Research on Failure Evaluation Method of Low-voltage Wire Harness in Automobile Collision

In order to obtain a more accurate failure evaluation method of low-voltage Wire Harness in the process of automobile collision, three kinds of low-voltage wiring harnesses with 16mm², 25 mm and 35mm cross-sections are used in existing vehicles, and three kinds of extrusion heads, M10 bolt, 5mm steel sheet and 20 mm steel sheet, are designed according to the extrusion conditions of the whole vehicle. The results show that the extrusion speed has little influence on the extrusion failure of the wiring loom, and the deformation compression ratio of the wire harness can be used as an evaluation index for the design of the early-stage collision protection of the wire harness. Through further comparison of CAE simulation and test results, the CAE modeling form of double-layer hexahedral element is determined, and considering the CAE calculation efficiency of the whole vehicle, the grid size of 3 mm CAE element is selected. Finally, the CAE analysis method and quantitative index of impact failure evaluation of low-voltage wire harness are given.

The collision damage of the circuit system not only affects the normal work of automobile safety-related parts, but also easily leads to the damage of circuit parts caused by short circuit or open circuit of the circuit system, and even accidents. In order to prevent such accidents, the collision protection of circuit system should be fully considered in the whole vehicle research and development process.

At present, the research,we Kable-X does, on the collision protection of automobile circuit system in the industry mostly focuses on the high-voltage circuit system of hybrid electric vehicles or pure electric vehicles, and it is divided into high-voltage components and high-voltage wire harnesses for research. The impact damage evaluation of high-voltage components is mainly judged by whether the shell of the components exceeds the fracture strain of the materials used, while the failure risk evaluation of high-voltage wire harnesses is mostly subjectively judged by whether it will be obviously squeezed, and there is no clear quantitative evaluation index. Jieli and Zhuxi Production team used CAE analysis method to evaluate the collision between high-voltage components and high-voltage wire harness, in which the high-voltage wire harness was modeled by tetrahedral solid element to judge whether it was squeezed or not. Zeng Zejiang used hexahedron+tetrahedron solid element to model the high-voltage Wiring Harness, and also used whether there was extrusion or shearing risk to judge the risk. These studies don't involve too much in the low-voltage wiring harnesses widely used in vehicles, and only make qualitative judgment according to whether there is extrusion or shearing risk in the collision process. These low-voltage wiring harnesses are not only functionally related to the high-voltage modules and high-voltage wiring harnesses, but also their failure often leads to major safety accidents, which needs to be paid enough attention.

The failure of low-voltage wire harness in vehicle collision condition includes cutting and extrusion. Cutting failure is mainly caused by the point-to-surface contact or line-to-line contact between the sharp edge of the part and the harness, which leads to the harness being cut or even cut off; Extrusion failure is mainly caused by the parts extruding the wire harness along the collision direction, which leads to the damage of the insulation skin of the wire harness and the partial exposure of the metal wire. Cutting risk is mainly avoided through the early layout design. This paper focuses on the extrusion failure limit that the wire harness can bear when colliding and extruding, in order to obtain the corresponding design evaluation index.

The most representative types of wire harnesses are selected from the circuit systems to be protected for the extrusion failure test. The cross-sectional areas of wire harnesses are 16mm, 25mm and 35mm, respectively. The evaluation indexes of other wire harnesses can refer to the research results of these three types of wire harnesses.

According to the contact type, the extrusion form is simplified into three types, namely M10 bolt, 5 mm steel sheet and 20 mm steel sheet are selected, which respectively represent small-area round surface contact, narrow surface contact and wide surface contact. Three kinds of extrusion heads are combined with three kinds of linear shapes, and at the same time, different extrusion speeds are taken into account to form a test matrix. In order to reduce the amount of tests, according to the mutual influence of various parameters, some combination items are selected for extrusion tests, totaling 18 groups of tests, and other combinations can be considered as basically covered.

The basic process of the test is to connect the wire to the resistance on-off block of the digital multimeter, and squeeze the wire harness with the selected extrusion head on the static press. One end of the multimeter is connected to the press equipment, and the other end is connected to the core metal of the wire harness. When the multimeter beeps, the press stops loading and the test is terminated. At this time, it can be considered that the extrusion head has touched the metal wire inside the wire harness, and the wire harness can be judged as being extruded and failed. In order to reduce the error, each group of tests is conducted for 5 times, and the average value of the results is taken as the test result.

During the extrusion test of 25mm low-voltage wire harness, the data of wire harness deformation and extrusion force were mainly collected, and the force-displacement curves of each wire type were basically the same under the same extrusion head and different extrusion speeds, which indicated that the extrusion speed had little influence on the deformation and extrusion force of wire harness, that is, in real vehicle collision, the deformation evaluation index of the same wire harness did not change with different vehicle collision speeds.

Under the same deformation condition, all kinds of harnesses have the smallest reaction force on 5 mm steel sheet, followed by M10 bolt, and 20 mm steel sheet has the largest reaction force. This shows that with the increase of the extrusion contact area, the resistance of the wire harness also increases. In actual collision conditions, the larger the extrusion area of the surrounding parts of the same wire harness is, the smaller the deformation of the wire harness is when it is compressed. Therefore, in the preliminary layout, the wiring harness should be arranged in the plane area with large contact area with the surrounding parts as much as possible, that is, the friendly interface of the wiring harness.

In addition, the extrusion reaction force of 25 mm2 is the highest, and that of 35 mm2 is the lowest, which shows that the wire diameter of the wire harness is not directly related to the extrusion pressure of the wire harness.

By counting the extrusion deformation of the harness in 18 groups of tests, it is found that the deformation of the harness with different cross-sectional areas is very different, and it is difficult to find a uniform evaluation rule. After fully studying the data, the deformation compression ratio is defined as δ = T/D (where T is the compression in the direction of the harness extrusion, and D is the remaining height in the direction of the harness extrusion), and the deformation compression ratio of the harness is counted. Only one group of data has a deformation compression ratio of 65%, and the deformation compression ratios of other groups are all concentrated in 70% ~ 70%. Therefore, the deformation compression ratio of wire harness can be used as the judgment basis to evaluate the extrusion failure of wire harness.

Considering certain test errors and the engineering feasibility of harness safety protection in actual collision, the target value of harness failure evaluation is defined as the compression ratio of the extruded harness ≤70%, and it can be used as the collision safety design value of most low-voltage harnesses.

In the forward development of the whole vehicle, in order to accurately judge the failure risk of the wire harness in the early stage of the project development, besides the above failure limit value as an evaluation index, a suitable CAE simulation method of the wire harness is also needed. Therefore, in this paper, a variety of CAE modeling methods are studied in combination with the above-mentioned harness tests.

Three methods can be considered to simplify CAE modeling of low-voltage wire harness structure. Method 1, adopt single-layer hexahedral element and add Beam element to the inner core of wire harness; 2. Single-layer hexahedral element with reinforced rubber material properties; 3. Double-layer hexahedral unit is adopted, the inner hexahedral unit is attached with appropriate strength material to simulate the inner core metal wire, the outer hexahedral unit is made of rubber material to simulate the insulation layer, and the outermost shell unit is added to simulate the wiring harness sheath or define the contact with the surrounding parts in CAE.

The test data of combination 10, combination 14 and combination 18, that is, 25mm wire harness, are randomly selected, and the above modeling methods are studied respectively, and the research results are compared with all 18 groups of test results, so as to finally select the CAE modeling method of wire harness with certain universality.

CAE analysis and comparison of different modeling methods under M10 bolt, 5 mm steel sheet and 20 mm steel sheet extrusion head are carried out for 25 mm2 wire harness. The results show that the stiffness of wire harness is obviously higher when method 1 is adopted, while the stiffness of wire harness is lower when method 2 is adopted. The CAE analysis results are closest to the corresponding test curves when method 3 is adopted, so the modeling method of method 3 is applied to the comparative study of test results.

The CAE simulation results show that, in all kinds of indenter tests, the results of 4 mm grid size are far from the experimental results, and the CAE simulation results of 2 mm unit have the best fitting degree with the experimental results, and the CAE simulation results of 3 mm unit are also close to the experimental values. Considering the unit number, scale and calculation efficiency of the actual vehicle collision model, 3 mm mesh size is selected for CAE modeling of the vehicle harness.

According to the above analysis, the CAE simulation and test results are compared and analyzed by using a 3 mm unit with double hexahedron. Among them, the maximum and minimum curves of extrusion force and the average curves of five test values under the same displacement condition were selected for each group of test results. Except for the data deviation of combination 1, combination 16 and combination 22, the other simulation curves were close to the average curves of test results. The CAE simulation result of combination 1 is on the high side, which indicates that under the same displacement condition, the extrusion force of the harness of combination 1 is higher than the test value, that is, the stiffness of the harness in this simulation is on the high side. If the target value ≤70% obtained from the previous test is used for the preliminary design, the CAE analysis may not reach the target value, but the later test verification has exceeded the target value. This requires that the simulation result should not only be evaluated by the target value in the preliminary design, but also the corresponding engineering experience should be carefully combined. For combination 16, the CAE simulation result is low, so it can be considered that if the CAE analysis of the early design does not exceed the standard, the later test will generally not exceed the target value. Compared with the test results, the CAE simulation results in Combination 22 show that the stiffness of the wire harness is higher before and lower after, and it can be generally considered that the deviation between the stiffness and the test is not significant, and the early CAE design analysis mainly focuses on when the wire harness is crushed and fails.

However, the stiffness of the second part of the combined 22 CAE simulation results is low, so when the target value of ≤70% is used for CAE design evaluation, the risk of the results can be controlled. Of course, in order to prevent a few omissions, it is still necessary to carefully analyze the CAE results in combination with engineering experience. By analyzing all the benchmarking results, it can be basically judged that this CAE simulation method and the target value of wire harness evaluation obtained from the test can be used in the design of wire harness collision protection.

In this paper, from the perspective of integrated development of circuit safety performance in vehicle collision, the failure evaluation criteria of 12 V low-voltage wiring harness in collision are studied, and the following conclusions are obtained:

A. the damage deformation of the same wire harness and the extrusion force of surrounding parts will not be different because of the collision speed;

B. The extrusion force of the wire harness is different under the action of different extrusion heads. Under the same extrusion displacement condition, the larger the extrusion contact area is, the greater the extrusion force the wire harness can bear. This also indicates that if the wire harness can make plane-to-plane friendly contact with the surrounding parts during layout, the risk of the wire harness being damaged by extrusion will be greatly reduced;

C. The deformation compression ratio when the wire harness is crushed and damaged can be used as the failure evaluation index of the wire harness collision protection, and its target value can be defined as ≤ 70%;

D. It is recommended to select 3 mm double-layer hexahedral unit structure for CAE modeling.

The research method in this paper is also of reference significance to the protection research of high-voltage wiring harness, cooling pipeline and other line systems in new energy vehicles.