# Fracture Behavior of Two Biopolymers Containing Notches: Effects of Notch Tip Plasticity
## 2 Materials and Methods  
### Materials, Testing Specimens, and Experimental Results  
In total, 3 groups of experiments were performed in the present investigation. The first group dealt with the standard tensile tests, aiming to obtain the stress-strain curves of the two biomaterials being analyzed. The second group, however, corresponded to fracture tests on cracked specimens, aiming to measure the corresponding fracture toughness. The last group corresponded to fracture tests on notched specimens, aiming to experimentally investigate the weakening effects of stress concentrations on the fracture behavior of the two materials, particularly their load carrying capacity. The two materials used in this research are referred to as BC (bone cement) and DM (dental material). BC required 40 g of sterile powder (composed of Polymethyl-methacrylate 83.9%, Benzoyl peroxide 2.8%, Barium sulfate 9.6%, Gentamicin sulfate 3.8%) per 16.4 gr of sterile liquid (composed of Methyl-methacrylate 85.3%, Butyl-methacrylate 13.2%, \(N\),\(N\)-dimethyl-p-toluidine (max 1.5%) and Hydroquinone (20 ppm)), the supplier being Groupe Lepine (France). DM was generated by combining 500 g of Polymethyl-methacrylate powder per 250 mL of liquid Methyl-methacrylate (supplier: Acropars Pharmaceutical Industry, Tehran, Iran). In order to generate the two biomaterials, there are two common methods. In the first method, the powder and the liquid are manually mixed for 60 s at 23 \({}^{\circ}\)C. In the second method, known as the vacuum mixing method, powder and liquid are poured in molds and mixed in a vacuum chamber pressurized at 0.7 bar [31]. In the current study, the second method was used due to its ability to minimize the porosity of the material. It consists of three stages: during the first 15 s, the material was pressurized without any mixing; in the next 30 s, mixing and pressurizing were applied simultaneously; finally, during the last 15 s, the material was left out under constant pressure without mixing. To avoid bubble generation, the mixing process was performed in the vacuum chamber. After the mixing process, the samples were subjected to room temperature for 20 min for the curing process to be completed. Finally, the specimens were removed from the molds. These molds had been previously made of Poly Lactic Acid by 3D printing, with the required geometries to obtain Brazilian disk (BD), rectangular (DM), and semicircular bend (SCB) specimens (BC material). In the final stage of specimen generation, computer numerical control (CNC) machining was used to introduce U-notches with 1, 2, and 4 mm radii to BD samples. Sharp cracks were also introduced to the rectangular and the SCB specimens using a razor blade, with the aim of measuring the fracture toughness of the two biomaterials. The geometry of the resulting specimens is shown in Figure 1. The thickness of the UNBD specimens, the cracked rectangular samples, and the cracked SCB specimens was equal to 6 mm, 3 mm, and 6 mm, respectively. The tip-to-tip length of the central slot of the UNBD specimens was 25 mm, and the crack lengths of the rectangular and SCB samples were 8 mm and 10 mm, respectively. Additionally, and according to ASTM-D638-IV [32], 3 specimens (per material) were manufactured with the geometry described in Figure 1d, with the purpose of conducting standard tensile tests, which were performed with a displacement rate of 0.5 mm/min. During the tensile tests, digital image correlation (DIC) was used to measure the displacements, and a 10 kN load cell was used to measure the loads. A summary of the tensile test results is given in Table 1, while Figure 2 shows two examples of the obtained load-displacement curves. These reveal linear-elastic behavior in the BC material and evident nonlinear behavior in the DM.