Performing Microscope-Mounted Y-Shaped Cutting Tests

Y-shaped cutting measures failure energy and a critical surface creation link scale in soft solids. Incorporating this technique into a microscope facilitates unraveling the microstructural mechanisms that govern these quantities. In contrast to the large planted crack typical of front-wheel loading, this protocol uses plate-induced stretch localization to reduce the field-of-view needed to image the interior failure process.

The approach may provide insight into the failure of soft synthetic materials and biological soft tissues. To begin, replace the original stage-mounted slide holder with a custom sample holder and attach the assembly to the microscope. Set the angle of the cup by loosening the angle adjust thumbscrew and then moving the linear slide.

Set the angle after measuring it with a protractor and tighten the angle adjust thumb screw. The angle between the leg and the sample midplane theta can be adjusted from 8 to 45 degrees. Set up the two vertical pulleys behind the apparatus.

Prepare a thin rectangular sample of polydimethylsiloxane, or PDMS, by either cutting it from a larger sheet or using a mold of the correct dimensions. The dimensions may vary, but a width of 1 1/2 centimeters or less for a sample with a thickness of three millimeters or less is recommended to start. Use a razor blade to cut the sample three centimeters lengthwise along the center line to create the Y-shaped sample.

This link may vary but the legs should be long enough to accommodate the tabs, yet short enough to leave an uncut sample for measurement. Using a marker or ink, place two marks centered and separated by approximately one centimeter on each of the thin legs and the body of the sample to measure the applied stretch in each of the three sample legs under load. Use adhesive-like cyanoacrylate glue to attach a 3D-printed or laser-cut tab to the end of each leg.

Measure and cut two lengths of thin fishing line. Approximately 30 centimeters of line is needed for internal routing through the mechanism to the external set of pulleys. Attach five-gram weighing plates to the end of the lines passing through the external pulleys and tie the other end to the tab on each leg.

Clamp the base of the sample using the sample holder thumbscrew and route the line for each leg through each side of the pulley system. Take a picture of the sample from the top while the sample is under negligible weight by holding a camera against the underside of the angle adjustment mechanism. Make sure the camera is parallel to the sample plane to minimize off-angle effects.

Add the desired preload weight of 75 grams to both ends of the fishing line near the external pulleys. Increase this quantity to 150 grams or decrease it to 50 grams to change the tearing contribution if desired for this combination of material and sample geometry. Align the fishing line from the lowest pulley with the Z plane of the sample legs using the Z component of the three-way micro adjustment stage.

Take a second picture of the sample after the weight is added. Approximately position the anticipated blade tip close to the objective's field-of-view. Place the razor blade into its corresponding blade clip and secure the blade in place with a set screw.

Slide this clipped razor blade into the blade clip mount attached to the load cell. Select the 2.5x microscope objective or as high as 20x, if closer images are desired, and use the transmitted light setting, augmenting the light behind the sample if needed. With the blade in place, focus the microscope on the nearest surface of the blade using the vertical adjust system if necessary to bring the tip to the appropriate working distance for the objective.

Carefully align the razor blade within the microscope's field-of-view using only the X and Y directions of the three-way microadjustment stage. Focus the microscope on the sample and align the crack tip with the razor blade by translating the microscope X/Y stage. This ensures that the midplane of the sample aligns with the midplane of the angle adjustment mechanism.

Open the code used for the load cell data acquisition and start recording the load cell data by clicking the Start Recording button. Translate the sample toward the razor blade for one centimeter or more at constant velocity using microscope stage control. Simultaneously gather images using the microscope's imaging interface.

When the microscope X/Y stage stops, click the Stop Recording button to stop recording the data and automatically save a text file of the load and time response. The force-time curve for polydimethylsiloxane using an ultra sharp blade is shown here. The elastic loading, cut initiation, steady-state cutting, and unloading regions of the curve are labeled in the graph.

This data illustrates a high initial force, as is typically required for cut initiation, followed by a constant force, indicating steady-state cutting. The cutting force is the maximum value of the force within this steady-state regime. The red circles shown here correspond to specific images obtained by the microscope.

A yellow circle has been added to facilitate the observation of the speckle pattern motion, and these numbers indicate image timestamps and seconds. The measured steady-state cutting force combined with the experimental testing parameters of leg angle theta, sample thickness T, preload f of pre, and blade radius yield the steady-state cutting energy according to the equation shown. Here, we successfully replicate the cutting energy previously reported in the literature for these conditions.

By incorporating Y-shaped cutting into a microscope, we enable the quantification of microstructural contributions to soft-solid and soft tissue failure via fluorescent probes, autofluorescence, and full-field strength techniques.