Study on the cause of early fracture of the frame

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Research on the causes of early fracture of heavy dump truck frame

I. preface

dump trucks are often used in mines, construction sites, ports and other places because of their complex operating environment. Most of them are used under the harsh conditions of uneven roads and serious overloading. The early fracture of the frame structure of the heavy dump truck of an automobile company was found in the process of product development, test and user use, which was mainly located at the place where the width of the main frame changed. Therefore, the finite element analysis of the frame is carried out by using the finite element analysis method, and compared with the experimental results, the cause of the early fracture of the frame is found out, and the improvement scheme is put forward

II. Establishment of finite element analysis model

(I) establishment of model

the frame of the heavy dump truck is of side beam type, which adopts multi-layer structure, and the layers are connected by riveting or bolts. Its structure is shown in Figure 1. This form of structure is different from the frame with continuous cross-section, and its force transmission is discontinuous. When establishing the finite element analysis model, appropriate simplification should be carried out according to the actual structure. The finite element analysis model of frame is generally divided into beam element and shell element model. The beam element model simplifies the frame structure into a group of frame structures composed of beam elements, and reflects the actual structural characteristics of the frame with the section characteristics of the beam elements

Figure 1 Schematic diagram of frame structure

this method cannot analyze the stress concentration of the frame in detail, and cannot reflect the connection mode of longitudinal and transverse beams of the frame. The shell element model will use the shell element to discretize the frame, which overcomes the shortcomings of the beam element model because it significantly improves the stiffness and strength properties of the polymer, and has high accuracy. Therefore, in order to improve the accuracy of analysis and calculation, the finite element analysis model with shell element as the basic element is adopted

the frame is 7.5m long and 0.93m wide, with 2 longitudinal beams, 6 transverse beams and 1 outer beam. Considering the complexity of the geometric model of the frame, first establish the surface model of the frame in UG software, import it into hy2permesh software for lattice division and other pre-processing, and then submit it to NASTRAN for solution. All layers of the frame are connected by rivets, and the ACM unit in hypermesh2 connectors can be used to simulate rivet connection. The model of the front suspension spring adopts two Bush units on each side longitudinal beam, and each Bush unit is connected with the frame through multiple MPC units; The model of the rear suspension spring adopts a bush unit on each side longitudinal beam to connect with the rear axle of the frame. The whole frame is divided into 228254 shell units, 1480 ACM units and 8876 MPC units. The material is 16MnL and the young's modulus is 2 × The National Engineering Laboratory of polymer surface material preparation technology has obtained the official approval of the national development and Reform Commission and recognized 105Mpa, with a Poisson's ratio of 0.3

the discrete finite element model is shown in Figure 2

Figure 2 partial finite element model of the frame

(II) determination of boundary conditions

since the frame is supported by the wheel through the leaf spring and suspension during normal operation, the boundary conditions are simplified to restrict all degrees of freedom at the grounding of the front and rear suspension spring units, so that the frame forms a simply supported beam structure

(III) load loading and calculation conditions

1. load simplification and loading

the external load acting on the frame is simplified as equivalent load and added to the corresponding parts of the frame, and the mass of the body and the assemblies on the frame are simplified as uniformly distributed load acting on the corresponding parts of the frame. According to the actual stress condition, add 7T uniform force to the place where the cab is placed, and add 33T uniform force to the rest. As shown in Figure 3

Figure 3 load diagram

2. Calculation condition

according to the relevant standards of vehicle electrical measurement and the stress condition of the vehicle during actual operation, the static analysis of the frame should consider bending condition and bending torsion condition. The four wheels of the vehicle are in the same horizontal plane when it is loaded under bending conditions and produced by Jinan assaying professional manufacturer; The bending and twisting condition is the stress condition when the right front wheel or the left rear wheel of the vehicle is lifted for a certain distance. The stress distribution after solution is shown in Figure 4 and figure 5

Figure 4 stress distribution diagram of bending condition at variable width

Figure 5 stress distribution diagram of bending and torsion condition at variable width

3. modal calculation

free modal analysis of the frame can calculate the natural frequency and vibration mode of the frame, so as to further study the dynamic characteristics and stiffness of the frame. The first nine natural frequencies of free vibration are extracted by Lanczos method, and the frequencies of each order are shown in Table 1. Table 1 first to ninth order natural frequencies and vibration modes of the frame

4. Electric measurement experiment of static stress of the frame

electric measurement experiment of strain was carried out on the frame. The arrangement of strain gauge is determined according to the actual position where early fracture occurs during the use of the frame and the place with large stress in the finite element analysis. See Figure 6 for the main position of the arrangement of strain gauge, and positions 83, 85, 86, 88, 89, 90 and 92 in the figure are the patch positions of strain gauge. When the frame is deformed, the flange edge of the longitudinal beam is in a unidirectional stress state, and the direction is along the axis of the longitudinal beam. Therefore, the single-chip strain gauge should meet the test requirements

Figure 6 layout position of some strain gauges

III. analysis of calculation and experimental results

(I) analysis of calculation and experimental results

calculation and experimental results show that the stress of the whole frame does not exceed the allowable safety stress under two working conditions. The place with the greatest stress is the bending joint of the third cross beam and the longitudinal beam, that is, the positions of strain gauges 88, 89, 90 and 92 in Figure 6. From the finite element analysis and experimental test results at the measuring points, it can be seen that the stress values at these four points are relatively large. The experimental results and finite element analysis results are consistent with the early fracture in the actual use process, which shows that the finite element analysis model is basically correct

it can also be seen from the finite element analysis and experimental test results at the measuring points that the maximum stress value at measuring points 88, 89, 90 and 92 is 14811mpa under the combined bending and torsion condition of the frame. The material of the frame is 16MnL, and the allowable stress is 211 ~ 238mpa, which is far greater than the maximum stress at the measuring point. The static stress is not enough to cause the frame fracture. The fracture should be caused by the dynamic alternating stress produced by the car during driving

through the analysis of the calculated results and measured values, it is considered that the main reasons for the stress concentration near the measuring points 88, 89, 90 and 92 are: the frame structure is wide in the front and narrow in the rear, while the front end of the subframe is just nearby, so the torsional stiffness of the front and rear ends of the frame is different, and the difference is large. The third cross beam is just connected at the bending of the longitudinal beam, which will not only improve the torsional rigidity of the frame, but also hinder the transmission of torsional deformation along the longitudinal beam when the frame produces large torsional deformation, resulting in high stress at this place, and eventually lead to the fracture of the structure

(II) improvement

based on the above static calculation results and analysis, the frame structure is improved: the length of the variable width area of the frame is extended by more than 1m; At the same time, move the position of the third cross beam slightly backward, so that it is not at the bending position of the longitudinal beam. Figure 7 shows the improved bending and torsion working condition. From the figure, we can see that the stress at the third cross beam decreases a lot, and the stress distribution at the bending of the longitudinal beam becomes uniform. After the improvement, the actual use results show that the frame has no early fracture problem

Figure 7 stress distribution diagram under bending and torsion conditions at the changed width after improvement


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