From Gears to Bearings: How Residual Stress Affects the Fatigue Life of Automotive Components?

A car is composed of tens of thousands of components, among which key moving parts such as gears, springs, and bearings endure repeated alternating loads during service. Statistics indicate that over 80% of automotive component failures are due to fatigue. The initiation and propagation of fatigue cracks are fundamentally governed by the actual stress state at critical locations of the parts. Residual stress—a type of internal stress "quietly embedded" during manufacturing—stands as one of the key variables determining the actual stress state. It can serve as an "invisible shield" to extend component lifespan or act as a "slow poison" accelerating failure. Understanding the dual nature of residual stress is an unavoidable challenge in automotive component design and manufacturing.

1. Origin and Mechanism of Residual Stress

Residual stress refers to the stress that remains within a component after the removal of external loads. Automotive components are subjected to or have their residual stress state altered in nearly every processing step, including heat treatment (carburizing quenching, induction hardening), machining (grinding, cutting), surface strengthening (shot peening, rolling), as well as forming processes such as welding and casting.

The impact of residual stress on fatigue life primarily lies in its superposition effect with external loads. When a residual compressive stress exists on the surface of a component, it counteracts the working tensile stress, reducing the actual stress level borne and thereby delaying crack initiation and inhibiting crack propagation. Conversely, the superposition of residual tensile stress and external loads accelerates crack formation and propagation. This fundamental principle underlies the manufacturing and service processes of various automotive components such as gears, springs, and bearings.

2.The Influence of Residual Stress on Typical Automotive Components

Gear

The gears in automobile transmissions and drive axles commonly use carburizing and quenching processes. After carburizing treatment, a hardened layer with high hardness and residual compressive stress is formed from the surface to the inside of the gear, which can significantly improve its ability to resist crack initiation and propagation. On this basis, shot peening further introduces high amplitude residual compressive stress on the surface of the tooth root, thereby increasing the bending fatigue limit of the gear. After strengthening treatment, the maximum residual compressive stress of carburized gears can reach 1170-1600MPa. After shot peening, the fatigue crack source of the gear shifted from the surface to the subsurface, indicating that residual compressive stress effectively protected the surface area most prone to crack initiation.

However, residual stress is not always advantageous. After gear quenching, there may be compressive stress near the surface, while residual tensile stress may occur in the tooth core and tooth end face. If the grinding process is improper, excessive grinding heat will generate high tensile stress in the subsurface, which becomes a hidden danger for crack initiation. Therefore, the "distribution design" of residual stress in gear manufacturing is more critical than simply pursuing surface compressive stress.

Spring

Springs are one of the earliest and most successful application areas of shot peening technology. Automotive suspension springs, valve springs, etc. are subjected to high cyclic alternating stresses during service and are extremely sensitive to surface stress states. After stress shot peening strengthening, a high residual compressive stress can be formed on the surface of the spring, and the fatigue life is greatly improved accordingly; After special shot peening treatment, the design stress of the leaf spring can be significantly increased compared to traditional designs. The fatigue life of spiral springs using new surface treatment methods can even be increased by many times.

However, there are also risks associated with managing residual stress in springs. Research has shown that if the shot peening process is not appropriate, it not only fails to generate effective surface compressive stress, but may also introduce tensile stress, seriously weakening the fatigue life of suspension springs. Adding a heat setting process after secondary shot blasting will actually reduce the strengthening effect of residual stress. This indicates that the "process window" for residual stress in spring manufacturing is narrow, requiring precise control of shot peening parameters and coordination with subsequent heat treatment processes.

Bearing

The failure mode of bearings is mainly rolling contact fatigue, and their lifespan is highly sensitive to residual stress states. Laser heat treatment and laser shot peening techniques can induce high amplitude residual compressive stress on the surface of bearing steel, effectively suppressing fatigue crack initiation and slowing down crack propagation rate.

 It is worth noting that the influence of residual stress is coupled with design parameters such as bearing clearance. When the clearance increases, the contact area decreases and the average bearing life is shortened. This means that the optimization of residual stress cannot be carried out in isolation from the overall design parameters. In addition, during the quenching process of bearing steel, residual stresses are also generated due to uneven temperature and volume changes in microstructure transformation. The precise control of the heat treatment process directly affects the distribution of the final residual stress field.

3.Advantages and disadvantages of residual stress

Based on the above analysis, the impact of residual stress on the fatigue life of automotive parts presents a distinct duality, and its advantages and disadvantages depend on three key dimensions: stress properties (compressive stress or tensile stress), distribution characteristics (size, gradient, depth), and service conditions (load type, environmental medium).

The beneficial effect of residual compressive stress has been repeatedly verified in gears, springs, and bearings. The residual compressive stress field introduced by shot peening can reduce the initiation of microcracks under fatigue stress and suppress their propagation, significantly improving the fatigue fracture resistance of parts. In crankshaft manufacturing, the residual compressive stress introduced by fillet rolling significantly increases the bending fatigue strength compared to unreinforced crankshafts.

The harmful effects of residual tensile stress cannot be ignored. The residual tensile stress generated by grinding processing may lead to cracking after several hours or even days after the completion of part processing, with concealment and hysteresis; The residual tensile stress caused by improper quenching process is the fundamental reason for the fracture of shock absorber piston rod; There is a significant residual tensile stress in the interference fit assembly area of the engine cylinder block, which becomes the direct cause of inducing cracks and leading to cylinder block cracking.

The double-edged nature of residual stress is also reflected within the same component. After heat treatment, the surface of the gear exhibits beneficial compressive stress, while the core may have harmful tensile stress, both of which together determine the fatigue behavior of the gear; The residual compressive stress field will also relax during fatigue loading, and its strengthening effect is not permanently unchanged. This means that the design of residual stress must consider the evolution law of the entire service life.

Conclusion

The impact of residual stress on the fatigue life of automotive parts is essentially a "stress state engineering" problem. It is different from the chemical composition or macroscopic geometric shape of materials, and is an implicit quality element that determines the visible lifespan despite being invisible. For key components such as gears, springs, and bearings, the importance of residual stress lies not in whether it is "present" or "absent", but in what it is and where it is - whether it is compressive or tensile stress, distributed on the surface or in the center, and whether the gradient is gentle or steep.

One of the core capabilities in manufacturing high-quality automotive parts lies in actively regulating residual stresses: implanting beneficial residual compressive stresses in hazardous areas through processes such as carburizing, shot peening, and rolling, avoiding harmful tensile stresses by optimizing grinding parameters and heat treatment systems, and achieving accurate characterization and process monitoring of residual stresses through detection methods such as X-ray diffraction. Only in this way can each component operate safely and sustainably throughout its service life under the dual requirements of lightweight and high reliability.