Investigation on the Effect of Different Pre-Cracking Methods on Fracture Toughness of RT-PMMA

Abstract In this study, eight techniques including coping saw, metal slitting saw, razor blade, cubic boron nitride (CBN) disc, scoring, die-cutting and guillotining, diamond disc, and laser cutting methods were used to produce pre-cracked fracture toughness (Kc) test specimens made of poly(methyl methacrylate)/graft-acrylonitrile butadiene styrene blends. The influences of notch shape (radial, rectangular, or angular-shaped and variety of thicknesses), pre-cracking method, chip removing or non-chip removing, and the contact or non-contact methods on the results obtained in fracture toughness tests were investigated. The results were analyzed by two methods, quantitatively and qualitatively, by comparing the obtained Kc results and studying the SEM and optical microscopy images, respectively. The results indicated that the different conditions of a produced pre-crack including; geometry of pre-crack due to geometry of tools, residual stress due to pre-crack creation, heat affected zone, damage of crack tip, and producing crazing around the crack tip could affect the fracture toughness. The maximum difference resulted from different pre-cracking methods was equal to 0.24 MPa.m0.5 and the lowest value of fracture toughness Kc, 2.53 MPa.m0.5, belonged to the scoring method but the highest value, 2.77 MPa.m0.5, belonged to the metal slitting saw method. Also, the results indicated that the effects of notching on toughness of RT-PMMA had a low notch sensitivity and the differences between minimum and maximum Kc values was found about 9%.摘要 本研究采用8种切割工艺制备聚甲基丙烯酸甲酯/接枝丙烯腈-丁二烯-苯乙烯共聚物共混物的预制断裂韧性（Kc）试样，所涉工艺包括曲线锯、金属开槽锯、剃须刀片、立方氮化硼（CBN）圆盘、刻痕法、模切、剪切法、金刚石圆盘锯以及激光切割法。本研究探讨了缺口形状（径向、矩形或角形）、缺口厚度多样性、预制裂纹工艺、是否去除切屑、接触式与非接触式加工方式对断裂韧性测试结果的影响。本研究分别采用定量对比所得Kc结果、定性分析扫描电子显微镜（SEM）与光学显微图像的两种方式对测试结果进行了分析。结果表明，预制裂纹制备过程中的多项因素会对断裂韧性产生影响，包括由刀具几何形状决定的预制裂纹几何形态、预制裂纹制备过程中产生的残余应力、热影响区、裂纹尖端损伤以及裂纹周边产生的银纹。不同预制裂纹工艺带来的最大结果差值为0.24 MPa·m^0.5；其中断裂韧性Kc最低值为2.53 MPa·m^0.5，对应刻痕法，最高值为2.77 MPa·m^0.5，对应金属开槽锯法。此外，结果显示，缺口加工对室温聚甲基丙烯酸甲酯（RT-PMMA）韧性的影响表现为低缺口敏感性，Kc值的极差约为9%。