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Summary

The presentation discusses the concept of fatigue in metals, a progressive and localized structural damage that occurs under cyclic loading. It explains the causes, primarily related to surface finishing, and introduces the SN curves used to evaluate fatigue resistance. The presentation details the stages of fatigue, from initiation to propagation, and the role of material properties. It also highlights the effect of shot peening treatment on gear fatigue resistance, showing that introducing compressive stresses can significantly improve fatigue life. The analysis concludes that shot peening improves the fatigue life of treated gears by 36%.

Takeaways

• 😀 Fatigue failure is a progressive, localized, and permanent structural damage process caused by cyclic loading from various sources like mechanical, acoustic, or thermal.
• 😀 90% of mechanical failures are caused by fatigue mechanisms, even though the stresses involved are lower than the material's yield stress.
• 😀 Fatigue failure typically begins at the material's surface, where imperfections like inclusions or extrusions lead to crack propagation.
• 😀 Cyclic loading parameters include stress range, amplitude, mean stress, and stress ratio, which help classify loadings as reversed, fluctuating, or static.
• 😀 SN curves (or Wöhler curves) show the relationship between applied stress and the number of cycles until failure, helping define the material's fatigue limit.
• 😀 For ferrous materials like steel and titanium, the fatigue limit is well defined, while for non-ferrous materials, the limit is not clearly established.
• 😀 Fatigue can be classified into high-cycle (over 10^7 cycles) and low-cycle (less than 10^7 cycles) types based on the number of cycles before failure.
• 😀 The fatigue process includes three stages: initiation, propagation, and final fracture, with cracks often initiating at the surface and growing in a shear direction along slip planes.
• 😀 The crack propagation rate can be predicted using Paris' law, which relates crack growth rate to stress intensity factor variation and material constants.
• 😀 A case study demonstrated that applying a laser surface fusion treatment followed by shot peening significantly increased the fatigue life of gears by approximately 36%, thanks to induced compressive residual stresses.

Q & A

What is the definition of fatigue failure in metals?

Fatigue failure in metals refers to progressive, localized, and permanent structural damage that occurs when a material is subjected to cyclic loading. This damage happens even when the applied stress is lower than the material's yield strength.

Why is surface finish important in fatigue failure?

Surface finish is crucial because fatigue failure typically initiates at the surface of the material. Any discontinuities such as inclusions, extrusions, or intrusions can promote the propagation of cracks, accelerating the fatigue process.

What are the main parameters used to describe cyclic loading in fatigue?

The main parameters are the stress range, stress amplitude, mean stress, and stress ratio. The stress ratio is the ratio between the minimum and maximum applied stress and is used to classify the type of cyclic loading, which can be reversed, fluctuating, or static.

What is the significance of S-N curves in fatigue analysis?

S-N curves are crucial in fatigue analysis as they represent the relationship between the applied stress and the number of cycles to failure. These curves help determine the fatigue strength limit, which is the stress level at which a material can withstand infinite cycles without failure.

How is high-cycle fatigue different from low-cycle fatigue?

High-cycle fatigue involves a large number of cycles, typically more than 10 million cycles, while low-cycle fatigue occurs with fewer cycles. Low-cycle fatigue is usually associated with higher stresses and more severe loading conditions.

Can fatigue failure occur below the yield strength of a material?

Yes, fatigue failure can occur even when the applied stress is below the material's yield strength. This is because fatigue failure is driven by cyclic loading rather than static stress levels.

What are the three stages of fatigue in materials?

The three stages of fatigue are initiation, propagation, and final fracture. In the initiation stage, cracks begin at the surface; during propagation, cracks grow according to the Paris law; and in the final fracture stage, rapid crack growth leads to the material's failure.

What role does the Paris law play in fatigue crack propagation?

The Paris law relates the rate of crack growth to the variation in the stress intensity factor. It is used to predict the propagation of fatigue cracks, accounting for material properties and cyclic loading conditions.

What is the effect of shot peening on the fatigue resistance of materials?

Shot peening improves fatigue resistance by inducing compressive residual stresses on the surface of the material. These compressive stresses counteract tensile stresses that might otherwise accelerate crack growth, thus enhancing the material's fatigue life.

How did the shot peening treatment affect the fatigue life of gears in the case study?

In the case study, gears treated with shot peening showed a 36% increase in fatigue life compared to untreated gears. This improvement was attributed to the induced compressive stresses, which reduced crack propagation and extended the gears' service life.