Resumen de On the fracture behavior of ductile polymer films: notch quality, essential work of fracture, j-integral, and crack tip opening displacement
Polymers have a wide range of applications. Their ability to be shaped into multiple forms makes it possible to obtain polymer films, which are highly used in industries such as food and pharmaceuticals. Characterization of polymer films using the classical mechanical properties is well-defined; however, this is not the case for the fracture properties.
The overall aim of this work was to analyze in full-detail the fracture behavior of ductile polymer films by applying the appropriate fracture mechanics approaches.
The first part of the thesis aimed to investigate the existing relationships between the specific essential work of fracture, the J-value at crack initiation, and crack tip opening displacement at crack initiation values and find clear evidence that makes the specific essential work of fracture the specific energy at crack initiation, that is, an initiation value. To do this, the digital image correlation technique was applied in double edge notched tension specimens. Applying this technique makes it possible to measure the displacement and ligament length at the same time, allowing that the essential work of fracture, the J-integral, and the crack tip opening displacement fracture approaches to be performed in a single set of double edge notched tension specimens.
The second part of the thesis was focused on analyzing the influence of the notch quality on the fracture behavior. The specimen preparation, particularly the notch sharpening technique, has been studied in more depth. With the purpose of seeing the influence of the notch sharpening procedure, several techniques have been used, including femtosecond laser ablation, razor blade sliding on specimens at room temperature and frozen by liquid nitrogen, saw cutting, plastically deformed saw cutting, scalpel sliding and drilled holes used as notches. The results revealed that all of the femtolaser sharpened notches had very consistent, sharp notches without plastic deformation in front of the notch tip. Instead, the razor blade sliding technique (the generalized way to sharpen notches in ductile polymer films) produced very sharp cracks, as sharp as the femtolaser ones, but the different compressive component of the sliding force that was applied by the operator generated notches with different levels of plastic deformation ahead of the notch tip.
As different notch sharpening techniques produce different levels of plastic deformation at the crack tip, differences would also be expected in the shape and size of the stress-displacement curves. In the third part of the thesis, the shape and size of the stress-displacement curves that were notched in distinct ways were analyzed to clarify their EWF behavior. In the propagation region, it is also identified the parabolic shape of the stress-displacement curves, which modeled relates the specific non-essential work of fracture with the stress at crack initiation and the extension ratio during the crack growth.
