CN109141849A - A method of improving boom structure fatigue life - Google Patents

Abstract

The invention proposes a kind of methods for improving boom structure fatigue life, and the residual stress by handling local weld seam is realized, comprising the following steps: the stress envelope for obtaining weld seam determines the local danger position of weld seam under corresponding dangerous working condition according to stress value；It obtains under corresponding dangerous working condition, the welding residual stress distribution map at local danger position；It is estimated in conjunction with welding residual stress distribution map, and to influence of the residual tension to fatigue strength at local danger position, the processing method and technological parameter of welding residual stress is determined according to welding residual stress, and handle the local danger position.The present invention is by determining the maximum local danger position of weld seam residual tension and handling for the local danger position, to accurately improve the fatigue life of the boom structure.

Description

A method for improving fatigue life of boom structure

technical field

The invention relates to the field of boom structure evaluation and detection, in particular to a method for improving the fatigue life of a boom structure.

Background technique

The boom is an important part of construction machinery, which is connected with the frame, rocker arm, oil cylinder and shovel (digging) bucket. It is generally welded by two boom plates, support plates and elliptical cylindrical beams. It is the key support and main stress structure in the working device of construction machinery. In addition to bearing a large static working load during the working process, it also needs to Bear a certain impact load. The welding seam of the boom structure is affected by the welding process and is a part that is more prone to cracking. There are two methods for solving this problem in the prior art: one is to enhance the strength design of the weld from the design point of view; the other is to improve the strength of the weld by processing after the welding is completed. From the design point of view, there are relatively many problems involved, so the post-weld treatment method is a more reasonable choice. The complexity of boom structure welding leads to a certain residual tensile stress in the weld and its vicinity after the boom is welded, and the existence of residual tensile stress will reduce the fatigue strength of the weld.

At present, the treatment methods that can be used for the residual stress of the welding seam of the loader arm include welding toe grinding, ultrasonic impact, etc., and the treatment position is the entire welding seam. Then, the current processing method does not achieve the two principles of precision and accuracy, but blindly processes the entire weld, the time required for the production cycle is far greater than the local processing, and the production time is greatly increased at the same time. Costs will also increase. Therefore, how to accurately improve the fatigue life of the welding seam, that is, the power arm structure by processing the local part of the welding seam, is a problem that needs to be solved in the prior art.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method for improving the fatigue life of the boom structure. By determining the local dangerous position with the largest residual tensile stress of the weld and processing the local dangerous position, the fatigue life of the boom structure can be accurately improved. . To achieve the above object, the present invention adopts the following technical solutions:

A method for improving the fatigue life of a boom structure, comprising the following steps:

Step A: Obtain the stress distribution diagram of the weld, and determine the local dangerous position of the weld under the corresponding dangerous working condition according to the stress value;

Step B, obtaining the welding residual stress distribution diagram at the local dangerous position under the corresponding dangerous working condition;

Step C: Estimate the influence of the residual tensile stress at the local dangerous position on the fatigue strength in combination with the welding residual stress distribution diagram, and judge whether the local dangerous position needs to be treated with welding residual stress; if so, go to step D; otherwise , then it ends;

Step D: Determine the processing method and process parameters of the welding residual stress according to the welding residual stress, and process the local dangerous position.

Preferably, in step A, a network model of the boom mechanism is established by the finite element method, and material properties, parameters, loads and constraints are defined; The stress distribution map.

Preferably, in step A, the dangerous condition includes a limit positive load condition and a limit eccentric load condition.

Preferably, in step B, the welding residual stress distribution map at the local dangerous position is obtained by the finite element method; the specific steps include: establishing a network model of the boom mechanism by the finite element method, using the finite element welding software to simulate, setting The welding parameters are simulated and calculated to obtain the welding residual stress.

Preferably, in step D, the processing method of the welding residual stress includes an ultrasonic impact method.

Preferably, the process parameters include a force value during impact; the force value ranges from 320 MPa to 390 MPa.

Preferably, in step D, the processing method of the welding residual stress includes welding toe grinding or shot peening.

Preferably, in step D, the processing method of the welding residual stress includes TG remelting.

Preferably, in step C, if the maximum value of the welding residual tensile stress at the local dangerous position exceeds the comparison value, the welding residual stress is processed.

Compared with the prior art, the present invention has the advantages that: the present invention analyzes the weak dangerous position of the welding seam through research, and then processes the residual stress there. In the process of residual stress treatment, precise control of process parameters not only ensures the release of residual tensile stress, but also avoids damage to the boom structure. Therefore, the present invention replaces the method of processing the entire weld in the prior art by processing the residual tensile stress of the weld at the dangerous position, and accurately improves the fatigue life of the boom structure.

Description of drawings

1 is a schematic diagram of a boom structure in the prior art;

2 is a flowchart of a method for improving the fatigue life of a boom structure according to an embodiment of the present invention;

3 is a schematic diagram of a load under a limit positive load condition in a method for improving the fatigue life of a boom structure according to an embodiment of the present invention;

4 is a schematic diagram of a load under a limit eccentric load condition in a method for improving the fatigue life of a boom structure according to an embodiment of the present invention;

Figure 5 is a schematic diagram of a local dangerous position of the weld in Figures 3 and 4;

Fig. 6 is the welding residual stress distribution diagram at the second fillet in Fig. 3;

Fig. 7 is the welding residual stress distribution diagram at the first fillet in Fig. 4;

Among them, 1- boom plate, 2- beam, 3- lug plate, 4- front side above beam, 5- rear side above beam, 21- first fillet, 22- second fillet.

Detailed ways

The method for improving the fatigue life of the boom structure of the present invention will be described in more detail below with reference to the schematic diagrams, wherein the preferred embodiments of the present invention are shown, and it should be understood that those skilled in the art can modify the present invention described herein and still realize the present invention. Advantageous effects of the invention. Therefore, the following description should be construed as widely known to those skilled in the art and not as a limitation of the present invention.

In this embodiment, taking the welding of a certain type of boom as an example, as shown in FIG. 1, the boom structure is composed of a boom plate 1, a beam 2, a support plate and a rib plate welded. The analysis of the weld between the boom and the beam 2 shows that residual tensile stress will be generated at the weld after welding.

As shown in Figure 2, a method for improving the fatigue life of the boom structure is realized by processing the residual stress of the local weld, including the following steps:

Step A: Obtain the stress distribution diagram of the weld, and determine the local dangerous position of the weld under the corresponding dangerous working condition according to the stress value;

Step B, obtaining the welding residual stress distribution diagram at the local dangerous position under the corresponding dangerous working condition;

Step C: Estimate the influence of the residual tensile stress at the local dangerous position on the fatigue strength in combination with the welding residual stress distribution diagram, and judge whether the local dangerous position needs to be treated with welding residual stress; if so, go to step D; otherwise , it ends.

Step D: Determine the processing method and process parameters of the welding residual stress according to the welding residual stress, and process the local dangerous position.

In this embodiment, in step A, the dangerous condition includes a limit positive load condition and a limit eccentric load condition. During the use of the boom structure, the dangerous working conditions are divided into extreme positive load and partial load conditions. The extreme positive load condition is when the boom is in the state of extreme rise; the partial load condition is based on the stability of the whole machine itself and the maximum output of the oil cylinder. The most dangerous stress situation obtained by comprehensive analysis of the pressure of the tire, the friction coefficient between the tire and the ground, and the size parameters of the whole machine. After determining the ultimate positive load and eccentric load conditions of the whole machine during the working process, finite element analysis or theoretical calculation can be used to determine the stress distribution map and local dangerous position of the boom weld under dangerous conditions. In this embodiment, finite element analysis is used to establish a network model of the boom mechanism through the finite element method, and material properties, parameters, loads and constraints are defined; finite element analysis is performed on the finite element model combined with the working conditions to obtain the corresponding dangerous work conditions. The stress cloud diagram of the condition, that is, the stress distribution diagram.

Under the extreme positive load condition, the force of the boom plate 1 and the lug plate is shown in Figure 3. Among them, the applied force loads at the holes on the left and right boom plates 1 are 211KN and 90KN respectively; the applied force loads at the two holes on the lug plates are 216.5KN and 211KN respectively. The finite element calculation limit is used. The stress state of the weld under the working conditions, extract the stress value with the larger stress of the weld, among which the stress value at the 5 rounded corners above the rear side of the beam, that is, at the first rounded corner 21, is the largest, about 145MPa, which is the maximum stress value of the weld. Locally hazardous location.

In the extreme eccentric load condition, the force of the boom plate 1 and the lug plate is shown in Figure 4. Among them, the load at A is about 1028kN, the load at B is about 646kN, the load at C is about 800kN, and the load at D is about 789kN. The finite element analysis is used to analyze it. According to the finite element analysis results, it can be seen that the stress value at the 4 transition fillets on the front side above the beam, that is, the second fillet 22, has a maximum stress value of about 228MPa, which is a local dangerous position of the weld.

To sum up, the local dangerous positions of the boom welding seam are the two transition fillet positions above the beam 2, that is, the first fillet 21 and the second fillet 22, as shown in FIG. 5 .

In this embodiment, in step B, the welding residual stress distribution map at the local dangerous position is obtained by the finite element method; the specific steps include: establishing a network model of the boom mechanism by the finite element method, and using the finite element welding software to simulate, The welding residual stress is obtained by solving the simulation calculation by setting the welding parameters. , among them, the finite element welding software includes simufact welding and Abaqus. After the boom structure is welded, there will definitely be a certain residual tensile stress at the weld and its vicinity, which will reduce the fatigue strength of the weld. The value and distribution of the residual stress of the weld can be obtained through the residual stress test, and the welding residual tensile stress and distribution at the local dangerous position of the boom can also be obtained through the finite element simulation method. For this embodiment, the finite element simulation method is used to obtain the welding residual tensile stress and distribution at the most dangerous position of the boom, and the welding process simulation calculation is performed according to the actual welding process parameters, and the residual stress of the welding seam is obtained after the simulation is completed. As well as the stress field distribution, the stress distribution diagrams of the residual tensile stress at the two dangerous fillet parts above the beam 2, that is, the second fillet 22 and the first fillet 21 are shown in Figures 6 and 7 . In Figure 6, the abscissa indicates that the arc length of the second fillet is 120mm. After the second fillet is expanded, the second fillet starts to end, and between each point on the second fillet and the welding toe, along the second fillet The distance of the rounded corner, that is, the arc length corresponding to each point on the second rounded corner. In Figure 7, the abscissa indicates that the arc length of the first rounded corner is 120mm. After the first rounded corner is expanded, the first rounded corner starts to end, and between each point on the first rounded corner and the weld toe, along the first rounded corner The distance of the corner, that is, the arc length corresponding to each point on the first rounded corner. In this embodiment, the welding toe located on the upper surface of the beam 2 is the starting point, that is, the coordinate of this welding toe is 0.

In step C, if the residual tensile stress of the weld in the dangerous position is very small, after the residual tensile stress is processed, it is not enough to significantly affect the fatigue life of the weld; if the residual tensile stress of the weld in the dangerous position is large, then The residual tensile stress can significantly improve the fatigue life of the weld. According to the above analysis, it is determined whether to carry out the residual tensile stress relief process, that is, step D. The influence of residual tensile stress on fatigue strength can be calculated according to the relationship, and the specific value depends on the design strength and actual strength of the boom structure. In this embodiment, the dangerous position is the two fillet transition areas above the weld beam 2. According to the residual stress simulation results, it can be seen that the residual tensile stress at the four fillet weld toe parts is relatively large, and the maximum value is about 310MPa, which is conservative. According to calculation, the influence of residual tensile stress on fatigue strength is calculated by 0.1 times of residual tensile stress, which can reduce the fatigue strength of weld by about 31MPa. The existence of residual tensile stress has a significant impact on the fatigue life of weld, so the residual tensile stress Elimination will significantly enhance weld fatigue strength. Combining with the working stress state of the weld, it is determined that the two transition fillets above the beam 2, namely the first fillet 21 and the second fillet 22, are the best processing positions to improve the fatigue life.

In step D, the processing method of welding residual stress includes ultrasonic impact method. The process parameters include the force value during impact; the force value ranges from 320MPa to 390MPa. Treatments to reduce the residual tensile stress include ultrasonic impact, weld toe grinding, and methods of forcing plastic deformation of the weld to release the residual tensile stress. For different parts and different parts, the optimal treatment process is different. The selection of the optimal treatment process needs to comprehensively consider factors such as the implementation effect of the treatment process, the convenience of implementation, and the stability of the treatment process, and select the best treatment method. When the residual stress of the weld is processed, the parameters of the process need to be precisely controlled to ensure that the residual stress is processed without causing damage. Considering factors such as ease of operation and stability of the treatment process, in this embodiment, the treatment process is ultrasonic impact. The parameter that needs to be precisely controlled for ultrasonic treatment is the force value at the time of impact. In order to deal with the residual tensile stress, plastic deformation of the weld and its vicinity needs to occur. Therefore, the control of the force value needs to be around the yield limit of the base metal, and the specific impact Select the position where the weld toe is close to the base metal. The control of the force value in this example needs to be around the yield limit of Q345, and its yield limit has been tested to be 389MPa. In other embodiments other than this embodiment, the method for processing the welding residual stress also includes welding toe grinding, TG remelting, or shot peening.

After step D, set the fatigue life effect verification; specifically, apply different levels of stress on the treated power arm structure to obtain the first average life value of the corresponding stress level; apply the corresponding level of stress on the untreated power arm structure On the power arm structure, obtain the second average life value of the corresponding stress level; then analyze and compare the first average life value and the second average life value. . The effect of reducing the residual tensile stress in the dangerous part of the boom weld and improving the fatigue life can be determined by formulating the corresponding test samples for ultrasonic impact treatment and then performing fatigue test verification. Effect. For this example, the method of testing is selected to verify the effect of reducing residual tensile stress and improving fatigue life. The effect is verified by performing fatigue test after ultrasonic impact treatment of welded joints. The test results are shown in the table.

Table 1 Average life of samples treated with different stress levels and life of original samples

According to Table 1, it can be seen that the ultrasonic impact treatment process has a significant impact on the improvement of the fatigue life of the weld. The fatigue life of the treated samples is greatly improved, and the fatigue strength is increased by about 25MPa, and the increase rate is about 22.7%.

Since the fatigue failure of the welding seam of the boom structure mostly originates from the stress concentration of the welding seam or the parts where the load stress of the welding seam is large, the fatigue life of the boom welding seam is improved and improved, and the residual stress is only carried out for the parts that are most prone to failure. deal with. When dealing with the residual stress of the weld, the process parameters of the treatment are also very important to the treatment effect. The treatment process needs to ensure the release of the residual stress without causing damage. Therefore, the treatment process still needs to be relatively accurately controlled.

Therefore, in the method for improving the fatigue life of the boom structure provided by the embodiment of the present invention, the present invention makes a weak position by studying and analyzing the weld, and then processes the residual stress there. In the process of residual stress treatment, precise control of process parameters not only ensures the release of residual tensile stress, but also avoids damage to the boom structure. Therefore, the present invention replaces the method of processing the entire weld in the prior art by processing the residual tensile stress of the weld at the dangerous position, and accurately improves the fatigue life of the boom structure.

The above are only preferred embodiments of the present invention, and do not have any limiting effect on the present invention. Any person skilled in the art, within the scope of not departing from the technical solution of the present invention, makes any form of equivalent replacement or modification to the technical solution and technical content disclosed in the present invention, all belong to the technical solution of the present invention. content still falls within the protection scope of the present invention.

Claims (9)

1. a method for improving the fatigue life of boom structure, is characterized in that, comprises the following steps: Step A: Obtain the stress distribution diagram of the weld, and determine the local dangerous position of the weld under the corresponding dangerous working condition according to the stress value; Step B, obtaining the welding residual stress distribution diagram at the local dangerous position under the corresponding dangerous working condition; Step C: Estimate the influence of the residual tensile stress at the local dangerous position on the fatigue strength in combination with the welding residual stress distribution diagram, and judge whether the local dangerous position needs to be treated with welding residual stress; if so, go to Step D; Otherwise, end; Step D: Determine the processing method and process parameters of the welding residual stress according to the welding residual stress, and process the local dangerous position. 2. The method for improving the fatigue life of a boom structure according to claim 1, wherein in step A, a network model of the boom mechanism is established by the finite element method, and material properties, parameters, loads and constraints are defined; Finite element analysis is performed on the finite element model in combination with the working conditions, and the stress distribution diagram of the corresponding dangerous working conditions is obtained. 3 . The method for improving the fatigue life of a boom structure according to claim 1 , wherein, in step A, the dangerous working conditions include an extreme positive load condition and an extreme eccentric load condition. 4 . 4. The method for improving the fatigue life of boom structure according to claim 1, characterized in that, in step B, the welding residual stress distribution map at the local dangerous position is obtained by the finite element method; The network model of the boom mechanism is established by the method, and the finite element welding software is used for simulation, and the welding parameters are set for simulation calculation to obtain the welding residual stress. 5 . The method for improving the fatigue life of a boom structure according to claim 1 , wherein, in step D, the processing method for the welding residual stress comprises an ultrasonic impact method. 6 . 6 . The method for improving the fatigue life of a boom structure according to claim 5 , wherein the process parameters include a force value during impact, and the force value ranges from 320MPa to 390MPa. 7 . 7 . The method for improving the fatigue life of a boom structure according to claim 1 , wherein, in step D, the processing method for the welding residual stress comprises welding toe grinding or shot peening. 8 . 8 . The method for improving the fatigue life of boom structure according to claim 1 , wherein, in step D, the processing method for welding residual stress includes TG remelting. 9 . 9. The method for improving the fatigue life of boom structure according to claim 1, characterized in that, in step C, if the maximum value of the welding residual tensile stress in the local dangerous position exceeds the contrast value, then the welding residual stress is processed .