Ultrasonic Welding of Thermoplastic Composite Coupons for Mechanical Characterization of Welded Joints through Single Lap Shear Testing

The overall goal of this procedure is to produce thermoplastic composite joints for basic mechanical testing using simplified energy directors, rapidly defined optimal welding parameters, and displacement control for consistent weld quality. This method helps answer key questions in the area of joining of composites such as what is the strength that can be expected from a welded joint and from a plastic composite. The main advantage of this technique is that it produces consistent high-quality welded coupons after fast definition of optimum welding parameters.

For this procedure, have a microprocessor controlled ultrasonic welder that can operate at a constant amplitude. The welder must output process data such as dissipated power and displacement of the sonotrode versus time to a computer. This demonstration uses a custom-built jig designed to accurately position and clamp single-lap sheer samples.

Turn on the ultrasonic welder and log into the computer. Before each experiment, log the following parameters:room temperature and humidity, the welding setup reference, the sonotrode type, the sample number and material, the width and thickness of the top and bottom samples, and the thickness of the energy director. Start the data acquisition software and open a new session.

This demonstration uses a 40-millimeter diameter cylindrical sonotrode. The next steps are to add the samples and the energy director. First, clean the samples and the energy director.

Then, tape the flat energy director to the bottom sample. It should cover an area slightly larger than the area to be welded. Then, place and align the bottom sample into the jig.

And, clamp it down by tightening the top screw. Next, secure the other end of the energy director to the base of the setup using tape. Then, place the upper sample into the clamp, and align it.

Now, tighten the top screw. Then, position the clamp for the top sample into the sliding platform and tighten both screws. Before proceeding with the weld, tighten all four screws once more.

To achieve the highest weld strength, you have to determine the optimum duration of the vibration phase based on the displacement of the sonotrode. This duration must be determined for each combination of welding force and vibration amplitude. Begin by setting the control mode of the ultrasonic welder to the differential displacement.

Next, input the welding force and the vibration amplitude. Please note that in this machine's setting, the amplitude is expressed as half the peak-to-peak amplitude value. Now, input the sonotrode's displacement or travel at the end of the vibration phase.

Use a value equal to the initial thickness of the energy director, which in this case is zero point two five millimeters. Then, input the solidification force and the solidification time. In this example, these values are 1, 000 Newton and 4, 000 milliseconds.

After inputting these settings, the system is ready to make ultrasonic welds. Put on protective eyewear, soundproof headphones, and start the process. After completing a weld, take note of the following output parameters:vibration energy, maximum power, and welding distance.

Then, remove the coupon from the welding setup, and label it on both ends with an identification number, using a paint marker. Now, export the welding data to a spreadsheet, and plot the power and displacement versus time during the vibration phase of the process. Please note that this curve plots the downward displacement of the sonotrode relative to its position at the onset of the vibration phase.

From the curves, identify the displacement in the middle of the power plateau. This displacement value is the optimum travel that controls the duration of the vibration phase for high-strength welds. Use this optimum travel value in every subsequent weld for the same welding force and amplitude.

After completing all the welds, test their single lap sheer strength by following the ASTM D1002 Test Standard or a similar standard, and use a universal testing machine. Composite samples made out of carbon fiber reinforced polyetherimide with a nominal thickness of one point nine two millimeters were welded using the described method. Flat polyetherimide energy directors with zero point two five millimeter thickness were used.

The optimum travel value for a 300 Newton welding force and 86.2 microns peak-to-peak amplitude was calculated as 40%of the initial thickness of the energy director. To validate the calculated optimum travel, samples were welded at different travel values and subsequently tested. Five welds were made for each value.

The apparent lap sheer strength peaked at 40%of the initial thickness of the energy director, which was, indeed, the calculated optimum travel value. Fracture surfaces of the tested samples showed complete welded areas, which demonstrates the flat energy director's ability to weld the intended welding area. Finally, displacement controlled welding resulted in highly consistent weld quality, as shown by the low dispersion in strength value.

A different strategy, such as time-controlled welding is expected to result in high scatter, as suggested by the significant overlapping of vibration times for different travel values, and, hence, different strength levels. After watching this video, you should know how to make ultrasonic welded joints in thermoplastic composites with optimum quality. This method is intended for basic mechanical testing, with simplified energy directors and simplified process parameter definition.

Once mastered, the definition of optimum welding parameters and the welding of a batch of samples can be done in less than two hours if performed properly. Following this procedure, other methods, such as cross-section microscopy can be performed in order to answer additional questions, like thickness of the weld line, or, extension of heat affected zones.