Stress Level and Stress Concentration Factor

 

Principle: The magnitude of stress endured by steel is a crucial factor affecting fatigue performance. When the stress level exceeds the material's fatigue limit, fatigue cracks are prone to initiate and propagate. The stress concentration factor also plays a significant role and is related to the component's geometric shape, surface roughness, and internal defects. For example, in steel components with sharp notches or holes, the stress concentration factor is high. Even under a relatively low nominal stress, the local stress may exceed the fatigue limit, leading to fatigue failure.

 

Application Scenarios and Cases: In mechanical manufacturing, for instance, in an engine's crankshaft, there is usually a sudden change in shape at the connection between the journal and the crank, which results in stress concentration. If the surface machining accuracy of these parts is insufficient and there are rough surfaces or micro - cracks and other defects, during the engine's operation, the alternating bending and torsional stresses borne by the crankshaft can easily initiate fatigue cracks at these locations. To reduce stress concentration, measures such as using transition fillets are adopted in the design, and the surface is finely machined and strengthened (such as shot - peening treatment) to improve fatigue performance.

 

 

 

 

 

 

 

KR/R E43 extra strength property
Grade	Mechanical Property			Charpy V Impact Test
	Yield	Tensile	Elongation	Degree	Energy 1	Energy 2
	Min Mpa	Mpa	Min %	-40	42	28
KR/R E43	355	490-620	21

 

Loading Frequency and Loading Waveform

 

Principle: The loading frequency can affect the fatigue life of steel. Generally, a lower loading frequency allows fatigue cracks more time to propagate because, between each loading cycle, the material has more time for microstructure adjustment and damage accumulation. The loading waveform (such as sine wave, triangular wave, square wave, etc.) also has an impact on fatigue performance. Different waveforms imply different stress - change rates and stress - peak - holding times. For example, in the case of pulse loading (square - wave loading), the longer holding time of the stress peak can lead to the accumulation of local plastic deformation of the material, accelerating fatigue failure.

 

Application Scenarios and Cases: In fatigue - testing equipment, researchers can control the loading frequency and waveform to simulate different actual working conditions. For example, when simulating the fatigue conditions of a bridge structure under the combined action of wind load and vehicle load, the loading frequency can be determined according to the actual wind - speed - change frequency and vehicle - passing frequency. If the wind load is a relatively stable periodic load (similar to a sine wave), and the vehicle load is an intermittent pulse load (similar to a square wave), then in the fatigue test, it is necessary to comprehensively consider the influence of these two different waveforms and frequencies of loads on the fatigue performance of steel.

 

 

 

 

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