The Relationship Between Mechanical Properties and Fatigue Fracture of Structural Materials

Material Fatigue

Fatigue refers to the phenomenon where the structural performance of a material (typically metal) deteriorates under cyclic stress or strain, ultimately leading to failure. Fatigue failure is one of the most common forms of failure. According to data from the literature, fatigue failure accounts for 60-70% of failed components in various machines. Fatigue fracture failure is fundamentally a low-stress brittle fracture, where noticeable plastic deformation is difficult to observe because it is primarily localized plastic deformation, often occurring at inherent defects in the structure. Although frequency has some impact on fatigue failure, fatigue failure is mostly related to the number of cycles.

Classification of Fatigue Failure

1. By stress characteristics:

   • Mechanical fatigue caused by mechanical stress.

   • Thermal fatigue caused by thermal stress (cyclic thermal stress).

2. By number of cycles:

   • High-cycle, low-cycle, and ultra-high-cycle fatigue.

3. By nature of load:

   • Tension-compression fatigue, torsional fatigue, and bending fatigue.

4. By working environment:

   • Corrosion fatigue, low-temperature fatigue, and high-temperature fatigue.

The strength of a material or structure before it undergoes fatigue damage is generally defined as the "fatigue limit."

1. Impact Fatigue

Impact fatigue refers to fatigue caused by repeated impact loads. When the number of impacts N is less than 500-1000, the fracture mode of the component is similar to that of a single impact. When the number of impacts exceeds (10^5), the fracture is considered fatigue fracture, characterized by typical fatigue fracture features. In design calculations, when the number of impacts exceeds 100, the strength is calculated using methods similar to those for fatigue.

2. Contact Fatigue

Contact fatigue is the process where a component experiences local, permanent, cumulative damage under cyclic contact stress, leading to pitting, shallow or deep flaking on the contact surface after a certain number of cycles. This is a typical failure mode for gears, rolling bearings, and camshafts.

3. Thermal Fatigue

Thermal fatigue refers to fatigue of a material or component caused by cyclic thermal stress due to temperature cycles. Temperature changes cause cyclic volume changes in the material, generating cyclic thermal stress or thermal strain when the material's free expansion or contraction is constrained.

Thermal stress occurs mainly in two scenarios:

• The expansion and contraction of the part are constrained by external constraints, creating thermal stress.

• Even without external constraints, inconsistent temperatures within different parts create a temperature gradient, causing inconsistent expansion and contraction and resulting in thermal stress.

Cyclic temperature changes not only generate thermal stress but also cause changes in the material's internal structure, reducing strength and plasticity. Under thermal fatigue, the temperature distribution is not uniform, and where the temperature gradient is large, plastic deformation is severe, and thermal strain is concentrated. When thermal strain exceeds the elastic limit, thermal stress and thermal strain no longer have a linear relationship, requiring thermal stress to be calculated according to elastoplastic relationships. Thermal fatigue cracks begin at the surface and extend inward, perpendicular to the surface.

4. Corrosion Fatigue

Corrosion fatigue refers to fatigue caused by the combined effects of corrosive media and cyclic stress (strain). Stress corrosion refers to the corrosion failure caused by the combined action of corrosive media and static stress. The difference is that stress corrosion only occurs in specific corrosive environments, while corrosion fatigue occurs in any corrosive environment combined with cyclic stress. Stress corrosion cracking has a critical stress intensity factor KISCC; when KI < KISCC, stress corrosion cracking does not occur. However, corrosion fatigue does not have a critical stress intensity factor; as long as cyclic stress continues in a corrosive environment, fracture will always occur.

Fatigue Life

When a material or mechanical component fails, the total life typically consists of three parts:

1. Crack Initiation Life: Extensive engineering practice shows that the crack initiation life of mechanical components accounts for most of the fatigue life during actual service (up to 90% of the total life).

2. Crack Propagation Life: In most cases, when a microcrack reaches a certain size (about 0.1 mm), it begins to propagate steadily along the cross-section of the material or component.

3. Unstable Propagation to Fracture Life.

Forms of Metal Fatigue

The fatigue of metal materials mainly includes the following forms:

• General plastic deformation.

• Plastic deformation under low-cycle fatigue.

• Plastic deformation under high-cycle fatigue.

• Microscopic plastic deformation of crystal lattice size under ultra-high-cycle fatigue.

Factors Influencing the Fatigue Strength of Materials and Structures

1. Mean Stress:

   • As mean stress (or statistical stress) increases, the dynamic fatigue resistance of the material decreases.

2. Stress Concentration:

   • Components often have steps, holes, or keyways that cause sudden changes in cross-section, leading to local stress concentration, significantly reducing the fatigue limit of the material.

3. Residual Stress:

   • Residual stress significantly affects the fatigue strength of notched components.

4. Size Effect:

   • The fatigue limit of materials decreases as the sample size increases.

5. Surface Condition:

   • Surface roughness and mechanical processing marks affect fatigue strength.

6. Environmental Factors:

   • Fatigue performance is influenced by environmental factors such as corrosion fatigue, low-temperature fatigue, high-temperature fatigue, different pressure environments, and humidity.

7. Load Type:

   • The order of fatigue limits under different loads is: rotational bending < planar bending < compressive load < torsional load.

8. Material Defects:

   • Cracks often initiate on the surface and are influenced by inclusions and other factors.

9. Processing Methods:

   • The preparation process of fatigue test samples, including machining and heat treatment, significantly impacts fatigue performance.

10. Material Properties:

• The fatigue strength of high-cycle fatigue is related to the hardness of the material, while toughness is an important indicator for medium- and low-cycle fatigue.

Scatter in Fatigue Test Data

The scatter in fatigue test data (or results) is primarily caused by test equipment and sample preparation.

Development of Structural Fatigue Design Methods

1. Safe-Life Method:

   • Designing with stress below the fatigue limit, assuming no defects in the structure.

2. Fail-Safe Method:

   • Designing with stress related to the residual strength in the presence of plane defects.

3. Safe Crack Method:

   • Allowing for a predictable growing crack.

4. Local Failure Method:

   • Widely applied in France, this method addresses issues in metal fatigue analysis.

For steel materials, in the absence of fatigue test data, the S-N curve can be approximated using the material's ultimate tensile strength.

In material and structural fatigue analysis, it is crucial to draw conclusions from experiments rather than blindly trusting elastoplastic calculations to ensure data reliability.