Beginner’s Guide to Unidirectional Composite and Composite Damage

3. Introduction to Composite Damage

Like any structure, composites take damage as well due to the manufacturing process or in-service. Composite material behavior is different compared to other common materials. It has bilinear behavior; just take a look at the stress-strain diagram in the figure below to better understand. When a composite material is subjected to a loading condition that passes the elastic area, it reaches the “damage initiation” point; if the loading condition continues, the damage will propagate, and we have the “progressive damage” area, and the damage might propagate till the composite fails. To run a correct composite analysis, it is crucial to know all composite damage types and their effects on composite materials.

Stress-strain curve for composite materials

In the FEM simulation, we must define the “damage initiation” point and the “progressive damage” area; to do so, there are different theories that come in, such as Hashin damage criteria, Tsai-Wu, Tsai-Hill, Puck, … .

3.1. Causes of Composite damage

In comparison to metals, composites’ damage mechanisms are less well known. Composite materials and structures are susceptible to defects, whether they appear during material processing, component fabrication, or in-service use. Understanding how the damage or defect affects the structural integrity of the composite component is crucial to determining how critical the defect is. If we can somehow determine these damages and tend to them in time, the maintenance cost would be severely decreased. Also, our design will be better and well-optimized.

The composite damages can be classified into two levels:

3.1.1. Composite damage during manufacturing

Anomalies caused by processing errors like porosity, microcracking, and delaminations are examples of manufacturing damage. Inadvertent edge cuts, scratches, surface gouges, impact damage, and damaged fastener holes are also included. Manufacturing flaws consist of:

• Voids
• Air bubbles
• Delamination
• Resin starved areas
• Resin rich areas
• Wrinkles
• …

Composite damage during manufacturing [Ref]

Porosity, or the existence of a void in the matrix, is the most typical one. Incorrect or ineffective curing parameters may be to blame for the porosity. Another production flaw is the inclusion of foreign bodies in the matrix, which can be anything from greasy fingerprints to a backing film.

3.1.2. In-service damage of composites

It’s true that the composite structures have sufficient strength and stiffness but like other structures, they may get damaged during the service. The damages could be caused by impact, low resistance to the service environment, overloading, Staff carelessness, etc.

In-service damages include:

• Impact damage
• Fatigue
• Matrix cracking
• Delamination
• Fiber breackage
• …

Service flaws in composite structures are typically caused by impact damages. Delamination is the most frequent impact-related damage. Delamination occurs when layers in a laminated composite are split, creating a mica-like structure with a considerable loss in mechanical properties. Delamination is the separation of the laminate at the boundary between two layers as a result of shock, impact, or repeated cyclic pressures. Individual fibers can pull away from the matrix, for instance.

1. Matrix Cracking

Matrix cracking occurs when tiny cracks form within the resin, which holds the reinforcing fibers together. These cracks typically form due to:

• Tensile stresses: Forces that stretch the composite perpendicular to the fibers.
• Temperature fluctuations: Sudden changes in temperature can cause the resin and fibers to expand or contract at different rates, leading to cracks.
• Impacts: External hits or blows can also cause cracks in the matrix.

Matrix cracking compromises the integrity of the composite by weakening the resin and reducing its ability to transfer stress between the fibers.

2. Delamination

Delamination refers to the separation of layers (plies) in laminated composites. This can be triggered by:

• Out-of-plane stresses: Forces acting perpendicular to the composite layers.
• Impacts: Strikes can cause layers to peel apart.
• Matrix cracks: Cracks in the resin matrix can spread, leading to layer separation.
• Manufacturing defects: Poor bonding between layers during production can make the composite more prone to delamination.

When delamination occurs, the load-bearing capacity of the composite significantly drops, making it a critical failure mode.

3. Fiber Breakage

Fiber breakage happens when the reinforcing fibers inside the composite fracture due to excessive stress. This typically results from:

• High tensile loads: Stretching forces that exceed the strength of the fibers.
• Impacts or overloading: Forces beyond what the material is designed to handle.

Once the fibers begin to break, the composite’s ability to bear loads is significantly reduced, leading to potential structural failure.

4. Fiber Pull-Out

Fiber pull-out occurs when the fibers are pulled from the resin matrix rather than breaking. This can happen if the bond between the fibers and the matrix is weak, which may result from:

• Manufacturing defects: Inadequate bonding between fibers and resin during production.
• Mechanical stress: Load-induced stress that weakens the fiber-matrix bond.

Although fiber pull-out can dissipate some energy during an impact, it also signals the material’s inability to effectively transfer loads, leading to failure.

5. Interfacial Debonding

Interfacial debonding refers to the loss of adhesion between the fibers and the resin. This can be caused by:

• Mechanical loads: Forces that weaken the bond between fibers and matrix.
• Thermal stresses: Temperature changes can lead to differences in expansion between the fibers and resin, causing debonding.
• Environmental factors: Exposure to moisture or chemicals can also weaken the bond.

When debonding occurs, the composite loses its ability to transfer stress effectively, leading to a reduction in its mechanical performance.

6. Porosity

Porosity refers to the presence of air pockets or voids within the composite material. These voids often result from manufacturing defects and can:

• Weaken the overall structure.
• Increase susceptibility to other types of damage, such as cracking or delamination.

7. Moisture Absorption

Composites can absorb moisture over time, which can lead to:

• Swelling and warping: Changes in the material’s dimensions.
• Loss of strength: Water ingress can weaken both the resin and the fibers, compromising the material’s load-bearing capacity.

8. Chemical Exposure

Certain chemicals can degrade both the resin and fibers in composites. This degradation can lead to:

• Loss of strength: Reduced ability to bear loads.
• Cracking: Chemical reactions that weaken the matrix or fibers.

9. UV Radiation

Ultraviolet (UV) radiation from sunlight can break down the resin in composites over time, leading to:

• Cracking: Surface cracks due to resin degradation.
• Loss of mechanical strength: Weakened structural integrity.

Composite Damage types [Ref]

Analysis of composites and determining the defects and damages so that the structure can keep working properly and lower the cost of maintenance is the engineer’s duty. With the help of Computer-aided engineering (CAE) and the finite element method (FEM), the composite analysis is done in the best way possible. Composite simulation is a special skill that can be achieved by our training packages.

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3.2. Factors influencing damage in composite materials

The damage susceptibility and severity in composite materials can be influenced by a wide range of factors, broadly categorized into:

Material factors:

• Fiber type and properties: Different fibers (e.g., carbon, glass, aramid) have varying strengths, stiffnesses, and fracture toughness, impacting damage resistance.
• Matrix type and properties: Resins differ in toughness, adhesion to fibers, and environmental resistance, influencing damage initiation and propagation.
• Fiber-matrix interface: Strong bonding between fibers and resin is crucial for stress transfer and resisting delamination.
• Microstructure and defects: Imperfections like voids, porosity, and resin-rich areas act as stress concentrators, promoting damage initiation.

Manufacturing factors:

• Processing parameters: Cure temperature, pressure, and time greatly influence fiber alignment, void content, and matrix properties, affecting damage resistance.
• Surface quality: Poor surface finish can act as initiation sites for cracks and delamination.
• Presence of impurities: Contaminants can weaken the bond between fibers and matrix, promoting debonding and matrix cracking.

Loading and environmental factors:

• Type of loading: Static, dynamic, fatigue, and impact loads affect damage mechanisms and severity differently.
• Magnitude and duration of loading: Higher loads and longer durations increase the risk of damage initiation and propagation.
• Temperature: Elevated temperatures can soften the matrix, weaken the bond, and accelerate damage mechanisms.
• Moisture absorption: Water ingress can plasticize the matrix, reduce strength, and trigger internal stresses leading to damage.
• Chemical exposure: Exposure to aggressive chemicals can degrade the matrix or fibers, causing localized damage and strength reduction.

Design factors:

• Geometry and thickness: Sharp corners, thin sections, and complex geometries concentrate stress, increasing damage susceptibility.
• Fiber orientation: The arrangement of fibers within the composite influences the direction and resistance to specific types of damage.
• Presence of holes and fasteners: Stress concentrations around holes and fasteners can promote damage initiation and propagation.

Understanding these factors and their interactions is crucial in selecting appropriate composite materials, optimizing manufacturing processes, and designing structures with good damage resistance for specific applications. Additionally, ongoing research explores innovative materials and design solutions to further enhance the damage tolerance of composite materials.

Now, it is time to say how to do a Composite Damage modeling in Abaqus. Here our focus would be on unidirectional composites.

4. How to do Composite Damage Simulation in Abaqus

When you want to do composite damage modeling in Abaqus, I think the real challenge lies in the material modeling and damage modeling; the geometry could be difficult sometimes but the real deal is in these two. So, let’s see how to model a single-layer unidirectional composite and laminate in Abaqus.

In Abaqus software, there are several ways to simulate composite materials. Here, we introduce the best one.

You can simulate composite materials using one of the three elements Solid, Continuum Shell, or Shell. Remember that unidirectional fiber composites are typically assumed to be orthotropic.

Example Table you could use. click on them to have access to their tutorial.

4.2. Composite Elastic Properties

First, you need to define the Elastic properties of the composite. When you do that, you need to define the type of elastic properties, Engineering Constants, Lamina, and Orthotropic.

In the table below you’ll see the difference between them.

4.3. Composite Layup Tool

After that, you need to use the Composite layup tool “”. When you use this tool depending on your element type (Shell, Continuum shell, or solid) you need to choose one of three options you see in the picture below.

Composite layup element selection

If you select Solid or Continuum shell, the settings would be as follows, which I explain to you in an example:

Imagine we have a 4-layer composite laminate like this [90^o, 30^o, 40^o, 45^o]. we have 2 elements in the thickness as depicted below and the thickness of the composite is 6 millimeters.

Composite laminate orientation

Now, we need to set these layers in the Edit Composite Layup window as depicted below. In element number 1 the thickness of lamina 90 and 30 are 1 and 2 millimeters, respectively; so, the whole thickness of element number 1 is 3 millimeters and therefore, the element relative thickness of each layer in this element equals 1/3 and 2/3, respectively. Same logic for other layers.

Edit Composite layup window for Solid elements

After that, you need to assign the material orientation, and the composite material modeling is completed. See it in an example: Abaqus composite modeling.

Now, to apply the damage, first you need to know which criteria you want to use. Here is the list of some damage criteria in Abaqus:

• Johnson-Cook damage model
• Tsai-Hill failure criterion
• Tsai-Wu failure criterion
• Puck criterion
• …

In the Abaqus simulation, regardless of which criterion we choose, we must define the “damage initiation” point and the “progressive damage” area. Next, we need to define the analysis step, boundary conditions, mesh, and start the job. 

Don’t worry, we have tutorial packages to help you out how exactly you should procced in these simulations:

5. Conclusion 

This article focused on unidirectional composites, materials where fibers are aligned in one direction, and the types of damage they can sustain. Understanding these materials is crucial due to their widespread use in industries like aerospace and automotive, where their strength-to-weight ratio is highly valued but their susceptibility to damage poses challenges.

The article began by discussing the properties of unidirectional composites, emphasizing their high stiffness along the fiber direction and their vulnerability in other directions. It then covered different types of composite damage, including matrix cracking, delamination, and fiber breakage, which can occur during manufacturing or in-service. The importance of simulating composite behavior in software like Abaqus was highlighted, with an explanation of how elastic properties and damage initiation criteria are defined to predict damage accurately.

In conclusion, this article provided an overview of unidirectional composite materials, their key properties, common damage types, and how engineers can model and manage damage using tools like Abaqus. Understanding these factors helps improve design and maintenance strategies for these essential materials.

Now, if you need to know how to model damage in composite materials, especially in unidirectional and laminate composites, you should see it in action, which I recommend our complete tutorial. Trust me you won’t regret it.

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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