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What methods are used to predict fatigue crack growth in aerospace structures?

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Fatigue crack growth is a major concern for aerospace structures, as it can lead to catastrophic failure and compromise safety. To prevent or mitigate this problem, aerospace engineers need to use reliable methods to predict how cracks will propagate under different loading and environmental conditions. In this article, we will explore some of the common methods that are used to predict fatigue crack growth in aerospace structures, and their advantages and limitations.

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Alain Forget

Supervisor, Liaison Engineering (MRB) and Engineering Support at Bombardier Aerospace
Pratham Pathak

??15K+ @Linked{In}?? || B.Tech ?? || Final Year UG ?? || Computer Science and Engineering Student ?????? || IIMT…

1 Linear elastic fracture mechanics

One of the most widely used methods to predict fatigue crack growth is based on linear elastic fracture mechanics (LEFM), which assumes that the material behaves elastically near the crack tip and that the crack growth rate depends on the stress intensity factor (SIF). The SIF is a measure of the stress concentration at the crack tip, and it can be calculated from the geometry, size, and location of the crack, as well as the applied load. LEFM can be applied to predict fatigue crack growth in metals and alloys that have a small plastic zone near the crack tip, and that obey the Paris law, which relates the crack growth rate to the SIF range. LEFM is relatively simple and robust, but it has some limitations, such as neglecting the effects of crack closure, plasticity, and environmental factors.

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Alain Forget

Supervisor, Liaison Engineering (MRB) and Engineering Support at Bombardier Aerospace
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To predict fatigue crack growth in aerospace structures, engineers use mathematical models, computer simulations, and experimental data. Analytical models like the Paris Law help estimate crack growth based on stress levels. Computer simulations, called finite element analysis, simulate how cracks evolve under different conditions. Empirical methods rely on real-world testing to understand how cracks propagate in specific materials. Emerging approaches, including probabilistic methods and machine learning, offer more advanced predictions. These methods collectively ensure the safety and durability of aerospace structures by anticipating and managing the effects of fatigue over time.

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Pratham Pathak

??15K+ @Linked{In}?? || B.Tech ?? || Final Year UG ?? || Computer Science and Engineering Student ?????? || IIMT COLLEGE OF ENGINEERING || 18M+ Content Views ?? || Aviation Enthusiast ??
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One thing I've found helpful is recognizing that linear elastic fracture mechanics (LEFM) is commonly used to predict fatigue crack growth in aerospace structures. LEFM analyzes the stress and strain distributions near a crack tip to predict crack growth based on stress intensity factors. Actually, I disagree with assuming uniform material properties. Considering the effects of material nonlinearity and variations in material properties across the structure enhances the accuracy of fatigue crack growth predictions. An example I've seen is the use of Paris Law, a LEFM-based empirical equation, to model fatigue crack growth rates in aerospace structures.

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2 Elastic-plastic fracture mechanics

Another method to predict fatigue crack growth is based on elastic-plastic fracture mechanics (EPFM), which accounts for the plastic deformation near the crack tip and the crack opening displacement (COD). The COD is a measure of the crack opening under load, and it can be used to estimate the effective SIF range, which accounts for the crack closure effect. Crack closure occurs when the crack faces come into contact under compressive load, reducing the effective SIF range and slowing down the crack growth rate. EPFM can be applied to predict fatigue crack growth in materials that have a large plastic zone near the crack tip, and that exhibit cyclic hardening or softening. EPFM is more accurate and realistic than LEFM, but it requires more complex and nonlinear models and experimental data.

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3 Damage tolerance analysis

A third method to predict fatigue crack growth is based on damage tolerance analysis (DTA), which evaluates the residual strength and life of a structure with an existing or assumed crack. DTA considers the effects of loading spectrum, stress distribution, crack geometry, material properties, and inspection intervals on the crack growth behavior. DTA can be used to design and maintain aerospace structures that meet specific safety and reliability requirements, and to optimize the trade-off between weight and performance. DTA is a comprehensive and practical method, but it requires a lot of data and analysis tools, and it may not account for all the possible failure modes and scenarios.

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4 Numerical simulation

A fourth method to predict fatigue crack growth is based on numerical simulation, which uses computational methods and software to model and solve the governing equations of fracture mechanics. Numerical simulation can incorporate various factors that affect fatigue crack growth, such as material nonlinearities, environmental interactions, residual stresses, microstructural features, and multi-axial loading. Numerical simulation can also simulate complex geometries, boundary conditions, and crack configurations that are difficult to analyze analytically or experimentally. Numerical simulation is a powerful and versatile method, but it requires a lot of computational resources, validation, and calibration.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What methods are used to predict fatigue crack growth in aerospace structures?

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5

1 Linear elastic fracture mechanics

2 Elastic-plastic fracture mechanics

3 Damage tolerance analysis

4 Numerical simulation

5 Here’s what else to consider

Aerospace Engineering

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What methods are used to predict fatigue crack growth in aerospace structures?

1

2

3

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5

1 Linear elastic fracture mechanics

2 Elastic-plastic fracture mechanics

3 Damage tolerance analysis

4 Numerical simulation

5 Here’s what else to consider

Aerospace Engineering

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