Research On Fatigue Property Of Aluminum Alloy Cruciform Joint Using Structural Stress Method

The traditional nominal stress method cannot reflect the influence of geometry on fatigue strength, and the hot-spot stress method shows bad convergence with the grid density. The structural stress method shows well me sh insensitivity because it takes advantage of elementary structural stress theory to calculate the hot spot stress, and uses a single master S-N curve to calculate the fatigue strength of complicate welded structure, so this method has considerable practical value. This study uses structural stress method to study the geometrical influence of cruciform joints on its fatigue strength. The main emphasis is laid on using finite element calculation to analyze the relationship between geometry of cruciform joints and the stress distribution. Also, the design rule for load-carrying cruciform joint is provided. The main work is listed below.Programming the post-processing of structural stress calculation according to the force equilibrium and moment equilibrium principle, and fitting the constants according to the low-frequency fatigue test results of 7N01 non-load carrying cruciform joints. Figure out the master S-N curve based on structural stress method. Compare the master S-N curve with the nominal stress based S-N curve to illustrate its prominent linear relationship.Analyze the influence of weld quality, plate thickness and weld toe geometry on fatigue life of non-load carrying cruciform joint. The small initial defect(about 0.01T) will reduce only 5% of the total fatigue life, while the initial crack with the size of 0.1T will reduce about 50% of the total fatigue life. Analyze the influence of plate thickness on fatigue life, and find that when the nominal stress equals 80 MPa, the fatigue life will be reduced by 60% if plate thickness increases from 6mm to 20 mm, since the increasing plate thickness will raise up the stress concentration factor of weld toe, making the stress intensify factor much higher. However, the attachment thickness has little influe nce on the fatigue life. If the weld toe is increased from 6mm to 12 mm, the fatigue life will decrease about 8%.Using the fatigue test and finite element analysis to figure out the master S-N curve for weld root failure of load-carrying cruciform joint, and find out that the master S-N curves for the two failure mode are significantly different. The first possible reason is that the material propert ies of the crack propagation path for the two failure mode are different. Also, the curvature radius of the t wo stress concentration sites is different.Increasing the lack-of-penetration(LOP) size will significantly reduce the fatigue life of both the weld toe failure and weld root failure. When the plate thickness equals 12 mm, and the nominal stress equals 80 MPa, the fatigue life of the weld toe failure and weld root failure will be reduced 35% and 96% if the LOP size increased from 4mm to 12 mm. Similarly, increasing the weld toe size will significantly increase the fatigue life of both the weld toe failure an d weld root failure. The fatigue life of the weld toe failure and weld root failure will be increased 1.75 times and 26 times if the weld toe size increased from 6mm to 14 mm. The reason for this difference is that although increasing the LOP size(or reducing the weld toe size) will increase the SCF of both the weld toe and weld root, the crack propagation of only the weld root failure is much reduced.Compare the fatigue strength weld toe failure and weld root failure, and obtain the critical weld size with equal failure possibility. When the plate thickness equals 6mm, 8mm, 12 mm and 20 mm, the critical weld size equals 1.3, 1.1, 1.3, 1.7 times of the plate thickness, respectively. If the weld size is larger than the critical size, the fatigue failure tends to occur at the weld toe instead of the weld root.