Design and Improvement of the Connection Between the Steel Wire Rope and Shell for Cobalt-60 Afterloading Source Whip

This article studied a novel connection method between the steel wire rope and the outer shell of a cobalt-60 afterloading brachytherapy source delivery whip. The performance requirements for the connection between the steel wire rope and the shell are described by analyzing the structural characteristics and usage environment of the source whip. Multiple joint forms and process ideas are proposed and explored. After repeated testing and improvement, a set of embedded adhesive bonding connection methods is finally proposed. The small-diameter cobalt-60 afterloading radiation source whip produced by this method meets the requirements of experimental verification and clinical use, effectively solving the problem of connection between the radiation source shell and steel wire rope.


Introduction
The Cobalt-60 afterloading therapy machine is a Class III large medical device used for intracavitary radiotherapy and brachytherapy for cancer patients [1].It usually consists of a Cobalt-60 radiation source, a shell, and a control system.The source whip, which is the core component of the Cobalt-60 afterloading therapy machine, mainly consists of the source core, source shell, steel wire rope, and the anode connector, and its whole structure looks like a whip.It is used to transmit the radiation released by the radiation source to the treatment machine.
Currently, although these components can provide high-precision and high-quality radiation therapy, and accurately control the radiation dose and irradiation area during treatment, in the treatment of cancer patients, the source whip is exposed to radiation for a long time, and it will also rub back and forth on the inner wall of the source delivery pipe when receiving and delivering the radiation source.Moreover, the joint of the source whip not only suffers from radiation and friction, but also frequently bends back and forth in the source applicator bend, and the connection location is subject to alternating stress for a long time, which is prone to creep, fatigue, and even fracture.
If the source whip is damaged or worn, it may cause problems such as radiation source failure, unstable radiation dose, or radiation leakage, which can be harmful to patients and medical staff.Therefore, quality control and process improvement of the source whip are particularly important.Generally speaking, the service life of the source whip is about 40,000 times, and no abnormal conditions such as deformation, fracture, loose or falling off of the wire rope are allowed during the life cycle.The harsh working environment and the special nature of the radiation source put extremely high requirements on the safety and reliability of the entire source whip.How to process a source whip with a small outer diameter and a thin outer shell has become a current research hotspot, and the study of the connection between the steel wire rope and the shell will also have an important impact on the production of the source whip.

Connection Structure and Design Scheme of the Source Whip
The source whip shell is an external layer used to protect and fix the radioactive source component.It is usually made of metal or alloy materials with strong corrosion resistance and high-temperature resistance.It can effectively prevent the component from being contaminated and damaged by the external environment, and ensure the stability and safety of the component during treatment.In this study, the outer diameter of the source whip shell should not exceed 2.4mm, and the materials of the shell and wire rope are both 06Cr19Ni10 (i.e.304 stainless steel).Due to the aperture limitation of the applicator (a device used for radiation therapy, mainly used to introduce radioactive isotopes or linear accelerators into human tissues) [2,3], the source whip requires a rigid section length of no more than 7mm and meets the requirements of small diameter, short joint, large load-bearing tension (about 130N), and long service life (6 years or 40,000 times) [4][5][6].With a known safety factor and the joint of the source whip being subjected to alternating stress and radiation for a long time, two connection schemes have been designed in this section to ensure a firm and reliable connection between the wire rope and the shell and have been tested and verified one by one.

Connection Structure and Design Scheme of the Source Whip.
The shell of the source is connected to the wire rope by welding [7], and the joint form is shown in figure 1.The production process is as follows: First, the plug is welded to the steel wire rope to form a firm welded seam 2. This requires professional welding equipment and technology to ensure the strength and stability of the welded seam, thereby ensuring the smooth loading process in the subsequent steps [8].Then, the shell is turned into the heating chamber and the cobalt source particles are loaded into the shell by a dedicated robot.The entire loading process needs to ensure that the cobalt source particles are positioned correctly and evenly distributed, while also avoiding unnecessary contact and collisions between the shell and the cobalt source particles to prevent damage to the source whip.Finally, the plug with the steel wire rope is inserted into the shell hole, and the welded seam 1 is formed by welding with a robotic arm.Similarly, strict quality control and inspection are required throughout the process to ensure the safety and effectiveness of the source whip [9].

The Embedded Adhesive Press-fit Method.
The source shell is connected to the steel wire rope by embedded adhesive bonding and pressure, and the joint form is shown in figure 2. The production process is as follows: first, fill the appropriate amount of adhesive in the inner hole of the shell to enhance the bonding strength and durability [10].Then insert the steel wire rope into the inner hole, and use a special tool to apply appropriate pressure on the pressure fitting area to press the shell and the steel wire rope tightly after the adhesive is solidified and stable.Next, transfer to the heating chamber and use a special manipulator to load cobalt source particles into the shell, and then insert the plug into the shell hole.Finally, the seal is welded by a special welding machine to ensure that the joint is completely sealed and prevents radioactive source leakage.This manufacturing process is simple, the connection is reliable, and the cost is low, making it easy to promote and apply.

Strength Analysis of the Fusion-welded Joint.
Given the yield strength of the 06Cr19Ni10 shell material is and the maximum load the source whip needs to withstand with a safety redundancy factor is , assuming the size of the welded foot is , according to the calculation formula of static load strength of the welded joint, the following assumptions are made:  Welding residual stress has no effect on the strength of the joint [11][12][13];  The joint is fully penetrated, and the local stress concentration caused by geometric discontinuity is neglected [14][15][16].
Combining the above conditions, we can obtain:   ( )     ( ) 0.6 0.6 102.5 61.5 MPa   The above formula shows that L represents the length of the weld.In actual situations , its strength far exceeds the requirements for the use of the weld.

Strength Analysis of the Adhesive-bonded and Pressure-connected
Joints.Based on the information mentioned earlier that the shell and wire rope are both made of stainless steel, during actual processing, the German PK7000 acrylic structural adhesive with an average strength of 2 20 N mm is used.Assuming that the diameter wire rope is in complete adhesive contact with the inner wall of the shell, then: ( ) 1.1 1.9 131.32 Here 1.9 is the safety factor of the adhesive.After calculation ( ) , it indicates that the strength of the pure adhesive joint meets the requirements for carrying the tensile load.Experiments have shown that pressing the joint with a tool after adhesive bonding is beneficial for increasing the contact area and improving the quality of the joint [17].Moreover, pressing itself is also a reliable mechanical connection method, which can fix the source shell and wire rope by applying pressure without generating high temperatures.The process is simple to operate and has little impact on the physical properties of the materials.The left side of the figure shows the welded joints designed according to scheme one, numbered 1# , 2#, 3#, 4#, and 5# respectively.The right side shows the pressed joints designed according to scheme two, numbered 6#, 7#, 8#, 9#, and 10 # respectively.Their appearance dimensions were measured and inspected by local macroscopic magnification to select the best specimens.Finally, 2#, 3#, 5#, 6#, 9#, and 10 # specimens were selected for reliability operation tests, each with three pieces.

Experimental Method and Requirements.
The running test will be conducted on the afterloading therapy machine, and the test operation of the source whip will be consistent with the actual operating conditions, which means that the source whip will be sent out from the storage source tank by the driving mechanism, enter the inner hole of the source applicator through the source delivery pipe channel (including bends), stay for 10 seconds, and then return to the original path under the drive of the driving mechanism.Repeat the above operations until the test times exceed 45,000 times.Check and record the status of the source whip every 100 times during the test, including wear connection joints and surface quality, etc.After the test is completed, it is required that there should be no looseness, breakage, significant deformation, and damage on the source whip joint, and no obvious bending deformation or damage on the surface of the shell.Products that meet the above conditions are considered qualified.

Analysis of the Cause of Weld Joint
Fracture.The broken weld was observed under an optical microscope, and it was found that the base metal near the heat-affected zone had shrunk and the weld surface did not exhibit a metallic luster.Analysis indicates that there are three possible reasons:  The rotational speed of the flat welded position during the welding process is manually controlled by the welder, so the uncertainty caused by human factors may affect the formation of the weld [18,19];  During the welding process, although small parameters were selected for the heat input control, the local heat input was still too high for the 1.1mm diameter test specimen, and the peak temperature in the heat-affected zone was in the sensitization temperature range due to a large amount of local heat input [20], which promoted the precipitation of 23 6 Cr C carbides in the 06Cr19Ni10 stainless steel parts ( 0.08%

 =
) at 600~850℃, resulting in intergranular corrosion in the sensitized region of the heataffected zone, and the material becomes brittle and the strength decreased [21][22][23];  The source shell is rigid while the steel wire rope is elastic.Although the strength calculation of the welding joint meets the requirements for use, there is no ductile transition zone at the junction position.During the test, the welding position frequently bent inside the hole, and the stress accumulated at the welding joint section due to fatigue effects.In addition, the steel wire rope is twisted from multiple strands of steel wire and cannot form a whole, which ultimately leads to fracture at the weak point of the joint [24,25].

Analysis of the Reasons for Joint
Wear.After conducting a metallographic macroscopic inspection of the radioactive source shell [26], numerous grooves and scratches were found on the outer surface of the shell, especially severe wear at the front end.Additionally, the spherical weld seam at the front end of the shell was worn down, resulting in a significant flattening of the adjacent base material and the loss of a significant portion of the deposited metal at the welded joint.
Analysis of this phenomenon indicates that the main reason for it is the high-hardness wire winding inside the radiation source channel (i.e. the wall of the delivery pipe), while the material of the source package shell is austenitic stainless steel with relatively low hardness.During the transmission and delivery of the radiation source, the shell will rub back and forth against the inner wall of the delivery pipe and the bending of the source applicator, leading to the appearance of scratches on the outside of the shell.

Test Results
In the test of design scheme 1, when the test run of 2# source whip reaches 6300±100 times, there are two strands of steel wire lose from the weld where the steel wire rope of the test piece is welded with the outer shell, as shown in figure 4. In order to rule out the possibility of this happening by chance, source whip was selected for the same test, and at 14200±100 times of the test run, three strands of wire rope became loose, indicating that there were certain defects and shortcomings in the design scheme.In the subsequent tests, the shell of the 3# and 5# source whips fell off at 9600±100 times and 17400±100 times during the test run respectively, and the tests had to be terminated.Therefore, design scheme one for the source whip has been deemed a failure.
In the test of design scheme 2, the 6# source whip was tested and operated approximately 41,800±100 times.After observation, the source whip joint had no looseness or breakage, and there were no significant deformations such as bending on the surface of the shell.However, severe wear occurred at the welding position between the welded head and the shell, and small holes appeared as shown in figure 5.In the remaining tests, source whip showed severe wear at its welding location after being tested for 45,000±100 cycles but did not exhibit any signs of damage.Source whip showed severe wear at the welding location and had some damage after being tested for 43,200±100 cycles.

Improvement Plan.
Based on the comparative tests in section 3.1, the sample with adhesive pressure joint showed better performance.Therefore, the joint between the source shell and steel wire rope will continue to use the adhesive pressure joint.Considering the wear resistance of the source shell, the structure has been optimized: the diameter has been slightly increased to 2.4mm (from the original 2.2mm), and a boss has been added to allow direct contact between the boss and the straight section of the source pipe and the applicator, which can effectively avoid direct contact and wear at the sealing position outside the bend.The improved structure is shown in figure 6.

Experimental Validation.
The improved source whip was remanufactured according to the improvement plan and 15 samples were produced as shown in figure 7. The fatigue resistance, wear resistance, and reliability of the improved source whip were revalidated through the operational tests in accordance with the requirements described in Section 2.3.2.After the tests were completed, each sample was inspected individually, and no abnormal signs such as looseness or fracture were found.The wear degree was significantly reduced, and all performance indicators met the requirements for use in the afterloading machine.

Discussion an Conclusion
This study investigated a cobalt-60 afterloading source whip for intracavitary radiotherapy in cancer patients.The source whip consists of a steel wire rope and a shell, with cobalt sources and a gasket sequentially loaded inside the shell hole from the inside to the outside, and the outer edge of the gasket is transitionally matched with the inner wall of the shell hole.The connection scheme of the source whip has been improved and tested extensively, with one end of the steel wire rope axially connected to the anode joint and the other end connected to the shell.The performance of the improved source whip was verified through a series of tests and evaluations.
The new type of source whip can be used for intracavitary treatment with small-diameter applicators.It not only reduces the pain of patients but also effectively avoids the tedious process of frequent source replacement.It has the characteristics of excellent therapeutic effect, safety and reliability, and easy use and maintenance, and has broad application prospects with good market forecasts.The 15 newly produced source whips based on the improved design scheme performed well in the experimental tests, without any abnormal situations such as radioactive source detachment or blockage, fully meeting the usage requirements.The source whip effectively solves the technical problem of connecting the steel wire rope with the radioactive source, providing an effective solution for the connection problem of the cobalt-60 source whip used in afterloading intracavitary radiation therapy.

Figure 1 .
Figure 1.The welding of the source shell and the steel wire rope.

Figure 2 .
Figure 2. Adhesive and press-fit joint between the source shell and the steel wire rope.
(where 1.1 is the diameter of the wire rope), it indicates that when the size of the welded foot is 3 a mm = , under the action of the rated load 13 P kg =

2. 3 . 1 .
Preparation of Specimens.The test samples of the source whip were prepared according to the process flow described in Scheme 1 and Scheme 2, as shown in figure3.

Figure 4 .
Figure 4. Diagram of looseness at joint weld.

Figure 5 .
Figure 5. Diagram of wear at the welded position.