During the implant resistance welding process,
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 is applied to push the parts together and
molecular diffusion occurs at the melted surfaces of the parts, creating a 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 current (AC) and
direct current (DC) both work in this process. The energy produced is calculated using the following equation: E=I^2Rt Research has shown the input variable with 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 or 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. Longer heating times at lower currents do not heat the joint surface as evenly. This can lead to the fiber reinforcement to move within the melted matrix. If the current is too high, however, it can result in residual stresses and warpage. For a given constant
electrical power, the temperature of the material surrounding the implants is directly dependent on the weld time. The longer weld time, yields a higher temperature. The
lapped shear strength and the weld time are also correlated. Initially, there is a
positive correlation between weld time and strength. However, the strength peaks for a certain weld time, and beyond this optimal weld time, the strength decreases.
Pressure Pressure is applied to the joining surfaces to prevent deconsolidation, allow intermolecular
diffusion, and push air out of the joint. The pressure can be applied using displacement or pressure control. Pressure also ensures good contact between the implant and the bulk material, in order to increase
electrical resistance. The pressure on the implant must create good contact without being 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. == Strength Testing ==