Teorias de falha por fadiga (parte 1) -- Elementos de Máquinas

Summary

This lecture on mechanical elements focuses on the theory of fatigue failure, with an emphasis on cracks and their propagation. The instructor explains the stages of crack formation, propagation, and sudden rupture, using the analogy of a windshield cracking from a stone impact. The lecture covers the impact of different stress types (alternating, repeated, and mean) on fatigue, and how correction factors such as material properties, size, surface finish, and temperature influence the fatigue limit. Students are encouraged to apply these concepts in practical exercises to understand fatigue behavior and improve their understanding of mechanical failure in materials.

Takeaways

• 😀 Cracks (trincas) are a form of fatigue failure, starting as small marks or cracks that can either develop during manufacturing or over time during use.
• 😀 Cracks generally go through three stages: initiation, short-duration growth, and propagation, often undetected, especially when internal.
• 😀 Fatigue failure often leads to a sudden rupture, similar to a windshield cracking after a stone impact, where a small crack eventually causes the entire piece to break.
• 😀 There are three types of stress: alternating stress, repeated stress, and mean stress, each with different characteristics and implications for material failure.
• 😀 Alternating stress has a mean stress of zero, while repeated stress has a non-zero mean stress, affecting material performance differently.
• 😀 The fatigue limit of a material is estimated through theoretical and experimental data, and must be corrected for factors like load type, size, surface finish, temperature, and reliability.
• 😀 Material strength (e.g., steel, iron, aluminum, copper) is a key factor in determining the fatigue limit. For example, steel with a tensile strength less than 200,000 PSI uses a correction factor based on its strength.
• 😀 The size effect requires calculating an equivalent diameter (d) to account for stress distribution in the material.
• 😀 Surface finish plays a crucial role in fatigue strength. Materials with different surface treatments, such as cold-rolled or forged, require different correction factors.
• 😀 Temperature effects on fatigue strength are corrected using specific formulas, with materials exhibiting different behaviors at temperatures above or below certain thresholds.
• 😀 Reliability factors, such as project reliability percentages (e.g., 50%, 90%), influence the adjusted fatigue limit for a given material.

Q & A

What are the three stages of crack propagation described in the lecture?

The three stages of crack propagation are: 1) The initial stage, where a small crack or mark forms, 2) The short-duration stage, where the crack propagates, and 3) The sudden rupture stage, where the crack reaches a critical size, leading to catastrophic failure.

How is fatigue failure explained in the context of the car windshield analogy?

In the car windshield analogy, a small stone hits the windshield, creating a tiny crack (the initial stage). Over time, this crack slowly propagates (the short-duration stage) until it suddenly breaks the windshield completely (the sudden rupture stage).

What are the different types of stress mentioned in the lecture?

The lecture mentions three types of stress: 1) Alternating stress, where the average stress is zero (maximum stress equals minimum stress), 2) Repeated stress, where the mean stress is non-zero, and 3) Static stress, where the stress remains constant.

What is the fatigue limit, and how is it estimated?

The fatigue limit is the stress level below which a material can withstand an infinite number of loading cycles without failure. It is estimated through theoretical and practical testing, and correction factors are applied based on material properties, loading conditions, size, and surface finish.

What are the factors of correction used in determining the fatigue limit?

The correction factors include: 1) The effect of loading (e.g., pure bending or normal force loading), 2) The effect of size (based on the material's dimensions), 3) The effect of surface finish, 4) The effect of temperature, and 5) The effect of reliability.

How does the tensile strength (ST) influence the fatigue limit of a material?

The tensile strength (ST) influences the fatigue limit by determining the stress levels a material can withstand before failure. The fatigue limit is often a fraction of the material's tensile strength, and different materials (steel, iron, aluminum, copper) have specific formulas for estimating the fatigue limit based on their tensile strength.

What formula is used to calculate the equivalent diameter (de) of a component?

The equivalent diameter (de) is calculated using the formula: de = √(A95 / 0.076), where A95 represents the area in which 95% of the component is subjected to the effective stress.

What are the correction factors for surface finish, and how are they determined?

Correction factors for surface finish depend on the manufacturing process, such as whether the surface is ground, machined, or forged. Specific values for these factors are given in the lecture, and they vary based on the type of surface treatment and the material being used.

How does temperature affect the fatigue limit, and how is it factored into the calculations?

Temperature affects the fatigue limit by altering the material's behavior under stress. For temperatures below 440°C (or 890°F), the correction factor is 1, while for temperatures above 440°C, a formula is used to adjust the fatigue limit based on the specific temperature range.

What role does reliability play in estimating the fatigue limit of a material?

Reliability is a factor that adjusts the fatigue limit based on the probability of failure. For different reliability levels (e.g., 50%, 90%), correction factors are applied, which decrease the estimated fatigue limit to account for the likelihood of failure over time.