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Mechanics of dynamic fracture in notched polycarbonate

机译：缺口聚碳酸酯中动态断裂的力学

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Fracture toughness of brittle amorphous polymers (e.g. polymethyl methacrylate (PMMA)) has been reported to decrease with loading rate at moderate rates and increase abruptly thereafter to close to 5 times the static value at very high loading rates. Dynamic fracture toughness that is much higher than the static values has attractive technological possibilities. However, the reasons for the sharp increase remain unclear. Motivated by these observations, the present work focuses on the dynamic fracture behavior of polycarbonate (PC), which is also an amorphous polymer but unlike PMMA, is ductile at room temperature. Towards this end, a combined experimental and numerical approach is adopted. Dynamic fracture experiments at various loading rates are conducted on single edge notched (SEN) specimens with a notch of radius 150 μm, using a Hopkinson bar setup equipped with ultra high-speed imaging ( ＞ 10~5 fps) for real-time observation of dynamic processes during fracture. Concurrently, 3D dynamic finite element simulations are performed using a well calibrated material model for PC. Experimentally, we were able to clearly capture the intricate details of the process, for both slowly and dynamically loaded samples, of damage nucleation and growth ahead of the notch tip followed by unstable crack propagation. These observations coupled with fractography and computer simulations led us to conclude that in PC, the fracture toughness remains invariant with loading rate at J_(frac) =12±3kN/m for the entire range of loading rates (j) from static to 1 × 10~6kN/m - s. However, the damage initiation toughness is significantly higher in dynamic loading compared to static situations. In dynamic situations, damage nucleation is quickly followed by initiation of radial crazes from around the void periphery that initiate and quickly bridge the ligament between the initial damaged region and the notch. Thus for PC, two criteria for two major stages in the failure process emerge. Firstly, a mean stress based defect initiation is suggested. The value of the critical mean stress for defect initiation under dynamic loading is found to be 115 ± 5 MPa, which is significantly higher than its static value of 80 MPa. The critical normal plastic stretch needed for crazes to nucleate from the nucleated defect is estimated to be about 1.78 ± 0.2.
机译：据报道，脆性无定形聚合物（例如聚甲基丙烯酸甲酯（PMMA））的断裂韧性在中等速率下随加载速率而降低，此后突然升高，在非常高的加载速率下接近静态值的5倍。比静态值高得多的动态断裂韧性具有诱人的技术可能性。但是，大幅增加的原因尚不清楚。受这些观察的推动，目前的工作集中在聚碳酸酯（PC）的动态断裂行为上。聚碳酸酯也是一种无定形聚合物，但与PMMA不同，它在室温下具有延展性。为此，采用了组合的实验和数值方法。使用装备有超高速成像（> 10〜5 fps）的霍普金森棒装置，对半径为150μm的缺口的单边缘缺口（SEN）标本进行各种载荷率下的动态断裂实验。断裂过程中的动态过程。同时，使用经过良好校准的PC材料模型执行3D动态有限元模拟。通过实验，我们能够清楚地捕获过程的复杂细节，无论是缓慢加载还是动态加载的样品，其损伤核化和缺口尖端之前的增长以及随后裂纹的不稳定扩展都是如此。这些观察结果与分形图和计算机模拟相结合，使我们得出结论，在PC中，在从静态到1的整个加载速率（j）范围内，加载强度（J_（frac）= 12±3kN / m）时，断裂韧性都保持不变。 10〜6kN / m-s。但是，与静态相比，动态载荷下的破坏起始韧性要高得多。在动态情况下，损伤成核后迅速从空隙周围开始产生径向裂纹，这些裂纹开始并迅速桥接韧带在初始受损区域和切口之间。因此，对于PC，出现了故障过程中两个主要阶段的两个标准。首先，提出了基于平均应力的缺陷引发方法。发现在动态负载下引发缺陷的临界平均应力值为115±5 MPa，明显高于其静态值80 MPa。裂纹从有核缺陷中成核所需的临界正常塑性拉伸量约为1.78±0.2。

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