User:Benatar.1/WE7406 Article 3

= Implant Resistance Welding =

Article Draft
The current article has good information on implant resistance welding, however it only talks about the subject in the context of pipes. This is not the only application of implant resistance welding, so I chose to write about implant resistance welding of thermoplastic composites. I also added sections on failure modes and strength testing of joints. My thought was that the articles could be combined to create a more comprehensive article.

Electro-Fusion Welding
Electrofusion welding is a form of resistive implant welding used to join thermoplastics and thermoplastic composites. Electrofusion welding of thermoplastic composites has been researched by many and has been used in the aerospace industry.

Process
A current is applied to a heating element implanted in the joint, this current flowing through the implant produces heat through electrical resistance, which melts the matrix. Pressure applied to push the parts together and molecular diffusion occurs at the melted surfaces of the parts, creating a joint.

Implants
The implants serve as the source of heat to melt the thermoplastic, the heat is created through electrical resistance as a voltage is applied to the implant. Stavrov discusses two types of implants used for the heating source in resistance welding of thermoplastics and thermoplastic composites. These two types are carbon fiber and stainless-steel mesh. The carbon fiber type implants can be further separated into unidirectional and fabric type implants. Because the unidirectional type carbon fibers do not transfer heat across the fibers easily, the carbon fiber fabric works better to evenly heat the entire surface. According to Stravrov, this difference affects the performance of the resulting weld, the welded joints using the carbon fiber fabric can have 69% higher shear strength and 179% more interlaminar fracture toughness, when compared to unidirectional carbon fibers. For carbon fiber reinforced thermoplastics, the carbon fiber heating element matches the reinforcing material, avoiding the introduction of a new material. It has been shown that welded joints with stainless steel mesh implants tend to have higher strength than welds using carbon fiber implants and results in less air trapped in the joint. Stainless steel wire can be placed in between two layers of resin, to avoid leaving spaces in the holes of the mesh. However, there are reasons to avoid using stainless steel in favor of carbon fiber including, increased weight, the metal acts as a contaminant, possibility of stress concentrations, and possibility of corrosion.

Energy Input
The amount of energy input into the system (E) depends on the resistance of the heating elements (R), the current applied to the heating elements (I), and the amount of time the current is applied (t). Alternating and direct current both work in this process. This is shown by the equation below:

$$E=I^2Rt$$

The current can be alternating or direct. The connection to the power source is important, the pressure on the implant must be great enough to create heat through resistance, but not so high that it severs the implant. This is achieved with pressures of 4 to 20 MPa for carbon fiber and 2 MPa for stainless steel mesh heating elements. Research has shown that the input variable that has the most impact on the performance of the resulting joint is the current. The same amount of energy can by input into the part by applying a low current for a long period of time as if a high current is applied for a short amount of time. In general, a higher shear strength of the joint is achieved using the method with a higher current for a shorter time. Lengthened heating times at lower currents do not heat the joint surface as evenly and this can lead to the fiber reinforcement to move within the melted matrix. Residual stresses and warpage are possible consequences of the current being too high.

For a given constant electrical power, the temperature of the material surrounding the implants or is directly dependent on the amount of time power is applied (the weld time). The longer weld time, yields a higher temperature. The lap shear strength (LSS) of the joint created and the weld time are also correlated. Initially, there is a positive correlation between weld time and LSS until the strength peaks for a certain weld time, and beyond this optimal weld time, the LSS decreases.

Pressure
The surfaces of each part being joined must be pressed together. This pressure prevents deconsolidation, allows intermolecular diffusion, and pushes air out of the joint. The pressure can be applied using displacement or pressure control.

Strength Testing
Lap shear strength testing, in accordance with ASTM D 1002, is a method of destructive testing used to determine the strength of electrofusion welds of thermoplastic composite materials. For this test, two rectangular samples of the composite are lapped at the ends and joined at the lap interface using resistance implant welding. Then, a tension strength test is performed on the welded sample, with the joint surface being loaded in pure shear, a machine pulls the sample until it fails and measures the maximum load. The lap shear strength (LSS) is the maximum tensile load imparted on the sample by the machine divided by the lapped area.

Failure Modes
Interfacial failure or tearing is when the resin or laminate in immediate contact with the heating element on either side is pulled away, leaving the mesh or fabric heating element exposed. This type of failure is associated with low LSS of the sample and can occur as a result of inadequate heat input into the weld. Another failure mode associated with low LSS is cohesive failure, which is a failure of the welded material, either the melted base material or resin surrounding the mesh. Cohesive failure is observed in samples with too much heat input during welding, which deteriorates the thermoplastic. Samples with high LSS generally fail due to debonding of the reinforcing fiber-matrix surface or other base material failure, known as intralaminar failure.