## 4 Results and Discussion  
### Detailed Aftermath of the Piston Rod Fracture  
In the first case, the piston rod fracture occurred as the piston moved to the BDC (bottom dead centre) position. Once the piston rod was completely broken, the piston and rod relied on their inertia to decelerate and continue to decelerate towards the BDC. During this time, the crosshead was still being driven towards the BDC by the crank linkage, causing both to move away from the break position. As the crosshead passed the BDC and moved towards the TDC, it collided with the piston rod, with the fracture position being the first point of contact, producing a large impact signal. The uneven surface of the fracture caused the contact path to frictionally slip to one side, resulting in an oblique plane that is not perpendicular to the horizontal axis. The impact caused a secondary fracture at the thread connecting the prefabricated fracture rod to the piston rod, and the fractured part fell into the crosshead slide and was jack-knifed to the underside of the oil scraper by the crosshead movement, as shown in Figure 10. In addition, the noncoaxial impact caused the piston rod to bend approximately \(2^{\circ}\) to one side, creating friction with the surface of the stuffing box. The stuffing box structure cut away part of the piston rod surface, leaving visible signs of wear. What is more, the impact caused significant deformation in the rear half of the preload nut, which was connected to the crosshead. The secondary impact and friction generated significant kinetic energy which was converted to heat at the point of contact, resulting in black carbonised hydraulic oil on the surface at the point of severe impact and friction. It is worth noting that, if the compressed gas were flammable and explosive, the point of impact could act as an ignition source, and sparks from the tiny metal particles could ignite the compressed gas and cause an explosion, which could be fatal to the compressor.  
In the other case, the fracture occurs as the piston moves to the TDC. After the fracture occurred, the piston and piston rod moved under inertia to the TDC, where they came to rest under the cushioning effect of the polyurethane pad. Fortunately, in this case, the piston and piston rod run beyond the TDC position, and therefore no secondary impact is generated at the fracture position. In practice, however, without the cushioning effect of the polyurethane rubber seal, the piston would collide with the cylinder wall, which, in severe cases, can lead to cylinder rupture and flammable gas leakage, resulting in an ignition explosion. The fracture morphology of the prefabricated fracture rod, shown in Figure 11, was retained. The crack in the piston rod develops gradually from the surface on one side of the piston rod. After the initial crack is formed during the fatigue fracture process, a slight increase in the crack length is formed as the number of cycles increases. When the fatigue crack has expanded to about 1/2 the diameter and the remaining section is so small that it is close to plastic instability, the material will rapidly undergo plastic tearing. The remaining sections can be pulled off one by one with only a small number of cycles. At this point, it is almost irrelevant to the alternating stress fatigue mechanism, but should be part of the plastic fracture process, very similar to the mechanism of shear lip formation in a ductile fracture.  
Figure 10: Prefabricated fractured rods after the secondary fracture. Schematic diagram of the secondary collision at the fracture (**a**); bending and wear of the piston rods due to collision (**b**); photographs of the prefabricated fracture rod after the secondary fracture (**c**).