Comment concevez-vous les joints et les connexions structuraux pour éviter les défaillances dues à la fatigue?

In general, it is right, the load (stress) range and the stress ratio are important factors in structural design with respect to fatigue resistance. However, it is not right to say "...the lower the stress ratio, the shorter the fatigue life." In practice, higher the stress ratio faster is the crack growth, hence shorter the fatigue life. The stress ratio also is not accounted for in the fatigue calculations of the welded joints due to the presence of weld residual stresses. In this case, whatever the load sequence is, it is accepted that the maximum stress is equal to yield stress and the stress ratio is always positive, and could be quite high, greater than 0.5, and is already included in the design SN curves for welded joints.

Adopt design strategies such as ensuring smooth transitions fillet radii to mitigate stress concentrations. Opt for materials with favorable fatigue resistance and maintain a smooth surface finish to discourage crack initiation. Emphasize high-quality welds, considering factors like penetration and toe profiles. Apply appropriate safety factors in design, address misalignments, and distribute loads evenly to prevent localized stress. Regular inspections are crucial for early detection of fatigue-related issues, and employing Finite Element Analysis (FEA) aids in simulating and analyzing joint behavior. Taking a comprehensive approach considering material properties, geometry, and loading conditions is vital for creating resilient joints.

Not sure if the entry wording maks sens that the common cause of connections failure is fatigue, mainly if they are subject to cyclic loading. There are many failure modes and fatigue is one of them but main reason why connections fail is because they were either wrongly designed or fabricated.

Integrate crack arrest mechanisms such as crack stoppers, crack arrestors, or sacrificial features into joint designs. These features can effectively halt the propagation of cracks, preventing catastrophic failure and extending the fatigue life of the structure. Consider the use of resilient materials with high damping capacity and energy absorption characteristics in joint designs. These materials can help dissipate energy during cyclic loading events, reducing stress concentrations and enhancing fatigue resistance. Optimize surface finishes and treatments to minimize the likelihood of fatigue failure. Smooth and polished surfaces can reduce stress concentrations.

From multiple fatigue failure investigations over 30 years, I have found that many factors apply. I explained these in the ASI Webcast Seminar "Reducing the Risk of Fatigue Failure of Fabricated Steelwork," which ran in November 2023. The highest proportion of fatigue failures is, however, due to inadequate analysis and design, with designers not assessing (or mitigating) stress concentrations or missing the degree of cyclic loading that will occur.

Here, also the strain-based and stain-energy-based methods should be added for low-cycle fatigue calculations. The stress-based method (comprising SN curves) is suitable for high-cycle fatigue only, beyond 10 thousands cycles, and cannot be applicable when cyclic strain is present in the critical location and fatigue life is rather low.

Employ advanced analysis techniques such as finite element analysis (FEA) or computational fluid dynamics (CFD) to simulate fatigue behavior and identify potential failure locations in structural joints. Take into account environmental factors such as temperature variations, corrosion effects, and exposure to aggressive chemicals or pollutants that can influence fatigue behavior. Implement protective measures such as coatings, surface treatments, or material selection to mitigate environmental degradation and extend joint longevity. In a project involving the design of welded steel structures subjected to cyclic loading, we meticulously followed fatigue design codes such as BS 7608 to ensure the structural integrity of welded joints.

Employ advanced welding techniques such as friction stir welding or laser welding for superior joint strength. Ensure consistent and high-quality welds to resist fatigue. In the automotive industry, laser welding was used to enhance the durability of chassis connections. Subject prototypes or components to dynamic testing under simulated conditions. Analyze the performance of joints and connections under realistic loads. In structural engineering, shake-table tests were conducted to assess earthquake resistance in building connections. Maintain comprehensive documentation of joint design and performance. Continuously update design standards based on lessons learned from previous projects.

It is very important to add that there are methods for fatigue resistance improvement, like grinding, peening etc. However, they are not advised to be taken into account in the design stage. They might be needed at later stages of structures life to improve the fatigue resistance if it appears to be not enough due to different circumstances of structure usage.

Don’t overlook the effects of temperature changes on the joint load. This can be most critical with steel bolts and thick aluminum flanges.

When designing a welded attachment to a fatigue-prone structure, the guidelines below are recommended: 1. Add Radiuses to Brackets: A radius of 150 mm or greater (according to Eurocode 3) is recommended to ensure smooth transitions and reduce the likelihood of crack initiation due to stress concentrations. 2. Ensure a minimum clearance of 10 mm away from the edges of the welded attachment (according to BS7608). This clearance helps prevent stress concentrations near the edge, minimising the risk of fatigue crack initiation. 3. Implement 100% NDT on the welds to detect potential defects and toe grind the welds to remove stress concentrations in areas prone to fatigue cracking, especially in the tension zone.