Defects and damage in composite materials and structures can be classified into different categories based on their origin, nature, and location. Here are some of the common types:

Manufacturing Defects

These defects occur during the manufacturing process and can be attributed to various factors, such as improper curing, inadequate tooling, or contamination. Some common manufacturing defects include ⁴:

• Delamination: Separation of adjacent layers of composite material, often caused by improper bonding or resin flow.
• Resin-starved areas: Regions with insufficient resin, resulting in a dry and brittle structure with reduced strength and stiffness.
• Resin-rich areas: Excess resin accumulation, leading to an imbalance in the fiber-to-resin ratio and potentially affecting mechanical properties.
• Blisters and air bubbles: Trapped air or volatiles within the composite, creating voids that can weaken the material and promote crack propagation.
• Wrinkles: Distortion or unevenness in the composite layers, caused by improper layup or handling during manufacturing.
• Voids: Empty spaces within the composite, reducing its density and overall strength.
• Thermal decomposition: Degradation of the resin due to excessive heat during curing, leading to reduced mechanical properties and potential delamination.

In-service Damage

This type of damage occurs during the operational life of the composite structure and can be caused by various factors, such as impact, fatigue, or environmental degradation. Some common in-service damage includes ⁴:

• Environmental degradation: Deterioration of the composite due to exposure to moisture, UV radiation, or chemicals. Moisture absorption can cause plasticization of the matrix, reducing its glass transition temperature and leading to dimensional changes and reduced mechanical properties. UV radiation can cause photodegradation of the polymer matrix, resulting in surface embrittlement and erosion. Chemical attack from fluids like hydraulic fluid or fuel can also degrade matrix properties.
• Impact damage: Localized damage caused by collisions or impacts, often resulting in delamination, matrix cracking, and fiber breakage.
• Fatigue: Accumulation of microscopic damage under cyclic loading. This damage typically progresses from matrix cracking to crack coupling, delamination, and finally, fiber breakage. Fatigue damage is particularly critical in areas of stress concentration, such as holes or ply drops.
• Cracks: Formation of cracks due to stress concentrations or overload. Cracks can propagate through the matrix, fibers, or along the interface between them.
• Debonding: Separation of the fiber from the matrix, weakening the load transfer between these constituents.
• Fiber fracturing: Breakage of fibers due to excessive stress, significantly reducing the tensile strength of the composite. Fiber breakage is considered the most critical type of damage in composites ⁵.
• Erosion: Gradual wearing away of the composite surface due to environmental factors like rain, sand, or hail.
• Lightning Strike Damage: Aircrafts are vulnerable to lightning strikes, which can cause both direct and indirect effects on composite structures ². Direct effects include fiber vaporization and resin decomposition, while indirect effects include electromagnetic interference and induced currents.

Classification of Defects by Stress State

Defects in composites can be categorized based on their effect on the stress state of the material ⁵:

• Intralaminar Matrix Cracks: Cracks within a single ply, parallel to the fibers. These cracks reduce stiffness and provide paths for moisture ingress.

• Interlaminar Matrix Cracks (Delaminations): Separation between adjacent plies. Delaminations severely reduce compressive strength and can lead to buckling or catastrophic failure.

• Fiber Fracture: Broken fibers, often due to impact or overload. Fiber fracture significantly reduces tensile strength and can compromise the overall structural integrity.
• Design Variance: Deviations from the intended design, such as misaligned plies or an incorrect stacking sequence. The effect of design variance depends on the specific deviation and often requires case-by-case analysis.

Microscopic Failure Modes

At the microscopic level, composite materials exhibit four primary failure modes ⁵:

1. Fiber Failure: This can occur in tension (brittle fracture) or compression (microbuckling or kinking).
2. Matrix Failure: Tensile matrix failure involves cracking perpendicular to the load, while compressive matrix failure involves shear banding.
3. Fiber-Matrix Interface Failure: This involves debonding, or the separation of the fiber from the matrix.
4. Delamination: Separation of adjacent plies, driven by interlaminar stresses.

Failure Mechanisms

Different failure mechanisms play important roles during the service lives of composite materials ⁶. These mechanisms include:

• Fracturing of the reinforcement: This can involve fiber breakage or the weakening of fibers due to fatigue or environmental degradation.
• Partial detachment of the interface: This refers to debonding between the fiber and matrix, which can reduce load transfer and promote crack growth.
• Nucleation and growth of voids: Voids can form during manufacturing or in service due to impact, fatigue, or environmental degradation. These voids can act as stress concentrators and accelerate failure.
• Coalescence of voids in the matrix: As voids grow and coalesce, they can form larger cracks that compromise the structural integrity of the composite.

Damage criteria are required to indicate the onset of failure and damage orientation ⁶. These criteria can be based on stress, strain, energy, or other relevant parameters.

Various methods and techniques are employed to identify defects and damage in composite materials and structures. These methods can be broadly classified into destructive and non-destructive techniques. Destructive techniques involve damaging the material to assess its properties, while non-destructive techniques (NDT) allow for inspection without causing any damage. NDT methods are widely used in the industry due to their ability to preserve the integrity of the component.

Surface Inspection Methods

These methods are primarily used to detect surface and near-surface defects in composite materials.

• Visual Inspection: Visual inspection is the most basic and accessible NDT method ⁵. It involves examining the composite structure for any visible signs of defects or damage, such as cracks, delaminations, or discoloration. Visual inspection can be enhanced with the use of magnifying glasses, borescopes, or other visual aids.

‍Advantages:

• Simple and cost-effective.
• Can be performed quickly.
• No specialized equipment required.

Disadvantages:

• Limited to surface defects.
• Subjective and dependent on the inspector's experience.
• May not detect small or hidden defects.

• Thermography: Thermography is an NDT method that uses infrared radiation to detect subsurface defects in composites ⁵. An infrared camera captures the thermal patterns on the surface of the material, and any variations in temperature can indicate the presence of defects. Thermography is particularly useful for detecting delaminations and disbonds.

‍Advantages:

• Can inspect large areas quickly.
• Non-contact method.
• Can detect subsurface defects.

Disadvantages:

• Limited penetration depth.
• Affected by surface emissivity variations.
• Sensitive to environmental conditions.

Internal Inspection Methods

These methods are used to detect defects within the composite material, including those that are not visible on the surface.

• Ultrasonic Testing: Ultrasonic testing (UT) utilizes high-frequency sound waves to detect internal defects in composites ⁵. A transducer sends sound waves into the material, and the reflected waves are analyzed to identify any anomalies. UT can detect various defects, including delaminations, voids, and inclusions.

‍Advantages:

• Can detect internal defects.
• Provides quantitative data on defect size and location.
• Can be used on complex geometries.

Disadvantages:

• Requires a coupling medium (e.g., water or gel).
• Challenged by complex geometries and rough surfaces.
• May require skilled operators for interpretation.

• Radiography: Radiography uses X-rays or gamma rays to penetrate the composite material and create an image of its internal structure ⁵. This method can detect defects such as voids, inclusions, and fiber misalignment.

‍Advantages:

• Can detect a wide range of defects.
• Provides a permanent record of the inspection.

Disadvantages:

• Radiation safety concerns.
• Limited ability to detect low-density defects.
• May require access to both sides of the component.

Other NDT Techniques

In addition to the methods mentioned above, several other NDT techniques are used for defect and damage identification in composites. These include:

• Acoustic Emission (AE): AE is a passive NDT technique that detects stress waves generated by growing defects in a composite material ⁵. Sensors placed on the surface of the structure capture these waves, and their analysis can help locate and characterize the damage.

‍Advantages:

• Can detect active damage.
• Can monitor large structures.

Disadvantages:

• Requires loading of the structure.
• Complex signal interpretation.
• Sensitive to noise.

• Shearography: Measures surface deformation under stress to detect disbonds and delaminations ⁵.

‍Advantages:

• Full-field inspection.
• High sensitivity to delaminations.

Disadvantages:

• Requires stressing of the part.
• Sensitive to environmental vibrations.

• Eddy current testing: Detects fiber orientation and breakage in carbon fiber composites ⁵.‍

Advantages:

• Sensitive to surface and near-surface defects.
• Can be used on complex shapes.

Disadvantages:

• Limited to conductive fiber composites.
• Affected by lift-off variations.

• Microwave NDT: Detects moisture ingress and disbonds ⁵.‍

Advantages:

• Can detect moisture and other dielectric variations.
• Non-contact method.

Disadvantages:

• Limited penetration depth.
• Affected by material variations.

• Finite Element Method (FEM): FEM is a computational technique used to simulate and analyze the behavior of composite structures under different conditions ⁶. It can be used to predict stress and strain distributions, identify potential failure locations, and assess the impact of defects on structural integrity.‍

Advantages:

• Can model complex geometries and loading conditions.
• Provides detailed information on stress and strain fields.
• Can be used for design optimization and damage prediction.

Disadvantages:

• Requires accurate material properties and boundary conditions.
• Can be computationally expensive for large models.

The selection of the appropriate NDT method depends on various factors, such as the type of defect, material system, component geometry, and inspection requirements. The following table provides a general guideline for selecting suitable NDT methods based on common defect types:

‍

Several case studies and real-world applications demonstrate the effectiveness of damage identification methods in composite materials and structures. Here are a few examples:

• AH-1W Helicopter Engine Inspection Pedal: Researchers used infrared thermography and 3D analysis to detect unknown defects in the engine inspection pedal of an AH-1W helicopter ⁷. The analysis helped identify the defect's location, size, and severity, allowing for efficient repair planning.
• Aircraft Wing Panel: Researchers employed a reconstruction algorithm for the probabilistic inspection of defects (RAPID) to locate damage in an aircraft wing panel ². The results showed that RAPID could accurately detect and locate the damage.
• Ceramic Plates: Deep learning models based on acoustic noise data were used to detect cracks in ceramic plates ⁸. The models were trained on noise curves generated by impacting the plates and showed promising results in crack detection.

These examples highlight the importance of damage identification methods in ensuring the safety and reliability of composite structures in various applications.

When composite materials components are damaged or defective, engineers and technicians need to determine the size, shape, depth, type, and extent of the anomaly and the appropriate restitution approach ⁴. A typical repair procedure involves the following steps:

1. Locate the damaged area.
2. Assess the extent of damage.
3. Evaluate the stress state of the damaged area.
4. Design the repair scheme. This involves selecting the appropriate repair method, materials, and procedures based on the type and severity of the damage, the structural requirements, and the operational environment.
5. Remove damage and repair structure. This may involve removing damaged material, preparing the surface for repair, and applying reinforcement patches or fillers.
6. Fabricate and prepare the repair scheme.
7. Apply the repair scheme.
8. Conduct post-repair quality checks. This includes inspecting the repaired area for proper bonding, surface finish, and dimensional accuracy. NDT methods may be used to verify the effectiveness of the repair.
9. Document repair procedures.
10. Monitor the repair region. This involves periodic inspections to ensure the long-term durability of the repair.

The selection of the repair scheme is influenced by several factors ³, including:

• Repair facility capability: The availability of equipment, tools, and skilled personnel.
• Types of damage: The specific type and extent of damage.
• On- or off-structure repair: Whether the repair can be performed on the aircraft or requires removal of the damaged component.
• Damaged component accessibility: The ease of access to the damaged area.
• Repair criteria: The structural and operational requirements for the repaired component.

The development of generic repair types in composite structures describes repairs for generalized defect/damage types, such as intralaminar matrix cracks, delaminations, and broken fibers ³. These generic repair types provide standardized procedures for common damage scenarios, simplifying the repair process and ensuring consistency.

Defects in composite materials can have a profound impact on the structural integrity of the component or structure ³. The severity of a defect is determined by the stress state it imposes on the local composite structure. Several factors influence the impact of defects:

• Location and orientation: Specific locations can either amplify or mitigate the stress severity caused by a defect. Similarly, the physical orientation of a defect can make it either severe or benign.
• Frequency of occurrence: Multiple occurrences of the same defect can compound the stress on the structure and increase the risk of failure.
• Component's load path criticality and stress state: The interaction between the defect's location and orientation and the component's load path and stress state can either increase or decrease the defect's criticality.
• Ability to detect and classify: Proper detection and classification are crucial for identifying defect criticality and determining appropriate repair actions.

The field of defect and damage identification in composite materials and structures is constantly evolving. Some of the future trends include:

• In-situ Structural Health Monitoring (SHM): Integrating sensors within the composite structure to continuously monitor its condition and detect damage in real-time ⁵. This allows for early detection of damage, potentially preventing catastrophic failures and enabling proactive maintenance.
• Machine Learning and AI: Utilizing machine learning algorithms to automate defect detection and classification, improving accuracy and efficiency ⁵. Machine learning can analyze large datasets from NDT methods to identify patterns and anomalies, potentially leading to more reliable and objective damage assessment.
• Advanced Signal and Image Processing: Developing sophisticated techniques to analyze data from NDT methods and enhance defect characterization ⁵. This includes techniques like 3D visualization of internal structure and improved signal processing algorithms to extract more information from NDT data.
• Nonlinear Ultrasonic Techniques: Employing nonlinear ultrasonic methods to detect micro-damage and early signs of fatigue ⁵. These techniques are more sensitive to subtle changes in material properties and can potentially detect damage before it becomes visible with conventional NDT methods.

Despite the advancements, some challenges remain in the field:

• Complex Material Behavior: Composites exhibit complex behavior due to their anisotropic nature, making it challenging to interpret NDT data accurately. The interaction between different failure modes and the influence of fiber orientation and stacking sequence can complicate damage assessment.
• Environmental Factors: Environmental conditions, such as temperature and humidity, can affect the performance of NDT methods and influence the results. Temperature variations can affect the thermal properties of composites, influencing thermography results. Moisture can affect the propagation of ultrasonic waves, potentially leading to inaccurate readings.
• Cost and Accessibility: Some advanced NDT techniques can be expensive and require specialized equipment and expertise, limiting their accessibility. This can be a barrier to wider adoption of these techniques, particularly for smaller companies or those with limited resources.

Identifying defects and damage in composite materials and structures is crucial for ensuring their structural integrity, performance, and safety. Various NDT methods, such as visual inspection, ultrasonic testing, and thermography, are employed to detect and characterize these anomalies. The selection of the appropriate method depends on the specific application, defect type, and inspection requirements. Case studies and real-world examples demonstrate the effectiveness of these methods in various industries, from aerospace to automotive.

Future trends in the field include in-situ SHM, machine learning, and advanced signal processing. These advancements promise to enhance the efficiency, accuracy, and reliability of damage identification in composites. However, challenges remain in accurately interpreting NDT data, addressing the influence of environmental factors, and ensuring the cost-effectiveness and accessibility of advanced techniques.

Continued research and development in this area are essential to improve the reliability and safety of composite structures. This includes developing new NDT methods, improving existing techniques, and integrating advanced technologies like machine learning and SHM. By addressing these challenges and embracing new innovations, we can ensure the continued success and widespread adoption of composite materials in various applications.

To reduce the occurrence of defects and damages in composite materials and structures, virtual production software like AddPath can be used to identify potential faults and errors before the manufacturing process ⁴. This software allows engineers to simulate the manufacturing process and optimize parameters to minimize the risk of defects.

References

Heslehurst, R. B. (2014). Defects and Damage in Composite Materials and Structures. CRC Press. Taylor & Francis Group. 6000 Broken Sound Parkway NW, Suite 300. Boca Raton, FL 33487-2742. © 2014 by Taylor & Francis Group, LLC. CRC Press is an imprint of Taylor & Francis Group, an Informa business. No claim to original U.S. Government works. Version Date: 20140114. International Standard Book Number-13: 978-1-4665-8048-0 (eBook - PDF) ⁹

Works cited

1. Defects and Damage in Composite Materials and Structures - 1st Edition - Routledge, accessed on March 6, 2025, https://www.routledge.com/Defects-and-Damage-in-Composite-Materials-and-Structures/Heslehurst/p/book/9781138073692
2. Common Type of Damages in Composites and Their Inspections, accessed on March 6, 2025, https://www.scirp.org/journal/paperinformation?paperid=74281
3. Understanding Defects and Damage in Composite Materials and Structures - Addcomposite, accessed on March 6, 2025, https://www.addcomposites.com/post/understanding-defects-and-damage-in-composite-materials-and-structures
4. How to Identify the Defects and Damages in Composite Materials and Structures?, accessed on March 6, 2025, https://www.wevolver.com/article/how-to-identify-the-defects-and-damages-in-composite-materials-and-structures
5. How to Identify the Defects and Damage in Composites Materials ..., accessed on March 6, 2025, https://www.addcomposites.com/post/defects-and-damage-in-composite-materials-and-structures
6. Application of the Finite Element Method in the Analysis of Composite Materials: A Review, accessed on March 6, 2025, https://www.mdpi.com/2073-4360/12/4/818
7. Detection and Analysis of Aircraft Composite Material Structures Using UAV - MDPI, accessed on March 6, 2025, https://www.mdpi.com/2411-5134/9/3/47
8. Damage detection of composite laminates based on deep learnings - Frontiers, accessed on March 6, 2025, https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2024.1456236/full
9. Defects and Damage in Composite Materials and Structures, accessed on March 6, 2025, https://structures.dhu.edu.cn/_upload/article/files/f6/62/f5c6159f4c86ae7a86fbd6b48811/de2f3e84-c3f9-42d4-833d-21279796173a.pdf