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A brief discussion on the failure of rectangular connector structure frame with different degrees of cracks

An airborne electronic device has completed environmental testing, self-inspection found that the airborne electronic device of the rear rectangular connector structure frame bottom edge of the crack, the failure analysis of which is the bracket can not effectively fix the product, resulting in displacement during the vibration, the product rear rectangle and power-up rectangular cable products unreasonable collision caused. Through strength simulation, the cause of the failure and the subsequent possible adverse consequences are simulated and verified.

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Airborne electronic equipment; vibration failure; finite element analysis

C.I.C. Classification Number :V243

Introduction

It is well known that airborne electronic equipment is complex and powerful, and is an important part of the aircraft. However, airborne electronic equipment is often subjected to a variety of vibration, shock and other loads during daily flight and combat [1-2], and if the airborne electronic equipment has poor environmental resilience, it is easily damaged when encountering extremely harsh weather conditions, so the strength of the environmental resilience of airborne electronic equipment will directly affect the strength of the aircraft’s combat capability; and previous examples show that more than 40% of the failures of mechanical equipment More than 40% of the failures of mechanical equipment are caused by mechanical vibration [3-4].

It can be seen that only by effectively improving the environmental adaptability of airborne electronic products can the level of aircraft combat effectiveness be effectively improved, which is also the inevitable requirement of modern warfare for our airborne electronic products [5]. Therefore, it is necessary to control at the source of engineering design and optimize the weak parts of the structure to achieve reliable operation and excellent life of the equipment [6].

Airborne electronic devices are generally installed in standard cabinets/racks, which generally contain sophisticated electronic components and more complex structures inside, and require specialized mechanical environmental adaptations in the design of the cabinet/rack structure [7-8]. A mature airborne electronic product passed commissioning and routine testing, but during an external self-inspection after completion of vibration testing, cracks were found along the lower edge of the frame of the rear rectangular connector structure of this airborne electronic device.

1 Fault phenomenon

The airborne electronic equipment was subjected to functional vibration test, and the test directions were X, Y and Z. The airborne electronic equipment had normal functional indicators during the test, but during the subsequent self-test, cracks of different degrees were found on the lower edge of the rear rectangular connector structure frame.

1.1 Fault analysis

First of all, the fault was reproduced (to confirm the whole process, the bracket installation is tight, no loose situation; after the addition of power rectangular cable installation is tight, no loose situation).

(1) the use of the original bracket and spare parts products for bottoming out test, the product is installed on the sensor to monitor the vibration, vibration X direction and Z direction, the product frequency is less than 100 Hz, and the fourth fixed frequency 83 Hz superposition, vibration displacement noise, “clang clang” metal clanging sound.

(2) Using the new bracket and spare parts products for bottoming out test, the product is installed with sensors to monitor the vibration, vibration X direction and Z direction, the inherent frequency of the equipment in about 160 Hz, vibration displacement noise is small.

In summary, it is found that the original bracket compared with the new bracket, the overall equipment in vibration when the inherent frequency is reduced, and the 4th fixed frequency superimposed, will cause the product to produce a large level of vibration.

1.2 Analysis of failure causes

(1) the new bracket vibration inherent frequency is greater than 100 Hz, vibration displacement and noise is smaller; the original bracket vibration first-order inherent frequency is lower than 100 Hz, will be superimposed with the vibration excitation of the fourth fixed frequency 83 Hz, resulting in vibration displacement and noise is larger, thus causing the rear rectangular connector fracture.

(2) The original bracket characteristics: loose rear pin cone assembly, low spring strength of front hook assembly, deformation of front and rear base mounting holes, and broken guide parts, all of which had an effect on the bracket vibration characteristics.

(3) By replacing the original bracket rear spring pin, front hook assembly, front and rear bases one by one, the inherent frequency can be raised to normal level, so that the vibration displacement and noise can be reduced, indicating that this bracket problem is not the effect of a particular part, because the bracket has been used for a long time, the force parts are worn and the fastening performance becomes poor, resulting in the reduction of inherent frequency.

Conclusion: Due to the long time use of the tooling bracket, the parts have more serious wear, in the vibration process, the bracket can not effectively fix the product, resulting in displacement during the vibration, the product after the rectangle and after the addition of power rectangular cable products unreasonable collision, which in turn leads to cracks in the rear rectangular connector.

2 Strength Simulation

To further verify whether the mounting bracket loosening affects other structural components except the rear rectangular connector, the following simulations were performed [9].

This airborne electronic equipment vibration environment is characterized by strong vibration peaks iterated over a broadband random, and its vibration profile [10] is shown in Figure 1.

Firstly, the simulation analysis of the rear rectangular connector was carried out as shown in Table 1.

As can be seen from Table 1, the rear rectangular connector imposes a downward displacement constraint, and the maximum stress site is on the lower surface of the lower cavity, which is consistent with the crack displacement. When the displacement 0.2 mm, the maximum stress is less than the strength limit of the material 540 MPa, with a safety margin of 1.53 times; when the displacement 0.4 mm, the maximum stress is close to the strength limit of the material, with the risk of fracture. It means that the rear rectangle installation size within ±0.2 mm is normally stressed and will not produce fracture, if the size exceeds the tolerance range, the rear rectangle has the risk of fracture. After that, the simulation analysis of the whole airborne electronic equipment products in two states of normal and fault was carried out, as shown in Table 2 and Table 3.

From Table 2, it can be seen that the normal installation state, the maximum stress of the structural parts occurs in the front B-type handle hook, 136 MPa, the material of this part is aluminum alloy LY12-BCZYU, its fatigue strength is 140 MPa, the maximum stress is less than the fatigue strength, to meet the vibration requirements.

The maximum vibration deformation of the printed board is 0.057 mm, in accordance with the Steinberg printed board components vibration deformation resistance experience formula, the maximum vibration deformation of the components on the printed board allowed for 0.28 mm, the maximum vibration deformation of the printed board is less than the allowable value, to meet the vibration requirements.

As can be seen from Table 3, the failure state, that is, in the mounting bracket before and after tightening loose, after the rectangular connector restraint case, the maximum stress of the structural components occur in the chassis after the rectangular connector, 515 MPa, the material of this part is aluminum alloy LY12-BCZYU, its ultimate strength of 420 MPa, the maximum stress is greater than the ultimate strength, after the rectangular connector structural components will occur vibration failure.

The maximum vibration deformation of the printed board is 0.21 mm, according to the Steinberg printed board components vibration deformation resistance experience formula, the maximum vibration deformation of the components allowed on the printed board is 0.28 mm, the maximum vibration deformation of the printed board is less than the allowable value, to meet the vibration requirements.

The above simulation verifies that in the case of mounting bracket front and rear tightening and loosening, the maximum stress of the structural components occurs in the rear rectangular connector of the chassis, and its maximum stress exceeds the material limit strength, and vibration failure will occur. The maximum vibration deformation of the internal module of the chassis in this state is less than the permissible value, which meets the vibration life requirement and no damage will occur. It means that the mounting bracket front and rear tightening loose state will only cause damage to the rear rectangular connector, but will not affect other components.

3 Conclusion

This paper mainly discusses the fault problem of an airborne electronic device found in the process of self-test after the completion of the vibration test that the lower edge of the rear rectangular connector structure frame has different degrees of cracks, and the fault analysis was carried out by actual fault reproduction, and then the cause of the fault and the subsequent possible adverse consequences were simulated and verified by using strength simulation, etc.

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