Fatigue Assessment Of Welded Structure And Joint Of Loader Front Frame

Loader is a kind of earth rock construction machinery widely used in construction projects.As a typical welded structure,the front frame is the main bearing component of the loader.The front frame is prone to fatigue failure under the action of alternating load,so it is urgent and significant to carry out accurate fatigue assessment and performance improvement.First of all,according to the actual size of the front frame,the finite element model is established and the grid is divided.The finite element software ANSYS is used to simulate the service condition of the front frame,and the nephogram of deformation,stress and strain is obtained.Based on the program of ASME structural stress method,the structural stress,fatigue life and fatigue damage of all welds of front frame are calculated,and the position of dangerous welds is determined.The results show that: among all the welds of the front frame,the weld located at the intersection of the transverse plate on the top of the bogie and the steering cylinder has the largest damage,and the fatigue damage value reaches 0.066 under the alternating load of 240 MPa.The stress concentration of this weld is relatively serious and the stress is large,which is the main bearing part.Afterwards,in view of the dangerous weld,the measures to improve the fatigue performance mainly by ultrasonic impact are put forward,and the influence of ultrasonic impact treatment on the fatigue performance of welded joint is studied.The results show that: ultrasonic impact can introduce compressive stress in the thickness direction,the maximum thickness of the introduced compressive stress layer can be up to 2 mm,and the maximum residual compressive stress appears at the place 1 mm away from the weld toe,whose value is more than 400 MPa,close to the yield strength of the material;ultrasonic impact can also introduce compressive stress in the surface direction,the maximum compressive stress range is 7.5 mm away from the weld toe,and the maximum compressive stress is 170;The ultrasonic impact can form a transition arc with a certain radius at the weld toe,change the geometry of the weld toe,reduce the stress concentration and improve the fatigue performance of the join.Finally,the typical welded joint types in the dangerous weld are extracted,fatigue test and corresponding finite element simulation are carried out,and the local strain energy density method is used to post process the test and simulation data.The influence of size effect(thickness ratio of main board and web,plate thickness,penetration,weld leg size)on the fatigue failure position is analyzed with the local strain energy density as the parameter.The results show that:The local strain energy density method can be used not only in the fatigue assessment of weld toe failure,but also in the fatigue assessment of weld root failure,the fatigue cracks at different positions will affect the stiffness of the structure,and then affect the fatigue life of the structure;for cracks of the same length,the stiffness of the structure is smaller when crack at the weld root than that at the weld toe.The weld leg size and penetration of the welded joint will affect the fatigue performance of the joint.The strain energy density of weld root and weld toe decreases with the increase of weld foot and penetration.The strain energy density based fatigue failure transverse region is reasonable and feasible.Based on the calculation of strain energy density,it is found that when the joint penetration is from 0.0 to 0.2,the relative weld leg size corresponding to the failure transition interval is 0.64 to 1.12.