The extruder screw works under higher load stress, but the working speed is lower, so it runs under the action of high stress and lower alternating frequency.
Each time the extruder screw starts to run, a certain amount of plastic strain is generated. Each time the screw is obstructed by a hard object, a certain amount of plastic deformation will occur. This plastic deformation will damage the screw much more severely than when it is started. In addition, due to the plastic strain caused by the screw being squeezed and friction for a long time, the plastic deformation is repeated. Under the action of strain, the internal damage of the screw accumulates to a certain extent and then a crack is formed. When the crack expands to a critical size and the remaining section of the screw is insufficient to withstand the operating load of the screw, the crack destabilizes and expands and causes a sudden brittle fracture.
From the analysis of the macroscopic fracture morphology, the crack of the extruder screw is a low-cycle fatigue damage. Three areas can be clearly seen on the section: fatigue crack source; fatigue crack propagation area; and instantaneous cracking area.
Fatigue crack sources can occur at the root of the extruder screw, surface cracks and internal inclusions where stress is concentrated. The fatigue crack source of the extruder screw is very small, which is the core area of the fatigue crack.
Ammonia infiltrated hardened layer is cracked. Fatigue cracks slowly expand in this area. Initially, the surface of the fracture surface is rubbed smoother and brighter due to repeated alternating stresses, and then shell patterns appear. The shell pattern is generally perpendicular to the propagation direction of the crack, advancing to the entire hollow circle. The spacing between shell patterns (fatigue arcs) varies. Under the repeated action of alternating stresses of varying sizes, the macroscopic traces left by the discontinuous changes in the crack propagation process are caused by the load changes such as the start-up and shutdown of the extruder, and the hard object obstructing the extruder screw. Generally, the area of the fatigue crack propagation area accounts for 98% of the fracture area, which is in line with the basic characteristics of fatigue cracks. When the shell pattern (fatigue arc) on both sides of the circumference is close to tangent, and the stress of the crack propagation to the net section reaches the breaking stress of the screw, the screw is cut. The section has obvious steps, and the area of this part accounts for about 1% of the entire fracture area.
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