PSI - Issue 28

Bahador Bahrami et al. / Procedia Structural Integrity 28 (2020) 829–835 Bahrami et al./ Structural Integrity Procedia 00 (2020) 000–000

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Inclination angle of crack in SENB specimen

Kolosov constant Poisson’s ratio

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Stress components in polar coordinate system

Tensile strength

2. Experiments To perform the mixed-mode I/II fracture tests, a modified form of the single edge notch bend (SENB) specimen was used. The final geometries of the specimens are shown in Fig. 1a. According to Fig. 1a, the modified version of SENB provides the full range of in-plane mixed-mode conditions by changing the value of α i.e. the crack inclination angle. Note that in all situations the vertical position of crack tip remains constant with 7.6 mm distance from the bottom edge of the samples. To prepare the PMMA specimens, first, the polymer (powder) and monomer (liquid) were mixed with respective ratio 4 to 3. Then, it was vacuumed under the pressure of 0.7 bar for the duration of 60 seconds. Next, the mixture was molded into the shape of rectangles, and, after 20 minutes the specimens were removed from the mold. To achieve an appropriate surface finish as well as the desired thickness of t =6 mm, the specimens were eventually face milled. To introduce crack into the SENB specimens, first, a sawing machine was used, and then the crack front was sharpened for an extra 2 mm using a razor blade. After performing a detailed finite element (FE) study, four values of inclination angle α = 0, 25, 50 and 61 degrees corresponding to pure mode I, two mixed-mode and pure mode II conditions were selected for the specimens. The elastic modulus E , Poisson’s ratio ν and the tensile strength σ t of the material were measured as 2400 MPa, 0.3 and 40.8 MPa, respectively.

Fig. 1. a) Schematic of the modified SENB specimen. b) A sample fractured specimen with α =50°. Note that crack growth initiates at the crack tip and then extends towards the load exertion point. The three-point bend tests were conducted using the SATNAM STM-150 universal testing machine with a displacement rate of 0.5 mm/min. It should be noted that as the supports utilized for testing should not apply any friction on the specimens (Bahrami et al. (2019); Sedighi et al. (2020)), free-rolling metallic cylinders were used as bottom supports for entire testing. In total, 12 fracture tests were performed with the fracture loads presented in the results and discussion section of this paper. Fig. 1b demonstrates a typical fractured specimen that underwent mixed mode cracking.