A new technique for fracture analysis of connectin

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New technology of fracture dissection for engine connecting rod Abstract This paper introduces the new fracture dissection technology used in the manufacture of automobile connecting rod abroad, analyzes the mechanism and occurrence conditions of fracture dissection, discusses some key problems to be considered in the application of this technology, and finally introduces the connecting rod fracture dissection device

key words: connecting rod fracture splitting mechanism device

I. Introduction

connecting rod is a key part with high precision in engine, which is composed of connecting rod body and connecting rod cap. In the traditional manufacturing process, the manufacturing of connecting rod body and cover depends on two methods: 1) the whole forging of connecting rod body and cover → sawing and separation → machining of contact surface → assembly. 2) Connecting rod body and cover are forged respectively → contact surface machining → assembly. Using the above two processes, it is not only necessary to mill and grind the connecting surface of the connecting rod body and the cover, but also to drill and ream the bolt positioning holes and tapping holes on the connecting surface, or cut the end face teeth, drill and ream the positioning pin holes and drill the bolt holes, so that the connecting rod body and the cover can be assembled accurately in the future. For this reason, more processing machines are required, and a large number of processing hours are consumed after more than ten processes

in view of the shortcomings in the traditional manufacturing process of connecting rod, in order to reduce the manufacturing cost and man hour and improve the fitting accuracy, the fracture splitting process of connecting rod was proposed, and alfling applied for a patent in Germany in the mid-1980s. Since then, relevant research has made continuous progress in the United States and Germany. In the 1990s, the process entered the practical production stage in the industrial developed countries. The applicable blank has developed from powder forged connecting rod to medium and high carbon steel forged connecting rod. The manufacturers used cover the three major automobile companies in the United States, as well as famous enterprises such as German Benz and BMW. At present, the domestic manufacturer applying this technology is FAW Volkswagen engine factory, which has imported German technical equipment and is applicable to the new engine connecting rod with five valves per cylinder

II. Connecting rod fracture splitting process

for the forged connecting rod blank, before fracture splitting, the connecting rod big end hole shall be roughly bored, and then two opposed grooves shall be machined at the predetermined fracture, which are the stress concentration points, as shown in Figure 1a. Then, the connecting rod big end hole is sleeved on the two half mandrel of a device for fracture and sectioning, and the connecting rod is positioned and clamped. Then, using the impact force, insert the wedge used to expand the connecting rod into the above-mentioned semi mandrel oil pump to start oil supply. At this time, under the impact of the wedge, the big end hole of the connecting rod is broken at the groove and divided into the connecting rod body and the connecting rod cap, as shown in Figure 1b

a) slotted connecting rod b) schematic diagram of fracture and dissection process

Figure 1 Schematic diagram of fracture and dissection process of connecting rod

this new process enables the separated connecting rod and connecting rod cap to be assembled naturally and accurately at the fracture surface without machining the mating surface, so as to achieve the purpose of reducing processing procedures and machining machine tools. In addition, in addition to the high fitting accuracy of the split surface of the connecting rod, because the split contact surface is uneven, the contact area is greatly increased, thus improving the bearing capacity of the connecting rod

III. fracture splitting mechanism

1 Brittle fracture

the theoretical basis of this process is the brittle fracture theory in fracture mechanics. According to the fracture mechanics, there are usually three types of displacement on the crack surface during the fracture process, namely, open type, back and forth sliding type and plane shear type. When the object is pulled by normal stress perpendicular to the fracture plane, it belongs to open fracture, which is the form and condition of brittle fracture. Brittle fracture has the following characteristics: 1) the working stress during fracture is low, which is usually far lower than the yield strength of the material, and the plastic deformation is small; 2) The fracture is greatly affected by temperature; 3) The fracture direction is perpendicular to the normal stress

according to the above characteristics of brittle fracture, the fracture process of connecting rod is to open V-shaped groove on the inner side of connecting rod big end, and then apply normal stress perpendicular to the predetermined fracture surface to meet the occurrence conditions of brittle fracture, so that the first cover of connecting rod body can be separated without plastic deformation. Attention should be paid to the following issues: 1) blank materials; 2) The relationship between the shape of V-groove and the required stress; 3) Operating temperature

2. Blank material

because brittle materials are more prone to brittle fracture, it is suitable for connecting rods manufactured by fracture process. The following three materials are mainly used: 1) powder forging blank; 2) Malleable iron; 3) 70 high carbon steel. The blanks made of these three materials can achieve brittle fracture at room temperature, and the connecting rod big end hole does not produce obvious plastic deformation, with the deformation ≤ 40 μ m. After machining, the roundness error can be reduced to 3 μ m。 In addition, the 45 ~ 55 forged steel blanks can also be manufactured with the fracture technology, but the brittle fracture can be realized only when the temperature is - 40 ℃ to ensure that the deformation after expansion crack is small enough

3. Temperature effect

the fracture profile, like the stamping surface, is usually divided into three zones, which can be divided into fiber zone, radiation zone and shear lip from the fracture source outward (see Figure 2). When the radial area of the section is wide, it shows that the plasticity of the material is poor and the brittleness is large. On the contrary, the larger fiber area indicates that the material has better plasticity and toughness. How to enlarge the width of radial zone and reduce the width of fiber zone is the condition to realize brittle fracture

Fig. 2 Schematic diagram of fracture section

Fig. 3 shows the effect of temperature on the size of each zone of the three elements of fracture. The material is 40Cr. It can be seen from the figure that when the temperature is lower than room temperature, the radiation zone has increased significantly, which provides an experimental guarantee for the implementation of fracture section process of connecting rod at room temperature. Carbon steel above 70 with mechanical properties similar to 40Cr also conforms to the temperature zoning trend shown in Figure 4

Fig. 3 Effect of temperature on brittle fracture

Fig. 4 relation curve between fracture strength and crack depth

4 Relationship between groove depth a and fracture strength

it can be seen from Figure 4 that crack (V-shaped groove) depth and fracture strength σ C is inversely proportional, that is, for a certain stress value, there is a critical crack depth AC. when the crack depth is less than this value, the crack is stable. Only when it is greater than this value, the crack is unstable. The deeper the crack, the lower the critical fracture stress of the material. Depth a and σ The relationship is as follows:

when the structure with notch is subjected to external force, a stress controlled by stress intensity factor KIC (also known as material fracture toughness value) is generated near the crack tip. When the external force increases, the stress intensity factor at the crack tip increases. When ki reaches a certain critical value KIC, the crack becomes unstable and the structure brittle fracture occurs, that is, Ki ≥ KIC

for a certain material, KIC is a constant value, which can be obtained by looking up the table or determined by testing method (more accurate). The lower the KIC value is, the more brittle fracture will occur. Therefore, the test and calculation of KIC value should be the focus of studying fracture mechanism

according to KIC principle, the applied stress can be calculated for a given size notch. On the contrary, the critical crack size can be predicted for the member with a given applied stress. This principle provides a design basis for fracture dissection of connecting rod

IV. design of fracture sectioning device

a reasonable fracture sectioning device is the key factor to implement fracture sectioning. The design principles are as follows: 1) the fracture force can act suddenly and instantaneously, because the application speed of the force has an important influence on the realization of brittle fracture. 2) During this process, the connecting rod shall be firmly installed and cannot move. During the separation process, the connecting rod body and cover can only move in a straight line away from each other. Any relative rotation between the connecting rod body and cover will cause unnecessary plastic deformation, resulting in the failure of normal assembly in the future

the sectioning device mainly includes: fixed base, connecting rod cap support and connecting rod body support. Each support is equipped with a guide rail, which can make the support move perpendicular to the fracture plane along the guide rail within a certain range. It also has a parting surface and a gate valve mandrel that can be inserted into a wedge for separation

v. conclusion

because the new connecting rod splitting process has the advantages of less subsequent processing procedures, low manufacturing cost, high assembly accuracy of connecting rod body and cover, and high bearing capacity of the splitting surface, this new process has been rapidly promoted in foreign connecting rod production lines. With the development of China's automobile industry, the development and application of various advanced manufacturing technologies is an inevitable way to improve the quality of automobile products and reduce costs. Therefore, it is necessary to strengthen research and development in this field, so as to make technical reserves for the future promotion of this new process in China

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