The residual stress of stainless steel riveting screw is an inevitable mechanical phenomenon in its manufacturing process. It has a dual effect on fatigue life and service reliability, and needs to be comprehensively analyzed in combination with stress type, distribution characteristics and service environment.
The effect of residual stress on fatigue life is first reflected in the crack initiation stage. During the riveting process, the root of the stainless steel riveting screw is prone to residual tensile stress due to local plastic deformation. This type of stress may directly exceed the yield strength of the material after superposition with the external load, causing microcracks to initiate prematurely at the surface defects. For example, when the residual tensile stress reaches 30% of the yield strength of the material, the fatigue crack initiation life may be shortened by more than 50%. If residual compressive stress is introduced by optimizing the riveting process, the initiation can be suppressed by the "crack closure effect", and the fatigue limit can be increased by 20%-40%. This difference is particularly significant under high-frequency vibration or alternating load conditions, because the residual compressive stress can effectively offset part of the tensile stress cycle and delay the formation of the crack core.
In the crack propagation stage, the residual stress dominates the propagation rate by changing the amplitude of the stress intensity factor (ΔK). Residual tensile stress increases ΔK and accelerates crack propagation, while residual compressive stress significantly reduces the propagation rate by reducing effective ΔK or closing the crack tip. Taking stainless steel 304 as an example, when the residual compressive stress reaches -200 MPa, the crack propagation rate can be reduced by an order of magnitude. This effect is more prominent in the short crack propagation stage because the residual stress has a significant effect on the stress field in a small yield zone. In engineering, shot peening is often used to introduce a residual compressive stress layer on the surface of stainless steel riveting screws, with a depth of up to 0.2-0.3 mm, which prolongs the fatigue life by 2-3 times.
The coupling effect of residual stress and stress corrosion cracking (SCC) is a key factor affecting service reliability. In a chloride ion environment, residual tensile stress promotes anodic dissolution at the crack tip, reducing the SCC critical stress intensity factor (KISCC) by more than 50%. For example, a type of stainless steel riveting screw used in marine engineering suffered SCC fracture after only 6 months of service due to the failure to eliminate the residual tensile stress of riveting, while the life of similar stainless steel riveting screws treated with stress relief annealing exceeded 5 years. This shows that for high corrosion risk scenarios, harmful residual tensile stress must be eliminated by low-temperature annealing at 250-350℃ or vibration aging.
Uneven distribution of residual stress will also cause the preload of the stainless steel riveting screw to decay, thereby affecting the reliability of the connection. The residual tensile stress generated during the riveting process may cause the thread to yield locally, resulting in a decrease in the friction coefficient between the thread pairs and loosening under alternating loads. Experimental data show that when the residual tensile stress exceeds 20% of the material yield strength, the preload of the stainless steel riveting screw may decay to less than 60% of the initial value after 500 cycles. This decay will be further aggravated in high temperature or high humidity environments, and it needs to be compensated by optimizing the thread geometry parameters or using locking coatings.
The impact of residual stress on the fracture mode is directly related to structural safety. Residual tensile stress tends to cause fracture to initiate from surface defects, showing brittle fracture characteristics, with cleavage steps or river patterns on the fracture surface; while residual compressive stress may cause cracks to extend toward the center, showing ductile fracture, with cup-cone or fibrous fracture surfaces. In fatigue tests, residual tensile stress will cause more dense beach-like stripes on the fracture surface, while residual compressive stress will increase the roughness of the fracture surface, indicating that the crack propagation path is longer. This difference needs to be considered in the design of safety factors.
The detection and control of residual stress is the core link to ensure the performance of stainless steel riveting screws. X-ray diffraction can accurately measure surface residual stress with an accuracy of ±10 MPa, which is suitable for quality control; neutron diffraction can penetrate the interior of the material to obtain three-dimensional stress distribution, but the equipment cost is relatively high. In terms of process optimization, stress concentration can be reduced by adjusting the riveting force curve, using segmented riveting or floating mold technology. For formed stainless steel riveting screws, shot peening can introduce surface compressive stress of -150 to -300 MPa, while increasing the surface hardness by 20%-30%.
A full life cycle management mechanism for residual stress needs to be established in engineering applications. During the design phase, stress distribution should be predicted through finite element analysis to avoid sharp corners or thickness mutation structures; during the manufacturing phase, the riveting parameters should be strictly monitored, combined with real-time feedback from online stress detection equipment; during the service phase, ultrasonic phased array technology should be used regularly to detect crack initiation, and a residual stress relaxation model should be established to predict life. For example, an aircraft engine manufacturer increased the failure interval (MTBF) of the key stainless steel riveting screw from 2000 hours to 8000 hours by controlling the riveting residual stress within -100 MPa.
In summary, the residual stress of the stainless steel riveting screw has a significant double-edged sword effect on its fatigue life and service reliability. By introducing beneficial residual compressive stress through process optimization, eliminating harmful tensile stress by combining stress relief treatment, and establishing a full-process stress control system, the comprehensive performance of the stainless steel riveting screw can be effectively improved, providing guarantee for the long-term safe operation of high-end equipment.
