Automotive wire harness fatigue fracture analysis and treatment program

Wiring harness fatigue fracture is a headache problem, if there is no certain accumulation of failure experience, or there is a complete test verification program, the sudden nature of the problem will make people at a loss. First, what is the fatigue fracture of wiring harness? Although the stress value of the metal conductor (copper wire) of the conductor has not exceeded the strength limit of the material in the alternating stress (generated by vibration or moving parts), the sudden brittle fracture of the conductor occurs after long-term repeated stress cycles. For example, after the mobile phone charging cable tail is used for a long time, due to the bending of the interface tail harness in the process of plugging and unplugging, the outer sheath material is broken first (the outer sheath material is mostly TPE), and then the conductor copper wire fatigue fracture will occur if it continues to be used. Therefore, it is particularly important to study the flexural performance of the charging line to improve the life of charging line. At the fixed position of the vehicle harness fixing buckle, the conductor will form a depression when it is squeezed for a long time, the conductor monofilaments will move each other to produce friction marks, and the fracture of the conductor will generally be oxidized. 2. Harm of wiring harness fatigue fracture The breakage of telephone charging wiring harness only affects the charging of our mobile phone, and the configuration of another charging line will not affect our daily life. But as an automotive wiring harness, it is very different. As the connection carrier of high and low voltage electrical components, the automotive wiring harness carries the power of the whole vehicle, transmits signals, and dominates the safety and performance of the whole vehicle. The carrier of the car is people, especially the safety and customer's feelings are very important. Wiring harness fatigue fracture is one of the main forms of wiring harness failure. Vehicle door wiring harness, trunk wiring harness, engine wiring harness, speed sensor wiring harness, driving motor wiring harness of new energy vehicles (especially wheel motor), air pump wiring harness, air conditioning compressor wiring harness, etc., bear bending and vibration load in the working process, and also bear heat load in harsh environment. When the automobile wiring harness is subjected to the periodic motion of a moving part or a vibrating part, if the design is not reasonable, the bending resistance of the cable is not enough, the conductor copper wire is partially or completely broken, and the problem of wire breakage affects the normal operation and safety of the automobile. If the high-voltage wiring harness of new energy vehicles is suddenly short-circuited, there may be outgoing arc, which will cause different degrees of harm to the wiring harness, electrical appliances and even the whole vehicle. 3. Analysis of fatigue fracture of vehicle wiring harness 1. The processing technology of conductor material (copper) is not reasonably controlled. The copper wire will become brittle due to excessive oxidation during wire drawing or annealing, or during the vulcanization process after extrusion insulation, the surface of the conductor copper wire will turn black due to water ingress or reaction with acidic substances produced by high temperature decomposition, which will reduce the toughness of the copper wire. In general, 99.99% oxygen-free copper is used for automotive cable conductors. 2、 insulation or sleeve material insulator, sheath material, poor mechanical and physical properties, poor aging resistance, insufficient elongation, tensile strength, tensile retraction, insufficient support of the core and conductor by the insulator and sheath. The 3、 is subject to abnormal mechanical impact, such as collision, mechanical impact, short tension on the harness, too short brake alarm harness, and the direction will make the harness very straight. After a long time, the harness will be disconnected from the inside, and no problem can be seen from the outside. The 4、 cable has good bending resistance, but the harness layout structure can not meet the actual use requirements. The factors affecting the bending resistance of the cable include the diameter of the conductor, the stranding structure of the conductor, the cable structure and so on. The bending resistance of the cable is related to the monofilament diameter of the conductor, the stranding structure of the conductor, the cable structure, etc. The monofilament diameter of the new energy vehicle high-voltage harness conductor currently used is determined according to QC/T1037. Generally, the monofilament diameter is 0.21 mm. If the bending performance of the existing conductor is to be changed, a smaller monofilament diameter, such as 0.15 mm, can be selected. In general, the pitch diameter ratio of the primary strand should not exceed 25. In the case of multiple strands, the pitch diameter ratio of the strands should not exceed 30, and the pitch diameter ratio of the inner and outer strands should not exceed 20 and 14, respectively. The large pitch copper wire is much stiffer than the small pitch copper wire, and the minimum bending radius is also increased. In order to prolong the service life of the vibrating or moving part of the cable, the conductor must be as flexible as possible. The thinner the wire that constitutes the conductor, the greater its elasticity. However, the thickness of a wire is directly related to its mechanical strength and bearing capacity, and the bearing capacity of a thin wire is poorer. In order to solve this contradiction, a plurality of single wires can be bundled into strands first, the stranding direction of each strand is the same, and then the strands are wound on the conductor, and the stranding direction of adjacent layers of strands is opposite during stranding, so as to ensure the flexibility and mechanical strength of the stranded conductor.

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4. Methods to solve the problem 1. Avoid copper wire oxidation. When copper wire is oxidized, first of all, a reasonable process should be selected. If secondary annealing is not needed, the oxygen-free purity must be controlled to ensure the purity of copper wire. Second, in the process of vulcanization, the side effects of water vapor and vulcanization reaction (except for some defects in the extrusion process itself) are not obvious; the comb reaction caused by the decomposition of acidic substances at high temperature of the formula can be solved by improving the thermal stability of the rubber formula, in addition, it is more effective to avoid too high vulcanization temperature. Of course, this does not mean that the temperature is required to be below the vulcanization point. 2、 to control the stranded pitch diameter ratio The theoretical pitch diameter ratio should be properly smaller than the standard value, generally 10% ~ 15% lower than the standard value, so that the actual pitch diameter ratio will be close to the limit value specified in the standard. Similarly, the control of the strand pitch diameter ratio is the same. The 3、 controls the quality of insulation and enclosure. The content of insulation glue in insulation glue and sheath glue should be controlled above 35%, and the content of insulation glue in insulation glue should not be less than 42%, which is the basic requirement. The tensile strength is naturally affected, and the elongation and tensile retraction are related to the formula composition, rubber mixing process and vulcanization process. The tensile strength of a good sheath rubber should be more than 15 N/mm (the standard value is 11 N/mm), and the elongation can also be more than 500%. The 4、 optimizes the structure of the cable. When the flexible bending-resistant cable is bent (the cable core will bear the corresponding tensile force and compressive force, the insulated wire core at the top of the bend in the cable core bears the tensile force, the position of the insulated wire core moves outward (the insulated wire core at the bottom of the bend bears the pressure), and the position of the insulator (the closer to the bending edge) is closer (the greater the force). The greater the relative movement between the insulated cores-when the cable bend disappears and returns to its original position (the tensile force and compression force on the cable core disappear) (the position of the insulated cores should also be restored)-the greater the relative movement between the insulated cores when the cable is bent frequently-the cable returns to its original position after bending (the tensile force and compression force on the cable core disappear and returns to its original position) The sliding compensation between the insulated cores is poor In addition, the greater the tensile force on the cable core (the more unstable the cable core structure), the greater the probability of deformation of the cable core. The following is an optimization program: In order to improve the structural stability of the cable core in the cable, the PP filling rope, hemp rope, cotton rope filling strip and other filling materials used in the past are cancelled, and the insulating material is used to extrude the filling strip. The aramid fiber wire replaces the soft carbon steel wire rope and is placed in the center of the filling strip, and the aramid fiber wire filling strip is placed in the center of the cable core, so that the bearing capacity of the filling strip is improved, the tensile force of the insulated wire core is reduced, the overall softness of the cable core is enhanced, the structure is stable and round, and the cable core has better bending resistance and torsion resistance. The flexible bending-resistant cable core is generally not wrapped with tape, and the insulation core is in close contact with the sheath to prevent the sheath from bonding, which is not conducive to cable stripping. Generally speaking, talcum powder is coated on the outside of the cable core, but talcum powder is easy to absorb moisture and agglomerate, which is prone to uneven coating on the surface of the core, excessive accumulation in the core gap, and easy to splash and accumulate in the mold core, which leads to cable peeling. Dry talcum powder is easy to produce dust, which is not conducive to hygiene and cleaning, and is harmful to human body. Flexible and bending-resistant cables using talcum powder for insulation and sheath isolation, because the talcum powder coating is not uniform, the sliding compensation between the insulated cores is not balanced (therefore, when the insulated cores are extruded, the liquid release agent is coated on the insulated surface of the core-the insulated core layer is immersed in the release agent. The surface of the cable core is uniformly coated, which is beneficial to the relative sliding of the insulating cores and ensures that the cable core compensates the balance when bending, thereby further improving the bending resistance of the cable core. The insulation extrusion molding adopts an extrusion die to tightly wrap the insulation layer outside the conductor to enhance the bonding strength between the insulation layer and the conductor. During the extrusion process, if the insulation layer is scratched or broken down, the insulation wire core shall not be trimmed to eliminate the potential safety hazards of the insulation wire core as far as possible. The improvement of the outer sheath plays a very important role in the flexible bending-resistant cable. It not only protects the insulated core, but also bears a large load and bending stress. Therefore, the outer sheath of the flexible bending-resistant cable generally has extremely high tensile strength, tear strength, excellent wear resistance and aging resistance, such as TPU.

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