electromagnetic crimping of terminal-wire

In general, coil life can be increased with a field shaper as the mechanical loading of the coil can be significantly reduced. 76 Figure 5.1 CS sectional view of the assembly with different cross-sections (a) Circular (b) Rectangular and (c) Trapezoidal.

Terminal–Wire Crimping

Careful adjustment of pressure is essential in the pneumatic crimping tool as less pressure leads to reduced terminal deformation while excessive pressure leads to overcompression of the terminal leading to cracking and terminal failure. Pressed terminals are exposed to different types of vibrations, different electrical environment, temperature gradient and less disturbed area even though most of 60% of electrical breakdowns occur at connector junctions (Gissila, 2013).

High Strain-Rate Metal Forming Process

The magnetic field of the workpiece is opposite in nature to that produced by the coil. Material springback is the elastic recovery of the material after the stress is removed.

Figure 1.3 Explosive forming process b. Electrohydraulic forming

Motivation

After reviewing the literature, it was found that there may be one effective compression method that can solve all these problems that have been faced for decades using the EM forming process. No research work has been carried out in the field of electrical cable crimping using the EM forming process.

Objectives

Due to the advantage of this process, such as a contactless forming process with high strain rate, EM forming can play an important role in crimping wires to make highly durable electrical connections in the coming years.

Organization of the Thesis

In Chapter 6, the effects of three field shaper geometries, such as a single step, a double step and a taper, on the EM terminal wire crimp were studied. The comparison was performed by keeping the total field former length and effective working area of the field former constant.

Introduction

When the connector is connected to electrical equipment, a small compression force causes the connector to separate from the inner wire, and the low density of the wire in the connector often causes a fire due to overheating. At the end of this chapter, the important equations involved in the design of EM are discussed, followed by a research plan in the form of a flow chart.

Conventional Wire Crimping Process

Traditional crimping tools not only leave the marks of the dies on the connection surface, but excessive pressure sometimes leads to cracking that leads to mechanical breakage. As shown in Figure 2.2, performed crimp cross-section showing defects consisting of wire strands and terminals.

Figure 2.1 Different application of connector terminals

Different Types of EM Forming process

In tube EM forming, the cylindrical tube is uniformly expanded or compressed by applying magnetic pressure (Haiping and Chunfeng, 2009). b) Sheet metal forming. The deformed sheet can take the shape of the die or can be a free protrusion.

Experimental Work Carried Out on EM Forming

Tube Forming

The formability and the hardness of the aluminum ring at different discharge voltages were investigated. They found that the hardness of the ring increases with the increase in the discharge voltage.

Figure 2.4 Schematic diagram of EM forming process

Electromagnetic Crimping

The use of deeper or narrower grooves resulted in higher deformation of the tube and provided higher stiffness. For a higher effective deformation of the tube over the door, a reduced springback is an important measure.

Figure 2.8 Schematic diagram of an EM crimping process

Numerical Studies

Non-Coupled Approach

Loosely Coupled Approach

This process is repeated iteratively until the end of the process time as formulated by Haiping et al. The limitation of this process is the assumption of adiabatic condition, which is a major limitation of this process in which thermal conditions are neglected (Haiping et al., 2009).

Fully Coupled Approach

While the peak value of the magnetic pressure is inversely proportional to the length of the coil. The deformation of the workpiece is not taken into account for the calculation of the EM pressure.

Field-Shaper

Working Principle and Modeling

The main purpose of the field former is to withstand high mechanical load to increase the service life. When designing a field shaper, it should be considered that the length of the coil should be the same length as the total length of the field shaper.

Figure 2.13 Schematic diagram of a working field-shaper

Analysis of EM Forming Process

These equations help to design an EM generating coil and to understand the inner mechanism of the process. With the skin depth S (S1 and S2 for coil and blank) of the electric current, effective radii are defined as,.

Figure 2.15 Equivalent circuit of the EM forming system where,

Gaps in literature

Thus, another important conclusion was reached, that for maximum pressure, C should be as large as possible. The number of windings per unit length n/lo must also be higher, and low inductance and resistance of the machine unit will be important criteria.

Figure 2.17 Research plan in the form of flow chart

Materials and Experiment

Materials

EDX above the surface of the terminal Chemical composition Figure 3.1 EDX and chemical composition of the terminal.

EM Machine and Equipment for Post-Processing

For measuring and performing post-processing of samples, different types of equipment are used as shown in Figure 3.3, which are calibrated before readings are taken to avoid any errors. a) Oscilloscope for current measurement (b) RLC measuring devices. e) Universal testing machine (f) Vicker hardness testing machine.

Figure 3.3 Testing equipments used for post-processing

Experimental Procedure

EM Terminal-Wire Crimping Coil

After obtaining the most suitable coil, experiments were conducted at various discharge energies to study the influence of EM high strain rate deformation process on terminal wire crimping applications on multiple parameters discussed in the next section.

Figure 3.4 Different types of experimental coil used for the crimping process

Experimental Work Carried out on the Optimized Coil

At discharge energy of 4.1 kJ, maximum radial deformation of 3.4 mm was obtained and according to the standard of shrinkage of 35 mm2, a deformation of 3.34 mm is required to avoid damage inside the terminal (“Connectivity TE,” n.d.). Frequency was calculated as 20 kHz which remained constant throughout the experiments and value of current was found to be 127 kA for discharge energy of 2.8 kJ.

Figure 3.8 EM crimped samples at various discharge energy

Conventional Crimping

Result and Discussion

Cross-Section Analysis
Electrical Characterization
Mechanical Pull-Out Testing
Surface Roughness
Hardness Analysis
Temperature Measurement

As shown in Figure 3.13, it was found that EM shrunk samples gave a lower resistance value of 4.4 µΩ compared to conventional shrunk samples. The arrangement of the pull-out process of a wire-shrunk sample is shown in Figure 3.14.

Figure 3.11 Cross-section of crimped samples under an optical microscope It was found that compression done using EM process was more effective than conventional crimping process as compression of wire strands was higher, giving denser compa

Summary

The temperature above the EM pressed sample was found to be lower than that of the conventionally pressed sample due to minimal resistance change in the contact area and less heat dissipation, making it a more attractive option for the conventional pressing process. The temperature above the EM rolled sample showed 30.5℃, which was 4℃ lower than the conventionally rolled sample due to minimal contact resistance change.

Introduction

Methodology

Finite Element Analysis of Electromagnetic Crimping

Fully coupled EM module in LSDYNA

The explicit mechanical solver calculates the deformation of the conductor, and therefore the new geometry is used to calculate the EM field in a Lagrangian way. If the skin depth is small compared to the workpiece thickness, the penetrated magnetic field is often neglected, and then the magnetic pressure is given by a simple equation.

Figure 4.2 Flowchart of the EM-mechanical structural coupling process

Designing of EM Crimping Process

Numerical Model

Coil Modeling and Material Properties

The input current obtained from the EM machine setup was transferred as a current-time graph in the EM module of the software to analyze the dynamic plastic deformation of the terminal. In EM crimping process as the first period of the current is responsible for significant deformation (Haiping and Chunfeng, 2009).

Figure 4.3 Numerical simulation model developed in LS-DYNA Table 4-1 Dimensions of the coil and workpiece used in simulations

Numerical Simulations Results

The vectors of the magnetic field in the coil are shown in Figure 4.7 Magnetic field density vectors of the coil at 16 µs and 39 µs. Thus, it was found that the value of magnetic field increased with the increase of discharge voltage.

Figure 4.6 Current density vectors of the coil at 16 µs and 39 µs

Experimental Work

Results and Discussion
Terminal Radial Deformation
Cross-Section Analysis
Electrical Characterization
Mechanical Pull-Out Testing
Hardness Analysis

Thus, for discharge voltage of 11.25 kV the maximum radial strain for the threaded terminal was found to be 2.2 mm and for the plain terminal it was 2.1 mm terminal as shown in Figure 4.16. As shown in Figure 4.20, in the EM-shrunk sample, the hardness of terminal increases with the increase of the discharge voltage.

Figure 4.14 (a) 25 mm 2 copper wire, (b) Internal threaded terminal, and (c) Plain terminal

Summary

The deformation of the crimped aluminum terminal is found to be greater compared to the plain aluminum terminal by 0.5 mm for the maximum discharge voltage of 11.25 kV. The resistance value was observed to be 20% less in the threaded terminal compared to the plain terminal.

Introduction

Numerical Analysis

Modelling Process

A total of 8 contact parts in the model, need 18 contact pairs for all possible two surface combinations. In the finite element simulation, Johnson-Cook (J-C) constitutive equation was used to model the behavior of deforming aluminum terminal.

Analysis and Discussion

Current Density
Magnetic Field
Radial Deformation
Impact Velocity

As shown in Figure 5.5, tapered CS coil generates the maximum value of a magnetic field of 8 T followed by a rectangle CS and circular CS coil with the value of 7.1 T and 6 T. As shown in Figure 5.7, the resulting impact velocity of the terminal over the wire strands was found to be maximum for trapezoidal CS coil with a magnitude of 225 m/s, while for rectangular CS and circular CS coil the velocity was 207 m/s and 194 m/s.

Figure 5.2 Typical waveform of current for various discharge voltages 5.2.2.1 Current Density

Experimental Work

Deformation Measurement in the Samples
Contact Length Measurement and CS Analysis
Contact Resistance of the Crimped Junction
Hardness Analysis
Pull-Out Test

As shown in Figure 5.14, the value of hardness increased with the increase of discharge voltage. The maximum pull-out value was found to be 2237 N for a trapezoidal CS coil as shown in Figure 5.15.

Figure 5.10 Change in diameter for samples crimped using different coils, experimental and simulations results

Summary

The pull-out test value shows an increase in strength by 22.5% for a discharge voltage of 11.25 kV for a trapezoidal spiral coil compared to a rectangular CS coil and 40.7% compared to a circular CS coil. The simulation conducted in LS-DYNA and experimental work showed that the trapezoidal CS coil was the most suitable coil among the rectangular CS and circular CS coil.

Introduction

The comparison was performed by keeping the total FS total length and the effective working area constant. The simulation of EM terminal-wire crimping process was performed on LS-DYNA EM module software and the experimental work was performed by comparing the results obtained from the simulations.

Numerical Analysis

Current Density
Magnetic Field
Lorentz Force
Impact Velocity
Effective Plastic Strain

The highest amplitude of the impact velocity of the terminal across the wire strands was obtained. Because the velocity of the terminal strike across the wire strands increases with increasing current amplitude.

Figure 6.1 Cross-sectional view showing dimensions of (a) Single-step FS (b) Double- Double-step FS (c) Tapered FS

Comparison between Experiment and Simulation

Contact Length Analysis
Contact Resistance
Surface Hardness Analysis
Hardness along Cross-Section
Pull-Out Strength

The maximum contact length was found to be for single-stage FS, followed by two-stage FS and tapered FS. As shown in Figure 6.21, terminal breakage was observed at 10 kV for a single-stage FS.

Figure 6.9 Assembly of different types of FS (a) Single-step , (b) Double step and (c) Tapered FS

Analytical Calculation of Field Shaper Designing

In this case, inductance of the FS is equal to inductance of the working zone. It can also be seen that by reducing the length of the working zone, a remarkable increase in the magnetic pressure can be obtained.

Figure 6.22 Various dimensional parameters of a tapered FS In the end zone, magnetic field density (B 1 ) is given by,

Summary

The seven wire strands and the contact length of the terminal interface obtained for the single-step FS were found to be 9.8 mm, which were 0.8 mm and 1.1 mm more compared to the double-step FS and tapered FS. An analytical calculation of the field shaper is performed, which showed that single-stage FS is more efficient, followed by two-stage FS and conical FS.

Conclusions

It was observed that a single-step field shaper gave more terminal distortion due to less induction because the smaller mass volume resulted in lower EM losses compared to a double-step and tapered field shaper. In terms of working efficiency, the single-step field former was found to be the most efficient, followed by double-step and tapered field formers.

Future work

34;A Finite Element Analysis of Eectromagnetic Forming of Tube Expansion." Journal of Engineering Materials and Technology. 34;Effect of Field Shaper på magnetisk tryk i elektromagnetisk formning." Journal of Materials Processing Technology.