# Investigation of Mixed-Mode I/II Fracture under Impact Loading Using Split-Hopkinson Pressure Bar
## 2 Experiment
### Discussion of Experimental Results  
To better understand the dynamic mixed-mode I/II fracture, an in-depth analysis was carried out. The two specimen patterns are displayed in Figure 6. The fracture mode of crack initiation presents different forms when the angle between pre-crack and the axis of the specimen varies. The maximum initiation angle of the specimen-30\({}^{\circ}\) is 62.13\({}^{\circ}\), shown in Figure 6b, and the average initiation angle is 57.32\({}^{\circ}\), which indicates that the fracture mode of the specimen-30\({}^{\circ}\) is mixed-mode I/II fracture. Meanwhile, the crack propagation paths also demonstrate different features related to the fracture mode. For specimen-0\({}^{\circ}\), both the crack initiation angle and the dip angle are 0\({}^{\circ}\), the crack propagates along the symmetrical axis of the sample leaving a straight line before it stops in the rear area, showing the typical mode I feature. Figure 5: Crack propagation gauge (CPG) signal and derivative of CPG signal. (**a**) CPG signal curve of specimen-0\({}^{\circ}\). (**b**) CPG signal curve of specimen-30\({}^{\circ}\). For specimen-30\({}^{\circ}\), the crack propagation behavior becomes more active than mode I crack, a larger movement can be caught and a more irregular trajectory can be observed. Instead of extending as a straight line, the crack first kept propagating with the direction of the initiation angle, then adjusted the orientation and ran towards the symmetrical axis of the specimen shaping a curved path. Particularly, several deflection points can be seen on the crack path of specimen-30\({}^{\circ}\). Among them, the two crucial points are marked with A and B, respectively. It is acknowledged that the point A on both specimen-0\({}^{\circ}\) and specimen-30\({}^{\circ}\) is a crack arrest area according to the historical stress curve and CPG signal, which should be induced mainly by the tensile stress wave reflected on the bottom of the specimen. The main reason is that the time when point A was shaped is almost 400 us after the specimen subjected to the stress wave, which means the compressive stress wave has a limited function on crack propagation. When the extending crack approach to the bottom of the specimen, the reflected stress wave has a great effect on arresting crack. Whereas, deflection point B can only be found on the specimen-30\({}^{\circ}\) with mixed-mode I/II initiation, indicating the crack propagation of mixed-mode I/II is not a very consecutive process. The points might be dominated by the crack deceleration which gives running crack an opportunity to release the inertia effect caused by high-speed propagation and choose a new direction to develop. To verify this speculation, the cracks propagation velocity is focused. Figure 7 illustrates a very interesting phenomenon that the propagating crack does have distinct velocity deceleration between 7 th-8 th wires and 8 th-9 th wires, which forms exactly the area where point B is located. Furthermore, the velocity decelerations of all the specimens between the 16 th-17 th wires are confirmed with very low speed, about 31 m/s, which can be the direct proof that the crack arrest had occurred. Therefore, the crack propagation behavior before the 16 th wire can be defined as the whole propagation process with initiation, propagation, and arrest. It is worth mentioning that the average initiation time of the specimen-0\({}^{\circ}\) and specimen-30\({}^{\circ}\) are 208.12 us and 223.32 us, respectively. When compared mixed-mode I/II fracture with the mode I, the crack extending velocity of the mode I/II crack is more stable in a stone's throw after initiation. All the above indicates that the specimens with a dip angle of 30\({}^{\circ}\) are harder to initiate than mode I specimen with an angle of 0\({}^{\circ}\).