Composite materials have impressive strength and durability, but they aren’t invincible—especially when it comes to fatigue. Just like metals, composites can suffer from fatigue, leading to cracks and eventual failure over time. However, understanding fatigue in composites is far more complex, given the layered and varied nature of these materials, which require a deeper look into how they respond under repeated stress.
Fatigue analysis in composites differs from metals because composites are made up of multiple layers or fibers within a matrix. This structure means each composite type—whether it’s woven, short fiber, or laminated—faces unique fatigue challenges. Factors like fiber orientation, fiber length, and fiber distribution influence how and when each type of composite begins to weaken, making fatigue analysis a specialized task for each material.
In this blog, we dive into composite fatigue analysis, covering essential concepts and methods used to predict and study fatigue in different composite types. From continuous and discontinuous fibers to woven and short fiber composites, we’ll explain how to approach fatigue simulation and analysis using models like the Nouri and Avanzini models, and we’ll introduce Abaqus simulation techniques. You’ll learn about the specific damage mechanisms, subroutines, and modeling strategies that can help you analyze composite fatigue accurately and efficiently.
1. How to do Composite Fatigue Analysis?
Fatigue failure in composites (Composite Fatigue) is more complex than in metals because the materials are heterogeneous, and the mechanisms involve various forms of damage before failure. To solve composite fatigue analysis problems, we need to better understand different types of composite materials, and analyze their behavior under different loadings. Simulating composite materials in powerful software like Abaqus provides us with the advantage of analyzing their behavior at minimal cost.
First, let’s take a quick overview of fatigue phenomenon, then an overview of types of composite materials because we intend to tell you how to do a fatigue simulation in each one.
2. What is fatigue damage?
Fatigue refers to the initiation and propagation of cracks in a material due to cyclic loading. When a fatigue crack starts, it grows incrementally with each loading cycle. The crack continues to grow until it reaches a critical size. Finally, the crack propagates sufficiently within the structure, which leads to complete failure of the structure.
Figure 2: Failure of a crank arm due to fatigue
In the fatigue phenomenon, various factors affect the rate and growth of cracks, such as the average stress applied to the structure, the temperature, the environment, and the surface finish of the structure. While studying these factors provides us with a good understanding of fatigue, structural failure usually occurs suddenly due to fatigue, making it a dangerous phenomenon. Therefore, examining and analyzing fatigue in materials is of great importance. Since fatigue is a destructive phenomenon, one of the approaches to investigate structural fatigue is the Finite Element Method.
| You can find more info about what is fatigue and fatigue parameters and all info about it in this article “What is Fatigue Analysis?“ |
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2.1. Fatigue cycle number
To study fatigue, including short fiber composite fatigue, attention should be paid to the number of fatigue cycles. Fatigue is generally divided into two main categories:
- Low-cycle fatigue
- High-cycle fatigue
If the applied stress exceeds the yield stress, it results in low-cycle fatigue. On the other hand, if the stress is below the yield stress, it leads to high-cycle fatigue.
| You can find more info about what is fatigue and fatigue parameters and all info about it in this article “What is Fatigue Analysis?“ |
Did you know that a car’s bumper, despite its tough exterior, can be susceptible to fatigue over time? This is because they’re often made from short fiber composites, which are strong but can weaken under repeated stress. So, the next time you see a car enduring a bumpy road, imagine its bumper quietly undergoing a fatigue test! Speaking of fatigue test; how to do short fiber composite fatigue analysis? check out this link: “Short fiber composite fatigue analysis“.
2.2. Fatigue life
Fatigue life is another important parameter in the fatigue analysis of a structure. Fatigue life refers to the number of cycles that a specimen undergoes during testing until fatigue occurs. In the stress-cycle number diagram, there is a theoretical value where the component never experiences fatigue for stresses less than this value. This value is referred to as the fatigue limit.
Figure 3: Fatigue life curve(S-N curve)
3. Four categories of Fiber reinforced composites
Fiber reinforced composites are a type of composites that are made by combining fibers with a matrix, such as resin or polymer. The presence of fibers in the composite can greatly enhance its strength, as the tensile load is borne by the fibers, which have much higher tensile strength than the matrix. The mechanical properties of fiber-reinforced composites are strongly influenced by the type, volume fraction, length, and orientation of the fibers. Generally, fiber-reinforced composites can be classified into four categories: (More about Abaqus Composite and Composite Analysis)
Continuous aligned fiber composites
Figure 9: Continuous aligned fiber composites schematic
Discontinuous aligned fiber composites
Figure 10: Discontinuous aligned fiber composites schematic
short random fiber composites
Figure 11: Short random fiber composites schematic
Woven composites
Figure 12: Woven composites schematic
4. Short fiber reinforced composite fatigue
Generally, when damage occurs in composites, such as short fiber composite fatigue damage, these damages can be categorized into three groups:
- Fiber damage: Tensile loading in composites is borne by the fibers. When the loading exceeds the fiber’s tolerance limit, the fibers become damaged, resulting in damage to the composite.
- Matrix damage: In fiber reinforced composites, compression is usually tolerated by the matrix. When the compression exceeds the matrix’s tolerance limit, the matrix fractures and becomes damaged.
- Delamination: Delamination is one of the mechanisms of damage in composites. In this case, the layers of the composite separate, leading to damage to the composite structure.
In the short fiber reinforced composite fatigue process, similar to other composites, various factors such as fiber material, fiber volume fraction, temperature, and creep strain have an impact. However, one of the most important factors in short fiber composite fatigue is the fiber distribution within the matrix. If the fiber distribution is random, the behavior of the composite material is homogeneous and isotropic. Otherwise, the composite will be anisotropic.
Figure 13: Damage modes in composites
Composite fatigue simulation with UMAT subroutine tutorial package syllabus: Workshop 1: Composite fatigue analysis with UMAT subroutine in Shell Elements-part 1 (theory)
Workshop 1: Composite fatigue analysis with UMAT subroutine in Shell Elements-part 3 (Implement modeling in Abaqus software)
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Composite fatigue simulation with VUMAT subroutine syllabus: |
Simulation of woven composite fatigue in Abaqus tutorial syllabus: |
4.1. Short fiber composite fatigue behavior
As mentioned, the short fiber composite fatigue behavior depends on various factors, including fiber length, fiber distribution within the matrix, etc. Most studies have shown that longer fiber length leads to higher composite strength, and the strength directly affects the fatigue life of the composite. If the fiber distribution in the composite is not random, the composite is orthotropic, meaning it behaves differently from a composite with a random fiber distribution. There are various models available to simulate the short fiber composite fatigue behavior, and two of the most well-known models for short fiber composite fatigue damage are:
- The Nori damage model for short fiber composite fatigue.
- The Avanzi damage model for short fiber composite fatigue.
In the next section, we will examine the Avanzi model.
But if you need to learn these models with examples, I suggest to visit this link: “Short fiber composite fatigue“.
4.2. Discontinuous fiber composite fatigue analysis by Avanzini’s model
The Avanzini model for discontinuous fiber composite fatigue analysis has emerged from the development and modification of the Nori model. In this model, the fiber distribution is considered random, assuming a homogeneous and isotropic composite. In this model, the strain energy is calculated using equation 4:
By integrating equation 4, the thermodynamic force is obtained from equation 5:
In the above equations, γ represents the damage constant, and d is the damage variable. The damage variable d can be calculated using equation 6:
We can use the Avanzini model to analyze the short fiber composite fatigue in Abaqus software.
4.3. Short fiber composite fatigue simulation using UMAT subroutine
As mentioned earlier, with the increased use of composites, it is necessary to simulate composites under various loading conditions and working conditions, including short fiber composite fatigue. Abaqus short fiber composite fatigue simulation is complex and challenging because Abaqus does not have a default material called “short fiber composite.” Therefore, to simulate short fiber composite fatigue, the user material subroutine (UMAT) needs to be utilized. Another challenge in this simulation is fatigue cycle simulation. However, there is a solution to these challenges.
We introduce the ” Short fiber composite fatigue damage simulation in Abaqus with UMAT subroutine ” training package to you. This package provides answers to all your questions regarding simulating short fiber composite fatigue. In addition to comprehensive explanations about fatigue and composites, this package provides complete guidance on short fiber composite fatigue simulation using the Avanzini model. Furthermore, all the necessary files, including Abaqus files and the umat subroutine, will be made available to you.
5. Woven Composite Fatigue
The accurate prediction of fatigue failure in woven composites requires advanced modeling approaches. Two primary failure models used in fatigue analysis for woven composites are the Maximum Stress Fatigue Damage Model and the Modified Hashin Fatigue Damage Model. These models provide different ways of understanding and predicting how woven composites respond to cyclic loading, each with its own strengths and limitations.
5.1. Maximum Stress Fatigue Damage Model
The Maximum Stress Fatigue Damage Model is a straightforward approach to failure prediction. This model assumes that failure occurs when the material’s maximum stress surpasses its ultimate strength. Because it relies on peak stress as a failure indicator, this model is relatively easy to implement and is suitable for simpler applications. However, it does not account for the complex microstructure of woven composites, which can limit its accuracy in predicting long-term fatigue behavior under varied loading conditions.
5.2. Modified Hashin Fatigue Damage Model
In contrast, the Modified Hashin Fatigue Damage Model is a more detailed and sophisticated approach. This model considers the microstructure of the composite, evaluating different damage mechanisms within the fibers and the matrix. By incorporating fiber tension, fiber compression, and various stress interactions, the Modified Hashin damage criteria provide a more accurate representation of fatigue behavior in woven composites. This complexity allows it to offer improved precision for design optimization, helping to enhance the durability and performance of composite structures.
6. Lamina Composite Fatigue Analysis
In laminated composites, fatigue damage often occurs in the form of delamination, where the adhesive or cohesive layer between the composite plies fails under cyclic loading. Fatigue failure of the cohesive layer between composite plies is caused by repeated loading that leads to crack growth within the adhesive or interlaminar region. This results in delamination, weakening the overall structure.
Fatigue failure in materials occurs when repetitive or fluctuating stresses, below the ultimate strength and often below the yield limit, lead to sudden and unpredictable failure, making it a significant concern in engineering due to its potential for catastrophic consequences. The reinforced part of the fiber-reinforced composites can be categorized as continuous or discontinuous, with the latter referred to as short fiber-reinforced composites. In this training package “Short fiber composite fatigue“, the fatigue of short (chopped) fiber composites is explained. Two fatigue damage models are presented for short fiber composites: Nouri fatigue damage model and Avanzini fatigue damage model. The Nouri’s model is applicable for composites with orthotropic behavior. But the Avanzini’s model has considered the fiber distribution in the matrix to be homogeneous and random. It has assumed the material behavior to be isotropic. Also, Nouri’s model was developed for strain-controlled test, but Avanzini’s model was developed for stress-controlled test. In this tutorial, we use the Avanzini’s model, which is base on this article: “Fatigue behavior and cyclic damage of peek short fiber reinforced composites”. This article has implemented the USDFLD subroutine, but we use the UMAT subroutine, which is more accurate than USDFLD since the material strength and properties reduction is smooth. A standard test specimen is used in this simulation to model such behavior. You will learn the details in the package.
It would be helpful to see Abaqus Documentation to understand how it would be hard to start an Abaqus simulation without any Abaqus tutorial.
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